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help with pipe sizing

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by Rob186, Aug 30, 2012.

  1. Rob186

    Rob186 New Member

    Joined:
    Jul 3, 2012
    Messages:
    39
    Loc:
    New jersey
    i want to make a dom. hot water coil but cant seem to find out what the gpms will be i want to use 3/8 i.d m copper 40-50 ft of it but dont know if it will have to big of a gpm loss if anyone knows the calculations for this can you post them i have a well so pressure is about 45-50 psi

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  2. BoilerMan

    BoilerMan Minister of Fire

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    Apr 16, 2012
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    Loc:
    Northern Maine
  3. ROVERT

    ROVERT Member

    Joined:
    Aug 7, 2012
    Messages:
    120
    If you have a well, you're pressure probably varies by a lot more than 5 psi (unless you have a constant pressure pump). 30 to 50 psi is the most common operating range.

    The calculations for flow rates are pretty complicated and require a number of inputs. That said, you can probably expect to get somewhere around 3 gpm out of your coil, probably a little less than a DHW coil in a typical oil boiler. If you want to be able to fill a bath tub as quickly with hot water as you can with cold water, you're going to be disappointed. For showering and faucets you'll probably be ok. Enemy number one will be scale build up. DHW coils are typically finned to aid in heat exchange. You're coil will have probably 2 to 3 times the surface area inside your coil for scale to build up as compared to manufactured DHW coil. I would expect your coil to suffer more quickly from scale build up. If you have soft water in your area or use a water softener, this issue will be greatly lessened.

    The above are just some thoughts based on my experience as a plumber. I have no experience with wood boiler/ storage/ DHW set ups. I've never made a DHW coil. I imagine that someone here has and can give you first hand experience.

    If I get bored later, I may even dig up some formulas and try to figure out your actual (theoretical) flow rate through your proposed coil.

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