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Thermostat and Heat Source

I have a very simple radiant setup in my basement in Southern WI. I installed everything, but I feel I could do better with the controls and heating. The latter is not my forte and Im having trouble finding pros willing to help.

1200 SF basement (5 loops) and additional 400 SF in an attached garage (on its own loop) = 1600 SF. Using a water/cryo-tec 100 mix. Insulated, 4" slab with 6 loops at 300' each. I have a single Taco 007 for circulation and am running it through an old conventional water heater set to about 160 degrees. This is supplemental heat, the rest of the house is forced air. Currently my thermostat is wired to the Taco. When it calls, pump turns on, cool water enters water heater and trips water heater thermostat, heating water. Room thermostat reaches set point, turns off pump and cycle begins again once temp drops. Frankly it didn't work too bad, but I know there are better controls and means of heating - that's what I am hoping the forum can help with.

Question 1 - Thermostat - What is the best radiant thermostat to use in this situation. Currently have a Honeywell RLV3120. I think I need something that can manage the cycle better for radiant with the slow heat up. I don't need programable, this system can be set for 70 degrees and left alone all winter. But it needs enough to run the pump too.

Question 2 - Heat - Although I didn't run my gas bill through the roof, the old water heater is likely not the best heating tool. I have a new tankless water heater for domestic that I installed - Takagi T-H3-DV-N - 15K-200K BTUs. Thinking maybe adding a heat exchanger to this would be a good idea. I understand the concept, but then I'm back to the controls - how does the thermostat turn on the pump and the tankless water heater? Also thought about using a little electric on demand water heater. I would add solar first can reduce the carbon footprint and avoid some sky high elec bills, these units aren't too spendy either.



  • Alan (California Radiant) Forbes
    A Taco SR501 will turn on the pump when the thermostat calls for heat and the on-demand will turn itself on when water starts to flow.
    Why not use a proper condensing boiler? People try to save money with on-demands for radiant, but regret it afterwards.
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
  • mattmia2
    mattmia2 Member Posts: 9,735
    1. Lowering the water heater temp to just as hot as you need it to produce the output to match the design conditions of the system will reduce the overshoot when the t-stat ends a heat call. outdoor reset would help a lot more.
    2. If you want to replace the water heater, get a real modulating-condensing boiler, it will have the controls you need built in and is designed to do this job. You can set up an outdoor reset curve and add an outdoor temp sensor to the boiler to make the supply water temp match the heating demand of the space so it will keep the temp a lot more even.
  • sellears
    sellears Member Posts: 3
    Thanks. Have done a lot of reading on the outdoor reset control - I think this would be very useful. Also read up on mod/con boilers and I see ALL pros agree that this is the way to go for heating. Then I looked at the price-tag! If this was whole house I would, but all I am doing is heating the basement slab for a warm basement and whatever heat moves up to the upper floors is a bonus. Follow up questions then.

    1 - is there a temp control I can retrofit to the water heater to interface with the outdoor reset?

    2 - I still need a thermostat, even with the Taco SR501 installed as noted above. Suggestions on a model there? Maybe it doesn't really matter with the Taco SR501 in place.

    Thanks for the guidance.
  • ratio
    ratio Member Posts: 3,649
    You'd need a(n expensive) mixing valve with ODR if you want to keep the water heater and add reset. Does the floor get warm when the stat calls for heat, & then cool off again, with long cycles of warmer & cooler?

    What I'd do if I wanted to stay on the cheap side is turn the water heater down to 120° or so & see how that works—maybe adjust it up or down from there a little. I'd bet that the heat loss of the basement (which determines how much heat it needs to stay at setpoint) doesn't change all that much from season to season, not like the above-grade parts of the building. During the coldest part of the year, aim for the stat to just call continuously & try to manage the space temp via the water heater setting, letting the thermostat take control when the outdoor temp rises. Like @mattmia2 said, that should smooth things out WRT overshooting.

    With the water temp lower, I think you'll find that the thermostat isn't playing such a big part. You're on the right track, too, with just setting the temp & leaving it, with radiant any changes to the space temp perforce means changing the temp of all those tons of concrete, verses just adding back in the heat that radiated out. Tekmar (& others I'm sure) make radiant stats that can use a slab sensor to manage surface temp as well as space temp, if you really want a new, radiant, stat. It would certainly help with overshooting, but with lower water temp it might not be necessary.

  • Mike_Breault
    Mike_Breault Member Posts: 35
    ODR on a water heater is not possible as ODR is a heating curve, and not DHW applicable, you always want hot water regardless if its 0 or 80F outside, and mixing control but that may require re piping (tekmar 356)

    good luck