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Heat exchange question

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by SE Iowa, Mar 20, 2008.

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  1. SE Iowa

    SE Iowa New Member

    Joined:
    Jan 17, 2008
    Messages:
    212
    Loc:
    SE Iowa
    I'm trying to do my homework on the addition of a wood boiler addition to my radiant system. To try to summarize, I have a Well-Mclaine instantaneous modulating boiler on my primary loop. There is one pump that circulates the primary loop and one pump that circulates from the loop out to any zone(s). When one of the zones calls for heat, the zone valve opens and causes the pumps to come on. Then the boiler starts to heat the water in the primary loop until the design temp is reached and then the boiler shuts off or modulates it temp down to just maintain the temp.

    The design temp for the primary loop is 136F. I was told that it should not go (much) above that. This would allow approximately 125F water to flow out to the zone. Each zone has a thermometer at the end of the loop so I can tell the return water temp from each line. There is some sort of feedback that if return water/primary loop temps get too high it will shut down the boiler.

    My understanding is that wood boilers have optimal burning conditions when the temp is around 180F, otherwise there is squelching. For example, large OWB's that smoke until the water temps get up to snuff. So, I obviously cannot directly pump from the boiler to the primary loop otherwise it would shutdown after reaching 136F. If I used a heat exchanger I could maintain the boiler at 180F and the loop at max of 136F. So how exactly do I do this? Are heat exchangers sized according to the max btu's exchanged or is there something else I could do?

    I've thought about using a small heat exchange coil inside a storage tank that would only have the capability of getting the temp up to 130F but that would seem extremely difficult to get the exact length of coil, plus the coil temp would fluctuate with the storage temps. So what does everyone else do? Obviously people have boilers in series with their gassifiers. Are your design temps much higher than mine or is there a way around all this. Baseboard loop temps are around 170F aren't they?

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  2. Vtgent49

    Vtgent49 New Member

    Joined:
    Jan 15, 2008
    Messages:
    102
    Loc:
    Central VT
    Hi,

    Some good info here: www.houseneeds.com go to the Heating University


    Note that they aren't OWB fans, so the real users on here are a decade ahead...

    Al
  3. Jersey Bill

    Jersey Bill Member

    Joined:
    Jan 21, 2008
    Messages:
    132
    Loc:
    Central NJ
    What I did, in a nutshell, is put in a recirculating loop back to the wood boiler with 3-way mixing valve to maintain boiler temp
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