Off peak electric vs. full time electric in-floor heat.

TomFritz

I currently have off peak in-floor electric heat. It is under concrete slab. It is controlled by the power company. Comes on at 2:00 AM and off at 10:00 AM. The rate is near half of what full time electric is. I monitored our indoor temperature and noticed that it did not start going up until around 6:00 AM. My question is, would it be more efficient to have the heat not be off peak, and rely on thermostatically controlled all the time? I do not have hourly power usage, only daily. We are located in central Minnesota, so we do have our share of cold weather.

Jamie Hall

That kind of lag from the power going on to a noticeable temperature change is not at all unusual for radiant heat and a concrete slab. It just takes that long for the slab to heat up.

Ideally you would be supplying enough power to the slab to hold its temperature even at all times -- but with the power company shutting off the power for two thirds of the day, that's not going to happen.

You would probably use pretty much the same amount of energy, all told, no matter how you timed it, since the energy use is related to the heat loss of the house. The difference would be more even temperatures in the space. So it's not a matter of efficiency as it is of perceived comfort -- and you almost certainly would spend more money on the power.

hot_rod

An electric boiler on a hydronic radiant system?
You could crunch some numbers and see what it would take to coast through some or all of the high peak rates. The heat load of your home at design, coldest day would be a starting number. The size of your slab?

Maybe bump up the supply temperature during the off peak times to add some more "flywheel" effect. That slab is an excellent thermal storage if you can get control over it.

The thermal mass of a slab looks like this. 29.4 btu to raise a cubic foot of concrete 1°

A buffer tank full of water could be another way to "bank" off peak energy

TomFritz

It is electric cable, no hydro.

hot_rod

One thermostat for the entire home?

TomFritz

My thought process is related to the fact that the cable is heating for up to 4 hours before any results are recognized. If the temperature was not allowed to get that cold, would that, in turn, save energy?

Hot_water_fan

My thought process is related to the fact that the cable is heating for up to 4 hours before any results are recognized. If the temperature was not allowed to get that cold, would that, in turn, save energy?

Nope - usage would be the same and you’d be shifting usage to more expensive hours that way. If you want a more responsive system, you might consider electric baseboard or something of that nature. 

Jamie Hall

TomFritz said:

My thought process is related to the fact that the cable is heating for up to 4 hours before any results are recognized. If the temperature was not allowed to get that cold, would that, in turn, save energy?

As I said, it takes that long for the slab to respond -- assuming that the power really is on (it probably is). That's the nature of a heavy slab, and nothing you can do can alter it.

TomFritz

Only one thermostat. Slab on grade, no basement, about 2600 sq ft.

Zman

Generally, heat produced is heat produced and you are wise to buy the energy when it is cheap. That being said, you may be overheating the slab on many warmer days and wasting energy by producing more heat than you need to coast through the day. You might consider a control strategy that uses a slab temp sensor, an outdoor sensor, and an SCR controller. With a setup like that, you would put less heat in the slab when the outdoor temp is higher. It would take a bit of noodling (not the kind that involves catfish) to figure out how to set it up. Tekmar probably makes a product that would handle the outdoor reset and floor sensor, the Tekmar could then send a 0-10 VDC signal to the SCR controller. You would need to find a solid controls tech to set it up, your local heating and electrical contractor will look at you like you have been doing the other kindle of noodling .
https://www.advancedenergy.com/globalassets/resources-root/white-papers/what-you-need-to-know-about-scr-power-controllers.pdf
https://www.watts.com/our-story/brands/tekmar

Dave Carpentier

I think you saying the 2am-10am is the regulated heating "on" time ?
Do you know if it stays on, adding heat to the floor) during that whole period ?
Is there a thermostat of any kind ?
How does the house feel in the evening ?

Adding more mass to the house would help with the flywheel effect to even out the peaks and valleys. A good number of gallon jugs of water (Walmart or other cheap source) set on the floor for example.