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Outdoor wood boiler with LP combi boiler as back up

bob eck
bob eck Member Posts: 930
Good evening.
Home owner wants to install a outdoor wood boiler (open system) with a LP gas combi boiler as back up heat.
Anyone have a piping drawing of how this should be installed.
Hot rod does caleffi address this in any of their Idronics books?
I thought of coming in from the outdoor wood boiler with a SS or bronze pump into a plate heat exchanger and then into a 80 gallon buffer tank (buffer tank will have a coil for preheating the domestic water during the heating season) and then out of the buffer tank to the heating zones. The condensing LP gas combi boiler will also be piped into the same buffer tank and it would run only when the tank temp goes below a certain temp to heat the house if they run out of wood.
The buffer tank would also work as P/S piping for the combi boiler.
Anyone had piping drawings of an installation like this?


  • hot_rod
    hot_rod Member Posts: 22,251
    Here are a few options. With OWF no need to have a return protection valve, in the one pic substitute the mix valve with a HX.

    Or add a buffer to the drawing with the OWF.

    Idronics 10 has some other piping. Also this NYSERDA link.


    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • leonz
    leonz Member Posts: 1,154
    edited January 2018
    Being a homeowner that burned wood for over 3 decades and having switched to a coal stoker:

    Is there room to install a coal stoker in the basement or where the existing boiler is?

    I would avoid the installation of a hand fired Forest Eater like the plague. The Forest Eater is only going to be 40% efficient at the most even with a gasser that has multi pass afterburners.
    The other thing is that they will smoke when idling losing all that heat value up the short flue stack.

    An Axeman Anderson S130, EFM DF520 or AHS S130 Coal stoker boiler is what I would suggest in this case and just put it in a small outbuilding on a concrete slab.
    The Keystoker coal stoker boilers will have the fire die in a few minutes in the event of a power failure so there is no way I would recommend them with my past experience.

    He is going to have to have a slab for the forest eater anyway and buying a shed and setting it on the slab with a bulk coal bin or a shed large enough to store bagged coal and the boiler would be MORE COST EFFECTIVE for him and you AND LESS WORK for both of you.

    Outdoor Water Stoves have to be fed multiple times per day and in the middle of the night if the load is high enough and you also have to treat the water to balance the PH and also have water samples sent the water stove company to have them tested to assure the water is being chemically treated properly.

    I bet the water stove dealer painted a real rosy picture telling him that he would only have to load it once a day too.

    You also need to have very dry wood <25% if a gasser is going to be the forest eater of choice.

    A coal stoker for you would be less work as long as you avoid the keystoker brand of coal stokers as the coal fire will die off in a few minutes after a power outage where this will not happen with an Axdeman Anderson, S130, S260 EFM DF520 or an AHS S130 or S260.

    Coal stoker boilers are closed systems and less work for more heat and they will not have to feed it at 3 AM in a blizzard.

    All you would need is to have boiler bypass valves to use one or the other boiler rather than tying them in series.

    I would wander over to the dealer that has been pushing the forest eater as an economical alternative to gas or coal to this fellow(do not tell him your a plumber) and look inside one of these things as most but not all of them do not use firebrick and many have stainless steel combustion chamber walls with water on the top, sides and rear wall where the tapping's are and where they typically mount the circulators and controls.

    Forest Eaters are also subject to cavitation corrosion as well when the fire is in direct contact with the water wall of the boiler.
    If they lined them completely with firebrick that would not be an issue.

    What is really hilarious is when one of these builders has youtube video that he personally and purposely changes the fire triangle to a model of his choosing to sell his forest eaters. I do not know if that video is still on the internet but it would not be hard to find.