Help me persuade cool air to rise

July 4, 2023 3:02 PM

My air conditioner in my house is currently working double overtime to cool the air on our main floor. But all the cool air is flowing down to the basement, where a) it is already wicked cold and b) there are no people. How can I get the cool air up from the basement?

We have a house with two levels, the main level where all the people are, and the basement, where there are usually no people. All the cold air flows to the basement because of physics, and the air conditioner, cruelly shackled to the main level, does not know this, and keeps blowing cool air at us, which immediately flows down to the basement.
What I want is a tube through the basement ceiling and a fan so that the air in the basement can be sucked up and rained down on the main floor. What does this look like in the real world, what is it called, and what might it cost, in a relatively affluent American suburb?

posted by Vatnesine to Home & Garden (23 answers total)

How is the cold air getting into the basement? Can you close that off?
posted by sriracha at 3:09 PM on July 4, 2023 [2 favorites]

How is it getting to the basement? Is it an open floor plan? Are doors left open to the basement?
posted by Silvery Fish at 3:10 PM on July 4, 2023

If the basement is uncomfortably cold, then it's probably colder than the air-conditioned air on your main floor. The cold air from the air conditioner won't be flowing down into somewhere colder still. If anything, the unpleasantly cold air in your basement should be pulling heat from the floor above and giving the air conditioner less work to do to achieve the target temperature.
posted by pipeski at 3:16 PM on July 4, 2023

Response by poster: I can't close off the basement, because we don't have a door at the bottom and we can't put one at the top. If I add a door to the bottom then I have to cut a hole in it so that the cat can still access her litterboxes, but that's doable. I don't want a permanent fixture but we could certainly jury-rig something. Definitely a good suggestion. Any ideas for semi-permanent doors?
posted by Vatnesine at 3:16 PM on July 4, 2023

I think this is the use case for which whole-house fans were invented - venting hot air out from the top floor through the attic and thereby slurping cooler air up to the higher portions of the house. Usually the idea is that you keep windows open to do this, but I imagine it would work just as well with your setup.

We also have a basement that's cold when the rest of the house is hot, and I promise you it's not because the a/c is escaping to the basement. We have heavy doors between the rest of the house and the basement that are always closed unless someone is walking through them. Basements are just cooler - closer to the earth, generally with fewer windows.

But if you already have an air conditioner, I'd spend a little bit of time trying to figure out whether you can rig up some fans to move the air around how you want before investing in something as major as a whole-house fan.
posted by potrzebie at 3:17 PM on July 4, 2023 [8 favorites]

How about a semi-heavy curtain on a tension rod at the top of the stairs? Pin back a bottom corner so the cat can see its way through.
posted by hydra77 at 3:18 PM on July 4, 2023 [11 favorites]

Maybe shut off AC to the basement, since it's generally not going to be too hot down there anyway, and then the air goes into the main living area
posted by mmf at 3:19 PM on July 4, 2023

I agree with hydra77’s curtain idea as a cheap and simple stop-gap solution. Even a old bed sheet hung up with tacks can help slow the air/temperature movement between the two spaces (and it’s okay to leave a small gap to help the cat find its way under, if necessary).
posted by mbrubeck at 3:22 PM on July 4, 2023 [2 favorites]

So if the vents in the basement are shut, wouldn't most of the cold air go to the upstairs?
posted by mmf at 3:25 PM on July 4, 2023

One thing to keep in mind if it's an unfinished basement or if you have gas appliances down there is you may not want to pump the air into your living space due to radon or other harmful gases. We have a similar problem and seal off the basement as much as possible due to living in a radon area and having a gas water heater.
posted by AaRdVarK at 3:28 PM on July 4, 2023 [1 favorite]

I mounted a fan like this in my stairwell, high enough that it won't hit my head. You can see that the mounting will allow it to be angled so that it blows cool air straight up the stairs.

It is remarkably effective. I'll probably add an outlet timer, or a temperature-controlled power source, so that it will work continuously.
posted by amtho at 3:29 PM on July 4, 2023 [2 favorites]

Friends of mine have one of these magnetic closure curtains (different brand) between their house and screened in porch. Both of their cats have learned to push through them and come and go as they want. You can also secure one corner up for a de facto cat door. It does a great job of blocking cold air.
posted by Silvery Fish at 3:47 PM on July 4, 2023 [3 favorites]

Hot and cold air stratify, the key is mixing it up and keeping it moving. Amtho has the right idea to use a fan and keep the colder air mixing with the warm.

I'm assuming by "shackled to the main floor" you mean this is some kind of local window or split unit so there isn't a return vent somewhere else in the house. I think using a large fan to shove the colder air back up into the main level would work great and function like your requested "tube".

But you'll need something that won't spook the cat.
posted by JoeZydeco at 3:54 PM on July 4, 2023

I live in a second floor flat that is entered from a first floor front door. The entryway and stairway up to the second floor are unheated. When you walk by the top of the stairs in winter, you can feel the cold draft from the unheated entryway.

Last winter we hung a heavy cotton shower curtain from a tension rod in the stairway, along the lines of hydra77’s suggestion. It wasn’t perfect, but it made a big difference, and was super simple and inexpensive to set up.
posted by Winnie the Proust at 4:04 PM on July 4, 2023 [1 favorite]

How about a semi-heavy curtain on a tension rod

My parents tacked quilts over open doorways to keep the AC in, it worked pretty well.

