The Air Side of Air Conditioning- Checking Static Pressure & CFM

  - with Darrell Udelhoven HVAC RETIRED 

- udarrell Darrell Udelhoven >Darrell Cynergy Home Energy Raters Public Radio Program - Listen

Free, for a while: whole house load-calc
Please Respond on my Blog
; let me know if you can't post: Ballpark Checking A/C Performance udarrell @ pcii. net
Static pressure is an indication of the operating condition of an HVAC system. Just as a doctor or nurse measures blood pressure to begin the examination, so should service techs, and duct renovation specialists measure static pressure "to begin their diagnostics procedures." The entire duct system must be leak sealed before final checking of duct Static Pressures. SIMPLE BTUH AIRFLOW MATH  |  AIR FLOW FORMULAS Figuring Delivered BTUH  SIMPLE BTUH AIRFLOW MATH | Formula for finding CFM Airflow

More authenticated survey test corroboration; the surveyed HVAC systems were systems that they had complied with their program’s energy standards and "all had received substantial incentive payments," but delivered an average of only 63% of their Rated Btu/Hr to the homes & 50% of the required airflow. http://bit.ly/1adKKG6 
70% of homes in California are operating at 50% capacity. - California Energy Commission
http://www.law.cornell.edu/uscode/text/47/396 (8) public television and radio stations and public telecommunications services constitute valuable local community resources for utilizing electronic media to address national concerns and solve local problems through community programs and outreach programs...
Example of typical furnace over-sizing; my former oil furnace & the other house here on the farm both had 140,000-Btu/hr input oil furnaces installed when new they were 80% efficient; 140,000 * .80% is 112,000 * an 85% nozzle is 95,200-Btuh.
The homes both load-calc Design at -15°F between 26,000 to 32,000-Btuh; therefore a 38,000- Btuh will easily handle the heat-loss at any extreme temperature. 95,200-Btuh / 38,000-Btuh is +2.5 times the proper Btuh sized furnace. 38000 * 2.5 is 95,000; left the fractions off the 2.5...

AIRFLOW DIAGNOSTICS and REPAIR PDF  Excellent & thorough

Why Look at your Ducts (Leakage up-to 33% of cooling cost) when replacing your AC System; a must viewing:

http://www.youtube.com/watch?v=IV0Rwv5gco4
More authenticated corroboration of the surveys that indicated HVAC systems delivered only 63% of its Rated Btu/Hr to the home & 50% of the required airflow. - We can fix these problems!
70% of homes in California are operating at 50% capacity. - California Energy Commission
The website of a friend in the desert area of California checks the delivered performance of air conditioning systems & confirms the findings above. http://www.perfectweather.us/

Local Contractor Locater Map - Talk with These Forum Contractor PROs in your area
Room Air Pressure Differentials in Pascals & IWC |Sizing Units' Tonnage to enough Airflow |Selecting Correct Sized Diffusers |*Video checking static ESP*Video 2 checking static ESP View!Got DSL 11/2010
*Video measuring airflow Velocity - Vane anemometer on a Return Air grille
Depending on the filter used; I'd use between to 50% & 55% for the free-air--area (sf) (Ak) of a clean FILTER, & 85 to 90% factor for an open grille area.
He programmed it in, because when I did the math using .90% for the grille I got 336.57863-FPM Vel *X 2.376562-SF free-air-area= 799.9-CFM, or 2-Ton of air flow.

A New Brand of Comfort and Efficiency Excellent linked articles by 'Doc'
Jan 2, 2013 1:33 PM, By Rob 'Doc' Falke

*Selecting Air Filters - Sizing the Filtering Area - Critically important to air flow!
CFM & FPM Velocity Sizing MERV 8 Pleaded Filters 
1” deep M-8 filter’s rated at 175 FPM medium and 350 FPM high; A 16X20 1" MERV 8, Rated @ only 780-CFM
Comparative Tests concerning Pressure Drops of HVAC Filters - Important Criteria!
At 300-fpm, the 4" deep Merv 7 has the same pressure drop at .10 of an inch, as the 1" fiber glass filter; all filter areas were 24X24.
 

*Basics Featuring the Testo 556 -*Video of a very thorough Air Conditioning BTUH performance test

COMFORT INSTITUTE Consumer Protection Division Found 04/14/12 What I've been saying for years
Do a Search for a Comfort Institute HVAC Contractor Member in your zip code area
Home Energy Audits - Rochester, NY Area Real Energy Cost Savings - Ted Kidd

A lot more Videos at bottom of this page |* HEAT PUMP DIAGNOSTICS

company logo is a triskelMO Home Energy Audits LLC

HVAC TALK HVAC Talk Community Ask the Experts FREE Forum - Save Money!

HVAC-TALK GENERAL DISCUSSIONS - OPEN FORUM
Importance of Two-Way Communications with HVAC Customers

ENERGY EFFICIENCY AUDITOR TALK FORUM - This is the future of the HVAC field of work

I found this new Video confirmation today of my mass communication ideas, 03/05/12; VIEW ALL of this New VIDEO:
THE CONVERGENCE OF COMMUNICATIONS & ENERGY a world changing video about everyone's future, by Jeremy Rifkin View this Video, it opens a future of unlimited economic opportunities...CFM Formula Per Room You should measure static pressure every time you service a unit?
 Designing Duct Systems Solving for ASP | Filter Rack SizingOpportunityMeasuring Static Pressure - Profits | Photos Diagrams | 50% Load -Home Air Leaks |#Evacuation Procedures Refrigerant piping System Evacuation Procedures - Pulling a Deep Vacuum pdf - Oil Furnace Airflow Problems Air Flow fixes
Heating Cooling Systems must be tested to measure real time BTU Capacity. It is common to find a 3 ton, 13 SEER, Heat & Cool System only producing 2 tons and 8 SEER. New installations are found with these problems, & even worse conditions.

Always begin with a thorough Home Energy AUDIT that shows all the options for lowering the heat-gain & heat-loss, then after reducing air infiltration, etc., have a room by room manual J heatload calc performed. A reduction in equipment sizing will usually greatly improve the duct system performance.

This is where the greatest savings in both heating & cooling will accrue; this will help in the down-sizing of equipment.     

"The 'proper heatload' on the evaporator coil must be established"
Check and thoroughly seal all the ductwork! For efficient operation, always check the return air temperature at the blower & at the Return Air Grille(s) to know whether it is drawing hot air from the attic or garage areas.

What I stated above, that ought to be done is far more important than SEER rating; as the above will determine the SEER achieved & the energy savings.
Then do a manual J room by room heat-gain calc with the option shown so you can do everything possible to reduce all sources of heat-gain & heat-loss, greatly reducing both heat & cooling BTUH equipment sizing.

