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Duct Losses vs Static Pressure

Duct Losses vs Static Pressure

Duct Losses vs Static Pressure

I am new to the HVAC realm and just visiting it at that.  I have a system that I am designing which is probably most similar to a car's.  I need to spec an Air Handler and my biggest concern as opposed to cooling power is the flow rate coming out at the end ports.  When I run through calculations using Equal Friction, I get worst case losses of just under 6 in H2O.  All the air handlers I come across have static friction ratings of .5 most commonly and with some changes that can get into the 1.x's but now where near my 6inH2O.

Does this mean if I use one of these AHU's with my system that I will not get the flow I need?  I dont really understand the implications of the static pressure of the AHU.  Is it strictly a "must supply more static pressure" than losses? Or is there another way to look at it that will mean my system could work afterall?

Thanks for any help,


RE: Duct Losses vs Static Pressure

Are you totalling the entire systems static losses?
As with pumping water, the system static losses will be the total of the worst case run.
This is usually the longest run, but not always, some shorter runs may have higher restriction fittings, etc...
Upsizing the duct, minimizing flex duct, full sweep elbows, turning vanes in square elbows, & more, can relieve losses.
The losses are based on a "per 100'" length.
a 50' duct at .08"/100' will be .5x.08= .04"WC lost in that portion of the duct.
Trying to find a blower to overcome 6" TSP will be an expensive task, & cost of operation of that blower will be expensive. It will also add to the load as the heat added to the airstream (if it is in the airstream) will need cooling as well.
How is this system similar to a cars? (Hot water/DX?)

RE: Duct Losses vs Static Pressure

Yeah, I determined the worst case, I will have to verify accuracy, but I did confirm some individual loss sections with some online calculators so I ran with my numbers and assumed that they are "fluids" accurate.  It is similar to a car in that it has qty 10 car-sized outlets into a small area with the outlets having minimal flow requirements.  There are also some slightly larger ducts within the system but my numbers determine one of the smaller ones with the large flowrate requirement to be my worst case run.

Im trying to get a handle on the whole thing, if my blower TSP is too low, what happens?  Does flow stall? Do I just get a lesser flow?  I am sure there may be different results for different situations, but I am just curious in general.

I going to make sure I accounted properly for the "per 100'" definition.  But anything else you can add would be greatly appreciated.

Thanks for the help, I really appreciate it


RE: Duct Losses vs Static Pressure

If your blower TSP is too low the flow will possibly match the intersection of the system curve with the fan curve.  You may also be operating in the unstable range of the fan curve.

You may look at resizing the supply duct to the nozzle outlets to reduce your critical static pressure.

RE: Duct Losses vs Static Pressure

At this kind of pressure, 6" wc,  you should be looking at backward-inclined blades or turbine fans which do not have the unstable regions of forward-curved and vane-axial fans.

How many cfm do you figure for the system?
How many tons cooling?

RE: Duct Losses vs Static Pressure

Well, at the risk of all of you thinking you're wasting your time with me, I did a little "tweaking", I had not normalized to 100ft, and I revisited some of my fitting coefficients, I think all is fairly accurate now and I get down to just above 2".

Total cfm we come up with is around 1600, 23K BTU/hr.  Largest duct is 4"dia with couple as small as 1.25"dia.  If anything here looks too wacky, just let me know.

RE: Duct Losses vs Static Pressure

I strongly advise you to recheck your calculations once again. (I would rather use a bigger size even for compressed air of 10 kg/sq.cm g pressure at this flowrate.)

1600 cfm for 23000 btu/hr is ok. For 1600 cfm, with a 4" size duct, you should get ridiculously high pressure drops, many multiples of 6.

RE: Duct Losses vs Static Pressure

...if your duct length is beyond 8'.

RE: Duct Losses vs Static Pressure

All of the 4" ducts are less than 7' and all the small ones are less than 4.5'.  I guess what is really confusing us/me is that we have done similar jobs in the past and the calculations also show greater duct losses, in the 3+ range, yet when the system has been installed, there is so much excess flow that they have to work very hard to downsize the systems performance even though the calculations show that based on ESP the unit isn't big enough.  Is there something I am not accounting for that can make up for the ESP, essentially boosting it's effect?  Most AHU specs I have come across have .5" ESP with some boostable to higher but values of less than 2"; does that sound correct.  My biggest concern here is the numbers, considering that in the past, very similar systems have been more than fine even though the numbers indicate otherwise.

Thanks again for all the help, if I seem frustrated, it's all at the numbers, I appreciate all the input.  And as you can see have a lot to understand with this stuff.


