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External Static Pressure vs. Total Static Pressure


External Static Pressure vs. Total Static Pressure

External Static Pressure vs. Total Static Pressure

I'm hoping that I can get some advice from this forum.  I recently built a house with two a/c units.  Upstairs I have a 3 Ton unit and downstairs I have a 5 Ton unit.  Both are Amana GUIC/D units.

Prior to my one year inspection, I had an independent inspection by an heating and air conditioning company.  The results of this inspection said that my Total Static Pressure on the 5 ton unit was over 1.0+ and on the 3 ton unit it was .72.  The specification sheet for the units says that the maximum External Static Pressure should be .6 and optimal pressure should be .5.  The inspector said that the returns on both units were insufficient.  There are three 10 inch returns on the 5 ton unit (1 upstairs and 2 downstairs), and one 12 ince return on the 3 ton unit.

I notified the builder who brought the installer out along with an Amana field rep.  They reported that their measurements indicated that the pressure was in the .2 to .3 range and that no action was required.

The contracted inspector provided these results for measuring Total Static Pressure:

3-Ton Unit
Total Static Pressure: .72
Static Pressure Return: .3
Static Pressure Supply: .2
Static Pressure Coil: .22

5-Ton Unit
Total Static Pressure: 1.0
Static Pressure Return: .5
Static Pressure Supply: .4
Static Pressure Coil: .20

My suspicion is that the installer measured only at the coil, which provided the results they indicated.  Again the spec sheet for the unit lables "External Static Pressure", which the inspector indicated was the same as "Total Static Pressure."

Three questions:

1.  Are they the same?
2.  Did the inspector calculate the TSP correctly?
3.  Any advice on how to approach the installer if there is an error?

Thanks in advance for any help.

RE: External Static Pressure vs. Total Static Pressure

Good luck getting someone to answer that question.
I have been in the trade for 16 years now, and have had this come up several times, the E.S.P. question that is. Rockhead (Mechanical) Feb 16, 2001

I also have a similar question regarding "External Static Pressure".   

 When viewing BLOWER DATA info on, for example, Trane XV80 gas furnace, External Static Pressure is always a factor on the chart, and is a fairly important one.

  Where, and at what point exactly, do you insert test tube in duct? Also, do you insert low tube in R/A duct at the same time as high tube in S/A duct? Is that ".45" you are seeing at that point, your ESP that you are trying to determine?
  This exact manner in wich this measurment is performed has always been a big "?" for me, as it seems everybody has a differing view as to how it is correctly performed.

            PLEASE HELP!     Thanks, Rockhead

IT HAS NOT BEEN RESPONDED TO YET   Let me know if I can help.
Nobody wants to answer it, probably because everyone has a differant view of it.
Here is a post I put up for responses back in February.

RE: External Static Pressure vs. Total Static Pressure

ESP is the static pressure "external to the air handler" needed to overcome only the duct friction losses.  You can measure this by using a Magnahelic 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.

TSP is the difference between the inlet and discharge fan static pressure.  This "delta P" is the pressure required to overcome all of the losses in the system - the external losses (duct) plus the internal losses (cooling coils, filters, and anything else in the air handler).  You measure the TSP using the same gauges as above and measure in the duct after the fan discharge (positive pressure) and in the air handler section where the fan is located (negative pressure).  The difference between the two is the TSP.

Air conditioner manufacturers already know what their internal losses are, but they have no idea what type of duct system will be installed with their equipment.  They then tell you their unit will produce a certain CFM at a maximum of "X" ESP.

I hope this helps.


RE: External Static Pressure vs. Total Static Pressure

Greetings Mr. Alexander,

I have been told by Air Balance folks that sometimes it is difficult to find a good spot to take pressure readings. This is due to the configuration of the ductwork and its proximity to the fan discharge..... the problem can be irratic readings on your instruments.....  Air Balance is sometimes a fuzzy science.

Sometimes.... sometimes..... Contractors find things that are maybe less than ideal and then say that needs to be corrected...... Mechanical Systems are typically very forgiving.... and maybe the correction is not needed.

Signs of inadequately sized return air ducts would be:  

Excessive Noise..... this is subjective: if it is too loud for you then it it excessive.....  

Low Air Flow.... you need some equipment to measure the air flow ( a pitot tube and manometer or a "hood" ) .... and again this is forgiving.... as long as your A/C units cool the areas required and they do not form ice on the indoor coil section then I would say you really don't have a problem.....

