Underfloor heating - can you warm up the floor only?

Submitted by FatFart777 t3_zrx1nb in DIY

We are planning to install underfloor heating in our new house. However, have a question about it.

When recently visiting friends who have underfloor heating, I noticed that the floor was nice and warm at first, but as soon as the room reached the desired temperature, the underfloor heating switched off and the floor tiles very soon became quite cold. The room remained pleasant and warm, but your feet were cold because of the cold tiles.

Can anyone share their experience on this subject? and is there a way to constantly keep the floor pleasantly warm without overheating the room?

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toalv t1_j15ch6j wrote

You run the underfloor heating off an independent temperature sensor installed in thinset under the tile, not a temperature sensor attached to the wall that might even be in another room entirely.

This takes a bit of work to figure out what temperature setting in the floor corresponds to a comfortable room temperature (usually a few degrees higher than the room) but the floor will be pleasantly warm at all times and not have the big temperature swings you noticed. You also need a controller per room or zone.

The way your friends have it leads to weird on/off switching behaviour of the floor heating as you noticed. It should be controlled based on the floor temperature, not the room.

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frzn_dad t1_j16qcrf wrote

On/off is cheaper because it just uses a basic on/off thermostat and zone valves.

When you want to have a floor setpoint you need additional analog sensors, modulating valves, an interface to set the temps, and controllers with appropriate i/o to monitor/control it all.

For most not rich people slippers are the right answer.

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AffectionateJump7896 t1_j18jnz1 wrote

For electric underfloor heating it does indeed tend to be a sensor in the floor, as the electric underfloor heating is about heating the floor for that nice feel on the feet. For wet underfloor heating I've more commonly see an air temperature thermostat - after all it's probably the main source of heating and the objective is to get the air temperature right. Some do have both air and floor sensors.

The answer here, OP, is to insulate the floor. What you're describing is classic retrofitted UFH, with inadequate insulation. The room gets nice and warm, the heating turns off, and the floor cools down fast. When the floor is insulated, the floor loses its heat to the air, so only if the air is too cool will the floor be too cool, and in that case, the floor will switch back on.

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El_Jefe-77 t1_j15cn3y wrote

If you’re talking about Ditra Heat or Strataheat or similar electric floor warming systems, they’re not designed to heat up the room. They provide something like 12-15 watts per square foot. I used Strataheat in a bathroom, about 525 watts for 35 square feet heated in a 100 square foot room (it doesn’t go under the vanity, tub, behind the toilet, etc). Controlled by a thermostat installed in the mortar bed under the tiles. It can get about 15 degrees above ambient. House has forced air heat, can’t speak to a build using the floor heat like a radiator.

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Master_Tape t1_j153q9e wrote

You can control it with an independent thermostat dedicated to the floor elements. Just set it to "on" instead of "auto".

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SuzieQbert t1_j152kk3 wrote

Not really possible, except maybe in an outdoor space because tile is a relatively effective conductor of heat, while air is an insulator. This means that if the tile and the air are the exact same temperature (22⁰C) and you will feel a chill from the tile while you were comfortable with the air. Unless you are constantly circulating the air in the room to replace it with cooler air, eventually the in-floor heating will have to turn off otherwise the air will continue to heat to the temp of the tile - which will be too warm.

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toalv t1_j15ded4 wrote

No, generally you set the floor to a few degrees higher than the desired room temperature and heat loss in the room means the ambient air temperature is a bit lower than the floor. Nice and comfy, and if they do shut off they go down to ambient and then turn back on.

The problem here is that the floor heating elements are being controlled via a temperature sensor on the wall, not embedded in the tile. It leads to big on/off temp swings at the floor.

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SuzieQbert t1_j15j3g4 wrote

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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Molrixirlom t1_j152m78 wrote

Keeping floor warm without overheating the room will physically not be possible (at least in a somewhat mondern house... or well... if you open the windows or turn on the AC or something it might work, but not recommended).

Most of my house has a ceramics floor. Bedrooms have vinyl and that feels a lot softer/warmer to the Touch even if the floor heating isnt on.

I do not know how houses are build in your region, or what alternatives are reasonable on the market, but in my area 100% of the houses in the last years are floor heated.

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OregonKlee8367 t1_j152v07 wrote

You could try to circulate the warm air away from the thermostat... Else the laws of thermodynamics won't play. Another easier method would be slippers for walking indoors without overheating the room or getting cold feet.

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Xeno_man t1_j156xgm wrote

For a heated floor make sure you use a stone like ceramic or porcelian tile. Tile will have a high thermal mass and take a while to heat up, but will also take a while to cool down. It's only natural for a heater to cycle on and off as it maintains a set temperature but a tile will hold it's heat for hours.