RE: Duct Losses vs Static Pressure

For the AHU contact your local supplier, Trane, York, McQuay etc.  They should be able to provide you with a fan that can accommodate the static pressure you are looking for.  Some older systems used to use high pressure duct >3"w.g. to a terminal that would reduce the pressure to <1" for the branch take offs.  (Saved in duct sizing initially but paid for energy consumption).

Anyway the major manufacturers should be able to accommodate you.

RE: Duct Losses vs Static Pressure

Let's step back a bit...
Your previous post said you had 10 outlets, correct?
Are you hoping to deliver similar amounts of air to each diffuser? Are there any balancing dampers on any of these branch ducts, or was this originally designed to deliver measured amounts based on the varying duct sizes/lengths?
(Figuring out how someone else designed an older installed system is like reading a dirty crystal ball...)

What is the reason for 4 Tons of airflow for 2 tons of cooling?
In my book 400 cfm per Ton is the rule. If you can get by with less air, you may be able to cut your TSP in half or more, & still deliver the conditioning required.
Even in high velocity, Space-Pak type systems, the airflow is lower (prx 250 cfm per ton I'm thinking) These system will run upward of 2" TSP & care less about velocity, static, & noise. That's why a typical high velocity system will have a 2HP blower for 2-1/2 tons vs. a 1/2HP blower in a conventional system.

What is the air velocity across the coil. If your moving the air too fast you'll loose the latent heat extraction & never control humidity. The coil will barely be wet & the drain won't be needed, conversly too slow a face velocity will lead to coil freeze up, no airflow & compressor shutdown on pressures etc...

good luck,

RE: Duct Losses vs Static Pressure

Hopefully I am looking at it correctly but the flow and cooling are, unlike I assume "normal" systems, not necessarily complimentary.  We basically have a heat load requirement based on rough accounting, but something we feel is fairly accurate for our purposes.  The flow is based on a requirement of one of the outlets.  I basically have tried to back-out what other flows will be based on that and they have of course all contributed to the final number.  I think we have gone about this the wrong way and might be fighting a losing battle, but I am trying to get it to work out regardless.  We do have dampers along the way and I think they will be our saving grace in the end.

As far as the coil velocity goes, that again, not something I have a good grasp on.  I have a system that I am looking at that will provide the airflow and cooling I need (strictly speaking in cfm and wattage specs), but with the ESP issue, I get concerned about the effects; in my understanding, if we are overloading the systems's pressure, we will influence the flow across coils and begin to address your concern there.  Maybe I'm wrong.

Can anyone give me an HVAC knowledge infusion?

Thanks 12000 BTU/hr again ... anything anyone can add is great

RE: Duct Losses vs Static Pressure

I would also be very careful with ductwork in and around the AHU as system effects (see AMCA handbooks) from poor ductwork may be causing problems.

The duct size and volumes make a lot of my flags go up at 3" sp. 7' of 4" duct handling 250 cfm is about 0.35" wc duct loss only. Coil is likely 0.5-1.0" wc, so we're still way under 3" wc.

Does the system have a filter? What kind?

RE: Duct Losses vs Static Pressure

The path I am using to back all my other info from ends with a 2.25" hose, fed by 4", fed by 12".  There are other 4"s that take off from the same pt, but that is my worst.  My 2.25" is the 112cfm but there are 2 of them and then I have 2 different (but identical to each other) ducts taking of from same "node".  There are also some fittings in there, so that's were my number is coming from (approx. 2.4").  Not sure yet on filter, but isnt ESP of an AHU outside the influence of a normally operating filter?  Correct me if wrong.

I am trying to basically "plug" an AHU in as long as it can give me what I need.

I hope Im not frustrating too many out there, bear with me.

RE: Duct Losses vs Static Pressure

Now you've gone & done it...
TSP & ESP, That is the question!

TSP stands for Total Static Pressure.
ESP stands for External Static Pressure.
Problems arise in asking "External to What?"

Some Mfgrs regard it as external to the AHU/System,
others say external to the blower.
I've found it difficult to get a Mfgr to commit to the ESP question & I've debated & changed my mind a number of times on this issue.

Since some AHU's have coils & filters built in, they can be considered part of the unit/system (that the SP is external to)
Some will take the filter at .1-.25 inch (clean to dirty) & throw in another .1-.2 for a damp/clean coil.

I usually give the coil to the system, but I think the filter should always be considered external to the system, as it is one of the most restrictive components (except for small duct & bad fittings)& it's pressure loss against the blower slides back & forth based on it's condition.

If anyone else can explain TSP & ESP otherwise, I'm willing to change my mind again.

Again, Good Luck!

RE: Duct Losses vs Static Pressure

If the fan has to get it moving (velocity pressure) and overcome friction/fitting losses (static pressure) the fan will have to do the pressure (total pressure), no matter what the fan purveyor says. Everything in the air path counts.

