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Thread: Help with Spa calculations

  1. Back To Top    #1

    Join Date
    Jul 2015

    Help with Spa calculations

    Hey everyone

    I have been trying to figure out how to answer this question that someone asked me. im not sure if there is enough given information to solve this but any help at all would be greatly appreciated.

    A: Indoor hot tub.
    223 sq ft, 2' 1" deep, 2,600 gal.
    Design temperature: 104 F.
    GPM: 120 Gal. (Turnover rate under 30mins.)
    Hot water temperature from Boiler is140 F.

    What is the delta T between the waterout and back into the hot tub to maintain 104 F?
    What is the BTU amount to maintain thistemperature?

    Same questions for pool B.

    B: Outdoor pool.
    1,500 sq ft, 3' 6" deep, 39,000gal.
    Design temperature: 97 F
    GPM: 257 Gal. (Turnover rate app 3 hrs)
    Hot water temperature from Boiler is 140F.

    Thank you

  2. Back To Top    #2
    Mod Squad Patrick_B's Avatar
    Join Date
    Jun 2011

    Re: Help with Spa calculations

    Welcome to the forum!

    Sorry to begin this with a question back to you, but are you sure those circulation rates are correct?
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  3. Back To Top    #3
    mas985's Avatar
    Join Date
    May 2007
    Pleasanton, CA

    Re: Help with Spa calculations

    I think you are really asking how much heat is lost in the spa per min or hr. That is very difficult to figure out and requires a lot of details about the weather at the time you are using the spa.

    However, if you want to know how many BTU is required to take the spa water from say 50F to 104F (54F rise), that is easy.

    For spa A, that is BTU = 2600 * 8.34 * 54 = 1,170,936 BTU

    If you know the BTU output of a heater, you can then calculate the input to output water temperature delta at a given flow rate. So if you have a 400,000 BTU heater rated at 80% efficiency, then

    dT = 400,000 * 0.8 / 60 / (120 * 8.34) = 5.33F

    Is the boiler a hot water heater and not a pool heater?
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  4. Back To Top    #4

    Join Date
    Mar 2007
    San Rafael, CA USA

    Re: Help with Spa calculations

    (typed this as Mark was posting so some duplicate information)

    (2600 gallons) * (8.34 pounds/gallon) = 21,684 pounds so that many BTUs to raise the water 1F
    (39000 gallons) * (8.34 pounds/gallon) = 325,260 pounds so that many BTUs to raise the water 1F

    As for how many BTU/hr it takes to maintain a temperature, that depends on the heat loss and that depends on many factors such as pool and air temperature, humidity (affects evaporation rates), and wind.

    It sounds like these are commercial spa and pool installations. This is primarily a residential pool and spa forum.
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