cold air return vent in basement

13 years ago

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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Comments (3)

alan_s_thefirst
13 years ago
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.
david_cary
13 years ago
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.
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What type of heat and how do you heat your water? Returning from the basement can be against codes. If you heat with gas or oil there is always the possibility to pull flue gassed through the water heater chimney or draft diverter if the return is in the same room as the furnace or there is insufficient combustion air in the space. Have a contractor check the pressure in the return duct. It may be possible to "redirect" where the air is being pulled from by blocking the basement return. Ideally, you want 60% of the return air to come from the high point in the system. Your description would indicate that is not happening. The only time you want to return from the basement is if you wife has cold feet in the winter and she is spending a lot of time in the basement. Good luck,
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The builder doesn't create the ideal situation for everything when the leave an unfinished basement 'roughed-in'. We have an architech friend who knows the yahoo that designs the homes we're buying. While they are reputable, they do have a lot of design flaws - and these are greatly exaggerated when trying to finish an unfinished basement that is roughed-in/half done. There is a possiblity of moving the duct into the main basement, but it would mean creating ductwork that goes perpendicular to the joists, thus lowering the ceiling in that area. It may not be a big deal, but it's something I was hoping to avoid to save money (wasn't planning on hiring an HVAC guy when the basement gets done). The bedroom has one suppy vent - and the basement area has one also. My contractor has already told me I'll probably need to put in electric baseboard heaters, as that's not enough to heat a below-grade basement. And the basement finishing bill keeps getting higher!!!
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They're selling snake oil. There are so manys thing wrong with this proposal. First off, why do you feel you're not getting enough airflow? Is the room they used for their demo hard to heat or cool? If the existing 370 cfm is enough to keep that room comfortable, then there is no reason to pay money to increase the airflow. "More" is not always better. Your air handling system was designed to move air as an enclosed system. OF COURSE it's gonna move more air if you rig it to operate with the fan cover door opened. But again, more is not always better. I'll bet you could stand in that back room and *hear* the air rushing and whistling out of that vent after they took the blower door off. Your system should be set up to move the required amount of air as quietly as possible. You're also risking damage to the blower by making such a drastic change to the static pressure. The fan is designed to work against some amount of resistance. Lessen or remove that resistance and you could burn your fan motor out. A return in the basement is going to bring extra humidty and musty odors into your home. The air conditioning unit would have to work harder to remove the extra humidity. It could actually cost *more* money to operate. And if the unit was not capable of removing the extra humidity, you end up with damp air in your living quarters. This results in a cold, clammy feel that makes you feel uncomfortable. If you have some rooms that are hard to heat or cool, have a reputable HVAC company *calculate* how much air needs to go to each room. It's not a guessing game, and more is not always better. I noticed that you called the original installers and they blew you off. Forget them, they don't sound very professional. Try asking some of your neighbors for referrals, or check something like "craig's list". There has to be a reputable company in your area.
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" It is a very hard design to heat or cool evenly." That is undouptedly true and all the more reason not to mess around with diy, roll your own and home made desighn changes to hvac system. On the surface your idea sound's like there is a free lunch to be had pulling cool air from basement. It's far more complicated than sticking a return in the basement. For every cubic foot of air sucked from basement,another cubic foot must replace it. That make up air can only come from outdoors (a terrible idea) or withing the building(which is exactly how every room in building is usually desighned). Trade schools spend hours teaching techs this so just believe me when I tell you for every cubic foot of air "falling down stairs"another cubic foot already "falls right back up them". So if you want to heat and cool basement,you should have ducts in it. If you don't care to have basement heated and cooled,make certain there are no leaks in existing ducts ( both supply and return ). Now so you don't go away empty handed,a simple test that might tell you if theres a leak in supply or return in basement. Hang a bed sheet,thin plastic or similar light fabric to cover doorway leading to basement. If sheet moves toward basement there's a leak in return. If sheet moves away from basement there's a leak in supply. If sheet doesn't move at all the bad news is there's a "REMOTE" chance leaks of exact same size exist's in both supply and return. I'm assuming your air handeler is in basement or main trunks run through it,if not,little I've said is applicial for your house.
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wyocounselor_hotmail_com
12 years ago
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?

cold air return vent in basement

Comments (3)

alan_s_thefirst

david_cary

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