Before you do anything else, educate yourself enough to "ensure that you request the proper things be done in the proper order of sequence." Checking ductwork & Airflow Checking Static Pressures  is Critically Important. As is knowing the operating feet per minute (FPM) velocity, the CFM & BTUH to each room along with the Total CFM airflow & BTUH.
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Simple easy anyone can do ways to check the performance of your central air conditioner so, if needed, you can call an Energy Efficiency HVAC Technician

If U want me to run a ballpark analysis of how your system is performing in respect to its 'Nominal Rated Btuh' we need at least the following numbers:

Performance Data Collection – Best Time to collect data is Late afternoon around 4:30 pm, when attic is HOT; also when outdoor temps are around 85 to 95-F, or above.

 *All U need is a good thermometer (digital reading in tenths preferable) & and indoor Humidity Gauge

1) Helpful; Tonnage & SEER of Unit & outdoor condenser model number: __________________

2) TXV or, orifice metering device? _______. Only if U know…

3) Outdoor condenser’s discharge-air-temperature ______-F

Subtract Outdoor air temperature: _______ F

Outdoor Condenser Air-Temp-Split _______F

4) Need the ‘Indoor’ percent of relative humidity - away from Supply-Air outlets ______

5) Indoor Return-Air Temperature ______ F

Subtract Indoor Supply-Air Temperature ______ -F

 Indoor temperature-split _______-F 

Need the above information for troubleshooting & performance analysis.

Example below:

A Goodman 2-Ton 13-SEER condenser, 800-cfm indoor airflow; 80-F indoor dry bulb & 50% relative humidity; Indoor temp-split 18 to 19-F.

@ 85-F outdoors; 103.9-F - 85-F outdoors or around an 18.9-F temp-split; 

@ Indoor 75-F & 50% RH condenser temp-split is only around 14.9-F.

In summer an all electric farm home TWO Half-Ton Window A/Cs & basement large dehumidifier:

June 30, 2012; Darrell’s electric meter; 62610 – 62140= used 470-KWh * .0985= $46.29
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In a recent survey conducted at the National Balancing Institute, less than 6% of the companies surveyed said they measured static pressures on a regular basis. In fact, many companies make the mistake of measuring static pressure only as a last resort when they're in trouble and have exhausted all other traditional testing.

Establishing a proper airflow should always be the first diagnosis that a tech does, because nothing will work properly nor can the system be charged properly, until an adequate airflow and heatload passes through clean evaporator coils and fins for absorption of heat into the evaporating refrigerant.


First, the piping should be checked for proper oil return to the compressor, if not adequate, do that before proceeding. Also, too many do not properly purge & evacuate contaminated central air conditioning systems.|

Download these Energy Saving PDF Graphics It will provide ways to cut your monthly Energy Bills, hopefully in HALF. Thanks TEDKIDD

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A Major Oil Furnace Airflow Problem Fix
Regal & Hallmark & nearly all Oil Furnaces - Installation manuals
http://www.boyertownfurnace.com/ProductDocuments/index.aspx
Download the installation & service manuals from ABOVE LINK BELOW LINK MAY NOT WORK!
http://www.boyertownfurnace.com/ProductDocuments/HallmarkONLYManual042909.pdf 
To find the information below; Use within the pdf search:  at least 6” above
Or use down arrow to P-8 & scroll down a-ways...

"If the oil furnace is used in connection with summer air conditioning the evaporator coil must be installed at least 6” above the oil furnace for proper airflow. Distances less than 6” will result in decreased airflow." 
Make sure outlet supply takeoffs are NOT blocked by the coil. In all cases, refer to the manufacturers’ data for static pressure losses to ensure the total system static pressure does not exceed 0.5” WC.

Dons 1.5 Ton Heil AC Data Chart The Condenser is ejecting HALF the heat of its Nominal Rating; evap-coil way under heat loaded!

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Home owners:

To ballpark efficient performance, you must know the Cubic Foot per Minute (CFM) airflow of your central system.

To ballpark CFM, I'd use an anemometer & measure the velocity out of those ducts.
If using cheap MPH only anemometer, as you know - multiply by 88 to get FPM.


Use the formula: register face velocity in FPM *X's area in sq.ft

6" rd metal duct 6*6=36*.7854=28.2744-sq.ins./144= 0.19635-sq.ft.
7" rd duct 7*7=49*.7854=38.4846/144= 0.2672541-sq.ft., etc.


Add together the number of branch runs to ballpark the actual CFM delivered to the rooms.
Air conditioners  & heat pumps need 400 to 450-CFM of airflow per-ton of cooling.

Importance of figuring static pressure & Amp draw changes in respect to CFM air flow changes:

  • SP2 (Static Pressure new) = {CFM-new / CFM-old}2 *X old SP
SP2= 1.5-Ton {600-CFM / 475-CFM|2 = 1.26157895 or 1.595567867 * .50" old SP= .80"SP new; too high
SP2= 4-Ton  {1600-CFM / 1350-CFM= 1.85185 * 1.85185= 1.4046634 *.55" old SP= 0.80" new SP2; too high
SP2= 5-Ton A/C {2000-CFM / 1650-CFM}2 = 1.21212 * 1.21212= 1.469237833 * old SP  .55" = 0.80" new SP;  too high.
It is always better to add SA branch runs & more RA area, plus "a lot more RA filter area."
Oil furnaces, ALWAYS RAISE the E-COIL,where possible, 6 to 8 or 10" above the Oil furnace!

OL 11 112 model Thermo Pride Oil furnace with E-Coil directly on top of furnace
+ long duct runs; HEATING 1000-CFM.
SP2= {1000 / 700}2
Above Oil furnace: SP2 ={rpm2 1000/700rpm 1}2= 1.42857 power2 = 2.04" X SP1 .5" = SP2 is 1.02" way too high!
Going from a quarter HP to a half HP motor & a 300 increase in RPM still won't get the needed air flow on the blower graph!

  • Fan amp-draw increases as to the cube of the cfm increase
  • Amp2 = Amp1 (cfm2/cfm1)3 cubed *x Amp1= Amp2

Required fan motor horsepower (hp) varies as to the cube of the rpm speed:

  • hp2 = {rpm2/rpm1}3 x hp1 = hp2
  • {840-rpm/700-rpm}3 =1.2 cubed = 1.728 * .25-hp= .43hp2 or, a .5, a 1/2 hp motor at 840-rpm SP @.
  • Because of A-Coil back-pressure is way high due to being set to close to the huge Oil Furnace HT/EX

Duct retro-work can solve the problem, increasing blower HP alone won't usually work well! A few calculations plus retro-work and presto, a matched airflow with your systems' heat absorbing coil capacities, delivering near its BTUH, EER, and SEER ratings at normal room temperature settings! (Most don't)

Figuring BTUH Performance

Get a low cost Testo Tester & ballpark figure actual BTUH & EER - the information on it:
http://www.amazon.com/Testo-605-H2-H.../dp/B000774B6A

Everyone, very low cost anemometer to get airflow FPM Velocities, get it:
http://www.amazon.com/Crosse-Technol.../dp/B0002WZRKE

This should be helpful: (Edited 4/5/10)
Duct system CFM X* 4.5 @sea-level, or use X* 4.25 if 1000' above sea-level, X* change in enthalpy = BTUH (Ballpark) Operating Performance.
"U Must Right Click Link,open in New Tab," look-up wet bulb enthalpy figures on chart," & figure enthalpy change.
Wet Bulb Enthalpy Chart
 
Rules of Thumb for Duct Systems  - Hart&Cooley

Google search Hart & Cooley, also this pdf might help you select the right diffuser for the particular application, & list (Ak) free sq.ft., area of the diffusers:
http://www.rileysales.com/hottips/resizingreganddiffuser.pdf

History tells us that our industry measured static pressure many decades ago. When and why did we lose the habit of testing this basic factor that affects the total efficient operation of the heat and air system? How and why did we lose touch with the air side of HVAC systems?

