Using an electric hot water tank heater for a radiant floor,

DYI2day

Before everyone jumps on me for using a electric hot water heater for a radiant floor let me explain. First, i wanted to keep my parts & installation costs down while waiting for the release of more efficient air to water heat pumps, a Renewaboiler, etc. Secondly, the radiant floor is relatively small at 560 sq ft in a ICF built extention to an existing house.
I have two zones that operate simultaneusly with a 28 gal A.O Smith electric HWT. The thermostat is a Tekmar564 and the controller is a Taco SR 501.
Knowing it's inefficiency, I've been experimenting with different scheduling scenarios attempting to get more bang for the buck. Two relatively short "on" periods (in 24hr) to keep the slab warm may not be as efficient as one longer "on" period daily. My thinking is because as the return water nears the tanks output temp the elements in the tank will be off while the pump still circulates hot water to the floor. Without a watt meter I'm winging it I know.
Sorry for the long rant but I do have a question. Currently, the thermostat controls the pump. As the cold water returns the HWT operates it's elements to heat the water.
Could I add a device or wire the controller for the pump to continue pumping while the system water is just above the HWT's element set temperature? Looking for increased efficiency. If anyone has something better to share, please do.

DCContrarian

First, I don't believe there is any electric water heater sold in the US that is approved for space heating.

I don't understand what you mean by "efficiency." Electric resistance heat is 100% efficient, all of the energy in the electricity is converted to heat. All of the heat in the water heater ends up inside the house. The only way there will be inefficiency is if the floor is poorly insulated and some of the heat ends up going someplace other than parts of the house you want heated.

I guess if your system is poorly controlled and ends up overheating you could argue it's inefficient, the whole point of heating is comfort and if you're not getting comfort for your money that's inefficient.

hot_rod

I doubt you would get much payback, but a outdoor reset control could be adapted that heats the water to the best temperature based on weather conditions. Those controls also help keep the floors from over-shooting the temperature.

A triac relay powering the element with a setpoint control would be an option. That is how many electric boilers modulate their temperature. Electric tankless water heaters also.


I've done plenty of electric HW tank systems.

At days end it is a 300 psi tested vessel, 150psi operating limit, 210°F temperature protection relief that I always leave in place, add a 30 psi pressure relief.

There are areas with cheap Kwh rates that make electric heat viable. @GroundUp does a lot of electrically powered radiant in the Minnesota area.

Hot_water_fan

No juice to be squeezed by monkeying with it. How many kWhs do you use a month?

Jamie Hall

Don't worry about @DCContrarian 's comment. We don't recommend using a hot water heater for space heating, but that's not because of a lack of approvals -- rather it's that fuel fired water heaters aren't usually all that efficient. An electric one, such as yours, converts almost 100% of the energy in the electricity to hot water, and you can't ask for much better than that.

The only thing you would gain from two cycles per day rather than one is more even temperatures in the floor -- and hence, in the space. There are ways to even the temperature out even more, using outdoor resets and temperature controlled mixing valves and two pumps rather than one, but... more even temperature would be the only advantage; I doubt very much you'd use less electricity -- and you might even use more.

PRR

There's no energy going up the stack or wasted in incomplete combustion. Do you have heat in places you do not want to pay for (pipe chases, attic)? Do you have to bring in other heat (fireplace, electric radiator) to be comfortable? If not, then it's as good as it gets.
Wild swings (off/on schedules) may be uncomfortable and thus bad. Steady heat, or wake/sleep cycle, are probably best.
Do you have any way to know when the element is "on"? With 120V stuff we can wire a lamp or old-fashion electric clock across the element, though 240V makes this trickier (100VAC-250VAC hour-meters are readily available).

hot_rod

The condition of the building dictates the heating costs. An energy audit with a blower door test and infrared scan may be worth the cost.

GroundUp

A given space requires a given amount of energy to maintain a given indoor temp with a given outdoor temp. Electric resistance is 100% efficient. Unless you'd be keeping the space cooler, thus requiring fewer BTUs, there is nothing to gain.

DCContrarian

GroundUp said:

A given space requires a given amount of energy to maintain a given indoor temp with a given outdoor temp. Electric resistance is 100% efficient. Unless you'd be keeping the space cooler, thus requiring fewer BTUs, there is nothing to gain.

Exactly correct. Put more succinctly than I managed.

DYI2day

Thank you for your responses, new to this and haven't figured out how to respond to individual comments yet.
What I mean by efficiency will help explain what I'm looking for. The COP of resistance heating is 1 to 1, that's not changing in my scenario. Only if I change the device heating my water will I attain more heated water for less kwh. Again, the electric HWT was a good, cheap fit till better technology is released in the U.S. Till then, what I'm focused on is utililizing the abundance of heated water in the tank before the pump shuts off circulation to the floor. The pump is contolled by the room thermostat program. By setting the "on" temp very high and the "off" temp very low I've insured the system only operates during the designated time I set. It irks me to have 28 gals. of ususal hot water in the tank because the pump is controlled by the room thermostat. Is there a device I could put inline before the pump and override the thermostat till the water is just above the tank element set temperature?
Presently, I do not know how long the elements are in the "on" state which is where the bulk of my system's energy usage is.
More stats include a total of approx 285' ft of 1/2" pex and my system piping to/from the manifold and the HWT is approx 3.2 gals. The HWT is 28 gals and the flow rate is 2.1 gals per minute.
The house is comfortable because the radiant floor extension has two 6'6"W x 8'H openings to the existing house heated by another source. The radiant floor does work well without hot/cold zones.
With a total of 3.2gals circulated through a 28gal vessel I would think there's usable hot water left when the pump shuts down. If the thermostat were programmed to go on/off 3x's a day there would be more gain from utilizing the remaining hot water in the tank in each interval.

Hot_water_fan

You’re at the point when changes made here will cost $ and not save anything. Until you get a heater with a COP > 1, I’d sit tight. The standby loss of the heater is extremely low and that heat goes into the house anyway. If you installed an electric boiler (COP = 1 as well) but without a tank, you’d see the same energy usage. 

The one thing I can think of: do you have time of use (TOU) rates? Shifting usage to those hours could save $. 

hot_rod

If you know or could determine the supply water temperature needed on the coldest day, set the tank about 5 degrees over that.
what temperature is the tank set at?
Is the radiant in a slab ? How thick?  Insulated below?

The higher you run the slab or supply to it the higher  the downward loss, so set the tank temperature as low as possible to get the job done

A reset  control varies the supply, so it lowers downward loss and also lowers operating cost

plenty of info and first hand experience on how ODR saves money and increases comfort

www.tekmar.com has essays on ODR

A water heater wrap could save a few bucks on the standby heat loss from the warm tank, that’s an inexpensive add on.

keep in mind the “useable” heat in the tank is still within the building, its not really lost. What ever heat it loses to the space lowers the heat load so to speak

unless the tank is outdoors🥴

Tell us the tank temperature and we can determine how many btus are “left” behind when the pump turns off. It’s not going to be a large amount , probably not worth spending money on to move it where it is not needed or wanted Since the thermostat is satisfied when the pump turns off.