Static, supply vs. return

[pstu]

If supply static pressure is twice the return static pressure, does that clearly tell me there are duct design shortcomings? The duct system is more or less the Texas flex octopus variety. The actual readings at 1400 cfm are 0.20 return and 0.40 supply. Is more info needed?

I have an HVAC pro I have worked with for a couple years, he does pretty good craftsmanship and isn't too weirded out that I have a manometer, data logger and some ACCA books. His company is NCI trained but doesn't use all the NCI methods. He has measured airflow and static pressure, that sets him above many local techs, however he kinda glazes over when I try to talk about finding duct flaws.

Thanks in advance -- Pstu

[Shophound]

More info would be good, such as:

Attic horizontal or closet upflow?

Air handler or furnace?

Air handler or furnace stated ESP?

Tonnage of condenser and evaporator? OEM matched?

Add-on filtration, such as CleanEffects, etc.?

If furnace, OEM specs for pressure drop over evap coil, wet/dry.

Any balancing dampers on system?

Type of supply registers...stamped steel angled blade or adjustable curved blade?

Size of return grill and ductwork.

Manual J sensible and total heat gain calculations.

General estimation of system's performance; i.e. are you comfortable or is it not combating the humidity capital of Texas conditions very well? :D

[Shophound]

I'd also want to know wet bulb/dry bulb entering evap (return air, allowing for gain) and then measure supply temp just past evap to know if I'm getting the sensible/latent ratio necessary for the structure. If not on track it could either be motor speed setting or restrictive ductwork design or installation. Refrigerant charge also may be a factor. If it's fixed orifice it's got to be dead on or it won't dehumidify well.

[pstu]

>>Attic horizontal or closet upflow?
>>Air handler or furnace?
Am. Std. AUD-080-R9V4 mounted horizontally in attic.


>>Air handler or furnace stated ESP?
I'm a little unclear on the definition. According to the "Service Facts" booklet between 0.3 and 0.9 ESP, airflow with current DIP switch settings will be between 1399 and 1402 CFM. About as flat a fan curve as you can get, provided ESP does not exceed 0.9. Does that answer your question?


>>Tonnage of condenser and evaporator? OEM matched?
Outside is a 3.5 ton Trane XE-1200 recently installed. Inside an Aspen BHL-32-48, I know that is not a proper match. While that may affect several things, I would be surprised and impressed if that were the source of high supply static.


>>Add-on filtration, such as CleanEffects, etc.?
None.


>>If furnace, OEM specs for pressure drop over evap coil, wet/dry.
Don't have that info, sorry. Though it may be in my possession, while I collect technical manuals as an asset I don't read them all.


>>Any balancing dampers on system?
Yes, on every trunk at the plenum. Also at many supply registers, those are cocked wide open.


>>Type of supply registers...stamped steel angled blade or adjustable curved blade?
A mix of good and bad. Ten plastic cheapos, 4 adjustable curved blade. I have the impression adding the curved blade registers (in the last week) has not made a difference in supply static.


>>Size of return grill and ductwork.
Two 20*30 on 18-inch flex, one 12*12 on 10-inch flex.


>>Manual J sensible and total heat gain calculations.
My whole house J7 model said 49,200 sensible BTUH and 59,100 total. Since I have attic ductwork, I figure a CWAG at non-modeled duct losses gives me a goal of 28,600 sensible BTUH for this one system (there is another, new A/S Allegiance 18 3.0 ton). On the one hand latent load is partially a CWAG in the first place, on the other hand I have 3 separate dehus in the house to fight humidity on those rainy April days.


>>General estimation of system's performance; i.e. are you comfortable or is it not combating the humidity capital of Texas conditions very well?

I believe all the other components of performance are working well. The main thing I am wanting to inquire about now, is whether my "fairly high" ESP of 0.60 or so, is because of poor design in the supply area. There seems to be plenty of duct area attached, 14 registers with 366 sqin total area in the ducts at the registers. 405 sqin in the 5 trunks attached to the plenum.

The other system in the house has only 291 sqin area of ducts at the registers, 310 sqin in its 6 ducts attached to the plenum. Its ESP at a given airflow seems to be no higher, leading me to think there is some matter about the duct "design" which makes it more capable. Would the absence of so many "Wyes" in the duct make it better for airflow?

Thanks -- Pstu

[mchild]

Have you read the return static without the air filter(s) in palce to see the what just the static of the return duct is? I bet that most of the return static is from the filter(s).