Not sure how pricey they are, but maybe check out PVC strip door curtains, they are used in commercial settings for walk in freezers and the like. Basically just overlapping vertical strips of clear vinyl you can walk on through.
posted by Poogle at 4:08 PM on July 4, 2023 [2 favorites]

Hey so this is nearly always (but not always) a result of poor insulation and/or improperly sized ductwork. Poor insulation is kind of self explanatory. If your house was built before about 1970, your ductwork may only be sized for heating; cooling requires more air movement, necessitating larger duct sizing. Keeping the fan on at the thermostat can help.

Some other things I might check:

System location and air return setup. If the system is in the basement and has an open return, it’s conditioning basement (cool) air, not removing heat from the upstairs. Attic vents or other exhaust on the top floor could help that.

Furnace/air handler filter. Clean filters will help airflow. I usually recommend about a MERV 5, changed as needed. If you require higher filtration than that, check your equipment manuals to see the highest rating it can go. More than that can restrict airflow, causing both airflow and mechanical problems. Just remember, unless you have a dedicated air cleaning/filtration system, the filter is there to protect the air handler, not improve air quality in the house.

Dampers: keep ‘em open. May seem counterintuitive, but the better the airflow, the more even the system will cool. If you do play with them, I highly recommend keeping track (notes/photos) of how you found them, and don’t close all the way. Too many closed dampers can mess up static pressure, could cause indoor coil freeze-up.

Outdoor coil. Make sure it’s clean. A good rinsing with the hose will help (aim the nozzle at a downward angle, so not to bend fins/channels. If it’s spine fin (looks like Christmas garland), let a pro handle it. Possibly easier to just let a pro handle it anyways, and usually fairly cheap as a clean and check.

After this stuff, we’re getting into less likely problems which are solidly in pro territory (sizing of either the system itself or the ductwork, fan speed issues, system charge, etc.).

In summary:

Probably insulation. Possibly ductwork sizing. Could be coil cleanliness. After that it gets esoteric.

A possible solution would be a minisplit upstairs. That would almost certainly be the easiest/cheapest solution if its ductwork or system sizing.

Without knowing your house, I’d say check the basics I mentioned, and if that doesn’t help, find a reputable, local company to come out, do a clean and check on the system, and see if they can offer any ideas.
posted by HVACDC_Bag at 4:12 PM on July 4, 2023 [9 favorites]

Also just want to mention that I’ve never seen/heard of a tube or other situation like the one you’re talking about. Whole house fans and/or other exhaust systems kinda fit the bill, depending on your geographical area and local weather conditions, but would require bringing in outdoor air somehow to balance indoor pressure and is probably more trouble than any number of other solutions.
posted by HVACDC_Bag at 4:31 PM on July 4, 2023 [1 favorite]

Assuming that your air conditioner doesn't involve the basement in any way (ie that there is no ducting at all to/from the basement and all outlet and return air is on the main floor), the key to me is that you need to isolate air-conditioned from non-air-conditioned spaces, or you are trying to air-condition twice the space.

A door would be best but, if that isn't practical, the vinyl strip curtain that Poogle suggested would be the easiest way to separate the spaces and you could cut a corner out to let the cat in and out easily.
posted by dg at 4:33 PM on July 4, 2023

I got a fan similar to amtho's, put it in an upstairs window facing out; it helps keep the 2nd floor tolerable, seems to increase air flow in the (small)house generally.
posted by theora55 at 4:53 PM on July 4, 2023 [1 favorite]

Put a box fan in an upstairs room, preferably the room furthest from the stairs, about 3-4ft away from an open window and pointed out the window. You may need to set it on a chair or box or table to get it at the same height as the window. Run it at full speed, this will create negative air pressure upstairs.

Then go open a window in the basement so the the negative air pressure upstairs will pull fresh air through the basement, including the cold air that’s down there. For this to work, there can’t be open windows anywhere else in the house. You should be able to feel a light breeze coming in through the basement window.

The basement may be too far for this to work, if there’s no breeze then instead open a window on the main floor.
posted by jpeacock at 5:24 PM on July 4, 2023 [2 favorites]

Note re: fan: some are much quieter than others, so notice that quality if you buy a fan.

Also: even if you can't attach one to a wall or ceiling, careful positioning of any large fan (box fan, etc.) should allow you to aim the air flow up or down the stairs. Yes, air will have to flow back the other way too, and that's fine - just figure out how to make that work. For example, your fan air could blow mainly along the floor/stair step level, letting the back flow of air happen closer to the ceiling.

This is not some kind of perfect airtight system, but it will work well _enough_ that y'all should feel a lot better. Plus: fans do use a lot less energy than air conditioning, so it's not that expensive.
posted by amtho at 5:33 PM on July 4, 2023

Just to answer the original question, you can search on "extractor fans" and find something close to what you want. Example. This is Amazon but Grainger and even Home Depot sell similar models.

From all appearances, these things can be loud. You could put the fan unit all the way downstairs and around a corner to minimize noise and then run the tube up the stairs. Perhaps there is one you can slow down with a variable speed control. A gentle breeze would be all you need.
posted by JoeZydeco at 6:18 PM on July 4, 2023

We did this in an apartment we used to have, where the doorway between the kitchen and the living room was just a big archway. Bought a big, cheap blanket and tacked/nailed it over the doorway. It worked really well, and you just pushed the blanket out of the way like a curtain to get through. You could cut a slit for the cat and teach it how to get through. I'd put it at the top of the stairs if I could, but the bottom should work okay.
posted by gideonfrog at 7:49 PM on July 4, 2023