DIY return air.

[Mike Jones]

Hi again everyone. I think I'm getting you all to think for me instead of thinking for myself, but here's the question. I have in the past been asked by the AC guy to create a plenum in a wall for a return grill. I have also seem some jobs with a joist cavity used for the return air. I am currently in a situation where I have a nice spacious area to create a plenum and frame for a 14 X 25 return air grill. The obvious answer would be to hire a sheet metal shop to custom build something but as this is in my home I was hoping to keep my costs down. Is there a problem with me fabricating a plywood assembly for this with a starting collar to adapt to the duct assembly? Any advise would be appreciated. Thanks.

[ted kidd]

All really bad practices. Panned returns are really leaky.

If you have anything other than sealed combustion appliances, unless you own a manometer and know how to perform worst case CAZ testing, best not to play around with your furnace. You don't want it depressurizing your combustion appliance zone.

Why would you be adding a return?

[Markus Keller]

A few thoughts on the limited info you have provided:
- What type of return are you trying to create, primary or additional? A 14x25 would be a primary return chase, not another return to improve air flow. Since your AC guy is telling you to add some sort of return, I'm guessing you have return air flow deficiency related to properly cooling the space.
- If you are trying to provide better return air flow for AC then, and if this is additional return, you would be better off installing several smaller return runs. At least one of which, if not more depending on layout, should be high returns to suck hot air off the ceilings. It doesn't matter how much cold air you blast in, if you aren't sucking out the hot air out, the space won't get comfortable
- I don't have an issue with return chases in wall or joist cavities. They can be constructed properly to be very air tight. You should however check local Codes since they may be restricted due to fire separation concerns.
- Absolutely DO NOT build a return air box out of plywood, osb, or other such materials. You may run into fire separation issues with local Codes. More importantly though, you run the risk of contaminating the air stream with mold and/or other organics under various scenarios.
- Since you presumably have an AC guy, when not have him design it and you build it. That way it will hopefully be properly designed, pay him something for his time; and you can install the components to save money.
Other than that you are probably just wasting everyone's time, including your own