Find the best way to save energy in your home, use your zip code. Profitability of Energy efficiency Upgrades | Hidden cost of Home energy Use

  DESIGN AND INSTALLATION OF RESIDENTIAL FLEXIBLE DUCTWORK SYSTEMS
http://www.dca.state.ga.us/development/constructioncodes/publications/1ONE.pdf

Look at the ducting, if it is not to code; make hard copies of this code & give it to whoever does the ducting work
Make sure they get redo it right!
Never have flex duct interiors commercially cleaned, I just viewed Home Inspection photos showing the interior damaged & insulation plugging the duct.
Home Inspectors warn people because the duct cleaner's tell them it won't damage the ducts. Some HI's look into the boot areas for clues of problems...

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Identifying your registers/diffusers & their (Ak) sq.ft. area, so you can multiply the FPM Velocity times the Ak (open air) to get the (CFM) Cubic Feet per Minute airflow from that register.

Have or do a manual J heat-gain calc for each room. If a room calls for 3,000-BTUH; first divide 12,000-BTUH by the CFM PER TON you want to use.
I.E., Wet coil, 12,000/400=30-BTUH per each CFM; Wet coil 12,000/425=28.235294; 3000/28.235-= 106.25-CFM;  Dry coil, 12,000/450= 26.6666-BTUH; 3500 / 26.6666= 131.25-CFM
If register/diffuser has the same (Ak) free-airflow-area, as the duct run!
Room calls for 3,500-BTUH, using 450-cfm per/ton dry coil or 26.6666-BTUH per CFM= 131.25-CFM.
I.E., 6" rd duct .6*6=36*.7854=28.2744sq.ins/144=0.19635-sq.ft.; 131.25-cfm / 0.19635-sq.ft= or 668.4-fpm velocity.

http://www.americanmetalproducts.com/lima/product_catalogs.html
Click on the categories to see the diffusers & Return-Air Grilles then find them on your downloaded pdf's engineering data.
Hart & Cooley: http://www.hartandcooley.com/grd/HC-100/residential/baseboard_registers/462.htm
Do a lot of Hart & Cooley engineering data searches, look at the registers & the Ak sq.ft. data to figure register's delivered CFM.
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Customers - A Simple FREE A/C Check you can do!
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Affordable Test Instruments Techs Must Own & Use!
All I had was the Sling Psychrometer & spinning it was a bit time consuming, but I used it religiously, it is information you need. 

The Testo 605-H2 Humidity Stick (wet bulb), displays relative humidity, air temperature and wet bulb temperature.

It is very affordable & because of its potential to help deliver tons of other data everyone should have one
!

For more information on it:
http://www.amazon.com/Testo-605-H2-H.../dp/B000774B6A

The other test data you need is the system's CFM airflow through the evaporator coil, then with software I have you can peg the BTUH the operating unit is delivering under those conditions.
Add to that a low cost Magnehelic gauge to read static pressures to compare with mfg'ers blower performance charts; plus a velocity meter & you have a ballparked CFM to plug into for the BTUH.

We could easily provide a detailed psychrometric print out what the operating system is delivering in BTUH, including condensate lbs/hr, & actual sensible & latent cooling BTUH & Ratio, every data detail imaginable. - Darrell
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Air Conditioning Performance Diagnosis using listed (CT) Condensing Temperatures
Using Goodman 16-SEER "Expanded Performance Data"

Find the correct (CT) Condensing Temperature with the following known mfg’ers data.

Outdoor Ambient Temp (OAT) 85-F; IDB 75-F; IWB 63-F or 50%-RH.
Listed pressure is 316-psig, or 99-F CT; that is 99-F -85-F is a 14-F SPLIT.

The delta T or temp-split should be within a 10-psig range or, +/- 2-F degrees; 97 or 101-F.

The mfg’ers Supply Outlet should be able to provide Contractors & Techs with those performance data charts. Goodman has their “Expanded Performance Data” on the Internet. BTUH = CFM X enthalpy difference from Chart X 4.5
  <-Click
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Home Energy Magazine Online September/October 1993
Raising Standards and Savings
New Group Hunts Bad Ducts

Does 40 billion kWh sound like a lot of energy? How about 4 billion therms? Researchers believe that's how much electrical and gas energy this country "could save by fixing inefficient ducts using current techniques." "Refining those techniques could reap savings of 90 billion kWh" plus 9 billion therms! Peak loads would be reduced too. To pursue these tremendous savings, national, state, and utility research laboratories, the U.S. Department of Energy, utilities, and energy service companies are collaborating. Their consortium is called "Residential Energy Efficient Distribution Systems," or REEDS.
These techniques, along with reducing air infiltration & heat-gain/heatloss calcs, ought to be taught in all our schools as part of the Science & math curricula. Half the heat-gain/heatloss can be due to a high Home Air Infiltration Rates!
ASHRAE standard 62-1989 is 0.35 ACH (Air changes per Hour) or 3-hours for a total interior, Air INFILTRATION Change.
Home Air Infiltration
Air Infiltration sources DTI Corp Catalog
DTI Corp Air Infiltratrion
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Years Ago it was the "U-tube" . . .

Years ago a method of measuring air flow and air pressure was required, and the "U-tube" was used. This instrument is still used today. It's a 1/4-in. tube formed into a 'U' shape. A scale is written on it in inches, and it's filled half-full with water. I still had one when I retired from  the business.

When pressure is collected in the duct with a static pressure tip. The pressure travels through the hose to the manometer, or pressure reading gauge. The distance the pressure moves the water is Inches of Water Column or (IWC).

Modern instruments usually use pressure sensors to interpret the readings digitally, which makes our jobs much easier. But the old liquid-filled gauges still do the job, if that's all you have to get started.

How long does it take to read the static pressure? About five minutes. Drill holes into the duct or equipment to insert your positive  static pressure tip just before the evaporator, the negative just before the blower, set up the pressure measuring gauge and read and record the Total Static of the system. Also, probe to read the Return & Supply separately to determine where work needs to be performed; just place only the positive before the evaporator & read gauge, then only the negative ahead of the blower & read static. That's all there is to it. You have to be super careful, drill should be fixed so it just goes through the metal.