So, if you don't have a noise problem and if you don't have a ice problem and if your equipment doesn't shut down on some type of equipment safety control and if your a/c equipment makes everything cool.... I would say you don't have a problem....

A home with 8 tons of A/C should be fairly  large.... estimates would vary from 600 square feet per ton to 425 square feet per ton..... so 3400 SF to 4800 SF....

I am familiar with this sort of equipment but I do not work with it daily so I could be off-base and would welcome to hear other thoughts on this subject.

RE: External Static Pressure vs. Total Static Pressure

Thanks for the info leatherneck, though this still doesn't explain ESP.   I understand "total static pressure", and, "delta P" and pressure drops accross water coils and the like.   Maybe External Static Pressure isn't realy a correct term to be used in this field? Iv'e never had anyone address or answer the ESP directly.

Thanks though,  Good Day Folks.

RE: External Static Pressure vs. Total Static Pressure

The E.S.P is not a wrong term in the trade, as the name implies; it is the pressure drop through ductwork (supply and return) only, and doesn't include pressure drop through unit components (heating coils, cooling coils, sometimes filters...etc.).
In other word, E.S.P. is the sum of the static pressure drop in stright ductwork, and the static and dynamic pressure drops in duct fittings (i.e. elbows, tees, transition pieces, air outlets,...etc).

RE: External Static Pressure vs. Total Static Pressure

Thank you all for the responses.  I've narrowed my complaint to the builder to 'excessive noise' caused by one of the returns (which is closest to the unit).  I'm still not convinced how to different people can come up with dramatically different results!

RE: External Static Pressure vs. Total Static Pressure

I think it is because some designers, engineers and PARTICULARLY contractors think that HVAC engineering is simple and the cost of an experienced designer isn't worth it.  Then you get someone that doesn't quite understand SOME of the more important parameters for design.  The truth is, the design of HVAC systems takes a lot of experience.

The sad truth is that even some HVAC engineers aren't inquisitive enough to find out exactly where something was derived from or what certain terminology means.  This forum and questions like yours will help many to understand engineering more completely.

RE: External Static Pressure vs. Total Static Pressure

Greetings Mr. Alexander,

I think you should stay focused on the excessive noise and ask for that to be corrected and use your static pressure readings as a reference.....  Your point of discussion, as an owner, is : A system should not be designed with excessive noise. The noise level can easily be reduced by increasing the size of the return air ducts / grilles..... this is easy before the system is installed, it may not be easy to correct now.... The contractor may say " The noise is not excessive ".... and then you can go round and round over this because  Noise is a VERY FUZZY busines....

Unless you have structural interferences you can correct the excessive noise problem by reducing the velocity of the air as it passes thru the noisey section..... and this is done by increasing the size of the duct / grilles.

Good luck and keep in touch

RE: External Static Pressure vs. Total Static Pressure

Remember, noise is not just related to velocity.  It is also related to vibration.  If the area where the noise is excessive is close to the fan inlet, you may need to line the ductwork with insulation to help absorb the noise.

If the noise is due to a balancing damper that is mostly closed to help balance the system, you can also try to line the duct but you may have less luck.

I know, lining the ductwork can get you into other problems, but a liner with biocide will prohibit bacteria and mold growth.  Otherwise there may be nothing you can do.

RE: External Static Pressure vs. Total Static Pressure

Bernoulli's Theorem
How pressure and velocity interact
static pressure + dynamic pressure = total pressure = constant
static pressure + 1/2 x density x velocity2 = total pressure = constant

General Concept:
The Bernoulli effect is simply a result of the conservation of energy. The work done on a fluid (a fluid is a liquid or a gas), the pressure times the volume, is equal to the change in kinetic energy of the fluid.

General Facts:
Where there is slow flow in a fluid, you will find increased pressure.
Where there is increased flow in a fluid, you will find decreased pressure.

In a real flow, friction plays a large role - a lot of times you must have a large pressure drop (decrease in pressure) just to overcome friction. This is the case in your house. Most water pipes have small diameters (large friction), hence the need for "water pressure" - it is the energy from that pressure drop that goes to friction.

Example: the showerhead
A showerhead (if you have a fancy one) has a number of different operation modes. If you go for the "massage" mode, you are moving a little water fast. For the "lite shower," you are moving a lot of water slowly. It takes the same amount of energy to move a little water fast as it does to move a lot of water slowly. This is the amount of energy you have due to your "water pressure."

