HVAC Design - Air Return

[Jeffery L. Haynes]

Have a Carrier gas furnace, model CNRVP3014ATAABAA, that is supplying
a two story home (800 down / 1200 up) with respective air return sizes of 14x14 and 20x20.

The owner complained of excessive noise from the air return......which I agree is loud.
The unit is housed in the laundry romm on the second floor with the return going directly to an adjacent wall that separates the laundry room and hallway.

The 20x20 air return is the only one upstairs that has to take care of 3 bedrooms plus the addtional rooms up there.

While I am no HVAC contractor it seems like this was a bad design.

Comments appreciated.

Jeff

[Charley L. Bottger]

Jeff in the 40 years I spent working in that field lack of and or inadequate return air was the most common flaw that I observed on any system. When trying to explain return air I like to use the bottle approach one can only blow so much air into a bottle and when the bottle get full it starts to whistle and makes noise. Increase the size of the return or get some ear muffs

[Jeffery L. Haynes]

I have heard of 200 sq. in. per ton for air returns, however I also know that many "rule of thumb formulas" used in the construction industry are just that.........you take a home that has 1200 sq. and its very open, then yes I would a 20x20 is adequate when strategically placed, however you take that same area and make a maze out of it, add interior doors that typically have a half inch gap...... than no matter what size your return is, it is still over taxed.

Trying to convey that point to the builder, especially a track builder, so that he will act is challenging; while I am not out to accomodate my client, I believe she does have a valid point regardless as to what the minimum code requirements state.


Jeff

[Brian A. MacNeish]

Originally Posted by jhaynes

I have heard of 200 sq. in. per ton for air returns, however I also know that many "rule of thumb formulas" used in the construction industry are just that.........you take a home that has 1200 sq. and its very open, then yes I would a 20x20 is adequate when strategically placed, however you take that same area and make a maze out of it, add interior doors that typically have a half inch gap...... than no matter what size your return is, it is still over taxed.

Jeff

Haven't heard the 200 sq in/ton before but have seen this many times.....400 cfm /ton. That unit appears to have a 2.5 ton coil installed and therefore would need 1000 cfm. Decent (unlined) duct design for 1000 cfm would require 160 sq in (8"X20") on supply and 196 sq in on return (8"X24").

From your #'s, the returns are oversized (576 sq in) and the noise being heard must be fan or vibration; should not be velocity noises from too small ducts. There may not be any duct liner (sound absorbing insulation) installed in the returns, allowing the fan sounds to be carried easily to the hall/rooms.....especially if the return ducts are short.

[Barry Adair]

If I'm understanding Jeff's post the return grill is a wall mount 20x20 into an approx. 3 5/8 stud bay, less than adequate, imo

[Charley L. Bottger]

Originally Posted by badair

If I'm understanding Jeff's post the return grill is a wall mount 20x20 into an approx. 3 5/8 stud bay, less than adequate, imo

That would be correct the surface of the return air grill means absolutely nothing, Its what's behind the grill that has to be caluclated. you could have the whole wall covered with a grill and connect it to a 6 inch duct and wait for the noise

[Brian A. MacNeish]

Originally Posted by badair

If I'm understanding Jeff's post the return grill is a wall mount 20x20 into an approx. 3 5/8 stud bay, less than adequate, imo

"a two story home (800 down / 1200 up) with respective air return sizes of 14x14 and 20x20"

When he didn't say "air return grilles or registers", I assumed he meant the return duct sizes.

[Barry Adair]

Originally Posted by Brian A. MacNeish

"a two story home (800 down / 1200 up) with respective air return sizes of 14x14 and 20x20"

When he didn't say "air return grilles or registers", I assumed he meant the return duct sizes.

That's why I posted my dimensions for clarification from him and you gurus...we taught you better than too *****-u-me anything out of this motely crew...

[Jeffery L. Haynes]

Barry,

You are correct, the unit is inside the laundry room and butts up against the wall (2x4 studs).. The grill is 20x20, at least the filter is that size, is located in the adjacent hallway near the ceiling. There really isn't any duct work per se other then the length it takes to run into the wall which is going to be maybe 5 - 6 inches at best.

I can not attest to any duct liner but even if I could, I don't think the noise is going to depreciate appreciably.

I will be speaking to the homeowner this evening about the write up's as well as the builder's obligation which is probably ZERO in regards to the unit.

Again, we are talking a large (national) track builder whose sole goal is to make money by simply cutting back on quality as well as quantity.

I was more curious about the design of the system but it appears that while there are obviously better designs, he does not and did not adhere to same.

Jeff

[Brian A. MacNeish]

Originally Posted by jhaynes

Barry,

You are correct, the unit is inside the laundry room and butts up against the wall (2x4 studs).. The grill is 20x20, at least the filter is that size, is located in the adjacent hallway near the ceiling.

There really isn't any duct work per se other then the length it takes to run into the wall which is going to be maybe 5 - 6 inches at best.

I can not attest to any duct liner but even if I could, I don't think the noise is going to depreciate appreciably.

Jeff

Sounds like it's not too small returns but......

20X20" is a standard size for many plenum take-offs from the HP/AC unit. That short of a return (even if oversized and with duct liner installed) has not a chance in hell of dampening fan noise!!!