Urgent return duct size question.

[Freeagent]

We are having 3ton heat pump installed to replace a 2 1/2 ton A/C.

Contractor is in presently installing new return duct. Original duct started at a 20x24 ceiling grille and ran through several elbows to air handler suction. It was mostly 20x24.

New duct starts after first original 20x24 elbow but then reduces immediately to 10x18. This runs about 16ft and then through an elbow into air handler return box.

It just looks wrong. Velocity is close to 1000 cfm.

To sum up, we have 1200cfm return through:

- 20x24 ceiling filter grille
- 20x24 elbow
- flex joint
- reducer 20x24 to 18x10
- about 16ft of 18x10
- elbow 18x10
- 17x17x20 box on end of air handler

The installer will be here first thing to continue, but if theis is wrong I would like him to stop until sizing is double checked.

Do we have a problem?

[pstu]

We are having 3ton heat pump installed to replace a 2 1/2 ton A/C.

Contractor is in presently installing new return duct. Original duct started at a 20x24 ceiling grille and ran through several elbows to air handler suction. It was mostly 20x24.

New duct starts after first original 20x24 elbow but then reduces immediately to 10x18. This runs about 16ft and then through an elbow into air handler return box.

It just looks wrong. Velocity is close to 1000 cfm.

To sum up, we have 1200cfm return through:

- 20x24 ceiling filter grille
- 20x24 elbow
- flex joint
- reducer 20x24 to 18x10
- about 16ft of 18x10
- elbow 18x10
- 17x17x20 box on end of air handler

The installer will be here first thing to continue, but if theis is wrong I would like him to stop until sizing is double checked.

Do we have a problem?

If your arithmetic is correct, 1000 fpm is way higher than ACCA Manual D says is acceptable. My copy of Manual D says 700 fpm is maximum.

Not many people get static pressure checked on their systems. So much would become clearer if they did. You might consider requesting your professional to do that, if the answer is out of spec for your air handler (furnace) that seems to me it would confirm you have a problem. Every pro I have heard with an opinion, says 0.5 inch w.c. is the desirable number, it could be higher but your furnace is only rated for a certain level.

Hope this helps -- Pstu

[cuchulain]

it's too small. To be honest even the 20x24 R/A grill is small for a 3 ton system.

[Freeagent]

Thanks for the replies.

I did ask the contractor if our existing ducts were adequate and they said they were. But they just checked the size of the main trunks for the 600cfm that each handles. I did my own check as best as I could and found that supply side to most distant register was about 0.4" (air handler has max of 0.8"). The return duct was short but with 3 elbows. I assumed that any changes contractor made to return would not increase the losses. But now I see what they are doing, I am sure we will have higher losses and noise levels.

Just wanted to be sure I was not wrong because I have to stop work on that section first thing tomorrow.

[HvacAndBeyond]

How did this end?

[Freeagent]

How did this end?

It has not - Discussion with contractor will take place about 8 hrs from now!

[Freeagent]

Contractor is insisting that 18x10 for return to unit is correct. Where can I get a reference that indicates this is not to industry standards?

[genduct]

Although the "rule of thumb" regarding 700FPM for return is true, it was there more because of noise. Pressure drop is your main concern and with what you describe there is a good chance your pressure drop will be reasonable.
You'll know when you take a pressure reading when it is running. In the mean time ask the installer why they ignored the well know rule of thumb that would mae the duct close to 2 sq ft area.

If pressure is about -0.1 then OK but if noisy tell him you don't want to live with the unnecessary noise. No you are not reassured by the sound of it running

[second opinion]

Contractor is insisting that 18x10 for return to unit is correct. Where can I get a reference that indicates this is not to industry standards?

is the final section of the return for the upstairs correct?

[pstu]

Contractor is insisting that 18x10 for return to unit is correct. Where can I get a reference that indicates this is not to industry standards?

The attachment is a scanned page from ACCA Manual D. That is a direct answer to your question. Copyright 2002, page 3-6. I believe there is a later version of this book.

It sounds as if your contractor believes this maximum speed is just optional. I do not know enough to argue that point. You might want to do some quick calculations and see if the return grille or filter face velocity limits are also exceeded.

Again, will they measure the external static pressure (ESP) of the finished job and make sure that is a good number? As I understand it one of the usual consequences of violating these ACCA rules is excessive ESP. This either consumes more electricity, or prevents rated air flow.

Hope this helps -- Pstu

[Freeagent]

is the final section of the return for the upstairs correct?

Thanks Genduct and secondop.

Our original return duct was very short - just a series of three large elbows about 20x24 from single ceiling filter/grille to old coil and Delhi air mover. New system starts after the first original elbow, reduces in size and runs more or less straight 16-20ft except for one z-fitting to jump over supply duct. Then at end it has elbow into a box that fits on end of new air handler which is installed horizontally. Picture attached - return comes down right side of unit, turns into box the will fit over filter end of handler. Supply will connect into tee on far side (not yet in place)

After further discussion, they agreed that size their guy chose was based on commercial (there main activity), but they now agree that for residential size should be larger. For practical reasons, it will be made 20x14 which gives us 615 fpm. This should reduce the suction sp.

Handler has variable speed ecm motor and is rated up to 0.8". I would like it to be 0.5" so as to allow for filter loading. Will have to recheck complete system, but at worst, we can reduce flow to if we run into problems.

Contractor has headed back to shop to make new ducts! I feel a bit better!

Thanks for the support!

[second opinion]

You are going to use a filter media cabinet at the end of the unit?

[genduct]

The 900 and 700 FPM is still good guide, in fact the Fitting Equivilant fitting values are based on these values and if you exceed these values you are supposed to adjust the FEL Values.
That being said the "mandatory" nature of having the 900/700 as the max has been removed from latest Ed of Man D. It was recognized that the "max" values were more a function of noise in traditional unlined sheet metal. Now with liner and especially duct board, the acoustic concerns are gone.