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charles2
12-19-2007, 09:41 PM
What is the minimum allowable square inches of return filter grille area for a 2.5 ton heat pump?

Mr Bill
12-19-2007, 10:15 PM
What is the minimum allowable square inches of return filter grille area for a 2.5 ton heat pump?

My guess the minimal would be 250 sq. in. but there are a some variables there also but personally
I would say on a 2.5 ton if you have at least 250 you should be ok, thats my best answer. :D

gonekuku
12-19-2007, 10:33 PM
I agree with Mr. Bill. That's a good FPM.

Tons 2.5
CFM/Ton 400
CFM 1000
SQ in 250
FPM 576

FPM = CFM / ( SQ IN / 144)

dan sw fl
12-20-2007, 06:04 AM
What is the minimum allowable square inches of return filter grille area for a 2.5 ton heat pump?

? Minimum allowable ?

Good Practice
400 FPM is standard Filter Face Velocity for low filter pressure loss.

AHU CFM = ~ 400 CFM/ ton * 2.5 Ton = 1000 CFM

Square Feet = CFM/ FPM

1000 CFM/ 400 FPM = 2.5 Square Feet .. 360 square inch

______
3 Ton .. ~ 1,050 to 1,200 CFM depending on climate ... 3 square feet ... 432 Square Inch ... 20"x20" or 24" x20" are frequently used

> 3 tons in residential use ... 2+ filters

dash
12-20-2007, 10:28 AM
Return grille with filter 300 fpm,without 500 fpm,per ManualD the industry standard for duct system design,less fpm is better.
2.5 tons,1000 cfms,18"X 30" filter grille,would be okay.

beenthere
12-20-2007, 11:52 AM
6 sq in free area per 1000 btu's.

glewis29
12-20-2007, 12:58 PM
Going to hijack this a bit...

When I got my new system put in last summer, they increased the unfiltered return size as much as they reasonably could, but it's still only a little over 2 sq ft for a 4 ton system. It's zoned, so most of the time it doesn't push that much air, but when it does, it gets pretty loud. Are there any major issues I should be concerned about that are worth looking into getting a bit more return area?

Thanks :)

beenthere
12-20-2007, 03:31 PM
Makes the system work harder, so it uses alittle more electric.
Does the zoning system have a bypass damper.

glewis29
12-20-2007, 03:46 PM
No, so if the pressure got to be too much that could definitely cause problems (as happened when they first installed and got one of the dampers wired backwards). Under normal operations though, the ducting only gets what I would consider a high pressure as dampers are opening or closing to match the furnace speed.

The system is a variable volume trane zoned system.

beenthere
12-20-2007, 03:53 PM
The harder a VS blower works, the more electric it uses. Let alone the low air flow takes longer to heat up the house meaning the HP is running longer then it should so it uses more electric.

glewis29
12-20-2007, 04:01 PM
Right, but is there any potential for damage due to strain on the motor, or anything else I should worry about? If I'm concerned, I suppose I could have them come out and measure the static pressure... what SP would I be looking for on the return side?

Thanks :)

captube
12-20-2007, 06:50 PM
Return grille with filter 300 fpm,without 500 fpm,per ManualD the industry standard for duct system design,less fpm is better.
2.5 tons,1000 cfms,18"X 30" filter grille,would be okay.

This is what i use with filter grills also.
Works out to 2 cfm per sq.in.
1,000/2=500sq.in.
18x30=1080sq.in./2=540 perfect.
Best to ck. the grill mfg.'s rating on their particular fin spacing for free sq. in.

Freezeking2000
12-20-2007, 07:09 PM
How about the return grill size for a 80K BTU furnace with a 50 degree rise?

