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Aquastat Wiring for Backup Heat

I have a wood fired gasifying boiler which has its own controller to control its own circulator, combustion air blower, etc. The boiler circulates heated water through a plate exchanger while the fire burns as needed. The boiler storage tank temperature is maintained between 160-180 degrees by operation of the blower. If the storage tank gets below 140 degrees then the circulator shuts down.

My house hydronic heat piping (the other side of the plate exchanger), has its own zones and circulators and domestic HW (Smart 80), which are controlled by the thermostats / zone controller.

I'd like to use a gas boiler (which also would have its own circulator and temperature limit controls) for freeze protection. I am thinking that I could use an aquastat at the wood boiler circulator supply / discharge to sense a low water temperature set point (100 degrees), to turn on the gas boiler until the tank reaches, say 130 degrees.

Q1) Can I use a Honeywell 6006C (Home Depot stock) and wire it like the attached drawing shows (bottom of sheet) to accomplish this?
Q2) How would the differential setting on the aquastat (adjustable from 5-30 degrees) work?

Thank you in advance, I sincerely appreciate the help.


  • njtommy
    njtommy Member Posts: 1,105
    Is your FPHX piped into the return line of your gas boiler now? Or where is it piped.

    If it's piped into your gas boilers return line you can just change the aquastat setting on your boiler or swap out the aquastat for one that will work for what you are trying to do.
  • QsinAK
    QsinAK Member Posts: 3
    Its piped on the exit of the plate exchanger supply (hot) side to the house. Supply and return of the gas boiler are tied into the piping, plumbed 4" apart with no bypass or check between them (plumber's recommendation). It seems to work fine but it takes over from the wood boiler.
  • QsinAK
    QsinAK Member Posts: 3
    Is anyone able to answer; will wiring the aquastat as shown at the bottom of the drawing, attached in the post above, work?
  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,646
    I assume the wood boiler is outdoors or in a remote building and you are looking for freeze protection for it. If so, is this to be automated? I'm not following your question. If the boiler circulator turns off at 140 and a gas boiler runs itself. Or are you looking for when wood system is down on temp to fire gas boiler to simply take over heat to the house? And wood boiler is fine without heat at all?

    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!