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Ideal thermostat for hydronic radiant floor?

AlexS Member Posts: 69
I used my old Lux thermostat when I set up my new radiant system, and it always seems to overshoot the temp by several degrees.  Its been warm enough here that we keep the system off unless we wake up to a chilly morning.  Is the thermostat overshooting because it has to bring the room up several degrees at once?  If it's always on, will it keep a more consistent temp?

I still need a second thermostat for another zone, so does anyone have a suggestion?  Should I use one with a floor sensor as well?


  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    As usual, it depends.

    With my old boiler, I had terrible overshoot problems. The reason was that that boiler did not modulate and did not have outdoor reset. And the heat emitter was a radiant concrete slab at grade. So changing the slab temperature was a 4 to 24 hoiur adventure, depending on how you measure it.

    So consider if the slab was cold and the house 1F below the setpoint. It would take about 4 hours or more to warm the slab and another couple of hours to raise the temperature of the house. Then the thermostat would shut off but the slab would continue to heat the house for another 4 hours or more. I used to set the thermostat to 70F, and when it went down to 69F it would call for heat. The house would drop another degree or two before the slab got heated usefully. Then the drill described previously. When the room got to 71, it would turn off and the room would go up to 73F or sometimes more. With outdoor reset and modulation, I have the thermostat set to 69F, and once in a while the room goes up to 70 or down to 68, but mostly it is 69 whenever I look at it. I do not attempt setback with this zone.

    If you have a slab, you might try to apply outdoor reset to your system. I do not know if it pays to retrofit such a thing; one of the professionals here could probably answer this.

    If you cannot modulate your boiler, and if you are lucky, you might be able to run it at a lower temperature. It may be oversized and maybe running it at a lower temperature would reduce overshoot. Might save a little fuel too. I lowered my old oil fired boiler from whatever it was to run between 130F and 140F. I did not know about accidental condensing and boiler protection then, but that old GE boiler was tough and did not rust out.

    At the other extreme, I have a zone that has baseboard heating with aluminum fins on copper tubing. It does not overshoot because it warms up fast and cools down fast. Cast iron baseboard and radiators are in between, and I have no experience with them.
  • Tim Potter
    Tim Potter Member Posts: 273
    Possible solution

    I have looked at this T-stat for my high mass radiant in-floor system. My system would REALLY overshoot before I converted to ODR, but I still have some overshoot that I am thinking about changing from round Honeywell's to this T-stat.

    Here is some info:

    Winter Park, CO & Arvada, CO
  • Tom Manton
    Tom Manton Member Posts: 30
    Uponor thermostats

    Uponor radiant themostats modulate the closer they get to the setpoint to minimize overshooting.
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    Uponor radiant themostats modulate ...

    Wow! How do they connect to a boiler? My boiler would not honor them because it has just on-off thermostat contacts. Would it not need a boiler controllor that hooked the inside thermostat attached to the modulation control of the burner?

    I had a proportional thermostat once for a large air conditioning system. It had a potentiometer driven by the bimetallic strip. That drove a three-way water valve that either made the chilled water through the valve go through the heat exchanger, or direct to the return. That valve was essentially a needle valve. It worked extremely well, but I would not want one in my house.
  • Tom Manton
    Tom Manton Member Posts: 30
    Uponor t-stat

    They are 2 wire just like a typical on off. I believe they are powered by impedence through the device being controlled (someone will correct me if I'm wrong) such as a motorized zone valve, for Taco zone valves they include a very small resistor that is place across the terminals on the zone valve, this small current powers the t-stat when it's not calling. Quite cool
  • AlexS
    AlexS Member Posts: 69
    I should have mentioned....

    Wow, did I leave out a lot of important info....

    I have a vitodens 100 mod con, the first floor is in a warmboard knockoff directly under hardwood, and the second floor is Uponor Joist trak mounted to the underside of the subfloor. (with hardwood up there also) Both zones use 3/8 pex supplied by manifolds, longest run is 200ft. 

    I didnt connect the outdoor reset yet, though one comes with the boiler. Will that do it?  I know it would lower the supply water temp since it's not that cold outside yet, but wont that take longer to heat up my room then?  Im just not ready yet to keep the system on for the winter, but I want some fast heat when we wake up to those chilly mornings. 

    Radiant just doesnt work that way, eh?  Should I just stick with the Lux thermostat, or would a floor sensing model like Honeywell Aube TH115 be a better idea?
  • CMadatMe
    CMadatMe Member Posts: 3,086
    Your Overshooting

    Because your delivering a higher water temp that is needed. By not installing the outdoor sensor the boiler is just riding the curve set on the control. Your just craming hot water into the radiant.