RE: External Static Pressure vs. Total Static Pressure

I too have been trying to find the "right way" to measure ext. s.p. and have found different answers. I have always taken measurements with one pito tube and the hi or lo on the guege open to the atmosphere. I then take my positive and negative numbers and add them together as if they were both positive. Now should these readings be before and after the fan or directly outside the unit for ext static pressure? I'v included an excerpt and link from an article in the NEWS published 5/21/2001 were the author does it totally different. He uses two pito tubes and measures supply and return simultaneously. This is a frustrating subject!


the next step is to check the external static pressure (ESP) of the duct system. For this you need a manometer, which can read the pressure in the ductwork. By reading the pressure in the ductwork, you can determine if the ductwork is too restrictive for proper airflow and which part of the ductwork is causing the problem.  Personally, I like to use a device called a Magnehelic gauge. These are made by Dwyer Instruments and can measure the pressure in the return side and the supply side simultaneously.  There are two different types of pressure in the ductwork: velocity pressure and static pressure. Static pressure is the one that helps us determine if the ductwork is restrictive.  To measure static pressure accurately, you need to use a device called a static pressure pitot tube. This is the pickup device that is inserted into the airstream. It is connected by a hose to the Magnehelic, which will give you the pressure at that point in the ductwork. Two pitot tubes are required.  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.  By measuring the static pressure in the supply and the return, it is easy to determine if the ductwork is excessively restrictive. Generally speaking, the total resistance to airflow in residential ductwork (not including high-velocity systems) should be in the neighborhood of 0.5 in. of water column or lower (0.5-in. wc).  Note, however, that this is a very generic number. Different air handlers perform differently, so don’t take this number too literally. Some machines won’t move 400 cfm/ton at 0.5-in. wc. If you have access to the application literature for the particular system you’re evaluating, this of course will be very helpful, but many times the paperwork is not available. The higher the ESP of the ductwork, the less air it will move. The lower the ESP, the more air it will move.  So, if I’m trying to diagnose a ductwork system that has low total cfm, I will measure the ESP of the system while the fan is running. If it is 0.7-in. wc, I have a pretty good idea that the ductwork is too restrictive for proper airflow.  The next step is to determine if the problem is in the return side, the supply side, or both. I will remove the pitot tube from the return and measure the supply only, making a mental note of the supply static pressure. Then I will reverse the procedure to determine the return static pressure.  Generally, the return pressure in a properly operating duct system will be much less than the supply pressure. See Table 2 for a guideline of return-to-supply pressure ratios. By comparing the individual return and supply pressures, I can determine which side is causing most of the restriction. Let’s say it’s the supply side causing the problem. I can insert my pitot tube into the supply plenum and watch the results of any modifications I try.

RE: External Static Pressure vs. Total Static Pressure

The contracted inspector provided these results for measuring Total Static Pressure:

3-Ton Unit
Total Static Pressure: .72
Static Pressure Return: .3
Static Pressure Supply: .2
Static Pressure Coil: .22

5-Ton Unit
Total Static Pressure: 1.0
Static Pressure Return: .5
Static Pressure Supply: .4
Static Pressure Coil: .20
The above appears wrong to me...or not understandable.
esp=static pressure of the return+sp of the supply.
sp of return duct is taken 6" below ahu.
sp of supply duct is taken 6" above ahu.

therefore esp of 3 ton unit=.5"
esp of 5 ton unit=.9"

total sp is sum of supply sp + inlet pressure in fan compartment of ahu.  the readings you supply above seem to show pressure drop across the coil..not the inlet pressure at fan the tsp by my definition is wrong.

but you were on the right track that the returns look a little restricted.

RE: External Static Pressure vs. Total Static Pressure

Greetings: Steve A, Leatherneck, Rockhead(???), Louie, et al!

Think I'll join the "fray".

I'm a "downsized" HVAC engineer (AHH: capitalism!... but it's GREAT!!) from one of the big 5 mfcts; now in private practice for a small firm. And by the way Louie: Hear! hear! on what you say about the need for educated designers in our field ... there is a definite dearth!!! ... which is why I was snapped up and allowed the priviledge of working from my home. You're right: this forum is a GREAT TOOL for education and for "chats by the coffee machine" which I can no longer have since the end of my "corporate" life.

