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Very cold in basement when AC is on

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[OP]
Sr. Member
Sep 24, 2006
819 posts
121 upvotes

Very cold in basement when AC is on

I've set up a home office in my basement but I find it gets very cold down there when the AC is on. I've closed all the vents down there but it is still uncomfortable. I've got my AC set to 24C and that's what it is on the main floor. I checked and it's 26C upstairs but 17C in the basement.

I'm thinking of trying these magnetic vent covers: https://www.homedepot.ca/en/home/p.magn ... 75443.html

Has anyone tried these? Does anyone have other ideas of how to minimize the cold air in the basement while the AC is on?

Thanks in advance
20 replies
Deal Fanatic
User avatar
Nov 24, 2012
5629 posts
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Space
Man id love if my basement was 17C, that would be prime sleeping space. Shut the basement vents?
Member
Apr 21, 2012
295 posts
55 upvotes
Toronto
cold air sinks, so to a degree, your basement will always be the coldest part of the house, I have those magnetic covers, they stop most of the flow from the vents, but my basement is still cooler than the rest of the house.
Deal Addict
Jan 1, 2007
1045 posts
102 upvotes
The ducts running through your plenum has small leaks at the connections to each other. Use aluminum tape and cover it all up, assuming you still have access. Run you hand across the duct joints to feel where the leaks are.

Or more expensive option, buy air handler to push air upstairs faster. Your AC may run less often. I have one and my first and second floor are never more than half or at most one degree difference. Basement is always warmer than upstairs when AC runs.
Deal Expert
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May 10, 2005
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Ottawa
When you close off just the basement vents, you have to open others. Typically, you open all the ones on the top level and 1/2 or a little more open the middle level and close the bottom level.
Balancing the duct is not an instantaneous result. It takes several days after the balance to feel the results and if that is not to your liking, you need to do it again till you get it to where it is comfortable for you.
The magnetic covers can work but you could also just lift the vents off and fill them with insulation or Styrofoam. The magnetic cover only stops air from coming out the grates in the vent and there will still be leakage around the edges. Stuffing the ducts will ensure no air gets out the ducts.
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[OP]
Sr. Member
Sep 24, 2006
819 posts
121 upvotes
Regarding the air handler, what do you during the winter months? Can you turn it off?
engmsf wrote: The ducts running through your plenum has small leaks at the connections to each other. Use aluminum tape and cover it all up, assuming you still have access. Run you hand across the duct joints to feel where the leaks are.

Or more expensive option, buy air handler to push air upstairs faster. Your AC may run less often. I have one and my first and second floor are never more than half or at most one degree difference. Basement is always warmer than upstairs when AC runs.
[OP]
Sr. Member
Sep 24, 2006
819 posts
121 upvotes
I assume the most efficient way of stuffing the ducts is to do it closest to the unit so minimal air goes to the basement?
Pete_Coach wrote: When you close off just the basement vents, you have to open others. Typically, you open all the ones on the top level and 1/2 or a little more open the middle level and close the bottom level.
Balancing the duct is not an instantaneous result. It takes several days after the balance to feel the results and if that is not to your liking, you need to do it again till you get it to where it is comfortable for you.
The magnetic covers can work but you could also just lift the vents off and fill them with insulation or Styrofoam. The magnetic cover only stops air from coming out the grates in the vent and there will still be leakage around the edges. Stuffing the ducts will ensure no air gets out the ducts.
Deal Expert
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May 10, 2005
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Ottawa
hank755 wrote: I assume the most efficient way of stuffing the ducts is to do it closest to the unit so minimal air goes to the basement?
There are several ways you can do ths. In the ductwork should be a damper and by turning it, you close off the flow of air but, that does not shut it off completely, there will always be some leakage. Your magnetic cover will also close of the majority of the flow but again, there will be some leakage. Removing the vent and stuffing the duct with whatever you want (insulation or foam cut to fit) is the most effective way of blocking all air.
You will never be able to completely close off the air unless you remove and completely seal the duct but that is not necessary if you prevent the outflow from the vent.
Oh and, keep the furnace fan on 24 hrs per day. That will help even out the air even when the A/C is not on as well. Just remember that you need to change the filter more frequently if you keep the fan on.
The Government cannot give to anybody anything that the Government does not first take from somebody else.
Deal Addict
Jan 1, 2007
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102 upvotes
hank755 wrote: Regarding the air handler, what do you during the winter months? Can you turn it off?
I forgot you have the more traditional design of furnace and AC. In my home, I have a boiler (does hot water as well) and AC. Both the boiler and AC coils run into the air handler, and thus I only have one running at a time. The air handler will move much more air than the furnace blower.

In the summer, my basement is the same temperature as the upstairs or warmer when the AC is running. What you want to do is ensure the furnace blower and ducts coming out are completely sealed tight in the basement. I went around the basement with aluminum tape and patches up all the leakages.
Deal Fanatic
Sep 16, 2004
9193 posts
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Toronto
When I lived at my brothers house I remember the same thing.

We just used comforters in the basement when watching movies etc.

We did stuff the basement ducts with paper and them put on those magnetic covers.

Upstairs was always hotter as didn't seem to be enough return air vents either.

Having a nice strong fan to push some of that cold air out of the basement and to the upper floors might work.

As others have said you need to tinker with vents at different levels.

Or simply run the fan alone with the A/C off while you're in the basement and see what happens.

IMO we need a separate ventilation system for A/C dropping air from the ceiling as cold air tends to sink.

Another ventilation system blasting hot air from the floors in winter as heat tends to rise.

Then off course you need a fan to pull all the hot air from the ceiling during winter back to the ground.

I guess keep the air moving/circulating.
Deal Addict
Jan 5, 2003
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Toronto
Pete_Coach wrote: Oh and, keep the furnace fan on 24 hrs per day. That will help even out the air even when the A/C is not on as well. Just remember that you need to change the filter more frequently if you keep the fan on.
+1

This will equalize the temperatures in your entire house, so 24C on your thermostat will be (somewhat) 24C throughout your house, rather than 24C only on the ground floor. Your AC will run less and you'll save money because your AC uses a lot more electricity than the increased electricity usage from running your fan more. On most thermostats, this will be setting fan to ON (rather than AUTO). Oh, and if your basement has return vents, then open them up (as well as the supply vents) so the colder basement air gets recirculated. Otherwise, even if you have your fan on 24/7, you're just recirculating 2nd floor and ground floor air, while the coldest air naturally goes down to the basement.
Deal Fanatic
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Dec 28, 2007
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Alberta
It's freezing in my basement also. Closing off the basement vents doesn't make much difference because the exposed ductwork is freezing cold and radiates into the basement.
Deal Expert
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Feb 8, 2014
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This problem is hard to diagnose over the internet, not knowing the design of your house or anything about it makes it impossible to give good advice.
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Deal Fanatic
Feb 4, 2015
5830 posts
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Canada, Eh!!
Need to have cold returns in basement close to floor so will take out cold air that has fallen to basement floor.

If basement finished and difficult to add floor level returns then have seen folks use foil flexible dryer duct attached to cold return in ceiling thru a hole newly created in storage closet or other such area not as readily visible.

Not ideal solution as 4" dryer duct not as good as HVAC duct work and depends on how many bends as well. Rona has 25' flexible foil duct for about $27
Deal Addict
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Apr 20, 2012
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Waterloo, ON
I read that is bad for your HVAC system to close vents because it creates pressure and makes it have to work harder. Is that not the case?

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