Programming in-floor electric radiant heat, is it worth it?

Does a programmable thermostat for an in-floor electric radiant heat system add efficiency, or is it faux efficiency?

My electrician installed in-floor electric heat in a second floor bathroom remodel I recently completed, and he used a non-programmable therm. I asked him to change it, and he pushed back that there is little efficiency gain because the thermal mass of the mud bed and tile create a long cycle time to raise the temperature. Better to leave the temperature set, and have short cycles throughout the day to maintain it, he argued. He admitted this was merely his opinion.

I have looked for studies on this and found little information so far.

The particulars of this project are that the home was built in the 20's, but it has blown-in cellulose (probably R-10) throughout the house and approx. R-40 in the attic, which is above the bathroom. In the bathroom, I removed the plaster, cleared out the cellulose to below the floor joists, then used closed-cell spray foam in the bathroom to achieve R-20 in the walls and rim cavities (balloon framing allowed access). The house's primary heat is radiant, with the bathroom above the kitchen. There is no radiator in the bathroom, but since this is Minneapolis, I installed an electric baseboard heater on a programmable thermostat in the room-- as a fail safe. The homeowners set their home to 65 while away or asleep, and 70 while active in the house.

My opinion is that the floor only needs to cycle up 10-15 degrees, and that through experimentation, the homeowners can start that cycle early enough in the morning to have desired temperature when they start to use the room. I also think the floor could cycle off in the middle of the evening and retain a comfortable temperature until past bedtime.

The in-floor electric is supplemental and primarily about comfort, of course, but I think it can still be operated in a more efficient manner through programming.

I'd love some data that supports or refutes my thinking.

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Comment by Morgan M Audetat on April 23, 2015 at 1:39pm

You would need a large bathroom to see the difference. We install them both ways depending on the occupants desire. 

Thermal mass has nothing to do with it. 

Comment by Dennis Heidner on April 23, 2015 at 1:23pm

The key is that this is the bathroom.  Occupancy is not constant.  It is more likely used in the morning and late afternoon.  If the house is in a cooling dominated zone - the heat on all the time adds to the cooling load.  For the bathroom the primary reason for adding floor heat  - is to avoid the ice cold feet after getting out of a shower or tub.  It doesn't take a lot of heat to do that.  The bottoms of your feet are not at the same temperature as your forehead.

FWIW,  I have a bathroom with in floor resistance heat.  It is a programmable thermostat.  Early on I followed a generic temperature setting the manufacturer recommended - and - didn't program it.  The increase in energy was measureable -- ESPECIALLY - in the summer time when the room was hotter than the rest of the house that had ventilation readily available and cooling if needed.  Simply opening the door in the bathroom seems like an easy way to cool it down - except if that floor is set on a non-programmable thermostat.  Cool air from outside of the room will indeed cause the floor heat to come on... and you loose.

Several years ago I measured the actual use - I then took temperature readings on the bottom of my feet and my wife's... I set the thermostat to match those temps (roughly).  I programmed it to not heat during the daytime.. just early morning and evening.   I also have periodically monitored the temperature in the room - it is far more comfortable when you move away from a fixed thermostat.

Did you insulate under the bathroom floor -- below the subflooring?

Comment by Charles Cormany on April 23, 2015 at 1:21pm

I have seen resistance floor heaters add nearly $100 per month to an electric bill. Since most folks want warm toes in the morning I would treat them like a radiant heating system and cycle them on several hours before they typically wake up and then cycle set them back during the day. If you have a smart meter check it out yourself by turning them off and seeing the meter stop spinning. They are a great Luxury feature but not a great heating solution.

Comment by tedkidd on April 23, 2015 at 12:14pm

In my experience electric in floor is not used to heat spaces, it heats occupants. Particularly their feet and ankles. This can often turn an unpleasant room into a wonderful room.

My parents have a 300 sf sunroom they use all winter. It has no heat other than the electric floor, which they have come on in the morning and go off in the evening. The room has a double wide sliding glass door connecting to their living room. In the morning the room can be a little chilly, but by leaving the slider open for 20 minutes the air naturally mixes. The room is tight and very well insulated, so it doesn't lose much temperature overnight. If the room you are putting floor heat in is leaky, poorly insulated, or otherwise going to fall below 50-55 degrees, you need a way to heat the room. 

I agree with them and you, there is no point to running that floor heat overnight. Electric floor heat is a great way to provide comfort to occupants DURING occupancy, it is a terrible way to "heat" a space. Even if you put reflective insulation below you are losing some heat to whatever is below. 

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