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Rehau dual sensing t stat

Gta Member Posts: 133
Hi I have rehau dual sensing t stat . It controls my radiant floor heat . My question is I would like a 2.0 c spread between it turning on . I have tried changing settings but it’s not responding.

I currently have it set to 20c and at 19.8 or so it turns on ... I would like to adjust it so it can fall to 18 or what ever I choose can anyone help ? Thx


  • Jamie Hall
    Jamie Hall Member Posts: 18,951
    At the risk of it's being a stupid question... if this is a radiant floor, why would you want it to have a wider swing? The whole point of a radiant floor (besides comfort!) is that it will maintain -- or try to maintain -- a constant temperature. Much more economical that way.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Gta
    Gta Member Posts: 133
    It runs almost constantly I tried everything to stop it .. heating bills are almost more than house payments... it’s close to zero outside and the bloody thing is constantly call for heat .

    .2-.3 below the set point and it’s running

    I’m wanting to have it fall by 2-3 c before it turns on

    I honestly hate this radiant system I’m tempted to turn it off buy a space heater and move it room to room.
  • neilc
    neilc Member Posts: 1,805
    Jamie and others will explain this better than I will,
    I think we need to discuss the boiler and settings,
    and building envelope,
    the boiler wants to run, steady state, at its lowest temp that keeps the floor and house warm,
    the efficiency is best there,
    then we look for where that efficiency is leaking out of the house.
    known to beat dead horses
  • Gta
    Gta Member Posts: 133
    How is it efficient to turn on then off constantly? It starts runs then stops cover and over .... I honestly bet it’s 200 times a day ... the zones are lucky to be off for min at a time .... the t stats are just too sensitive
    I have blocked the air passages they have with tape so they don’t fall so fast in temp

    I have added 5 k worth of attic insulation and floor joist insulation since fall and no difference😡😡😡🙄🙄🙄
    HVACNUT Member Posts: 4,594
    Turn off "room sensing" and use "floor sensing" only.
    Don't go wider than .1°C.

    How many loops? Is the loop serving the thermostat area working properly? TD, flow rate, actuator, mixing valve.

    Properly set, it should run near constant.
  • Gta
    Gta Member Posts: 133
    3 zones 3 separate pumps , no zone valves 23 kw boiler ... I just changed them to floor only 🤞🤞🤞
  • Jamie Hall
    Jamie Hall Member Posts: 18,951
    We need to step back a minute and think. What actually determines how much energy one uses? Two things: the efficiency of the boiler or other heat source, and the heat loss of the building.

    Taking the second one first. That one is the primary key to how much energy one is going to use. Anything one can do to reduce that heat loss is directly translated to the amount of energy used, so one should start there.

    The first one is a little more complicated, as there are so many different variables involved. We don't know, in this instance, what the efficiency of the boiler is. One hopes that it is a relatively recent installation, though, so it should have an efficiency -- if it is fuel fired -- of somewhere between 80% and 96%. If it is electric (which, since the OP quotes KW as the power, it likely is), the efficiency will be up around 100% somewhere. Then we need to look at the way that heat is transferred to the structure. In this situation, we are looking at a radiant floor installation. Properly controlled and piped, these are very very high efficiency, relatively speaking. But... they are also deceptive. They can only achieve anything even remotely resembling decent efficiency if they are controlled so that the circulator for the floor is running all the time, and the temperature of the circulating water and the flow rate are controlled -- preferably by outdoor reset trimmed by in-floor sensor, although straight in-floor sensor will do -- so that the temperature of the return flow which the boiler is seeing is as low as possible (which depends on the boiler -- mod/cons can go much lower than straight).

    So... our OP here should have the boiler controls and the mixing valve controls for the floor loops set so that this is accomplished. The floor loops will then deliver the amount of heat needed to the floor, and the boiler will run when (and, if it is a mod/con, at the firing rate) needed to keep that circulating water temperature correct.

    There is no efficiency loss -- particularly if the boiler is electric -- to having the pumps turn on and off, although it suggests that perhaps the controls are not correctly set up. This is also true of electric boilers (which, to make a ridiculous point, actually turn on and off 120 times per second, with the power coming to them).

    There is, however, with radiant floors, a terrific hit on efficiency if the floor temperature is allowed to drop, and then brought back up. There is also, unless the controls are very sophisticated, a tendency for the space temperature to overshoot, sometimes rather dramatically.

    So... bottom line for @Gta . Make sure your controls are set up to maintain as close to a constant floor temperature as possible, and that the circulating pump is running all the time, if possible, or if that is not possible that it runs frequently enough to keep the circulating water temperature within very close limits (half a degree is about as wide as you ever want to go). If there are no mixing valves or outdoor reset on the valves, that's going to mean those pumps are going to turn on and off a lot, but it won't hurt them -- or your efficiency -- but must be controlled by the floor sensors, not space temperature.

    And no setbacks!

    If, having done all that, you are still using more energy than you like -- you have to look at building heat loss.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England