While you can use certain laminates or vinyls for heated floors, they have low thermal masses and will heat up or cool down quickly.

It's the same reason we use bricks for houses. The brick would absorb the heat from the sun all day long and at night it would radiate the heat into the house all night.

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autoposting_system t1_j159fws wrote

Here's an idea: if the floor isn't intended to be the only heating, what you can do is set it to a higher temperature than the central or whatever heat you're using.

If the house doesn't reach the temperature the floor heat is set to, it'll stay on. Basically just underdesign the floor heat so it's not enough for the whole house.

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THEREALCABEZAGRANDE t1_j15eag4 wrote

Yes, but it requires a more sophisticated AC system and you have to have a traditional system in conjuction with the floor heating. You can leave the floor heating on low to keep the floor warm and move the excess heat out of the room with the traditional air mover HVAC. In a "dumb" system this heat will just be wasted, in a smarter system it can be diverted to other cold areas of the house.

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armouredqar t1_j15fqu2 wrote

The underfloor heating - was this electric set in under the tile, or liquid from a boiler?

If it's from a boiler, sounds like something set up incorrectly to me.

If it's electric as others have suggested - should be set to some lower output level and to come off and on more frequently, so that the tiles at least stay warm. Is this underfloor the only source of heat in the room?

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FatFart777 OP t1_j16jndp wrote

thank you! not sure what their was, but ours will be liquid. and based on everyone's help, we'll make sure to set it up correctly from the start.

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armouredqar t1_j16oqha wrote

I'm going to guess that their system was something different at core (like electric) or something very wrong with the set-up.

For yours I guess others have laid it out - the temperature of the water flowing through should be moderate. I've had a mixed system of rads and floor, and quite clearly distinctly different loops and temperature settings (note some rads can be designed for low temperature).

For our radiant flooring, the heated flooring sections are never cold - they move slowly in heat terms, so even if the heat was cut off it would take hours to get to 'cold'.

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Floopthecoop t1_j15qgf6 wrote

You can get 2 types. One is for heating the room. Generally the higher wattage cable. And one is for comfort for walking in tiles lower wattage cable. . Of course it will still heat room but is not meant to be the sole heating method. Also you usually get 2 thermostat sensors the one goes in the screed under the tile and you get it to operate off floor temperature. The display usually has another which operates off of ambient temperature of room and you can choose which one to use to switch on and off

https://www.theunderfloorheatingstore.com/electric-underfloor-heating/underfloor-heating-mats

See this link here. Lower wattage is just for ancillary heating of floor

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bombloader80 t1_j16rolw wrote

>Of course it will still heat room but is not meant to be the sole heating method.

Aren't all floor heating systems intended to be the sole heating method 😎

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SauciePicard t1_j16nbvd wrote

I have floor heaters and they’re always the same temp, don’t change based on room temp

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boomR5h1ne t1_j18m7dn wrote

You could go with IR electric mats. My father just installed some in his bathroom and an additional room they added. Only downside to it is you are limited to flooring with an R rating 1 or less so many LVPs will not work. Best to stick with tile.

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BafangFan t1_j15m0l0 wrote

Use water-pipe underfloor heating. Set the boiler or manifold temp to something like 75 degrees, and have it constantly circulate the water.

Control house temp by opening some windows.

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chrisbe2e9 t1_j160x2o wrote

Maybe not opening the windows, it was -22f this morning. windows are frozen shut!

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amberita70 t1_j1739ou wrote

Lol I have this type of heating and have opened the door when it's that cold out. I finally figured out the best setting for the thermostat so don't have to do it that much anymore. Mine is run in cement so you get the residual heat still coming from the cement when after the heat shuts off.

Before it stays cold all day then it gets way to hot by afternoon.

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Critical_Equipment79 t1_j16ojdk wrote

have the floor too hot to walk on and waste energy to open the window, genius

edit: just realised you meant faranheit

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FatFart777 OP t1_j16jeu3 wrote

thank you!

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Jish1202 t1_j1mkam6 wrote

You should be able to set the boiler or mixing setup on an outdoor reset curve and use thermostats with floor temp sensors and the floor temp will change with the OA temp and pretty much never shut off

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skwolf522 t1_j1ac665 wrote

This is why i didnt recommend radiant floor heating in my parents new construction build.

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BafangFan t1_j1akpn7 wrote

Ultimately I think radiant floor heating is the most comfortable type of heating you can have. No cold spots, no blowing wind from forced air, humidity remains higher, dust is lower, there is even vertical temp distribution from floor to ceiling.

It just has to be designed correctly with the insulation of the house taken into consideration.

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