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radiant floor heat nightmare

Joe Brix
Joe Brix Member Posts: 626
I guess the boiler keeps firing to high limit and tries to heat up a concete slab with 180 degree water for 12 hours. Not efficent. You have to get some controls in place to measure the slab and air temp. You probably should have constant circulation with a mixing valve control that will only run water at 120 degrees max through the tubing until the right indoor temp is reached, then start running slightly cooler water to just maintain temp. Look for a radiant heating pro that can look at your config and get some modern controls in there.


  • Keith_14
    Keith_14 Member Posts: 4
    radiant floor heat nightmare!

    My radiant floor heat is impossible to control. It is an old 70's copper system running on an old 70's boiler. My problem is; when I set the electric thermostat to 68* it heats to 68* and shuts off. Then it continues to get hotter until its about 75* at midnight. It is set for 65* at night?! Then in morning it's freezing and takes till about 7:00pm before it starts the vicious cycle all over again! How do you keep a radiant floor heated home consistent. I'm going through about $350 in oil a month! Probably from the combination of my residential hot water ALWAYS running.
  • GW
    GW Member Posts: 4,702
    time to get modern

    You need a digital room sensor (NOT a thermostat) to control your system via constant circulation w/ modulation. Try slipping in a Viessmann or a Buderus. Your problems will disappear forever.


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  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    Are you moving the thermostat up and down significantly each day? If so, try either none or slight setback.
  • Brad White_36
    Brad White_36 Member Posts: 30
    Thermal flywheel....

    Without the ability to detect and adjust to the rate of room temperature rise or slab temperature rise, this is so very common. Thermal flywheel is common when there is such a mass that it's output occurs hours after it's call for input and peaks well after the room setpoint is met. What is important to do is to stop the inflow of heat to the slab well before the setpoint is met and to start the heat input well before. (I mean like, duh :) )

    What you have is a great emitter that needs to be tamed -and thanks to modern controls, can be. You have an asset in that.

    Foremost you need to decouple the radiant floor loop from the boiler (hot water heater in your case??)

    Key aspects are:

    1. Constant circulation in the slab.

    2. Variable temperature to the slab, indexed to outside temperature and to the slab temperature and to the room temperature. This is done by controls that take all of these into account including supply and return water temperatures. The means and methods vary; 3-way or 4-way mixing valves are one way, injection of boiler water by variable pump or control valve -or brazed plate heat exchanger- is another.

    3. STRONGLY consider disconnecting the system from your DWH heater. That much oxygenated water cannot do you any good. Make it a sealed system with either a dedicated heater or heat exchanger. Stop the deterioration of the copper.

    4. Controls with PID anticpatory and self-educating characteristics.

    Tekmar among others have controls made to do just about anything you want.

  • hr
    hr Member Posts: 6,106
    Some improvements

    are possible as noted above.

    But if that slab is uninsulated, thick, and has wide tube spacing, all very common for that vintage system, you will never get perfect control.

    Another option would be an over the top retro fit. depends on how much you want to spend to get it control-able and efficient.

    hot rod

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  • soot_seeker_2
    soot_seeker_2 Member Posts: 228
    Taming a thermal flywheel


    You have gotten good professional advice from the others, and you may wind up needing radical solutions up to point of a new boiler and/or floor tubing, but the most simple and easy thing to try is a digital programmable thermostat.

    Getting a unit like a Tekmar with a slab sensor, PI logic, and cycles/hour modulation would all help, but, at least at first, just go down to the local big box or hardware store and invest $30-$90 in a consumer grade, four period, digital, progammable unit. You really don't have much to lose.

    One program, Five days and two days, or seven individual days days is a choice based on your lifestyle, but the most important thing is to have at least 4 cycles within the day.
    For your situation even more would be better if they exist.

    Install it, and then start playing around with the programs. Set up any time and temperature on/off points you like so that basically the boiler is on for twice a day for as long as it takes to charge up the slab. You may have to set it to run from 2-5 in the am for the house to be warm through the morning. You are really shooting to control run time rather than temperature set point.

    This solution really does not really qualify as "control" and you will not have constant even heat, but given the on once per day regime you are currently in, you will be in a 2x/day world for a very small investment. You may be a "do it once, do it right" kind of guy, but if you've been living with the beast you have, I sort of doubt it, and trying a simple kluge might help.

    At least you'll learn how to program one of the new tstats and if you do step up to one of the more sophisticated Tekmars, you'll have had the experience of walking through a menu and pressing small buttons.

    Good Luck
  • tim smith
    tim smith Member Posts: 2,765
    radiant over shooting

    I would think an in between option would be to add a 4 way mix valve w/ motorized operator. Tekmar reset control for weather response. Don't turn down your heat at night as you will always run into a heat up problem in the am. This is a step short of a new condensing mod. boiler. This I would think would cure your fly wheel over shooting problem and also give you more efficient heating. Good luck, Tim.
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