I MADE those "mfcts fan perf tables" (ESP) you all look at for about 12 years of my life. The reason so many opinions abound is, as Louie says, everyone thinks "It's not rocket science" (which it ain't); BUT it does take day in and day out exposure to be comfortable with the terminology.

As Leatherneck pointed out: ESP is the STATIC pressure across the unit ... inlet to outlet. And what is catalogued in the lab is per ASHRAE and ASME test procedures: a static pressure "rake" consisting of several STATIC pressure measurement points (various areas of the duct in a single plane ... to get an average) is positioned 3 to 5 "duct diameters" downstrean and upstream of the AHU and the delta-P or ESP is measured at various airflows; as determined by a calibrated ASME nozzle. The reason for more than one point of measurement is to obtain an "average"; anf the reason for the "3 to 5 duct diameters" is to eleiminate turbulent inlet and outlet effects of the unit; without inadvertently including duct friction loss ... if you get too many diameters downstream, there will be friction losses which will unfairly penalize the AHU's apparent performance. Leathernecj is right: You don't care about what's happening IN the AHU: it's a black box that can produce so many CFM against so much resistance: Think of a PUMP: suction as well as discharge.

Anyway, I don't want to "ramble" too much.

Getting back to Steve A's problem: The difference between lab catalogued performance and field measured performance CAN BE (but not always) like night and day... but what else can mfcts do?? Think of it: X number of static P points in the same plane vs ONE in the field. And how close to "90 degrees to the flow" is that probe you're sticking in the duct vs a lab setup?? The slightest deviation from 90 degrees and you're picking up some velocity pressure. And what of turbulence effects??? How many field installations have 3 to 5 straight runs of duct into and out of the AHU?? I think you may be starting to see the limitations of field tests vs thew standardized lab conditions under which data is catalogued.

Mfcts will often publish "base" unit performance with deducts for options like 1" filters or 2" filters, X kW electric heat, hi-eff coils, economizer dampers, wet coil (for cooling, dry coil (for heating), etc. Then "ESP" capability of a particular AHU is the "delta-P" measured from inlet to outlet LESS the above mentioned options ... in other words: what's left for the distribution system: the ducts, elbows, grilles, etc. We designers typically design to 0.08"/100ft for supply and 0.1"/100 ft for return of STRAIGHT pipe .. and "hope" everything else falls out in the "cushion". While this may work for residences and light commercial jobs, the analysis for extensive, high-pressure syatems like say the Wold Trade Center need to be more astute or exacting ... but still, there's "pad".

Often manufacturers will offer oversize drive kits (bigger motors and/or sheaves.. for more RPM) if the distribution system losses are really big.

Back to Steve's question: Max ESP published on the nameplate doesn't mean the unit will "blowup" if that value is exceeded ... it simply means that the standard drive can only do so much... beyond which you will need an "oversize" kit.

But more specifically: Using the fabulous Ductulator (which I hope Louie now love) tells me that the sound problem lies in excessive return velocity to the units. Assuming a general avg of 400 cfm/ton, the 3 ton should be moving 1,200 cfm. The ductulator tells me that with a 12" dia return duct, the velocity will be 1,500 fpm. "Industry" guidelines to avoid "sound" problems in residences (assuming you have reasonably unimpaired hearing)are: Main Ducts: 700 - 900 fpm; Branch Ducts: 600 fpm; Branch Risers: 500 fpm. So the velocity is too high .. no doubt thru the installer's unawareness of these guidelines or efforts to cut costs ... or physical constraints of your residence's construction. For a single return duct, the 3 ton would need a 15 1/2 to 17 1/2 inch diameter duct to satisfy the 700 to 900 fpm guideline. The 5 ton, with 3 returns of 10 inches (2,000/3 = 667 cfm/duct has a high velocity from a sound perspective as well; at 1,200 fpm per my ductulator. Those returns should be 12" to 14"....But would they fit? Or would you, the owner have objected to the higher cost??

You didn't say what your main supply duct sizes were ... and if they were round or rectangular there could be "challenges" here as well! All is not lost, though, as duct liners and stiffeners might yet be an option ... even "white noise" backgrounds or there are products on the commercial market that "cancel" air noise by exotic accoustic wave collision techniques.

It's a constant tug of war: keeping the designer, contractor and owner, occupant satisfied ... which makes this job a satisfying challege ... for those who like challenge!!