Was the supply reading taken before or after the evaporator?

[pstu]

It's been a long time since I did that. You should know my return capacity was more than doubled with addition of the 2nd and 3rd ducts, I now have 1344 sqin filter area, and an average air speed at filter face below 200 fpm. My return side is OK as far as I can tell, it only has about 0.20 in w.c. vacuum in the return plenum.

It's the supply ducting I am concerned about. If I could get supply plenum pressure down 0.10 or 0.20 (i.e. 0.30 or 0.40 pressure before the coil), then supply and return static pressure would be closer to equal. And ESP would be in the range a pro might design for.

Thanks -- Pstu

[mchild]

pstu,

I'm confused. In your original posting you stated your supply was .40. In this latest post you state you would like to get down to .30 to .40 before the coil. Am I reading this wrong or not understanding what you are saying. For my confused brain what is the supply reading and was it taken before or after the coil?

I tend to agree with you that your return seems relatively good. Just wondered how much was from filters and how much from duct. With the size of the return it seems like it should be even lower than your reading. Maybe there are a number of turns the air has to make.

[dash]

ESP wise I don't see what you currently have as a problem,the supply being highr then the return ,really doesn't matter.

Now if you lower it,you will reduce the the wattage being used to operate the indoor fan,and save a little.

If you want better filtration,then you need to reduce it for sure,and since the supply is high and return is relatively low,I work on reducing the supply.

As to whay it higher then the other system,it's likely due to Wyes, fittings at the indoor unit,or other turns/90° elbows.

[pstu]

I was doing arithmetic in my head and got it wrong, I should have said I would like to see supply static at 0.20 to 0.30. Sorry for the confusion.

Dash's opinion added to you other pros makes me feel much more relaxed about it all. The one thing it really means is I will hesitate to add any media filter to that system, I can grumble about .60 ESP but would feel worse if it were 0.75 or so. It's just a feeling really, I know the equipment is rated for that ESP and higher.

Best wishes -- Pstu

[davidr]

Did they record the pressure drop readings on the evaporator?
If so what were they, wet & dry.

Your supply ducts maybe fine it might be the evaporator eating up all that static.

[mchild]

Did they record the pressure drop readings on the evaporator?
If so what were they, wet & dry.

Your supply ducts maybe fine it might be the evaporator eating up all that static.

That's why I asked about whether the reading was taken before or after the coil. May be just a dirty coil.

[dan sw fl]

If supply static pressure is twice the return static pressure, does that clearly tell me there are duct design shortcomings? The duct system is more or less the Texas flex octopus variety. The actual readings at 1400 cfm are 0.20 return and 0.40 supply. Is more info needed?

He has measured airflow and static pressure, that sets him above many local techs, however he kinda glazes over when I try to talk about finding duct flaws.

Sounds proper to me.
Check your Total Effective Length (TEL) on Supply and Return
and you will realize that
Supply = 2 X Return ( or 3 x Return, 0.45/0.15) is common.

[Shophound]

>>Attic horizontal or closet upflow?
>>Air handler or furnace?
Am. Std. AUD-080-R9V4 mounted horizontally in attic.

Horizontal natural gas furnace. Got it.




>>Air handler or furnace stated ESP?
I'm a little unclear on the definition. According to the "Service Facts" booklet between 0.3 and 0.9 ESP, airflow with current DIP switch settings will be between 1399 and 1402 CFM. About as flat a fan curve as you can get, provided ESP does not exceed 0.9. Does that answer your question?


Are we determining CFM by dip switch settings, or has a manometer been used to obtain these readings? With dip switches (I'm mainly a heavy commercial guy so I'm not deeply familiar with resi equipment specs...nevertheless resi hvac still interests me a lot) I take it this air handler is variable speed? Variable only to maintain static setpoint, perhaps, since as you note the fan curve is rather flat.



>>Tonnage of condenser and evaporator? OEM matched?
Outside is a 3.5 ton Trane XE-1200 recently installed. Inside an Aspen BHL-32-48, I know that is not a proper match. While that may affect several things, I would be surprised and impressed if that were the source of high supply static.


