Single thermostat automatic changeover between furnace/wood boiler + freeze protection

danjayh Posted By danjayh, Oct 29, 2012 at 1:16 AM

  1. danjayh

    New Member 2.

    Aug 23, 2012
    I saw somebody asking about this, so I thought I'd post how I did it:

    I put one of these: that opens at a high temperature (140f if I remember right) on my supply line, and then ran the W wire from my thermostat through it. I then reprogrammed my thermostat (Honeywell RTH7600D) to 'thermostat controls fan' mode, so that every time it calls for heat, it also independently turns on the fan (which doesn't cause a problem for my particular furnace). So if the water temp is up, the signal to the propane furnace is cut, if it's down, the propane runs.

    If you don't have a programmable thermostat, you could order one of these:
    and use it to bridge the W line to the G terminal when the water temp is up, which would cause the fan to run when the thermostat calls for heat.

    If you wanted to get really fancy (I bought it but haven't wired it in yet), you could afford yourself some freeze protection by getting one of these:
    that opened at a fairly low temperature (say 45f) and using it to activate your fan when the water temp got too low (this would cause the boiler to rob energy from the house until the water temp came up).

    With all three of those you get:
    * Automatic changeover between propane and wood boiler with a single thermostat
    * Automatic freeze protection
    * Fan activation on heat call when using wood boiler

    All for < $30, which is not bad if you ask me.

    (PS - I have no association with Senasys, but they're the only ones who I found that make the variety of switches needed to accomplish everything)

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