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combining an antifreeze outdoor wood boiler with a modcon via a buffer tank

We are getting ready to install a system designed for 140F supply temps at design conditions with ODR control of the modcon. It is extensively zoned (with radiators on TRVs) and uses a buffer tank to limit short cycling and for hydraulic separation. We are considering using an indirect tank as a buffer tank, which would allow us to send the supply water from an outdoor wood boiler, which contains antifreeze, through the internal coil. This way, when the supply temp from the OWB is insufficient to keep the buffer tank hot enough, the modcon will turn on and take over.

My first question- Is this the best way to integrate the two systems? In my research, I have been unable to find an example that specifically addresses OWB with antifreeze and gas modcons using a buffer tank. Also, one drawback is that this setup does not allow the OWB to heat the DHW indirect tank which is part of the system.

my second question assumes the proposed setup is the best way to go. If so, how we can gain control over the OWB supply temps in such a system?

With outdoor reset, the temperature required of the buffer tank water is always changing. Is there a way to get the OWB supply water to be responsive to the ODR curve? I've read that most OWB manufactures recommend keeping the supply water above 165F to minimize expansion and contraction of the steel. If that's the case, perhaps we will need a mixing valve to keep the OWB from overheating the house. Then again, all radiation will have TRVs, so perhaps the higher water temp is a good thing, since we only need to keep the supply temp low to keep the modcon condensing. When the OWB is heating the buffer tank, the modcon will be off


  • hot_rod
    hot_rod Member Posts: 21,820
    Basically the indirect is a heat exchanger for you to keep the glycol separated, not a bad approach at all. It's a nice insulated container for that.

    Typically the OWF are a buffer tank also, how many gallons does it hold?

    The loads to the building should be run via ODR, not the wood fired device. Run the OWF hot and hard, store that energy at high temperature, and mix down from the indirect/ buffer.

    Don't get hung up on the OWF, it has one simple job, put heat energy into the "drum" of water. It should have it's own control to handle on/ off control. Everything downstream of that is where you want to take control. That can be from mild to wild depending on how many SWT you need.

    The first shot at dhw should be the hottest point in the system.

    You might consider pulling that, at least a pre-heat right off the glycol lines coming in. From there dhw flows through whatever is going to be your off season course for dhw. wood energy does what it can, back up source tops it off if needed.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • roundrightfarm
    roundrightfarm Member Posts: 54
    thanks hot rod,
    I'm looking at an OWF with 160 gallon capacity.

    Run the OWF hot and hard, store that energy at high temperature, and mix down from the indirect/ buffer.

    Do you mean mix down from the OWF 160g buffer or the indirect buffer (whose temp controls the modcon)? How do you mix down while communicating with the ODR to get the desired supply temp?
  • hot_rod
    hot_rod Member Posts: 21,820
    My thoughts,
    you want to keep the buffer as hot a the OWF can get it, to store the most amount of energy. So it will maybe it runs 180F when you are wood fired.

    All the heating loads pull from the buffer via a 3 way motorized valve running on ODR. So heating loads are supplied with the correct, and lowest possible temperature always. This leverages all the useable heat from the buffer. It will provide the best comfort, and possibly eliminate cycling of then load pump.

    A 3 way motorized valve is one of the simple way to do this.

    If you don't want to run out of heat when the fire goes out, chose a temperature where the mod con kicks in to help, or replace the wood fired energy, say 130F for example to assure that it will run in condensing mode for best efficiency.

    Is DHW via another indirect tank? That tank is wired to the mod con DHW priority connection, so whenever DHW calls the mod con steps to high fire output to quickly cover the DHW, and also summer time DHW.

    A goal is to have all the loads operate within 10- 15 degrees of one another so only one mixed temperature is required.

    Depending on how you plan on controlling all this, either fully automatic, or manual operation, I'd run the OWF with a differential control, as you don't want flow going out to that when the mod con is running, set the low limit at the same temperature that the mod con kicks into the system.
    Sensor 1 at the OWF top, sensor 2 at the indirect buffer.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Ironman
    Ironman Member Posts: 7,341
    Totally agree on hotrod's approach.

    I'd recommend on further thing: put an aquastat on the ODWB that won't allow its pump to come on until it reaches 150*+. That way the mod/con won't be back heating the wood boiler through the buffer tank.

    Something else: beware of the btu ratings of indirect tanks; they are what you get with a 77* temp rise (50* in, 127* out). Your btus from the wood boiler will be less than the rating because of a lower delta T. Go a little bigger if necessary on the tank.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • njtommy
    njtommy Member Posts: 1,105
    edited December 2016
    Is the OWB going to pre heat the DHW as well? Or just heat the tank? Most I've seen are set up to pre heat the DHW going into the tank. Then it just runs the pump for the indirect to maintain tank temp if temp drops in tank.
  • roundrightfarm
    roundrightfarm Member Posts: 54
    Thanks so much hot rod for the drawing. I've been trying to understand it completely but I'm still confused about the heatflow for the DHW.

    In the drawing, is the buffer tank held at a minimum temperature, say 140F+, by the OWB with modcon backup? How does this work in the summer?

    When the DHW priority zone has called for heat, can both the modcon and DHW circulators be running at the same time?
  • hot_rod
    hot_rod Member Posts: 21,820
    Correct, for summertime DHW both the mod con and the indirect tank pump would need to run, if there is no or insufficient wood fired energy available.

    The indirect could be a parallel load off the mod con, but then you would need a means for the wood fired boiler to heat the tank, without heating the mod con with wood fired heat.

    Ideally you do not want to heat the wood fired boiler with the mod con, and vice versa. Rarely would you put the two in series.
    A perfect design would move energy from the selected source, wood or mod con, directly to the loads. The shortest, simplest path.

    The piping I showed above is one of the simplest ways to accomplish that. There are many other piping options with 3 way diverter valves, primary secondary, etc. I think this is clean and understandable when you follow the various flow paths under all the possible flow options.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream