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HX Size for 750 sq ft.

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by leeeallen, Feb 10, 2012.

  1. leeeallen

    leeeallen Member

    Joined:
    Oct 21, 2009
    Messages:
    46
    Loc:
    s. Maine
    I have a living space above a garage that is approx 750 sq. ft. I want to run a feed and return from my OWB thru a HX to my living space. I'm not sure what size HX I need - I'm thinking 30 plates because it is super insulated. Any thoughts?

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  2. jebatty

    jebatty Minister of Fire

    Joined:
    Jan 1, 2008
    Messages:
    4,285
    Loc:
    Northern MN
    My thoughts are that to make the right choice you need to know the amount of heat (btuH) that you want the hx to deliver, which also relates to your heat loss calculation. With that info you also need to know the temperature of the water you need to deliver to your emitters and what the temperature drop will be at that rated output (delta-T). Next you need to know the number of gpm the emitters need to deliver their rated btuH output (delta-T x gpm x 500). Then you need to know at that gpm what the pump head will be (Side B of the hx and related plumbing), and then size a circulator to deliver those gpm at that pump head.

    Now do the same thing on Side A of the hx for your heat source to the hx, and look at the table for various plate hx's to size one that when supplied by your heat source (Side A - gpm, delta-T, pump head, circ) will deliver the heat you need (Side B). And remember to counterflow plumb the hx. Keep in mind that the temperature output of a hx to the system (Side B) will be less than the temperature input to the hx by the heat source (Side A). Typical difference is 10F (approach temp), although closer approach temps are possible.

    I have seen OWB supplied info on plate hx's that show very high btuH ratings, and upon closer look that info was based on 30 or more gpm flow rates, which in most home applications is realistically unachievable. My thought is that this results in a smaller hx, and therefore less expensive hx. On the other hand the cost to make such an hx work could be prohibitive or impossible in a particular heating system.
  3. leeeallen

    leeeallen Member

    Joined:
    Oct 21, 2009
    Messages:
    46
    Loc:
    s. Maine
    Thanks, Jebatty.
    In your formula (delta-t x gpm x 500) what does the 500 represent.?
    My plan is to put two old style cast iron rads (strategically placed in living space) to heat the area. These emit around 8-10000 BTUS each. I know from experience these are sufficient to easily heat the space.
    Also, can you give me some ideas on a controller that will be thermostatically controlled to shut off the circ pump on the b side of the HX?Thanks
  4. jebatty

    jebatty Minister of Fire

    Joined:
    Jan 1, 2008
    Messages:
    4,285
    Loc:
    Northern MN
    The 500 represents the conversion from btu/minute to to btuH: delta-T x gpm x 8.33 lbs/gal x 60 min/hr. The simplest would be a line voltage thermostat, like that used for electric baseboard heaters. Or, a low voltage (24vac) thermostat, and 24vac transformer powering a relay with a 24vac coil to switch the line current with the relay contacts for the circulator.

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