Room Air Pressure Differentials in Pascals & IWC | *** Sizing Units' Tonnage to enough Airflow New
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 into the evaporating refrigerant.
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?
Customers - A Simple FREE A/C Check you can
do!
--------------------------
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
========================================
Raising Standards and Savings
New Group Hunts Bad Ducts
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:
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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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.
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, you can begin to check the system's charge.
* There
is a TXV system that has very low airflow, approximately only 300-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 300-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.
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.
Find the Total Return and Supply Lengths by measuring duct, runouts, 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!
TEL ASP FR Chart Graph Too large a file loads slow using dailup - Save the pdf to a quick access PC folder for review.
The above is an example of the result being way off the graph!
It will deliver the required CFM with some blowers, but at a higher operating cost.
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 .50-ESP, leaving .05-ASP (Available Static Pressure).
According to the way I figure it, (with a .45-DPL), .05-ASP / 3.3-TEL = .015-Friction Rate is required, it's way off the Graph!
There 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) especially for the Return Air Side.
Otherwise, it needs a much shorter TEL to function to design!
Air Turning vanes in 90-ELLs greatly reduce the TEL.
Other changes could also help reduce the TEL.
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!
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 248 pascals, 0.5" IWC is about 124 pascals; 0.25" WC = 62 pascals; 0.125 = 31 pa.; 1 / 248 pascals =0.0040322 *X's -2.48 pascals = -.0099999" IWC or make it - 0.01" IWC for low Return Air room pressure differentials.
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.
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Man. D 0.03-IWC |
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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
is dirty or damaged.
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.
======================================================
*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!
============================
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.
To 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
=========================
"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
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
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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
"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.
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
Found this excellent HVAC-Talk
Thread today 02/27/09.
Skippedover post: "What
beenthere is telling you is that a residential blower is like a bank
account."
http://hvac-talk.com/vbb/showthread.php?t=224752
MY HVAC BLOG - YOUR QUESTIONS & COMMENTS WELCOME - Go to my blog for the required trouble-shooting Data to collect
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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)
