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I have two story house and downstairs is always colder. Head feels fine but feet are cold and you can feel cold coming from the floor. The house have no basement but is built on a concrete slab. The living room is on 2 inch raised floor. I have one opening and was able to see that the raised floor have perfect passages from one side to the other. There aren't any fire breakers. I realized that I could somehow direct hot air from some of the duct and make the floor warm.

Is something like this feasible or is there something that just do that or similar?

I found air radiant floor heating but that's inside concrete.

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  • Sure it will work, I find customers usually ask cost up front and operational. You lose some efficiency but since heat rises it’s not a waste. Cost of ducting to the space may be the high side of this project but the comfort of warm floors can be nice. One thing to note you will only want a fraction of the heated air running under the floor. the floors will get two hot to the point of uncomfortable if all the air is ducted through the floor. To prove this to yourself after your furnace shuts off touch the heat register it normally will not burn but will be uncomfortable for sustained contact. – Ed Beal Dec 22 '20 at 15:56
  • @Ed Beal, are there any examples on the internet on how to do it? – Grasper Dec 26 '20 at 1:48
  • There are logistical concerns, like how are you going to get hot air into every single joist bay? If you don't, you'll end up with warm and cold stripes on your floor. Granted, the warm areas will be nicer, but that would probably be even more annoying that just generally cold. – FreeMan Jan 21 at 15:10
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While this should work, I'd be worried about losses, including 1/2 the heat going down to the slab instead of up to the floor.

A simpler solution would be to insulate. If that involves ripping up the floor then it would probably not make sense. But you may be able to blow in insulation through the same holes where you are thinking of blowing in hot air, or possibly cut a few holes in hidden places - e.g., inside closets - where the fixup afterwards doesn't have to look perfect.

If you do end up blowing air under the floor, make sure that you have an easy way to turn it off in the summer - you really don't want air conditioning under your feet!

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    Plus 1 for italicizing the word should. – Steve Wellens Jan 21 at 18:07
  • What kind of insulation would you suggest? – Grasper Jan 25 at 13:58
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Using a floors bay as a duct isn't new. Many furnace and central air systems use the space between (2) joists/studs for the return air that goes back to the HVAC. It's usually closed off at the bottom with sheet metal and such.

Most municipalities will approve such a set-up. If you decide to construct this type of floor duct for heat you should consider:

where will the warm forced air exhaust? you stated the floor is open but unless you cut-in floor registers the warm air will ultimately find its way outside not to mention how the slab will cool the warm air. Either way I believe you will be using and wasting alot of energy.

wouldn't make more sense to route the duct directly into the room that is cold? Bring it through a wall low to the floor with a register?

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  • The warm air will just raise up through the carpet. It'll be like a radiant floor heating. – Grasper Dec 22 '20 at 2:51
  • “Not new” older than you consider: the romans did underfloor heating some 2000 years ago. – Solar Mike Dec 22 '20 at 6:50

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