cold air return vent in basement

rexgNovember 30, 2010

Currently finishing my basement and need to add a cold air return vent(s) before dry wall goes up but not sure what size. The basement is about 1100 sq. ft/ and has 5 4x12 supply registers. There is only one return air vent for the main level of the house and it is very large and centrally located. It handles that area well but no return vents were added to the basement when built. I had the furnace (dual fuel, high Eff. unit)services a couple of weeks ago and the tech suggested I had just one cental cold air return but did not suggest a size. The basement is well insulated and never drops below 55 degrees during the winter even with the supply vents closed and heats well when open but because I am dividing the area into 5 rooms (3 large rooms, one small and a bath) will just one work or do I need more and what size? The funance is centrally located so adding more than one should not be a problem but I do not want to effect the balance supply/return that now exist. The house is 1972 sq. ft. finished on the upper two levels and has a unit for each level. I do not need to a/c the basement in the summer since it stays around 70 degrees, I usually close the supply vents during the summer and run a large dehumidifier which keeps every thing dry and comfortable. Also it is easier to place the return vent up high then close to the floor except in the room next the furnace and there I can run a return vent about 6 inches thru an existing interior wall, which is better or will one down and two up work? Ceiling is 9 ft. high and all the supple vents are located in the ceiling. The tech said it was an easy job I could handle but what size return and what size duct work?

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You'll hear a lot of debate about this one - some will say you can't do it without calculating a Manual J etc but the reality is there's a lot of boilerplate hvac systems, at least where I am in Canada. I have two returns in the upper floor of my house only a couple of metres apart, and none in the finished basement and none in any of the bedrooms, which I've been told is an 'impossibility' - well here in Canada, it's not and the system works well enough - the master bedroom is a bit cooler with the door closed, but it's above a poorly insulated garage.

I see a lot of basements with the return in the ceiling. Warm air does rise, though, and in the summer you might want some of that cooler air in the basement circulating upstairs, so I'm planning to do exactly what you mentioned, put the return near the floor, backing onto the furnace room.

If there was some sort of big issue with changed airflow, you could probably block off an equivalent of the downstairs return vent in the upper return, to equalise the air flow, but to this layperson, the SUPPLY of cold air is not as much of an issue if you're not getting cavitation (a vacuum effect because the pump isn't able to suck in as much air as it's trying to deliver.)

Take some temperature readings over a few hours in different spots in the house, and make note whether the furnace is running a lot more than it was previously.

The 'duct work' typically is either the space between floor joists or a boxed duct around the same size, or, running down the wall, typical stud wall spacing, 16."

That's what I see here in British Columbia, again, I'm not an HVAC tech. Don't go excessively large. If you go through the furnace room wall you won't need much in the way of ducting and if you need to you can use thermopan (silver faced cardboard) instead of drywall or OSB to form the 'duct' from the floor up to the return (make sure you cut into the return, not the warm air section and make triple sure you're not going to damage any wires, gas lines, a/c lines etc.

As for the return, the standard size grille is about the width of the space between studs I guess so around 16?" If you need to tweak it, you can block sections of it or the upstairs one inside under the grille if you need to.

If you're careful, you're not going to damage anything irrevocably.

    Bookmark   November 30, 2010 at 7:31PM
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Faced with similar situation here. Drywall in a few days. I did 2 14x20s because that seemed enough for what I was trying to do and it fit in the studs.

I have a very large open area and then a midsize room so I put a return in each (total 1400 sqft). I have a 4 ton unit for the 1st floor but I don't currently anticipate ever running just for the basement although I put enough supplies in for 4 tons (7 6x16). I have enough access to up the returns if I need to but I may be making the basement open to the upstairs so in that case the returns are probably not an issue.

I think in your case, your HVAC needs are small. My seat of the pants guess is that you need a ton so I might make a return big enough for that - 14x20 should be fine. That would balance your supplies (roughly). You generally want more return area so if you can bump that up a bit it would help.

    Bookmark   December 1, 2010 at 4:57AM
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I presently have a cold air return duct running from one room upstairs under the joists downstairs to a panned section of the joists about 5 feet away. Do I need this section at all? It is an old piece of ductwork on a 1924 house. There are presently two large returns and one small one (that mentioned) upstairs and one small one downstairs. Is this enough? Should I run a panned section between the studs of new walls downstairs to pull more air in?

    Bookmark   May 16, 2011 at 4:03PM
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