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Please explain how two-stage T-stat works

Is this only one room(that gets cooler), out of other radiant zones? Is your radiant stat located in this room also?<BR>If it is,,,typically the radiant part of stat operates first, then if the room temp continues to drop, the supplementary(baseboard) section comes on as well.


  • Steve Garson_2
    Steve Garson_2 Member Posts: 636
    Please explain how two-stage T-stat works

    I have a room with radiant heat that works fine until the temp drops below 10*. I asked that the installer install a baseboard just in case the radiant wasn't adequate, which has a separate thermostat.

    I was advised here on-the-wall to install a two-stage thermostat, but I don't understand exactly how it would work: If the weather is cold enough to need the supplementary heat, then that heat will prevent the primary thermostat from running the circulator for the radiant, which will then take forever to work again. My intuition tells me that when the temps are so low, that the baseboard should come on and the radiant continue to run even after the room come to temp.

    How does this all really work?
    Steve from Newton, MA
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    A staged thermostat is all about the deviation between the desired setpoint and actual room temperature.

    Say you allow 1F deviation both above and below your desired room temp setpoint.

    You set AND LEAVE the thermostat at 68F with the heat turning on at 67F and off at 69F. As long as the room temp stays in this range only the first stage will operate. Let the room temp fall below 67F and the 2nd stage will kick in because the first stage alone is inadequate. BOTH stages will be running.

    Now say you raise the thermostat by more than 2F. Both stages will call with the 2nd stage dropping out once room temp is 1F below setpoint.

    Such is the simplest--but far from the only--method of staging.

    Things can get FAR more complicated when you're talking about staging supplemental baseboard in a space ordinarily heated by a floor. While not always true, the baseboard may well require a higher supply temp from the boiler even to provide the 2nd stage boost. Unless the system is designed to provide two independent supply temperatures, the first (floor) stage may be "locked out" whenever the 2nd (baseboard) stage is calling. If it's not "locked out", and independent supply temperatures are not provided, then the 2nd stage may never get hot enough because the 1st is dragging it down.
  • Steve Garson_2
    Steve Garson_2 Member Posts: 636

    Yes, this is the only room that is a problem. It has a cathedral ceiling with a tall north facing wall. The T-stat is in this room.
    Steve from Newton, MA
  • Steve Garson_2
    Steve Garson_2 Member Posts: 636


    Excellent explanation. In my case, the baseboard is electric. There is a relay allowing it's control with a 24-v thermostat, which at present, is a separate second thermostat. I manually turn it on for those two or three real cold days each winter.

    Steve from Newton, MA
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    In that case you'll need an appropriate relay (often called a "contactor" for an A/C condensing unit) to power the baseboard.

    If I properly recall your earlier question here and the electric B/B is already installed with the common wall control, I nearly beg you to use professionals for the job.
  • singh
    singh Member Posts: 866
    Question for all

    Does any one know how much the baseboard output is reduced , using it as a second stage source with radiant floors? Being that floors are warm and convective flow is interupted.

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  • andy_21
    andy_21 Member Posts: 42
    two stage stat

    Great explanation guys! can anyone recommend a stat for a Radiant floor w/ Hydro air back up. Also is there anyway to adjust the 2nd stage - 3-4 degree from set point, to allow the Radiant to try & achieve the desired temp & not be cut short
  • tim smith
    tim smith Member Posts: 2,360
    Tekmar 510

    has 2 stage heat w/ option for slab sense. Does not have aux fan switch for summer fan. Or just use a Honeywell TH8110U??? stat. Don't remember last digits for multi stage but good touch pad stat. Good luck, Tim
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