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
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
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

MO Home Energy Audits LLC

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 Sizing |  Opportunity |  Measuring 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.

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 ______

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.

June 30, 2012; Darrell’s electric meter; 62610 – 62140= used 470-KWh * .0985= $46.29
================================

========================== ===============
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!

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

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
  

• 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 hugh 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. Profitabiity of Energy efficiency Upgrades | Hidden cost of Home energy Use

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.
=============================================================

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 & heatgain/heatloss calcs, ought to be taught in all our schools as part of the Science & math curricula. Half the heatgain/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

--------------------------------------
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.

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

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.

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

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.

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-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!
============================
*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

"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.

Measuring Static Pressure is a sure Thing - by Tony Doyle

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
  

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