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Radiant Floor heats very slowly

I have an under floor (between the joists) radiant floor. It is 1/2 inch oxygen barrier pex on Joist Trak, 8” spacing. It’s 3/4” subfloor and basic oak flooring. The joist bays are insulated with reflective radiant barrier.
The house is single floor, 1200 sqft, modern, highly insulated. I have a one-zone system built by HydroSmart, utilizing a Thermolec 11kw electric modulating (37,532 BTU) boiler with outdoor reset, running to a 6-loop Bluefin manifold. The loops are all 275-290 feet and each loop is set at a flow rate of .5. It runs beautifully. It is quiet and keeps my house extremely comfortable. The pressure at the boiler is a constant 15psi, per the manufacturer.
My question is this: if I am going to be away for a few days, I set the “away” feature of my TekmarTstat. This sets the temperature at 62 degrees. About an hour or two before I return home, I use the Tekmar app to turn the heat back to its normal setting at 66 degrees. But it takes much longer than an hour or two to “move the dial” 4 degrees. Is that normal?
The water is leaving the Thermolec at around 100 degrees and returning at around 80 degrees.
The concept of “turning the heat up” is very different than when I had forced air or hydronic radiators. If people in my house are chilly I cannot simply turn up the Tstat and expect it to be warmer within 30 minutes or so. I want to know if this just means that I must adapt my thinking and planning or if something is wrong.


  • GordyGordy Posts: 9,264Member
    edited January 2
    Agreed, just allow some more time to come out of set back. 4 degrees doesn’t seem like a lot, but under floor radiant panels are not as responsive as an over the top assembly.

    If reflective barrier insulation is the only insulation in the joist bays you need batts also r-19. Unless you are referring to batt insulation with the foil face vapor barrier. Insulating the perimeter rim joist detail, and air sealing that area is a big help also.
  • parkingnoticeparkingnotice Posts: 9Member
    Thanks for the reassurance!
    The rim joist perimeter is heavily sealed. Every crack and crevice has been air-sealed.
    I don’t have the R-19 batts in yet. That’s next.
    The house stays very warm and comfortable even when it’s 10 degrees outside.
  • GordyGordy Posts: 9,264Member
    Once those batts are in it will decrease your response time.
  • djc2232djc2232 Posts: 106Member
    edited January 4
    Radiant is super slow to react. I never use setback on either of my thermostats. I heat 1620sq ft. two zones with staple up radiant. Bedroom zone is secured with heat plates, layer of reflectex insulation, then R21 batts. The living room zone is just staple up then layer of the reflectex with R11 batts. Most of the house is vinyl or laminate flooring. My boiler is a Weil Mclain ECO 70 LP. Not sure how the electric works but I have my ODR temps set so that anything really below 25 degrees OD temp the boiler is running non stop. Mostly at 20 percent firing rate.

    1. Dont use set back with radiant. Find a temp you are comfortable with and set it and forget it. Even when my wife and I are away for a few days I just leave it where its at.

    2. Im not too sure on how the electric boiler runs but with the LP gas boiler I want to run my with the LOWEST possible water temp to just hold the thermostat set temp.

    Get the batts in and you will see a huge difference. Also try to insulate your supply and return pipes with the black foam pipe insulation. I was surprised how much lower I could run my boiler when i did that.

    20 degree difference is quite a bit for return temp. Ive never seen that even on mine. But yes, with your set back its going to take some time to raise the temp back up. If i set my thermostat up one degree it would take about a day for it to get there.
  • GordyGordy Posts: 9,264Member
    edited January 4
    It seems that the OP is gone for extended periods. Days. In that case setback is beneficial in saving energy costs. Any time you decrease the delta of the envelope it drops the load.

    However on a daily basis not so much. Especially with a well insulated envelope,
  • Dave H_2Dave H_2 Posts: 330Member
    You definitely need to insulate under the radiant, when you come out of setback, your heat goes to the basement first (path of least resistance) and then when the basement finally gets hot, then the heat will travel to where it is colder.

    A joist-trak system is a quick responding system but without insulation (pushed up tight) your response time will increase.

    The other thing you have is the boiler on outdoor reset, the modulation of the water temp based upon outdoor temp will also slow the response down.

    Everything sounds awesome and it is working the way it is supposed to do.

    Don't change a thing except for adding insulation and know that it doesn't respond in an hour.

    Dave H.
    Dave H
  • Jean-David BeyerJean-David Beyer Posts: 2,622Member
    I have a neat thermostat, but since I have radiant heat in slab-at-grade, I almost never do setback. Only when I am away for many days (like two or three weeks). Then I put in 4 degree setback for the whole time, and set it to end the setback two days before I return. I have done this only once, and it worked just fine.
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