Solving the Mysteries of ESP - External Static Pressure & Duct System Design by finding TEL, ASP, & FR:

ESP is the static pressure "external to the air handler," which includes the entire ductwork. This is the reading that manufacturers' refer to on the equipment tag of their airhandler's performance data, usually .50"-ESP.

In other words, ESP is the sum of the static pressure drop in straight ductwork, and the static and dynamic pressure drops in duct fittings (i.e. elbows, tees, transition pieces, air outlets, etc.)

"Consider that on High efficiency furnace there is a Condenser & an Evaporator that adds to the Supply Air Side Static."

The Return External Static Pressure is measured as the air enters the return opening of the equipment, the Supply External Static Pressure is measured just outside the supply opening. Try to find the least-turbulent air to take the readings.

To avoid turbulence take the readings 3 to 5 duct diameter inches downstream of turbulent areas.
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The North Country Oil furnace A/C or heat pump scenario:

Any good Tech should already know what I am going to illustrate here; this is for Homeowners & any who can use the least costly way to deal with this too common a problem in the cold North Country.

We are moving into the cooling season; however, any changes in equipment should consider how every component matches with optimal airflow efficiencies.

My lowest cost solution to the cold climate oversized Oil furnace with a small tonnage A/C evaporator coil which requires nearly half the airflow as heating:

I ran a check on my late brother’s home & the summer cooling heatload is about 14,000-BTUH.

The original scenario, had much less airflow than required for 1.5T cooling; its 2-T A-Coil wouldn’t flow 1150-cfm for heating

I did a lot of other figuring; 45,000-BTUH output should heat that small single story home.

However, with the 140,000-BTUH input, 112,000 output Oil furnace installed; the nozzle size can be dropped from one gal an hour (we’ll use 139,000-BTUH input) to .75 @100-psi, the BTUH drops to 103,500 input, this furnace tested at .74% efficiency; *103,000 = 76,590-BTUH.

Using a 90F heating temperature rise, which Thermo Pride can stand; (90*1.06 here) 76,590 / 95.4F is 803-cfm.

Therefore, the 2-Ton A-Coil will handle the CFM; mounted +6 inches above the furnace, it has the flow capacity to work okay in both heating & cooling modes.

With a third HP belt-drive motor, you could simply adjust the RPM down by turning the adjustable motor pulley out enough to get to 700 or 600-CFM for the 1.5-Ton A/C.

The other solution is to install a multiple speed direct drive motor with the fan relay energizing the cooling speed tap.

This rather common situation in cold climates seems not to be properly addressed my Techs, & the HO does not know why things don't work well.

We should all work to improve efficient use of high cost energy ... so America & everyone wins.

Unless the proper CFM heatload goes through the evaporator coil it is nearly impossible to achieve an accurate & proper refrigerant charge, and BTU/HR along with efficiency will be way below Ratings! 

Take the static pressure measurements on both the Return and Supply Plenums of the furnace (with filter(s) in place.  

Drill two holes large enough to insert the static pressure tip, one on the supply side and one on the return.  Pressure measurements are then taken at each location.  The measurement on the return side will be negative with a positive reading on the supply but you disregard the positive/negative and just add the two numbers together. You have to be super careful, drill should be fixed so it just goes through the metal.

When using two tubes (neg. & Pos.) on a modern gauge you will read what the gauge indicates!

Once the ESP has been determined, look at the fan curve for that particular blower and determine the CFM from that chart.

If the air flow is not per manufacturers' recommendations, it is near impossible to get the refrigerant charge correct.

If you leave out the area up to & including the A-Coil where does that leave you? The area to the coil & including the coil can represent major Static Pressure problems!

Measure Total-SP (TSP) by using a Magnehelic gauge or manometer and inserting the tube in the duct in two places - after the fan discharge and at the return before it enters the air handler blower.

Formula for Changing pascals to IWC: The External Static Pressure for the furnace is 125 Pascals, 125 x.004 = 0.5" IWC - Inches Water Column.

Modern gauges have two tubes that add the Return negative-
side to the Supply positive-side; if yours doesn't, add the negative reading to the positive reading. However, to balance the Supply Air side pressures to the Return Air side you need the separate readings to know whether one or both are out of specification perimeters!

 
If the Supply Air side is too high say .8" that needs to be addressed by checking to find where the static increases are occurring on the Supply Air side. Then adjustments can be made on the RA side to reduce any - Negative restrictions, "to achieve the full airflow rating of the blower to meet the A/C airflow requirements."

Additionally, when the Return Air Filter(s) begin to load up the positive side Static Pressure or ESP will begin to Decrease as the RA Negative reading Increases. If the Return Air Suction side of the system is restricted or starved then the positive SA ESP will be “Reduced” as will the air velocity & total CFM airflow. The Return Air portion has a huge effect on the CFM airflow of the system, and many are too restrictive.
LOW AIRFLOW - this will help to open your eyes!

Therefore, it is essential to have a proper balance of pressures between the + SA & - RA sides of the duct system to achieve proper airflow to all of the rooms. Excellent diagrams pages on taking various Air Readings


Another Example of imbalance, - there are rooms with "Inadequate Return Areas," the static goes higher in those rooms & airflow is reduced, & air also leaks to the exterior outdoors. The amount of leakage will depend on the direction & velocity of the exterior wind on the side of the building.


Now, if there is "Adequate Return Area in several other rooms those rooms will get excessive airflow, however, the SA ductwork will be short changed & therefore the ESP reading before the E-Coil might be a little higher than it would be if the Return Air System were in proper balance with the SA in all rooms. 

How does the system "Regain Required Velocity" when flow through the A-Coil will be produced largely by static pressures. What is the “Total Static & Velocity Pressures” the Blower & its motor must overcome to achieve required airflow?  Optimal efficiencies will never be achieved until the proper Static Pressure is achieved!

Ideally, there would be return air filter grilles out in the system, and a nice, smooth transition between the coil and the furnace to drill into. However, real-life situations often force us to take pressure readings elsewhere.

For instance, on the return side of the system there's often a filter placed into the top or bottom of a furnace. The resistance of the filter must be included in the total resistance that the fan sees. The real-life solution is to drill into the side of the blower compartment and measure the negative pressure at that point.

On the supply side, the coil sits directly on top of, or, below the furnace. This isn't an ideal situation for measuring static pressure either, but in real-life fieldwork there are two main practices for measuring positive static pressure.

The first is to drill into the coil housing between the coil and the furnace blower. I hope you shuddered a little when you read that, because we all hate the hissing sound when we drill into a coil. But if you're careful, that doesn't happen. Here's how: drill a pilot hole with a sheath, or stopper on the end of your drill bit that only allows it to penetrate the coil housing only 1/4-in. Then look and feel inside to see if the coast is clear for testing.

Some coils have a secondary wall that you must also penetrate to get into the airflow; this must be drilled through also, go below it if possible.

