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Cold Air Return Installation

TonyBarr's picture


I am in the process of finishing my basement and would like to add a cold air return.  Fortunately, I have two ideal locations where the top plate of my stud walls run directly underneath a cold air return.  In one case, the wall is made from steel studs and the other is wood.  I plan on cutting through the top plate and into the cold air return duct and use the wall cavity as a duct.  I'll install a grill to the wall about 10" off the finished floor.

Question:  What type of hardware is available to accomodate this installation?  Particularly, the drywall opening for the grill?  Is there a mud ring of sorts that I can fasten to the stud wall so the drywall contractor can leave an opening?  Also, should I line the cavity with anything?

Thank you,


(post #110537, reply #1 of 9)

I found this and see that no one has responded. You will need to cut the top plate and then extend down from the existing metal duct work down to the basement. You should have a 10.4 by 4.5 roughly metal duct running in the wall to accomplish this. You will tie into the bottom of this with a new duct, wrapped with 1.5 inch fiber glass and then bottom out 10 inches from the floor. Your duct will have a cap on the bottom. You will then cut into the duct, on the finish wall side, with snips and then trim out the opening with a finish piece of metal L metal used in HVAC work. It bends into the cutout. You then have an extention from the basement to the return air. If you don't have any way though for air to get to the basement though only the upper floor will be working. Think of the return air as a bottle, you need air to be accessible to the R.A. duct for it to work. Hope this helps.

(post #110537, reply #2 of 9)

First of all, thank  you for seeking out my unanswered post.  Your response was very helpful. 

I was wondering if would be OK to simply cut through the top  plate and into the existing return air duct and then use the stud cavity as the new duct?  I would use the L metal as you suggested to make the opening in the wall for the drywall to fit around.  I know this is done on above ground floors, but is it not a good idea for basements?

Also, what is the best way to tie the new duct to the existing duct?

Thanks again,

Tony Barr

(post #110537, reply #8 of 9)

Well here goes. Your return air should be installed just as you would a new installation. The joints joining the return air duct should be tied into the existing duct work using good sheet metal installation methods. Look into sheet metal trade books for specifics. It goes without saying the the duct should be properly sized for the volume of air flow, seems that your existing R.A. duct was working so I will assume for this discussion that using the same size is acceptable. If you do cut the plate you will need to tied the plate back together using straps and nails. The new duct would tie into the bottom of the old duct using "S" clips. This is metal vernacular. Just think of the "S" on its side and flatened. This allow the new duct to be inserted into the old duct. Another way would be to cut the corner of the new duct so it could be reduced to fit. See the MALCO brand tool line. 

Now for the other part of this installation. Remember that where ever you put a return air duct air will be drawn to that location. I didn't think to ask if the existing basement had an air path already. I mean, it won't do you any good to put a R.A. in the basement if the door to that basement is closed. No air will flow back. I didn't think about asking if the existing basment had a water heater, furnace etc.. which would be affect by the depressuring effect of a R.A. duct. Just think about the the R.A. drawing air while a basment furnace and water heater were in operation. You do not want to create a reverse draft situation.


(post #110537, reply #3 of 9)

Do you have any combustion appliances in the basement (furnace, boiler, water heater)? If so, it's particularly critical that you balance the air flow. If you've only got a return, it can depressurize the space and cause any appliance that's not sealed combustion to backdraft. Think CO poisoning here.


Andy Engel, The Former Accidental Moderator

Andy Engel

Never try to teach a pig to sing. It wastes your time and annoys the pig.

(post #110537, reply #4 of 9)

The basement has five existing supply registers in the ceiling, two of which will be located in the same room as the cold air return.  Does this pacify your CO concern?

Thank you,

Tony Barr

(post #110537, reply #5 of 9)

BTW, the basement has a gas furnace, gas water heater and a gas drier.


(post #110537, reply #6 of 9)

Tony, Can you isolate the three gas appliances from the conditioned space,and supply them with a source of outdoor air? If so, you will prevent a lot of conditioned air from being sucked out of the house,(which means outdoor air replaced it), and you will have taken the backdraft possibility out of the equation.

You literally have three open flues introduced into your forced air balance, and wind, opened doors, windows, kitchen/bath exhaust fans etc.,

create both an energy loss and a dangerous potential.


Energy Consultant and author of Practical Energy Cost Reduction for the Home

Energy Consultant and author of Practical Energy Cost Reduction for the Home

(post #110537, reply #7 of 9)

Thanks Paul,

I do have a fresh air supply intak outside the house tied into my cold air return.  Will this help?


(post #110537, reply #9 of 9)

Tony, from an energy standpoint, every time any one of those three gas appliances fire, you will be drawing outdoor air into your return air system. The air intake sizing for makeup air for gas appliances is very important.

The air demand with all three firing at the same time is likely a very large volume of cfm., and I don't think you want that much air flowing through your central air system on its way to the burners/flues and out...

I don't know where you live, but if you have need for both a/c and heat, your utility bills can be greatly reduced by supplying outdoor air to all burners, unless you have sealed systems that in effect already do that.A clothes dryer alone can empty a house of one air change in one hour of operation.

From a safety standpoint,both Andy and I have a concern for your CO levels.As an example: If the air flow resistence is lower for the dryer fan blower to pull air down the water heater flue than through your duct/filter/coil/heat exchanger,more duct and supply grilles, then that is the path the air will take. Kick on the furnace and it only gets worse. You would be pulling both the heat and the fumes into the basement, which is now part of the whole air system.

If you can possibly enclose the appliances in a room or rooms with their own OD supply air, you would be well advised to do so. You need to check local code on sq inches of supply air. It will be based on total Btus of the burners, and source placement of the supply air. Be safe, my friend.


Energy Consultant and author of Practical Energy Cost Reduction for the Home

Energy Consultant and author of Practical Energy Cost Reduction for the Home