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Thread: BTU Math

  1. Back To Top    #1

    Join Date
    Oct 2008
    Location
    Terre Haute, IN
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    51

    BTU Math

    To my understanding a BTU is unit of energy to raise 1lb of water 1 degree F......But in terms of a heater/pool what is the math to find the time needed to raise the water say 1 degree....Was unable to find it on the net. If my pool drops "t" degrees during a cold week how long will it take with an "xBTU" heater to raise the temperature a "y" amount...This will help me figure cost to heat this water and time. I realize theres an efficiency factor of the heater as well. Anyone know of a chart/calculator??? This will also help me size my gas heater.

    As an alternative I've considered a fire burning pool heating system (not in city limits). Anyone know of a source for these?

  2. Back To Top    #2
    JasonLion's Avatar
    Join Date
    May 2007
    Location
    Silver Spring, MD
    Posts
    37,887

    Re: BTU Math

    The challenge is that it is tricky to find out how much heat the pool loses. Heat loss varies with water temperature, air temperature, wind speed, humidity, and other factors. Heat loss is both highly variable and difficult to predict accurately using math.

    If by "fire burning" you mean wood burning, I would look elsewhere. It takes huge amounts of wood to heat a pool, not something most people want to deal with.
    19K gal, vinyl, 1/2 HP WhisperFlo pump, 200 sqft cartridge filter, AutoPilot Digital SWG, Dolphin Dynamic cleaning robot
    Creator of PoolMath and Pool Calculator. Other handy links: Support this site, TF Test Kits, Pool School

  3. Back To Top    #3

    Join Date
    Mar 2007
    Location
    San Rafael, CA USA
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    12,085

    Re: BTU Math

    1 gallon of water weighs about 8.33 pounds. So a 15,000 gallon pool is about 125,000 pounds so 125,000 BTU/hr. would be 1F temp rise per hour. As Jason points out, this assumes no losses.

    In addition to our solar system, we have a gas heater which, unfortunately, needs to be used this time of year on cloudy days and for assistance given the cold nights. It's output is 200,000 BTU/hr (input is 250,000 BTU/hr so only 80% efficient -- yes, that's awful) and our pool is 16,000 gallons. In theory, this should cause a 1.5F rise per hour, but in practice we usually see the rise more like 1F per hour due to other losses (mostly heat loss from the pool at almost 0.5F per hour since it's warm near 88F -- some of this is lost in the ground from the pipes which are not insulated). I asked our pool builder to insulate the pipes but he said it wasn't worth it, but I'm not so sure. The gunite is relatively insulating, but the PVC pipe isn't and though the area is relatively small, it doesn't seem insignificant.

    Richard
    16,000 gallon outdoor in-ground 16'x32' plaster pool; Pentair Intelliflo VF pump; Pentair IntelliTouch i9+3s control system; Jandy CL-340 square foot cartridge filter
    12 Fafco solar panels; Purex Triton PowerMax 250 natural gas heater (200,000 BTU/hr output); automatic electric pool safety cover; 4-wheel pressure-side "The Pool Cleaner"

  4. Back To Top    #4

    Join Date
    Oct 2008
    Location
    Terre Haute, IN
    Posts
    51

    Re: BTU Math

    The 8.33lbs/gallon and knowing that BTU units is expressed in per hour is what I was missing. So I guess a very rough rule of thumb to sum things up is real world you get 50-80% outuput in heat based on what you put in. It varies based on variables per Jason's list and efficiency of your heating unit and pool/plumbing.

    Richard/Jason...I have enjoyed reading a lot of your posts. THANK YOU for your time doing posts and sharing your knowledge. I have more knowledge from this site and some of the main contributors (you and Jason) then all of my local PS owners put together!!! I won't rant much here but as a compliment I tested the waters of chemical knowledge with the manager of a local PS owner in town about recommended FC levels for my high CYA and he looked at me like I was crazy and changed the subject!!! Great Work!

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