The second way to gain access to the supply side air stream is to remove the high limit switch, and take a pressure reading there. Be certain the power is off when removing the switch, or you could get shocked. Also, because the hole is much larger than the static pressure tip, you must use a seal (a hole bored in a cover plate) to seal off the air pressure.

WARNING: on units with a Thermostatic Expansion Valve (TXV), you cannot use the suction pressure to check the charge; many appear to be doing this; it tells you nothing. "Only after you have verified that all the coils are clean & the airflow is right-on," can you begin to check the system's charge using Subcooling method with a Superheat check.
Always check the actual airflow CFM 'before' checking the charge, - get CFM Right First!

* There is a TXV system that has very low airflow, actually less than 200-cfm per-ton of cooling, they're only checking the suction pressure & saying the charge & everything is okay! * That system has a TXV & shows; 98-F condenser saturation temp & 97-F liquid line temp near E-Coil, a mere 1-F Subcooling, it's undercharged even with a mere 200-cfm per-ton cooling load! Unbelievable, but it's happening out there.

Use my Superheat Subcooling Charging page!

The Proper Steps

Let's measure static pressure step-by-step. First, read the nameplate data on the air handler or furnace to determine its ESP static pressure rating. This is often the maximum amount of static pressure or resistance that the fan can handle on the Supply and Return Sides and still deliver 400 cfm per ton on high speed. Residential Centrifugal Blowers begin to unload when static pressures get too high.

To measure in-duct air pressures, drill two 3/8-in. holes in the duct to insert the static pressure tip.

Third, connect the static pressure tip to the hose, and attach the hose to the pressure connection on the Magnehelic® gauge. The top (or high connection) is for positive or supply pressure. The bottom (or low connection) is for the negative or return pressure.

Fourth, level and zero the pressure gauge to ensure accurate repeatable readings. Then insert the static pressure tip into the duct with the tip facing into the airflow.

Fifth, read the pressure on the gauge, and record the reading on the supply side, then on the return side. These readings can be taken at one time, but diagnostic ability increases when each side is read separately.

Use a (+) sign before the positive or supply side reading to show where it was taken, and a (-) sign before the negative or return side reading.

Add the two pressures. Disregard the positive and negative signs before the pressures, because each pressure is an absolute number; each pressure is "seen" and affects the fan as a force, so they must be added together to determine the total resistance the fan sees. For example a +.23 w.c. plus a -.19 w.c. equals a total static pressure reading of .42-in. w.c.

The gauges used now days measure both + & - at the same time giving the Total SP reading or pressure drop across the various components.

Record the pressure readings on your diagnostic report or on your service ticket. Our techs often write the pressures on the cooling coil for future use. Any change in static pressure reveals a change in the system that should be addressed for the system to operate properly.

Most residential and light commercial systems five tons and under are rated to deliver 400-CFM at a static pressure of 0.50-IWC. Maximum static pressure increases with larger units.

Certain mfg’ers of equipment will rate the AHU with ESPs external to the built in components (coils & occasionally clean filters) & others will rate the blower alone. [Your job is to Figure the required blower ESP & CFM for your specific application.]

Now when you add all of the required elements to an air conditioning system you are adding pressure drops, which reduce velocity pressure, but increase static pressure, therefore, Total System Pressure (TSP) remains the same.

If the External Static Pressure is exceeded because of poor installation, or other conditions, then the CFM produced by the system will drop.

If you leave out the area up to & including the A-Coil where does that leave you? The area to the coil & including the coil can represent major Static Pressure problems, "especially in oil furnace applications." If there are any existing heating or cooling airflow problems, you can & should measure & record the static pressure on each branch run.
Darrell - Bloomington

====================

Be certain to always read the nameplate data to determine the ESP that the unit was manufactured for. Also, obtaining a copy of the manufacturer's fan curve data can be very helpful in interpreting static pressure and airflow readings

Measure the Components

Static Pressure Drop (PD) can be measured over each component of a system by taking a pressure reading on each side of the component, including a possible poor transition to the cooling coil. Subtract the readings from each other to determine to drop through that component.

Total Static pressure can be traced from the grilles all the way back to the fan. Try adding together the pressure drops over each of the components of the system. This isn't the usual procedure for measuring pressure, but it is a learning opportunity.

For example, a cooling coil may have a reading on one side of +.35-in. and a reading on the other side of +.20-in. Subtract and the the coil's pressure drop is .15-in.

Measure the Static Pressures on each side of the blower, add the - negative reading to the+ positive reading to get the Total Static Pressure.

When designing or redesigning duct systems; finding TEL, ASP, & FR:
Find the Total Return and Supply Lengths by measuring duct, run-outs, transitions, turning elbows with or without turning vanes, trunk take-offs, and boots, etc.


Once you have the correct Pressure Drops (PDs) for those lengths, use the manufacturer's nameplate pressure (IWC) and subtract all airstream device pressure losses (filters, wet coil, etc. all available in manual D) from that given value. That will leave the "Available Static Pressure" - ASP, for design purposes.

You can figure the Total Equivalent Length (TEL) by using the Manual D length additives for the various fittings, then use your Duct Designer Ductulator to properly size the duct system to meet the velocities and CFMs required in respect to the blower's Nameplate ESP and its performance graph data.

There are charts available to determine what the total pressure drop will be when you figure the total equivalent run of the longest Supply duct runs; ALL lengths of duct and ALL fittings for turning, reductions, etc., have a Total Effective Length (TEL) of duct - that must be added to the regular length of that duct run!

Designing or Redesigning Duct Systems Chart  Print

TEL ASP FR Chart Graph  Loads slow using dailup - Save both the pdf to a quick access PC folder for review

Variable Speed Motors and Static Pressure


The above is an example at .7" ESP which is too high for most residential air handler blowers!
It will deliver the required CFM with some blowers, but at a higher operating cost, use .5" ESP to subtract Device Losses.

If there are any ductwork air leaks, it will throw everything off.
Nearly all duct systems have a percentage of air leaks.
Check for & minimize air leaks.
Blower wheels, evaporator coils, etc., everything has to be clean.


Get copies of the ACCA Worksheet; below is my math involved with the linked Graph:
The .45-Device Pressure Loss (DPL) should be subtracted from the furnace nameplate max ESP which was .5"-ESP, leaving a mere .05"-ASP (Available Static Pressure).
According to the way I figured it, (with a .45-DPL), .05"-ASP / 3.3-TEL = .015-Friction Rate is required, it's way off the Graph!

In the above example, there should be around 30-DPL or .20-ASP / 3.3 -TEL = .06-FR -the minimum FR graphed.
Though many do use .05-Friction Rate (FR) or less especially for the Return Air Side.

Otherwise, it needs a much shortened TEL to function to design!
Air Turning vanes in 90-ELLs greatly reduce the TEL.
Large transitions on all take-offs, etc.
Other changes could also help reduce the TEL.
Oversized Return-Air Filter areas, etc.