I'm seeing that as a 3.5 ton condenser on a 4 ton evaporator, correct? What I see right off is not a static concern but one of latent capacity. It may be only 6,000 BTUH difference but it's going to be a warmer coil unless you play with fan speeds a little (and then you might not move enough air through the house to meet design load). You've said a TXV is installed; that will help the coil run more consistent superheat for certain, but it's still going to be better at sensible heat than latent due to more surface area (superheating will occur earlier in the coil, if I'm looking at it correctly).

According to the American Standard specs I found for your furnace, it can handle a four ton coil (at least an OEM coil) so coil size may not be a large factor in your supply static issue. A dirty coil will be of concern, however.




>>If furnace, OEM specs for pressure drop over evap coil, wet/dry.
Don't have that info, sorry. Though it may be in my possession, while I collect technical manuals as an asset I don't read them all.


Sans OEM specs pressure drop can be obtained with a manometer/Magnehelic. Static tip inserted immediately before, then after the coil, yields static pressure drop. Easier to do on a horizontal coil with a transition toward the furnace and a plenum downstream than on an upflow closet furnace. The static pressure immediately downstream of the coil will be the available static pressure for all the supply duct runs.



>>Any balancing dampers on system?
Yes, on every trunk at the plenum. Also at many supply registers, those are cocked wide open.


Balancing dampers add a small amount of pressure drop, even when open. Typically .03 static drop.




>>Type of supply registers...stamped steel angled blade or adjustable curved blade?
A mix of good and bad. Ten plastic cheapos, 4 adjustable curved blade. I have the impression adding the curved blade registers (in the last week) has not made a difference in supply static.


I brought this up for two reasons; one to address your static concerns, the other toward air distribution. The plastic cheapos are restrictive, the adjustable curved blade much less so. If the plastic cheapos are in place to control condensation on the register face, the condensation is an indoor dewpoint problem, not register face material choice.

Many, many residential supply registers installed in the ceiling use stamped steel angled blades. The direction of throw from these registers is entirely wrong. It deflects the air down into the occupied zone (defined as from the floor up to six feet, and one foot away from walls) and does a poor job mixing room air. A ceiling mounted curved blade register can be adjusted to throw air along the length of the ceiling toward outside walls, entraining room air into the primary jet as it does so. This jet will slow and drop as it nears the wall, away from occupants so no draft is detected.

Sidewall registers with their horizontal vanes adjusted between 5 to 15 degrees upward and also aimed toward outside walls will accomplish similar results. When either is done correctly the room will be very comfortable, with much more even temperature gradients and minimal stagnation. All of this is concurrent with properly sized, unrestricted return air provisions.



>>Manual J sensible and total heat gain calculations.
My whole house J7 model said 49,200 sensible BTUH and 59,100 total. Since I have attic ductwork, I figure a CWAG at non-modeled duct losses gives me a goal of 28,600 sensible BTUH for this one system (there is another, new A/S Allegiance 18 3.0 ton). On the one hand latent load is partially a CWAG in the first place, on the other hand I have 3 separate dehus in the house to fight humidity on those rainy April days.


With a J7 sensible calc of 49,200 and total calc of 59,100, that is a sensible heat ratio (SHR) of .83, which would be appropriate for Houston. Here in Fort Worth it's a bit drier, with SHR numbers over .9 to be common for design conditions. Manual S has tables stating where the supply air temperature delivered to a room should be when the SHR is known. I'll look at it again when I get to where it is, but going from memory it would appear your supply air coming off the coil should be around 54 degrees. If you're running warmer than this it could be for several reasons; one being the mismatch between coil and condenser.



>>General estimation of system's performance; i.e. are you comfortable or is it not combating the humidity capital of Texas conditions very well?

I believe all the other components of performance are working well. The main thing I am wanting to inquire about now, is whether my "fairly high" ESP of 0.60 or so, is because of poor design in the supply area. There seems to be plenty of duct area attached, 14 registers with 366 sqin total area in the ducts at the registers. 405 sqin in the 5 trunks attached to the plenum.


General consensus has residential TESP at around .50 - you're just .10 over target, going by your readings. You'll probably see a slight drop if you ditch the remaining plastic crap registers and go with adjustable curved blade.




The other system in the house has only 291 sqin area of ducts at the registers, 310 sqin in its 6 ducts attached to the plenum. Its ESP at a given airflow seems to be no higher, leading me to think there is some matter about the duct "design" which makes it more capable. Would the absence of so many "Wyes" in the duct make it better for airflow?

Thanks -- Pstu


The wyes add about 35 feet of total equivalent length (TEL) each if I recall correctly.