I have one thats got an 8" amd 8x24 and it is loud.

cbh04
12-20-2007, 07:53 PM
144sq ins./ton

charles2
12-20-2007, 09:10 PM
Let me ask my original question in a different way: What is the minimum allowable diameter for a round return plenum (before or after the grille) on a 2.5 ton heat pump?

tinmantu
12-20-2007, 09:41 PM
diameter depends on distance from intake register to the unit. to determine static pressure.

dan sw fl
12-20-2007, 09:49 PM
Let me ask my original question in a different way: What is the minimum allowable diameter for a round return plenum (before or after the grille) on a 2.5 ton heat pump?

That's not even close to the same question.

Charles II, AND How does one have a plenum BEFORE the Return Filter Grille?
Is that in Phantom space?

How long is the return plenum? ___ Are there any transitions or elbows? ___
What is the Total Equivalent Length ( T.E.L.) for your entire duct system?

How much noise would you like to hear?
Is it allowable for you to have an ~ NC 50 rating?

What does the blower performance curve look like?

OHhh maybe 14" diameter would be sufficient and tolerable in a humid climate?
But 14" is definitely NOT desirable !

charles2
12-20-2007, 09:51 PM
Alright, let's say 2 feet.

dan sw fl
12-20-2007, 09:53 PM
Alright, let's say 2 feet.

T.E.L. is likely closer to 200 feet

charles2
12-20-2007, 10:07 PM
Charles II, AND How does one have a plenum BEFORE the Return Filter Grille?
Is that in Phantom space?

How long is the return plenum? ___ Are there any transitions or elbows? ___
What is the Total Equivalent Length ( T.E.L.) for your entire duct system?

How much noise would you like to hear?
Is it allowable for you to have an ~ NC 50 rating?

What does the blower performance curve look like?

OHhh maybe 14" diameter would be sufficient and tolerable in a humid climate?
But 14" is definitely NOT desirable !

Dan, I don't know how they do things in SW FL, but up here (in NE GA), I've seen filter grilles before return plenums, and filters after return plenums (i.e. fitted tight to the air handler). I guess the confusion is whether we are talking about a filter grille or just a plain grille.

Anyway, I'm looking at a situation right now where the return plenum looks grossly undersized, and there are abrupt transitions between round and rectangular duct. I'm fixing to measure the diameter of the round section and post it. Wish I had a camera.

dan sw fl
12-20-2007, 10:34 PM
Anyway, I'm looking at a situation right now where the return plenum looks grossly undersized,

and there are abrupt transitions between round and rectangular duct.
I'm fixing to measure the diameter of the round section and post it.

Ask Santa, ... maybe she will get one at Best Buy or Circuit City
if out-of-stock at North Pole

Alternate plan .. borrow a relatively new digital camera
then you will be hooked

http://www.circuitcity.com/ssm/Sony-Cyber-shot-DSC-H7-B-8-1-Megapixel-Digital-Camera-Black-DSC-H7-B/sem/rpsm/oid/174597/catOid/-16941/rpem/ccd/productDetail.do

One can always dream ... my dream did come true

charles2
12-21-2007, 09:59 AM
Hey, I measured the round bottleneck in the return plenum late last night. It's actually an ellipse that measures 14" x 18". Since a 14" x 20" return for a 1.5 ton system is appropriate, do you agree that this ellipse is grossly undersized for a 2.5 ton system?

dan sw fl
12-21-2007, 03:17 PM
Hey, I measured the round bottleneck in the return plenum late last night. It's actually an ellipse that measures 14" x 18". Since a 14" x 20" return for a 1.5 ton system is appropriate, do you agree that this ellipse is grossly undersized for a 2.5 ton system?

1000 CFM
209.9 sq inches for 14 x 18 oval duct = 1.458 Sq.Ft

686.04 FPM is just a little of the high side for returns

.........
For NE GA, your A/C can be set-up for 350 CFM / ton

350 CFM/ Ton
2.5 tons
875 CFM
600.29 FPM ... sounds O.K. to me.

Although it is not exactly desirable, it sounds O.K.
as long as there are not any other significant restrictions
creating a T.E.L. >> 200 and filter is not > MERV 8,
then the blower performance may (likely) be adequate.

Change your filter at least 4 times a year! !!