    Get the boiler on reset, set the curve and let it do its work. The thermostat is not the cause of the overshooting, the higher then needed water temp is. I was running a radiant system on start up last week when it was in the 50's with 76 degree water satisfying a setpoint of 72 degrees. Have faith, the boiler will deliver let it do its job.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • AlexS
    AlexS Member Posts: 69

    So the system running longer is not a big deal, eh?  The boiler has been heating supply water to 125 degrees on the lowest flame...  After I hook up the outdoor reset, is the boiler going to cycle on and off even more and is it a big deal with these units?
  • CMadatMe
    CMadatMe Member Posts: 3,086

    If i get in my car and hit the highway and go 50 miles using my cruise control and my wife takes the town routes to go the same 50 who burns more fuel?

    You also don't need that 125 water temp everyday. Hook the sensor up, set the boiler to the 2 spot and let it fly. The 2 spot curve will max out at your 125.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • drhvac
    drhvac Member Posts: 190
    I have a very similiar problem

    with the overshooting, and it seems like after all my research, installing the outdoor reset to modulate the diverting valve is the fix to the problem. The higher water tempertures is what is causing the overshooting. what is the most economical control to do this? I have already gone for my lungs with the money I spent on this system. The thing I dont like about these controls though is it seems like all of them have the circulators running 24/7. I don't want that, I want my system to come on and off with a thermostat, and have the water temp modulate only then, on a call for heat. I suppose i could get one of these controls and just not use the cirulator and burner terminals, and have it come on and off the way it does now. There must be something that  just does the modulation part?,I think that and a t-stat designed for radiant would make the system run perfect.
  • kcopp
    kcopp Member Posts: 4,431
    tell us....

    more about your system. Need more info to give you a better answer.... Oil or gas. ? Boiler? Is it radiant? If so staple up or Slab? etc...kpc
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    circulators running 24/7

    I have a mod-con with outdoor reset. My main zone is concrete slab at grade. My circulator for that zone does not run 24/7, although I have set the reset curve for that zone (I have a separate reset curve for each of my two zones) set so tightly that the circulator runs for long periods. On a very cold day, it may run 18 hours. The reason it runs so long, and that I want it to run that long, is because the water delivered to the slab is so very close to that needed to make up for the heat loss. So it can take 12 hours to raise the temperature from 68.5F to 69.5F or whatever the dead zone of the thermostat actually is.

    In warm weather, I cannot make it run 18 hours because the heat loss is so much lower that the boiler will not modulate down far enough. So rather than have it rapid cycle, I have it deliver a little more heat so the boiler can just shut off.

    If I shared your objection to running a circulator for a long time, I would replace my two zoning circulators by a single ECM delta-P circulator and use zone valves, But continue to run the system as it is. That would save a lot of the energy I now use to run those circulators. I use circulators that are, I believe, 1/25 horsepower each. But these are low efficiency and actually use about 85 watts.
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
    No one mentioned this...

    Conventional "air temperature" sensing thermostats work fine on forced error systems. And they work well with convective (hot water baseboard) systems. But when it comes to mass intensive radiant heating systems (walls, floors, ceilings, radiators etc) these systems have a tendency to significantly affect the Mean Radiant Temperature, which is a mass temperature, and not primarily an air temperature. AIr temperature is a secondary effect.

    Honestly, there are no reasonably priced MRT thermostats on the open market, yet...

    So we have to use what we have, and my suggestion (based on personal experience with high mass radiant systems and the input of my customers with same) is to start LOW and go up SLOW, and when you finally achieve "comfort", turn it back 1/2 degree and let it soak for a few days. Start out at around 65 degrees F, and don't raise it more than 1 degree F per day until you achieve .

    Radiant heating systems are not conducive to large temperature swings (night/day set back) and unless occupants are going to be gone for an extended period of time (think weeks away from home)and shouldn't be run through deep set backs. If the home is set back for extended periods of time (like weeks), have a friend or neighbor turn the heat back up prior to arrival, or expect it to take days to recover fully.

    Let the outdoor reset controls take care of the conservation effort. If you DO need to turn your thermostats down to achieve significant savings, it indicates a need to spend some quality time on the end of a caulk gun around windows, doors etc. to eliminate uncontrolled infiltration.

    So, to review, conventional thermostats don't do a good job of controlling the MRT on true radiant heated jobs.

    Start LOW and go up SLOW.

    Don't do deep cycles for set back/set up.

    Enjoy the "Radiant Comfort".

    Utilize the controls that were provided with the heat source, reap the savings and enjoy the comfort.

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • Gordy
    Gordy Member Posts: 9,546
    edited November 2011
    What ME said.

     Oh so true. With high mass radiant. set it, and forget it till spring.

This discussion has been closed.