Return duct size question

13 years ago

Without getting into too many details, I think my return duct is a bit undersized since the suction is extremely strong on the 20x25 filter (they all warp inward). I measured today, and I can hold the filter about 3" away from the return boot and the negative pressure counteracts gravity and pulls it from my hands to lock it in place.

I've read that this situation is harder on the unit (perhaps just the compressor?).

My question is this: is that solely a factor of draw from the unit or is it also affected by the supply ducting to the various rooms.

More simply put, would closing off a couple of registers in unused rooms alleviate some of the wear to the unit in an under-sized return duct situation?

I know general advice (for a properly sized unit & balanced ductwork, i.e. "perfect" installation) you shouldn't close off registers. It can cause the same scenario, forcing the unit to work harder rather than the desired affect of simply re-routing air flow/pressure without consequence.

TIA

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Comments (4)

zippyhvac
13 years ago
Closing supply vents will not help, it'll just make things worse. Increased pressures are hard on the blower, not the compressor, and they can have a large affect on comfort and efficiency of the unit.

The situation you have described is very common due to "rules of thumb" that even 10 years ago allowed for too little return air.

In a perfect world, you could have the pressure measured at the unit both before and after the a/c coil. Too many hvac guys are loath to break out the tools to do this for you, the ones who will - will likely charge handsomely for their services.

If you've got good access to the area, adding a second - same-sized return might be an ideal solution - a side benefit is your filters will last longer and do a better job (excessive pressure can pull dirt all the way through a filter) It's pretty much impossible to go "too big" when it comes to return capacity.

A short-term solution might be one of the Honeywell retrofit 4" filters. They fit into a 1" filter rack, but give you a 4" thick filter (with a much lower pressure drop than a 1" filter)

Google the FC40R1011 for 20" X 25" sizing (you should be able to find genuine versions for under $40, and knock-offs for a few bucks less) A side benefit is they last a year in most residential settings.
cebury
Original Author
13 years ago
Thanks for the information! I will go ahead and order that Honeywell filter for now.

Last year, I added an additional supply vent to the system: a 30' run of 5" R5 ducting to our master closet (converted to baby room). Hesitant to add more to the pressure imbalance, I closed off an existing 5" ducting to the second bathroom, thereby hoping to not alter the supply/return balance. I wish I could've added a small return (instead or in addition to), that's probably all it really needed to establish some air flow into the closet.

>>> If you've got good access to the area, adding a second - same-sized return might be an ideal solution

How does one increase the overall return without expanding the hole in the underside of the roof where the return attaches to the HVAC unit?

Or are you suggesting cutting the existing return duct a few feet from the roof underside, and adding a WYE: one side to the existing return grate and the other side to any new returns I could add myself? I had considered doing this, but in my limited understanding figured the bottleneck would still exist, since ducting to the unit itself (before the WYE) hadn't changed. Was that an incorrect assumption (made before I discovered helpful people such as yourself here at GW)?

Here is the pic of the North attic including ducting, plenum, and just visible is the entire run of the return (attached to underside of roof and starting from ceiling).

This is what it looks like now, after I installed a radiant barrier protecting the ducting and insulation. The plenum is centered in pic, rather than off to the left.

Thanks again for your help!
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zippyhvac
13 years ago
Great job with the radiant barrier - it really looks nice!
Adding a second return lowers the velocity of the air at the filter - and also lowers the resistance that the blower has to overcome - increasing efficiency.

Every foot of flex duct increases the pressure the blower must overcome - by adding a second return of the same size, with the same size duct, it lowers the velocity needed to supply the correct cfm of air.

With what you have, I'd split the return as close to the roof as possible - then wye it, and route another 16" flex-duct to a return-air grille right next to your existing one.

So, we're talking a 16x16x16 wye, probably a (25') bag of 16" flex duct, a 20x25 "air can" and a 20x25" filter grille.

Not much you can do with the nursery, but you should make sure the door to it is undercut, to allow air to leave that room - depending on the layout of the room, you might be able to do a "passive return" with a couple of wall grates to an adjoining room that has better return access.

Here is a link that might be useful: passive air return
cebury
Original Author
11 years ago
Zippy/Others:

I finally had my gas-pack replaced with a 2stage Coleman (details in a different post last week) but want to know if it's a good idea to continue buying/using the 4" Honeywell filters, like the FC40R1011.

The return was sized up to 18" from the existing 14", but it still fits in the 20x25".