Look at the above TEL, FR, ASP Graphed Chart!
It would have to have a .20-ASP Available Static Pressure with a 330-TEL to get a (FR) Friction Rate of .06.

First find 330-TEL graph, then look at the ASP at the bottom the the FR shown.
Also, a .06-FR is the lowest Friction Rate shown on that graph; though many use .05-FR on the Return Side.
Once the ESP has been determined, look at the fan curve for that particular blower and determine the CFM from that chart.

In the heating mode, looking at the product data chart, what temp-rise does the mfg'er recommend?
Measure the temp rise, and see what you get.

Unless the proper CFM heatload goes through the evaporator coil it is nearly impossible to achieve an accurate & proper refrigerant charge, and BTU/HR along with efficiency will be way below Ratings!

CFM formula per room:
Use the load-calc for that room divided by the total system BTUH for cooling. Say that room's load calc calls for 3500-BTUH / 24000-BTUH = 0.1458333 * 800-CFM = 116.6-CFM to that room, a 6" metal duct should be around 600-fpm velocity. (24,000-btuh / Ave of 3500-btuh = 7 Runs.)

I.E.,  a 4-Ton System: 48,000-btuh @ 400-cfm per-ton= 1600-cfm | Load calc says room requires 3600-btuh / 48000=  0.075 * 1600-cfm = 120-cfm, or a 6" RD metal duct, close to 611-fpm velocity.

You must know & record the operating feet per minute (FPM) velocity & the CFM to each room & the Total CFM airflow! Every tech should be using an anemometer to check operating airflow velocities in FPM & then figure the CFM airflow's.

For Room Return Air balancing, i.e., -.01" IWC = approximately -2.48 Pascals, which is a more precise easier incremental scale to read.  One inch water column (IWC) equals rounded to > 250 pascals, 0.5" IWC is about 125 pascals; 0.25" WC = 62.5 pascals; 0.125 = 31.25 pa.; 1 / 250 pascals =0.0040322 *X's  -2.50 pascals = -0.01003657696655" IWC or make it  - 0.01" IWC for low Return Air room pressure differentials; - use pascals.

Here are some examples of residential or light commercial in Inches Water Column (IWC) Pressure Drops. Check the engineering data as you design, and you may be surprised. This is mainly only the example of the Device pressure losses.

Component
Pressure Drops IWC
Cooling coil
0.15 to 0.50-IWC
Pleated filters
0.10-in. to 0.45-in.
Electrostatic filters.
0.20-in. to 0.80-in.
RA Grilles and SA difusers
0.02-in. to 0.15-in.
Man. D  0.03-IWC
Transitions, Boots
0.05-in. to 0.35-in.
Elbows, - Use Turning Vanes!
0.01-in. to 0.10-in.
Disposable filters
0.05-in. to 0.30-in.

Diagnostics Clues

There are many combinations of different static pressure readings found over the years. The basic premise is that if static pressure is high, there may be blockage, or the duct may be undersized.

If static pressure is low, there's a good chance that there may be duct leakage, or that the fan blades are dirty, or motor malfunctioning.

Keep in mind that with about one in 20 field static pressure readings you'll get what we call a "goofy" reading (I realize that's not a very technical term, but it's accurate). Where do these readings come from? Pressure equalizes immediately, but air also "rolls" and "bounces" around as it passes through ductwork. Occasionally, the turbulence of the air inside will give a false reading. It will just be way too high, or way too low. Because perfect conditions don't exist in the field, the answer is to drill another hole a few inches away and try again.

My brother Don has a 2-ton A-coil with a TXV and a 1.5-ton Heil condenser, and because they mounted the coil directly on top of the oil furnace, it does not have enough air flow! The 140,000-btuh furnace has a belt-drive small Quarter 1/4HP motor. It needs a Third 1/3HP motor and the coil moved up 6" on rails with preferably a transition between the A-Coil and the top of the furnace so the air is directed into the entry area of the coil, thus eliminating the blocking of more portions of the velocity force!

My Scan of My ThermoPride OL 11 "Oil Furnace" Graphed Blower-Curve-Chart (Same as my brother's Oil furnace)
Thermopride OL 11 Graph ipg image - Thank you Dave Staso, CA. for the better expandable image!
"After it loads Right click "Show Original Images" - Move cursor arrow over graph - Click + when 'over graph' for expanded image," then print on the highest quality setting.
Notice at 700-RPM with a quarter 1/4HP motor, I checked his actual airflow at less than 300-CFM (no appreciable duct air leaks). the graph shows 5.24" SP.

Now, we switch to a 1/3HP motor @800-RPM, the graph shows 6.85" SP & only 400-CFM, not nearly enough airflow for 1.5-Ton of cooling. Therefore we have to raise the evaporator coil 6" above the furnace on rails & then check the airflow. Eliminating that restriction using a 1/3HP motor, hopefully that will be adequate at
+800-rpm & say +5.5" SP & +700-CFM. Formula: SP2= (SP2/SP1)2 X's SP1
======================================================
*You could ballpark the CFM using the static test & a air handler graph. You could measure the CFM delivered to each room with a hood Alnor Balometer, it's usually the best instrument to use, but not cheap. Measuring the air velocity is a bit tricky because you have to use the diffuser data which you don't always have available.

A rough ballpark formula to get the CFM: CFM = (velocity in (FPM) Feet per Minute times the square footage of the duct area, you have to have & use the diffuser data & get velocity there -for operating conditions.) Taking the manifold gage head pressure & gage condensing temp, is important data. Coupled with a TH condenser temp-reading, if the condenser gage pres/temp is too high compared to the TH reading, there may be non-condensables in the system.

Also, there is a legitimate formula I use to determine the operating BTUH it is delivering at all the data taken. All the mfg'ers ought to list the condenser temp-split (it varies with EER & SEER) just like they list the indoor split, it is valuable trouble shooting info.

You can also use the condenser temp-split (it contains both Latent & sensible heat) combined with the indoor data to plot the indoor CFM. I was never good at math, but those equations have to balance, & they do work!

============================
*You could ballpark the CFM using the static test & the air handler's graph. You could measure the CFM delivered to each room with a hood Alnor Balometer, it's usually the best instrument to use, but not cheap. Measuring the air velocity from diffuser's is a bit tricky because you should use the diffuser mfg'ers data which you should always have with you.

You can usually get the diffuser mfg'ers data, say its a 1.5-Ton system that already has 6"rd branch duct runs & to achieve enough CFM airflow, U need 700-FPM velocity in the ducts.
I would want to use a diffuser with a little more free open sq.ft area than the 6" duct  which area is 0.19635-sq.ft., say middle of the room in the ceiling;
Hart & Cooley 2-way curved blade 12x6 has Ak .235-sq.ft. Free-Area delivering 140-CFM at 600-FPM - diffuser face velocity.
This would help lower the velocity of the duct
through the diffuser & reduce air noise.
Throw is 7.5-ft toward each wall. Terminal velocity at the occupant level is 75-FPM.
One duct yields140-CFM times 5 outlet yields 687-CFM *X's 30-BTUH per CFM = 20,616-BTUH, about right.


