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Discuss Insulation in thermostat back box?? in the Central Heating Forum area at

  1. Edo72

    Edo72 New Member

    hello, I’m looking for some expert advice. A recently completed extension has wet unfloor heating installed. The stat is hard wired to the boiler with a probe into the floor. The stat is a SolFex slimline. The rest of the house also has wet underfloor and is on a Heatmiser RF system.

    The issue I am having in the extension is that the stat has trouble reading over 19 degrees C in the morning after a cold night. Yet the room and floor are really warm. Using an infrared thermometer gun this morning the floor temp was at 29 degrees (wooden floor so likely too high), the walls internally were in the 23-24 range and the room felt hot. Using the IR gun very close to the stat and on the air intake of the stat the temp dropped considerable. Temp in that area was in the 20 degree range and at the air intake sometimes 19 degrees.

    This suggests to me that as the stat is in a back box, there is less insulation around/behind it and therefore the stat is reading a lower air temp based on the outside temperature, causing the room to over heat. Obviously I don’t want to over pay for heating but also, the room is getting much hotter than it needs to be and on the verge of uncomfortable.

    The extension is wood frame with outside boarding, 100mm insulation, then another 25mm before the plaster board. The stat is in a normal sized back box.

    Any suggestions on a solution? Here are the potential options I thought of so far.

    - try to insulate the back box, to cut out the draught or some of the inside outside temp variance. Could I use insulating foam spray? The digital part of the stat fits to a plate via a connector, the plate fits to the metal back box. Loads of space in the back box other than the wires.
    - turn the stat down so that it cuts out at a lower temp and hope the room temp will be ok
    - replace the stat for a surface mounted or smart stat. Filling the back box with insulation. (Technically I may still be able to keep it wired?)

    Any advice is welcome. I am in the Guildford area if any great heating engineers are nearby and have ideas or want to help.

    The rest of the house on Heatmiser RF stats is working fine. These are battery surface mounted stats and therefore I assume not facing the same issue. The rest of the house is wood frame too. That sits at a constant 20 degrees really nicely, floor temp there is around 23-24 degrees in the morning.


    P.s. I have replaced the stat as I originally thought it was faulty but still has the same issue.
    Last edited: Dec 11, 2018
  2. Ben-gee

    Ben-gee Plumber Top Contributor!!

    How about just turn stat down? Effectively think of a systematic error, ie set stat to 6 degrees (or whatever it proves to be by experiment) less than you want the room.
    Stat at 14, room is actually at 20. Simple as that, don’t over complicate it!
    • Like Like x 1
  3. fixitflav

    fixitflav Active Member

    Yes, and if the stat is like mine, the knob is on splines. If so, pull the knob off, turn it so 20 lines up with marker, and push it back on.
  4. Edo72

    Edo72 New Member

    It’s digital however I think there is an adjustment option in the settings to recalibrate the temp to one that is closer to reality. I’ll try that - so it will still show 20 on the screen but in reality be bridging the gap between the back box temp and the actual room temp - might require a bit of trial and error.

    I’m getting the feeling that trying to insulate isn’t a good option?
  5. fixitflav

    fixitflav Active Member

    It might be. Depending on how much fixed wiring there is to spare, perhaps you can unscrew the mounting box from the wall, and leave the stat hanging, with say a few layers of newspaper behind it to keep it out of the hole and insulate it from the wall. If it's better like that insulation could be worth trying.
  6. YorkshireDave

    YorkshireDave Plumber

    I suspect you are right in your analysis. Unless the air being drawn through the stat is able to overcome the losses experienced by the local environment it will never quite work.

    Unfortunately the offset function (as alluded to by my esteemed colleagues) will not work effectively in this instance because the rate of change will just be nothing like the real environment.

    If it were mine, I'd be looking to relocate whichever component is doing the actual sensing to outside of the box via a simple 2 core cable. Typically they will use a bead thermistor in a sensor like this but it would be good to check with the manufacturer's tech support.

    If you can do it the issue will simply disappear.
  7. Edo72

    Edo72 New Member

    Actually that sounds like a better plan.

    Can anyone suggest a 230v thermostat with a remote (wireless) air sensor that I can place in a different spot in the room?

    As the SolFex has both a floor probe and air sensor I was expecting the floor probe to be driving the majority of the temp setting, however it doesn’t seem to be. Possibly a problem with the floor probe. If I can find a wireless remote air sensor combination then I can replace the SolFex slimline stat (thus not damaging the new plaster or moving the wiring) and take the reading from elsewhere in the room.
  8. YorkshireDave

    YorkshireDave Plumber

    Ahh. Why have you not excluded the air sensor from influencing control? Most of that nature allow that!
  9. Edo72

    Edo72 New Member

    You’d think right. But the SolFex slimline doesn’t allow the use of the probe only (as far as the manual says, the sensor options are built-in sensor or remote air sensor only - see attached jpg). I’d like to use probe only if I could.

    Unless the probe is also considered an air sensor and I am thinking it means an external room based air sensor? I’ll try switch it later and see if it makes a difference.

  10. YorkshireDave

    YorkshireDave Plumber

    To me that clearly indicates you can buy another sensor that is remote from teh electronics. That's your way to go.

    Do bear in mind tho, hysterisis in that case will quite wide so you'll need to put in some kind of offset so it turns off a bit earlier than reqd.
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