I hope this helps  (some??)

RE: External Static Pressure vs. Total Static Pressure

    I ain't an engineer so this will be very basic and to the point. 400cfm/ton at.05 static for residential.Short runs of flex for sound attunuation only.There are many residential tin benders out there that do not have a clue.Bottom line ,you get what you pay for. Ken The HVCA tech

RE: External Static Pressure vs. Total Static Pressure

Judging by the number of replies over such a long period of time, I'm wondering if stevealexander has been able to enjoy his comfort system at all!  I think that's why he has asked for help, his HVAC system is underperforming.  I'll put in my estimate of the situation and hope it proves to be of some help.

As a field service rep for a large HVAC manufacturer, I long ago gave up on trying to measure total ESP on residential applications.  External Static Pressure is literally everything external to the cabinet of the air handler and includes all resistances mentioned by the other respondents.  Since so many residential distribution systems are designed to fit tight spaces, it is very difficult to measure pressure properly in the duct.  As Goorah points out, the pitot tube or static sensing tip must be exactly 90 degrees to the airstream or velocity pressure will confound your reading and lead you down the proverbial garden path.  Residential duct installers often use stack 90 degree fittings with little or no throat, short radius fittings are everywhere, square heels, elbow plenums and on and on.  Aerodynamically speaking, you don't know what's going on inside the duct at the point where you should be measuring - just external to the cabinet.  This is where the air if often most turbulent and you will not be able to measure pressure accurately.  Just forget it!

The tried and true method of determining proper system airflow in residential duct systems is the standard heat rise test.  CFM = BTUH x Temp. Rise.  Believe me, this test can be difficult to perform too.  There are a bunch of things that need to be checked (filter clean, blower clean, evap coil clean, registers open, RA's clear, gas pressure adequate, etc). As long as you take the time to measure the temperature rise accurately and the actual BTUH is determined by clocking the gas meter (use different methods for oil and electric furnaces), this test will prove that the airflow through the air handler is adequate. Most manufacturers provide a CFM vs Temperature Rise charts with their heating products.  If your heating unit is operating in the temperature rise range that's printed on the unit rating plate you can conclude the static pressure must be 0.5" w.c. or less.  Granted, the test is not completely accurate, you will always be a few cfm out, but for all the reasons I mentioned, you cannot get an accurate picture by measuring duct pressures in residential applications.  

I have no doubt that stevealexander's units are running outside of the temperature rise range - this should have been checked first.  If a furnace cannot move enough air to heat a building, it won't move enough to cool it either.  Once the proper airflow has been provided, the system should be balanced.  This is the next most difficult problem in residential HVAC because so many systems are designed to be out of the way.  Some heating runs are hundreds of equivalent feet long.  A heating run comes directly off the plenum going to small powder room in the centre of the building while the master bedroom is kitty corner to the furnace.  Nuts!  Even with the the register closed and the duct damper closed, you still get enough leakage into the powder room to overheat it.  Meanwhile, the owner is looking a installing a window unit in the master bedroom because its too hot in the summer.  It's time to get the heating plant back into the centre of the system as it was once intended.  

You don't necessarily have to calculate the cfm.  Just do a heat rise test to confirm the furance is not overheating.  Here's a few tips.  A proper heat rise is done by using an accurate dual input digital thermometer using type K thermocouple probes meant to measure air temperature.  Take supply & return reading simultaneously.  With large trunk duct, take at least two readings and average.  With two or more supply (or return) trunks, take temperature readings in each and average.  Allow the furnace to operate for at least 10 MINUTES before taking any readings.  Be sure blower motor is on the proper heating speed (if in doubt, use high speed).  Be sure by-pass humidifiers are blocked off.  In stevealexander's case, check the manufacturers instructions to be sure return air instructions have been followed (most 5 ton air handlers require dual return air ducting or all the return air through the bottom of the cabinet).  If the temperature you measure is higher than it should be, it's almost always a distribution system problem.  Stevealexander should have the installer do these tests to his satisfaction.  Hope this helps.

RE: External Static Pressure vs. Total Static Pressure

The info received so far is good, but if you look at the amp draw you will get a good idea about the fan.  Higher static means the fan will more less air.  The fan moving less air does less work and draws fewer amps.  It is a shot in the dark, but if the amp draw is real low compared to FLA, there is too much static.  I would measure the temperature rise on the furnace as well.

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