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Single Thermostat, Two Heat Sources

I wanted ot bounce an idea off the collective experience here. We are installing a Propane Furnace with AC System in a new construction home with a hot water wood coil from a outdoor wood boiler.

I’m using a Taco 501 switching relay turn on the pump and close R and G for the furnace fan call. Wood boiler loop goes to a nearby hydronic separator and boiler pump runs continuously.

I wanted to use a single thermostat. My idea was to use a strap on adjustable aquastat set at maybe 150F on the boiler supply at the separator. The NC contacts allow a heat call to go to W1 on the furnace. NO contact to W on the switching relay. The swooned benefit is if water temp drops too low, it’s provides boiler protection and prevents cool air from blowing and furnace automatically picks up.

Also boiler is oversized, so this would allow the owner to add some storage, load up just enough wood for the load and storage capacity, and the propane picks up once he runs out of heat automatically.

Seemed like a simple and cheap solution and avoids having two thermostats on the wall.

Any thoughts or a better solution.

THnaks to the heatinghelp community for the separator idea/design. Should make purging dramatically easier and solves several issues and avoid two pumps in series. Boiler is 300’ away (sigh) on 1” PEX, but that’s enough heat. Only need 60-70k peak anyway.


  • mikeg2015
    mikeg2015 Member Posts: 1,194
    ... better setup would have been a single large boiler for this house with the wood boiler on a heat exchanger. Plus a furnace vs. a airhandler is maybe... cheaper to install after you add coil, piping pump and trim, and more efficient. But furnaces have vents, and gas line. But hey, not my call. It’s time & material but I think the owner crunched the numbers both ways and thought this was a little cheaper and it’s easier for us to troubleshoot and service, as 80% of our installs are furnaces, and maybe 19% heat pumps 1% are boilers.

    In total we have 2 gas furnaces with AC, 9 loops radiant floor heat in basement, 3 loops of underfloor heat for part of 1st floor addition, garage slab heat, future snow melt near garage, and a couple mini splits on top of all that. Heating the radiant and supplying hot water with two combi’s Quite the project.

    Long term, one boiler, one indirect and 2 hydronic air handler will be dramatically cheaper to maintain and replace than 2 furnaces and 2 boilers and you only have one gas connection and vent pipe.

  • GroundUp
    GroundUp Member Posts: 1,865
    The thermostat/aquastat arrangement works great for these applications. If this is a new build, why are there 1" pex lines underground instead of 1-1/4" like there should be? What is the actual heat load of the house (not heat loss, heat load) and how much storage do you intend to incorporate? Pressurized or non? Almost none of the current OWB systems in the market will benefit from storage as they are all dressed for onboard storage so I'm wondering what the theory is there; if the OWB is oversized as you say, storage will be useless. You've mentioned several different piping arrangements- which one is actually being used?
  • mikeg2015
    mikeg2015 Member Posts: 1,194
    Good to hear the aquastat is a common practice and should work well.

    We didnt’ install the wood boiler or have anything to do with it other than installing the coil and connecting to the thermopex in the basement. I agree. Should have been 1-1/4” and tied into the Combi with a heat exchanger and a coil added to the upstairs furnace as well.

    This model has 300 gallons internal storage. The irony is it’s too much storage in a sense, and the off cycles are too long because the load is too smal, especially in mild weather that the fire cools off and he has ot manually relight it. Thinking about that now, what he needs to do then is reduce the differential on the temp controller, if it allows that, so it cycles more often. What the controller needs is a either-or logic where it has a max time delay plus a min temp differential and then a max differential. Maybe that logic is in there already. I’ll check the boiler manual.

    My theory on more storage is that he could store enough so a single load could carry him though 24 hours. But he would need almost 5000 gallons of storage. SO maybe just changing the differential and living with short cycling in colder weather would work OK. I suspect it’s set with maybe a 30F differential, and he needs to be closer to 10-15F. The reality is that in mild weather he needs to not fire it at all.

    Heat loss of the house would require hours ot do a load calculation. Original house is a 2500swft vicorian with about 3000+ sqft of additional space plus finish basement going on. True Insulation value and air change rate would be a total guess.

    My gut says it needs about 120k peak on a design day not including the snow melt or garage. I think a room by room load call given the nature of the build out is as likely to be wrong as it is correct.