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Boiler cycling

alv54 | Posted in General Questions on

I have a 5-zone condensing high-efficiency boiler. It cycles way too much.

My question: Is it plausible to join 2 of the thermostat wires to run off 1 of the thermostats and send water to both zone?

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  1. Jon_R | | #1

    Install partial bypass valves on all (except zones that you sometimes want completely unheated) zone valves such that each zone gets some flow when any thermostat calls for heat. This retains effective zoning while greatly increasing minimum flow (perhaps 3.5x). If this isn't enough (unlikely), you can add a tank (thermal mass).

    1. alv54 | | #2

      thanks much Jon R my question is what does this entail regarding work and whose to do it as opposed to my sugestion of joining 2 thermostat wiring to act as 1 zone? providing my idea is even workable.

  2. Jon_R | | #3

    My entire house runs as a single zone with a single thermostat. So yes, balancing can be adjusted such that a shared thermostat works well enough. Try it for a perhaps 2x increase in minimum flow in your proposal.

  3. Expert Member
    Akos | | #4

    Should be no problem if you have a standard setup (low voltage thermostat C,W,R connection going to something like a TACO multizone relay module). Just jumper the W wire from the thermostat you want to use to the W wire of the 2nd zone. Most likely you can even keep the 2nd thermostat (either thermostat would be able to call for heat for both zones).

    One thing to check with condensing boilers is the return water temperature. Most I've seen have the heating loop way over pumped, almost no delta T between supply and return. Getting that return temperature down makes a big difference on efficiency.

    1. Jon_R | | #6

      Also check that outdoor reset temperatures are set as low as possible.

  4. Expert Member
    Dana Dorsett | | #5

    Have you done any of the napkin-math on the zone radiation vs. the minimum modulated output of your boiler to get a handle on why it's short cycling? See:

  5. tommay | | #7

    As Akos said, jump the t-stat wiring to operate both zones at once. If the boiler itself is cycling to much, check your differential settings on the boiler control.

    1. alv54 | | #8

      finally jumped the W wire effectively reducing thermostats from 5 to 3 resulting in longer run times and almost no short cycling. averaging run times now 15+ minutes.
      thanks all much appreciate your help.

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