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use 2-stage stat to override outdoor reset?

Nom_DeplumeNom_Deplume Member Posts: 51
Is there an easy way to wire a 2-stage stat such that, when the 2nd stage is called for (typically when room temp is more than say 3 degrees away from set point, e.g. during recovery after setback), this overrides outdoor reset and runs the boiler flat out until the 2nd stage is satisfied?

If stats had a 2nd stage output that is NC (normally closed), this would be easy, I could just run the outdoor sensor wire through this switch. In fact the stat output is NO, and switching NO to NC requires an external relay, which I am not eager to do unless I have to.

I'm thinking maybe there is a smarter way to wire it? Or maybe a commercial product that does what is needed?


  • SWEISWEI Member Posts: 7,356
    What kind of boiler do you have?

    many onboard controls have a boost setting for just this purpose.  Also what kind of emitters?  Setbacks are problematic with radiant heat, and often a bad idea.
  • Nom_DeplumeNom_Deplume Member Posts: 51
    edited October 2013
    HTP Elite FT

    HTP Elite FT. No boost input. (I didn't realize some boilers had boost inputs...makes a lot of sense..)

    Now speaking of boost, what would happen if I wired the 2nd stage signal into the DHW input (not currently used), and tie the DHW circ output and space heating circ output together? There's only one zone. Would that work?

    Radiators only, no radiant heat.
  • IronmanIronman Member Posts: 3,535

    Setback on a mod/con is counter-productive to the design and logic of the boiler. You will actually use more fuel while sacrificing comfort.

    Set it and forget it.
    Bob Boan

    You can choose to do what you want, but you cannot choose the consequences.
  • SWEISWEI Member Posts: 7,356
    Boost setting

    not a boost input.  I'm not familiar with the newer HTP controls, but it usually works something like this:

    The boiler controls monitor the firing time on a call for heat.  If the call is not satisfied after X minutes, supply temperature is increased by Y degrees.  Repeat until boiler high limit reached.

    Your radiators are a form of radiant emitter.  If your outdoor reset is properly dialed in, you won't even need a thermostat.  Set yours to 90F and play with the ODR settings until everything is comfortable.  That may take several weeks.  Then set the stat 2-3ºF above the room temp and leave it there as a high limit control.  If you fire up the wood stove or have a really sunny day, it will keep you from overheating.

    If you absolutely must use a setback, limit it to 2-3ºF, and allow at least an hour for recovery. 
  • NYplumberNYplumber Member Posts: 487

    Is this your home or a customer?

    Using the second set of contacts to energize the dhw setting and wiring the dhw to send power to the right places will work. However, if this is your home you would be much better off setting the heating curve via odr to match the home. When thats done, of you still desire to wire up the second stage to kick in when the door is left open do so. You can also wire the second stage to a counter and see how often its needed. That will help you dial in the odr.
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