A rough ballpark formula to get the CFM: CFM = (velocity in (FPM) Feet per Minute times the square footage of the duct area, you have to have & use the diffuser data & get velocity there -for operating conditions.) Taking the manifold gage head pressure & gage condensing temp, is important data. Coupled with a TH condenser temp-reading, if the condenser gage pres/temp is too high compared to the TH reading, there may be non-condensibles in the system.

Also, there is a legitimate formula I use to determine the operating BTUH it is delivering at all the data taken. All the mfg'ers ought to list the condenser temp-split (it varies with EER & SEER) just like they list the indoor split, it is valuable trouble shooting info.

You can also use the condenser temp-split (it contains both Latent & sensible heat) combined with the indoor data to plot the indoor CFM. I was never good at math, but those equations have to balance, & they do work!
======================================

Formula for finding CFM Air Flow from Velocity in FPM

If you can measure the air velocity coming from a known size duct or open area of a SA register, here is a rough ballpark formula to get the CFM:

CFM = (velocity in (FPM) Feet per Minute times the Square Footage of the duct area). To convert sq.ins. multiply by 0.00694 for sq.ft., or divide sq.ins. by 144.

Converting square duct
inches to round duct size, Figuring the Square Inches of Round Ducts, an 8" x 8" duct = 64-sq.ins. x .7854 = 50.26 sq. ins. You round off to 50 sq. ins. for an 8" duct.  Or, simply getting the square inches of round ducts: a 7" duct; 7" x 7" = 49 x .7854 = 38.48-sq.ins. or divide / by 144 = .2672222-sq.ft. X's a velocity of 500-fpm = 133.6-cubic feet per minute delivered to the room; 133.6-cfm x 30-BTUH per cfm ratio= 4,008-BTUH.
(12,000-BTUH /400-cfm per-ton = 30-BTU per cfm ratio | / 450 = 26.666-BTUH per-cfm)

Sized for in the chart below - BTU/hr per CFM figures "are figured for heatpumps at 450-CFM per ton of cooling."
Use 800 to 900-FPM MAINS' VEL. Use an optimum of 500-FPM VEL for Supply Branch Runs | Air speed Face of Return. 

Air Filter Rack Sizing
I realize this will never happen if you use the furnace filter size; however, Air Filter Rack Sizing for efficient operation - Size Gross Return Air filter grille area for 200-sq. ins. per ton; or 2-CFM per square inch of filter area. A 4-Ton (425-cfm-per-ton) with 1700-CFM /2=850-Sq.Ins.  For a 5-Ton system @ 2000-CFM, that would mean Two filter racks 25X20's each, I would go with Two 30X18" RA filter racks for 1080-sq.ins for a 5-Ton system.
  (Darrell - Bloomington)
=====================
Quick Check for Sizing Units to enough Airflow


Actually, even on service calls where there are cooling problems the ductwork should have a quick Manual D performed.

Then take the ESP static pressure & compare to blower graph or chart, also take the FPM duct velocity.

Then do a quick estimate of airflow per equipment tonnage.

3To find area of a round duct; Duct diam is 7"; 7"X7"= 49-sq.ins., X's .7854 = 38.04845-sq.ins divided/ by 144= 0.2672541-sq.ft. area X's FPM Velocity 600-FPM = 160.35246-CFM X30 = 4,810.5738 each 7" run X's 6 branch runs = 28,863-BTUH, or airflow for 2.4-ton. 

That would also be good for 2-ton; at 550-FPM velocity X's 0.2672541= 147-CFM X 30-BTUH Per-CFM = 4,410-BTUH each run X 6-runs = airflow for 26,460-BTUH.  (12,000-BTUH /400-cfm per-ton = 30-BTU per cfm ratio | / 450 = 26.666-BTUH per-cfm)

Never sell units requiring more airflow than the duct system will support! - Darrell udarrell
=========================
Ductwork Retrofitting - An Excellent Economic Opportunity - Don't Miss IT!

"More than 80% of the duct systems in residential and light commercial applications 'do not' work as designed." Do your service agreements include the duct system? If not, this is an important & significant business opportunity that you are missing!

Your best access into the duct renovation market is to include the duct system in your service agreements. What this includes is having the service tech measure static pressure on each service visit. Remember, this takes five minutes or less. If pressures are very high, or very low, send out a tech competent salesperson with a flow-hood and a manometer to identify the problem, and propose a bid to do the necessary ductwork fix.

One reason we have service agreements is to gain additional income from repairs, "so start repairing the real problem with the system," and not just the equipment. In most areas of the country there is very little competition for quality duct renovation and air balancing.

Prescribing HVAC repairs without competition from other contractors in a way that will greatly improve performance and efficiency, will delight your customers - that's our definition of opportunity.

Rob Falke is an owner of Saunders Air Conditioning, as well as Balancing Ltd., an air balancing company. Falke, an NEBB-certified air balancer, is also president of the National Balancing Institute, which provides air balancing training as well as complete air balancing business startup packages for HVAC contractors. To reach Rob, call the National Balancing Institute at 800/633-7058.

Diagnostics Clues

There are hundreds of combinations of different static pressure readings we've found over the years. The basic premise is that if static pressure is high, there may be blockage, or the duct may be undersized.

If static pressure is low, there's a good chance that there may be duct leakage, or that the fan is dirty or damaged.

Keep in mind that with about one in 20 field static pressure readings you may get what is called a "goofy" reading. Where do these readings come from? Pressure equalizes immediately, but air also "rolls" and "bounces" around as it passes through ductwork. Occasionally, the turbulence of the air inside will give a false reading. It will just be way too high, or way too low. Because perfect conditions don't exist in the field, the answer is to drill another hole a few inches away and try again.
==============================================================

