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Thread: HELP...Integrating Outdoor Wood Furnace with indoor Propane Boiler

  1. #1
    DIY Junior Member bkmeis's Avatar
    Join Date
    Sep 2013
    SW PA

    Default HELP...Integrating Outdoor Wood Furnace with indoor Propane Boiler

    (Radiant floor board heating) I have plumbed the outdoor water lines (from Central Boiler 1450 furnace) into the existing indoor (pressurized) heating lines.
    I have included ball valves to separate the two water systems when one is running. Additionally, Central Boiler requires installation of their thermostatic valve (which returns water before integrating into
    the existing system if the temp of the water is not above 150 degrees). I am having an issue when I open the valves to run the wood furnace, water from the in house system back drains out the wood furnace.
    I have yet to test the wood furnace until I resolve this issue. Any assistance is appreciated.
    I can add more info as you require.


  2. #2
    Like an engineer alternety's Avatar
    Join Date
    Apr 2006


    Personally I would not do what you are trying. For a whole bunch of reasons. There is nothing wrong with the basic concept - wood heat for radiant system. The devil is in the details.

    You really should have intermediate storage. A loop for the wood burner with antifreeze in it, a large insulated storage container, and a circulating pump between the boiler and storage. This can be unpressurized. And that will help avoid explosions if you are also not clear on relief valves or they get frozen.

    Then a heat exchanger in the storage tank piped into your existing system and a circulating pump driving that loop.

    Then all of the controls and electronics necessary to merge the functions. And something to avoid boiling water in your wood boiler.

    Assuming things freeze, you should not be using plain water in the boiler unless you drain it when it is not in use or make sure it is always keeping all the exposed water containing equipment warm.

    You migh consider bringing in someone with experience with this sort of thing to at least provide you with a diagram that will work. I am surprised the boiler vendor does not provide more information.
    Last edited by alternety; 10-27-2013 at 11:09 AM.

  3. #3
    In the trades Dana's Avatar
    Join Date
    Jan 2009


    Most wood boilers (and it's a boiler, not a furnace) are required to run at atmospheric pressure for safety reasons. All fossil-fired hydronic systems need to run at 10psi as a BARE MINIMUM to keep the micro-boil on the heat exchangers in the boiler from going macroscopic, creating a insulating layer of frothy water on the water side of the heat exchanger reducing it's heat transfer efficiency. (Some low-mass boilers need to run 15psi or so to keep the sizzle down, heat transfer efficiency up.)

    Furthermore, running at atmospheric pressure allows oxygen to get into the system, which will corrode iron pumps and other heating system components at a rate 100x that of normal 10-20psi operation.

    The solution will require some sort of heat exchanger & buffer tank to allow the radiation & fossil burner to run isolated from and higher pressure than the wood boiler side. Plate-type heat exchangers are fine, with a pump on each side (a bronze or stainless pump on the low-pressure/wood-boiler side), with the wood boiler dumping ALL of it's heat into the buffer tank. This is all do-able, but it's a real hydronic design problem, not a mere plumbing problem- you have to do the math on all of it to make sure it works efficiently, effectively and safely.

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