[pstu]

>>If the plastic cheapos are in place to control condensation on the register face, the condensation is an indoor dewpoint problem, not register face material choice.

Could you please explain to me how the plastic cheapos might control condensation on the register face? I never dreamed it would have such an effect.

I aim for indoor dewpoint around 57 and usually keep close to that, so am hoping this is academic rather than a practical problem.

>>I'm seeing that as a 3.5 ton condenser on a 4 ton evaporator, correct? What I see right off is not a static concern but one of latent capacity.
>>It may be only 6,000 BTUH difference but it's going to be a warmer coil unless you play with fan speeds a little
>>(and then you might not move enough air through the house to meet design load).

Funny thing is, that coil was originally attached to that ICP compressor of considerably smaller capacity. We now think it was a "light" 3 ton unit based on fan motor, while I believe the Trane is fully 3.5 ton. It appears they were going for SEER originally without too much regard for latent capacity. I don't mind having a good reason to slow down airflow from 1400 CFM, to perhaps 1200 CFM. That would be 350 CFM/ton and based on past experience would bring ESP down to .50-.55. That sounds like a double win.

>>Sans OEM specs pressure drop can be obtained with a manometer/Magnehelic. Static tip inserted immediately before, then after the coil, yields static pressure drop.

I can actually do that. Have placed a 3rd manometer takeoff after the coil, mainly since I was unsure earlier which would be the correct place for the high-side portion of ESP. Never used it before. Now all I gotta do is overcome my inertia and climb into the attic while the temperature is still bearable, and then read the manometer while a cooling call is ongoing.

My AC company placed an inspection door in the transition, I trust when they looked at it last week they ascertained the coil was clean.


Thanks for all the ideas. Hope I am keeping up with the questions you are asking, at least most of them.

Best wishes -- Pstu

[Shophound]

>>If the plastic cheapos are in place to control condensation on the register face, the condensation is an indoor dewpoint problem, not register face material choice.

Could you please explain to me how the plastic cheapos might control condensation on the register face? I never dreamed it would have such an effect.I think this is what some folks think will resolve sweating register faces vs. it actually being able to do so. If the face temp of the register is lower than the dewpoint of the room, it will sweat.


I aim for indoor dewpoint around 57 and usually keep close to that, so am hoping this is academic rather than a practical problem.
Funny thing is, that coil was originally attached to that ICP compressor of considerably smaller capacity. We now think it was a "light" 3 ton unit based on fan motor, while I believe the Trane is fully 3.5 ton. It appears they were going for SEER originally without too much regard for latent capacity. I don't mind having a good reason to slow down airflow from 1400 CFM, to perhaps 1200 CFM. That would be 350 CFM/ton and based on past experience would bring ESP down to .50-.55. That sounds like a double win.
Sounds about right according to fan laws:

2nd CFM = 1st CFM x square root of (2nd static pressure/1st static pressure)

What I'd want to monitor for tweaking latent capacity is the leaving air temperature at the evaporator. Your muggy climate would call for a leaving air temperature of around 54-55 degrees at 75 degrees/63 degrees wet bulb return air. With your stated dewpoint at 57 degrees it looks like you're about there, which gives you about 53% relative humidity in the house. Do you maintain the house at around 75 degrees, or is it a little warmer?


I can actually do that. Have placed a 3rd manometer takeoff after the coil, mainly since I was unsure earlier which would be the correct place for the high-side portion of ESP. Never used it before. Now all I gotta do is overcome my inertia and climb into the attic while the temperature is still bearable, and then read the manometer while a cooling call is ongoing.
I know what you mean about that inertia deal when it comes to attics. This past winter I put some time up in my own attic, sealing round pipe duct runs, sound attenuating each run with short lengths of flex, sealing plenums, take-offs, supply boots, and re-insulating with R6 with mylar jacket on remaining metal duct runs. Lots of work and some days I just had to drag myself up there, but it's paying off for us in better home comfort already.


My AC company placed an inspection door in the transition, I trust when they looked at it last week they ascertained the coil was clean.
Thanks for all the ideas. Hope I am keeping up with the questions you are asking, at least most of them.

Best wishes -- PstuYou're doing fine. It's enjoyable to "talk shop" with someone as interested in his home HVAC system as you are. Most people I know get that glazed over look whenever they ask me about what I do for a living and then I begin to explain it. :D