If the AHU purveyor gives you some flack that his is better than the other's, guess what he's selling............

Physics is independent of the manufacturer.

RE: Duct Losses vs Static Pressure

swisgr8, Let see if this makes sense to you. First let me say that when they say a fan has an external static(ESP) pressure, what they mean is that if there is no restriction to the supply air then the static pressure is equal to the velocity pressure which is equal to the total pressure. 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(TSP) Total pressure remains the same. SO lets take an example of an equal friction method of calculating a total system
1. 0.50" wg total external static pressure produces 1000 cfm
minus 0.10 " wg design supply side resistance (DUCT WORK)
=0.40 wg
2. -0.05 wg design return side resistance
= 0.35 wg static pressure remaining
3. -0.20 wg wetted coil resistance
= 0.15 wg static pressure remaining to produce 1000cfm
4. -0.05 wg resistance fo clean filter
= 0.10 wg remaining to produce 1000 cfm
5. -0.08 wg for dirty filter resistance
= 0.02 wg static pressure remaining for 1000cfm
Now if the external static pressure is exceeded because of poor installation, or other conditions then the cfm produced by the system will drop.

There are charts avialible to determine what the total pressure drop will be when you figure the total equivelent run of the longest supply and return duct.

Example: say on the return duct with a design pressure drop of 0.05 wg you determine that the total equivilent length(TEL) is 95-104 ft that is = to a pressure drop of 0.05
wg which is perfect, now for the supply duct, the design is 0.10 "wg so if the(TEL)is say130-150 ft the total pressure drop = 0.07 " wg

Rmember all lengths of duct and all fittings for turning , reductions, etc. havd a TEL of duct that must be added to the regular length of the duct run. hope this helps!

RE: Duct Losses vs Static Pressure

Great post IMOK2!
I'm printing a copy to hand out to a few I work with.

Not to pick nits with your example..

"return duct with a design pressure drop of 0.05 wg you determine that the total equivilent length(TEL) is 95-104 ft that is = to a pressure drop of 0.05"... supply duct, the design is 0.10 "wg so if the(TEL)is say 130-150 ft the total pressure drop = 0.07" wg"

The S.A. duct will have a pd of .13 to .15 call it .14,
added this to the R.A. pd giving a pd of .24 for duct system.
Unless these items have been included in the TEL calcs, you will have to add the pd's of the coils, filters, diffusers & grilles, dampers, etc... & TSP will be known. now you can select the blower.

Again certain mfgrs of equipment will rate the AHU with ESPs external to the built in components (coils & occasionally clean filters) & others will rate the blower alone.

good luck

RE: Duct Losses vs Static Pressure


When you mentioned taking the worst-case run for determining system requirements, that was one of the things I understood, but with respect to that, I was looking at balancing the system which I have also come across and makes sense to me ... I think.  What doesnt make sense is that I was just thinking about the whole "balancing act" (like that?) and from the way I unsderstand it, if your system is balanced, aren't ALL runs going to be approx equal?  Again, not questioning anything, just trying to make sense of this.


RE: Duct Losses vs Static Pressure

A balanced system is just one in which all outlet volumes are as shown on contract documents.

Balancing is done by introducing an artificial pressure drop with a damper in individual run-outs.

Balanced does not mean equal flow on every  branch.

RE: Duct Losses vs Static Pressure

I kind of hear you there, but I wasnt meaning equal "flow" but nearly equivelant losses.  Again I'm a newbie here, so just saying my point from my understanding.  I understand the field/real-world value of the dampers, but the balancing of losses makes sense to me in terms of the path of least resistance analogy.  If something can go through a less resistant path, than it will do so until the optional paths all offer a relatively close level of resistance.  And with respect to the design parameters, my understanding tells me that if you do not design to a balanced system then the numbers are really just arbitrary and the real system will not produce them without "fied" tweaking (which again I understand that is part of the whole game).

Please any and all correct my thought process if gone astray.   Not challanging your point willard3, just trying to understand it.

Thanks for your help,

RE: Duct Losses vs Static Pressure

I kind of hear you there, but I wasnt meaning equal "flow" but nearly equivelant losses.

This is equal friction design and is the basis of the Trane and etc wheels.

There are always physical constraints for ductwork size and aspect ratio. Air has lousy specific heat and it takes lots of it in big pipes to heat or cool. Ventilation also takes high air volumes.

HVAC systems are contained within other structures and all systems compete for volume.

Given the above, it's not always possible to have equal friction ductwork and compromises are made. It's the Engineer's job to make sure it works.

RE: Duct Losses vs Static Pressure

Gotchya ... thanks for all the patience and explanation

Appreciate it,

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