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SuzieQbert t1_j15j3g4 wrote

Reply to comment by toalv in Underfloor heating - can you warm up the floor only? by FatFart777

I suppose that could be true if the "underfloor heating" OP is talking about is simply a floor warmer, and not the heat source for the home. A low-power electric warmer in conjunction with forced air might accomplish what you're describing.

However, the phrasing of this post certainly implied to me that this floor would be the way that home will be heated. If the heated floor is what warms the house, by nature it must be connected to a thermostat that measures the ambient room temperature. In that case it will cycle on and off based on the air temperature reaching a certain level.

Which brings us back to the issue I described. It's comfortable to be surrounded by still air at 22⁰C because it is a poor conductor of heat so it doesn't remove much heat from your skin. Pick up a rock that's 22⁰C and it will feel cool to touch. Hop in a bath that's 22⁰C and you'll be pretty unhappy. This principle means that a tile floor will always feel cool unless it's warmer than the air in the room.

So, I suppose I could add to my earlier comment: what OP is hoping for could be accomplished by decoupling the two goals: warm floor through one mechanism, warm air through another.

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wotoan t1_j15w6v7 wrote

No, a whole house system can (and should) have floor thermostat elements for precisely this reason. Cycling an underfloor heating system on ambient air temperature alone with a naive (non PID) controller leads to these type of issues.

A stable underfloor heating system will have the floor at a constant temperature slightly above the desired ambient air temperature. Heat losses in the house lead to a steady state equilibrium. You can do that with an expensive controller and an ambient air sensor, or a cheap controller and a floor sensor. A cheap controller and an air sensor, like this case, will over and undershoot and be miserable. This can be moderated by high thermal mass systems but is not eliminated.

The problem here is how it’s being controlled, not any fundamental failure of underfloor heating.

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catawampus00 t1_j1619gm wrote

This is 100% accurate. The only other issue to consider is thermal mass. Tile laid over backer board on top of warmboard (aluminum/plywood subfloor) has a low thermal mass but fast thermal response. Tile laid over gypcrete or concrete is the opposite but is much more consistently “warm” to the barefoot.

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evergreensphere t1_j167ss7 wrote

He’s still right - tile at comfortable air temperature (or near it) will feel cold because of the heat transfer coefficient.

Walking barefoot on 74 degree marble will still feel cold, even though it might be heating the air to 72 degrees and the air feels perfectly comfortable.

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wotoan t1_j16nd0o wrote

No that’s not how they actually operate and are perceived. You’re so used to losing a huge amount of heat through your feet warm floors feel amazing.

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