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Would 2 be better than1?

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by sixroses, Feb 23, 2008.

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  1. sixroses

    sixroses Member

    Joined:
    Jan 31, 2008
    Messages:
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    Loc:
    alaska
    I have a flat plate HX. I do not know how many plates only the size. It is about 12x4x3. It is the interface between my storage/boiler water and the water that moves through my baseboards. If I was to plumb in a second HX in series would I pull more usable heat out of the water? I am thinking of only using it at temps below zero.? What kind of flow problems would this create?
    Steve

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  2. jebatty

    jebatty Minister of Fire

    Joined:
    Jan 1, 2008
    Messages:
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    Loc:
    Northern MN
    You can count the plates if you look closely. For 3" thickness, I estimate 20-24 plates. From your prior posts I think you have the GW 100 boiler. I don't know what the BTU rating is for that boiler, but that would relate to the size of hx you need.

    For example, I have a Tarm gasifier rated at 140,000 BTU. I use a 5 x 12 x 30 plate, which can pretty well pull out all the heat the boiler can deliver, that is, practically no boiler idling, and then idle periods of only 15-25 minutes. An additional hx for me would accomplish nothing regarding boiler efficient operation. On the heat side of the hx, water in at 100, water out at 150-160, with a Taco 007 with 3/4" line, about 5-6 gpm. Same case, on the boiler side water in at 165-180. If water in to the hx is 130 or higher, water out will be within 5-7 degrees of the boiler side to the hx. In this case boiler side water in 170-190, water out 165-185.

    Yes, two in series would pull more heat, which may or may not be usable, but then also would return cooler water to the boiler. The boiler would have to heat that cooler water, so you might not be gaining anything. In my estimation return water to the boiler should be in the 130-160 range for the boiler to operate efficiently. If it is less than this, I doubt your boiler can produce output of 160-190, which for me is pretty normal operating range. If you don't have a boiler mixing valve or other arrangement to insure that return water is in the proper range, and your return water is too cool, you should look at fixing that problem.

    A second hx in series also would reduce flow because of added resistance, how much I don't know. Reduced flow would reduce delivered BTU's.

    What is the water temp into the hx on the boiler side? What is the water temp out of the hx on the boiler side. Ditto on the heating side? This will tell you and us a lot. You can buy (check Ebay) metal meat probe thermometers which you can cable tie to the pipes, then wrap well with pipe insulation, to get fairly accurate readings.

    Since it looks like you need more heat when it is really cold, I hope you find a solution. Where I live in northern MN, our coldest periods normally only run from -20 to -30, lasting 2-5 days each, and we have had 5 of those so far this winter. Our coldest (about 10 years ago) where I live was -55. Hit that 3 nights in a row.
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