Measuring Static Pressure is a sure Thing - by Tony Doyle

Would you consider doing something that takes less than five minutes to do that could generate, oh, let us say, more than $90,000 in revenue? Read on and see how measuring static pressure on every service call could conservatively earn you $98,000 in new revenue this next year.
If you had three service techs doing three calls each that would total nine service calls per day. In a week's worth of service calls (45), if only 25% had high static pressure, (a safe bet), you would have a potential for 11 diagnostics a week. Of the 11 potential diagnostics, if only 3 of these customers bought one and only one of those customers agreed to a duct renovation you'd be on your way to $98,000 of new revenue this year.
Three diagnostic tests and one duct renovation job a week! That's all it takes. Let's review the numbers:
3 - Service techs.
3 - Calls per day.
45 - Service calls per week, measuring static pressure on every one of them.
3 - Diagnostics per week. ($395 for the diagnostic).
1 - Duct renovation job per week. (average $1,275 for each one).
40 - Weeks of service calls.
Results:
40 weeks x 3 diagnostics a week x $ 395 per diagnostic = $47,400
40 weeks x one duct renovation a week x $1,275 avg. renovation price = $51,000
Total new revenue: $98,400
The number of technicians, service calls, diagnostics sell price and average duct renovation price used in this example are conservative. Use the formulas in this example and estimate the potential for your company.
What else do you have at your fingertips that will bring in $98,000 or more in new revenue this year? By the way, you should be able to perform the diagnostic and duct renovation work at a gross margin of at least 70%. Even if your overhead is a whopping 40%, that drops about $30,000 to the bottom line. How are the plans for the new boat coming along?
- Tony Doyle
===================================
Supply Static Test Hole
When measuring supply static on a non-condensing gas furnace, if the area between the furnace outlet and the coil is hard to drill into, carefully remove and isolate the fan control/high limit switch on the furnace, put some electrical tape over the opening and poke your static pressure tip through the tape. The reading will be virtually identical to measuring just before the coil. If your reading seems unstable or out of line, you might need to measure it elsewhere. [IAQ – Indoor Air Quality]
========= 

TOOLS OF THE TRADE

There are many fair, good, and excellent quality digital manometers on the market. Some are more than $1,500 for use daily in commercial air balancing, but many fall into the $300 to $500 range. For less than $150 (excluding a drill) you can assemble the following items and have a terrific starter kit with everything you need to begin measuring pressures.

  • Dwyer Model 2001AV Magnehelic® Gauge
  • 8-ft. of 1/4-in. (ID) neoprene or rubber tubing
  • Static pressure tip
  • 1- 3/8-in. drill bit
  • 100- 3/8-in. plastic test hole plugs
  • 1- 6-in. x 1/8-in. stainless steel tube
  • 1- Drill bit sheath (for drilling into coil housings, and not the coil)
  • Optimizing the Evaporator BTU/hr Heat Input  Seal All Ducts First !

  • My scan & Dave Staso's image work on my Thermopride OL 11 Graph
Thermopride OL 11 Graph ipg image - Thank you Dave Staso, CA. for your image work!
"After it loads - Move cursor arrow over graph - Click + when 'over graph' for expanded image."

Example of improper Duct Design on an Oil Furnace Name Plate Max., .5" ESP:
Quarter HP belt drive blower with horrible big loss NO transition to the Cooling Coil
The Figures showed, it would have to be designed at .43-ESP, it shows 700-CFM on Blower Graph.

A Major Oil Furnace Airflow Problem Fix
Regal & Hallmark & nearly all Oil Furnaces - Installation manuals
http://www.boyertownfurnace.com/ProductDocuments/index.aspx
Download the installation & service manuals
http://www.boyertownfurnace.com/ProductDocuments/HallmarkONLYManual042909.pdf
If the oil furnace is used in connection with summer air conditioning the evaporator coil must be installed at least 6” above the oil furnace for proper airflow. Distances less than 6” will result in decreased airflow. Make sure outlet supply takeoffs are NOT blocked by the coil. In all cases, refer to the manufacturers’ data for static pressure losses to ensure the total system static pressure does not exceed 0.5” WC.
 
My page on the Oil furnace ESP problems should be read & carefully addressed by everyone that works on them.

http://www.udarrell.com/oil_furnace_heating.html

You never want to lose the velocity force or run the static way up before the airflow ever gets through the E-Coil!

Checking the SP after the E-Coil will show how much of the static is occurring before the Coil.

You want the velocity forces & static at the diffusers to get adequate throw and air movement in the rooms NOT before the E-Coil!

The "Total Pressure" required to move the required amount of CFM of air through the duct system is composed of Two Elements, "Static pressure and Velocity pressure." Velocity Pressure is pressure in the direction of the flow. It is the push force needed to bring the air up to the required FPM Speed. "Total Pressure" is the sum of the Static Pressure and Velocity Pressure at that specific point of measurement. The manometer can record Static, Velocity, and Total Pressures.

Manufacturers know the resistance inside the equipment. The  ESP reading tells us what resistance the coil,  ductwork, filter, and diffusers & grilles need to be designed at , so that the blower can deliver the required airflow through the Supply-Side and Return-Side duct systems.

Air moves in what is called "turbulent Flow," as it is constantly churning and mixing. Constrictions, duct changes of shape, turning, or banging against something that stops its velocity will greatly increase the force required by the blower.

This is why oil furnaces with the blower to the side and with huge round or octagon (eight sided) shaped heat exchangers that come close to the top of the furnace "can have tremendous increases 'in both' Velocity and Static Back Pressures." If the A-Coil is not mounted high enough above the furnace on rails on the plenum, the airflow system will never achieve the required CFM.

Let us take an engineer's view of this horrible scenario. The blower is set to the side at right angles to the huge H.E., therefore from the get-go, the velocity is blocked & turbulence is also maximized, thus generating tremendous forces that the blower is required to overcome.

Then here in Wisconsin where I now live, it appears many install the A-Coil directly on top of the furnace, therefore we have a major restriction along with most of the airflow hitting other than the entry area of the coil, another block to velocity!  This is the area where we want the back pressures to be as low as possible so that there will be more velocity pressures and static pressures at the supply diffusers. This will provide the proper throw into the rooms and proper mixing of the room's air.
- Darrell

============================
HVAC Techs
This should be helpful.
CFM X change in enthalpy X 4.5 = BTUH (Ballpark) Operating Performance
"U Must Right Click Link & open in New Tab"
Wet Bulb Enthalpy Chart

=================
  MY HVAC BLOG - YOUR QUESTIONS & COMMENTS WELCOME  - Go to my blog for the required trouble-shooting Data to collect
Evacuation Procedures - Pulling a Deep Vacuum pdf
Too many do not properly purge & evacuate contaminated central air conditioning systems.
First, the piping should b checked for proper oil return to the compressor, if not adequate do that before proceeding.
----------------
Determining which metering device the system has without physically looking
If you do not absolutely know whether the metering device is a TXV, or a fixed orifice device or cap tube. 

Hook up your manifold gauges, block off considerable condenser air intake for a short time.
If the suction pressure starts rising, you have a piston, or a cap tube.
If only the high side goes up, you have a TXV.
Double check using Superheat, if Superheat stays relatively stable when the pressures change it's a TXV.

Have things with you in your van or truck to block-off the entering condenser air for a short time.
Check every time you are not certain what metering device it has.
There will be a lot of guessing in the future
=============================================
DISCLAIMER:
I assume NO responsibility for how anyone uses the information I provide in posts or on my websites.
I provide information for better understanding, edification & communication between contractors, techs & customers'.
A licensed Contractor & licensed Techs should always do all HVAC/R work! You are responsible for your diagnostics and resultant decisions!  Darrell Udelhoven  Darrell's Refrigeration Heating and Air Conditioning - Federal Refrigerant Licensed - Retired Licensed Contractor
Darrell Udelhoven | Posted: 08/01/04 (Darrell - Bloomington)
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