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Water heating calcs

Discussion in 'Home Power and Microgeneration' started by Guest, Oct 21, 2004.

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  1. Guest

    Guest Guest

    I'm sure I saw this here a while back, but I can't seem to find it now I
    need it.

    I want to calc how much heat I need to raise the temp of the pool water.
    Once I know the max (winter) BTU's required, I want to evaluate the
    solar systems (tube vs panels vs direct vs glycol etc) and then compare that
    using an oil or LPG heater.

    eg lets say water is 10 deg Centigrade and I want 26 C to swim
    and say the pool is 50,000 litres and a solar blanket will cover the pool
    Daytime temp in winter is 15 deg.
    Given the required water temp is more than daytime temp, I'm not sure if
    this is possible, even if the walls & floor have insulation..

    If the above is true and I can't swim in winter, I'd like to figure the
    daytime temps vs the amount of solar heater required to get the pool to
    required temp.

    Bottom line is, I'm trying to figure out if Solar heating is the right
    From what I've seen it is more expensive to install than oil or lpg, but
    then the running
    costs are totally different! but If solar only gives me an extra few weeks
    on swim season
    then I'll prob put in a oil / lpg unit anyhow....

    Thx for any input.
  2. One calorie will raise one gram of water one degree centigrade. Not
    sure if this helps, as you need to decide how fast you want to raise
    the water temperature, and take cooling into account...
  3. Gymmy Bob

    Gymmy Bob Guest

    Best thing is to actually ask pool owners with the designs you envision.
    Nothing like the "horse's mouth"
  4. The calcs are fairly straightforward but solar depends heavily on the site and
    the amount of heat can vary based upon the weather. What you might consider is
    an idea based on an old plan shown in Popular Science. Polyethylene plastic
    sheeting was used to make a clear quonset hut like structure over the pool. It
    was kept inflated when in use with a box fan and allowed to deflate to a clear
    pool cover when not used. An advantage is the air inside the bubble heats to a
    more summerlike temperature and cold winds are kept at bay. The poly might
    last a season, but it is fairly cheap to replace if covering only a small pool.

    Solar blankets prevent evaporation and evaporative cooling, but are often used
    more to extend the life of pool chlorine by keeping it out of the ultraviolet
  5. Gary

    Gary Guest

    You might try downloading this free pool simulation software:

    I believe that it will let you define a pool and a set of collectors,
    and it will tell you what kind of pool temperatures you can expect.

    It will need a TMY weather file for your location, or one like it.
    You can get US ones at:
    Or, you might find one of the cities it provides close enough.

  6. Guest

    Guest Guest

    i work on pool heaters and they are typically in the 300,000 btu range. it
    will cost a bunble to run figure average house furnace is 60,000 byu in a
    new energy eff house. they can easily burn a torpedo tank of propane in 3
    weeks whe weather gets cooler in the fall.

  7. Guest

    Don't rule out a wood burner either. If you live out in a rural area, fire
    wood is probably cheap. In suburban areas shipping pallets are often free.
  8. From a previous email I sent to someone with a similar idea:

    A lot depends on the size of the pool and the temperature of the
    water. It takes a HUGE amount of heat to heat a full size pool full
    of water. You also need to be aware that copper and chlorine can turn
    women's hair from blonde to green, which could make you just a little
    unpopular. :) I think I'd skip the copper piping if possible.

    I think it is possible to do what you suggest, but unless you have the
    heat exchanger lower than the pool bottom, you'll need a pump. Hot
    water rises and having the coil above the pool will give hot water or
    steam in the coil and cold water in the pool. Cold pipes in the fire
    area will collect creosote, which can act as an insulation around the
    pipe, so instead of small copper tubing which turns hair green I might
    stick an unjacketed old hot water tank in a second 55 gal drum for use
    as a heat exchanger. That could solve the copper problem and small
    surface area for transfer problem.

    I don't think it would be enough to heat a 15,000 gallon in-ground
    pool, but the idea could help on a 5,000 gallon pool.

    Definition of one btu =a unit of heat equal to the amount of heat
    required to raise one pound of water one degree Fahrenheit at one
    atmosphere pressure

    So, if the water is at 50 degrees and you want to get it to 75
    degrees, each pound of water needs 25 btus. There are roughly 8
    pounds in a gallon (a pint's a pound, the world around) so each gallon
    needs 200 btus. 200 x 5000 gallons = 1,000,000 btus needed (one
    million btus)

    There are aprox 6200 btu in a pound of good dry wood, so you would
    need 161 pounds of wood in a 100% efficient setup. Since most stoves
    are rated at about 40,000 btu/hr we could use that as a starting
    guideline. At that rate, the heating would take about 25 hours with no

    In a real world situation, I'd guess 225 - 250 lbs of wood and a
    couple of days would be needed. You have to consider the
    inefficiencies of the heat transfer to water and the continual heat
    loss in the pool, where heat above the normal temperature of the water
    is lost to heating the ground and air, and in evaporating the water.
    I think you can see why I don't suggest this for a full size 15,000
    gal pool. :)

    Putting bubble wrap over the surface of the pool to prevent heat loss
    through evaporation is much cheaper and the solar gain makes it an
    easy way to add some heat to the pool, but using a wood fired hot
    water heater could be a useful way to crank the heat up a few more
  9. Guest

    In this case the OP was already going to put a solar cover over his pool,
    but wanted more heat to extend his pool season with solar panels, oil, or
    gas heat.

    When I suggested heating with wood, I was thinking of a commercial outdoor
    wood boiler with fan draft, water jacket, and pump that is typically
    used to provide hot water heat to a house. Unlike a 55 gallon barrel
    on top of a wood stove, these can put out 50k to 1.5M BTU/hour depending on
    the model. Yes, these burners smoke at startup, but since this application
    is heating a huge thermal mass, the stove won't fully damp down when
    heating the pool.

    If the OP has a source of cheap wood, I still think this is a viable
    alternative to oil or gas.
  10. Guest

    Guest Guest

    Hi Harry
    Many thanks for the info. Really helps put my thoughts on track.
    Still trying to work out the pounds etc but I'll figure it out.:) but
    just incase can you chk my math....

    50F -> 79F (ie 10C -> 26C) = delta of 29F
    Volume = 50,000l => 13,000gal
    8pounds / gal

    so (13000 x 8) x 29 = 3,016,000 btu's required @ 100% effeciency

    => 6200btus/pound wood
    3016000 / 6200 = 486 pounds (~ 221kg) of wood required for 3M Btu's

    => heater output 40,000btu's / hr
    3016000 / 40000 => 75.4 hrs (maybe a bit to long)

    => heater burn rate
    40000 / 6200 => 6.45 pounds / hr ( ~ 3kg/hr) (seems low?)

    I know all of above is at 100% efficiency with no losses etc...
    but it doesn't seem far out of the ball park except for the last number
    3KG / hr is a couple of smaller logs, or 1 bigger one.....seems out of
    Our LOPI wood heater we bought last year certainly burns more than that!
    (but admittidly, it's not heating water only air)

    Currently I'm thinking it might be worth investigating further & cal losses
    as 10C (50F) is the coldest winter daytime temp we usually get...and once
    I'm thinking we could maintain the temp with less than above, even if I
    closed the
    pool for 2-3mth over winter, the above sounds feasable to keep the pool open
    spring & autumn (fall). (approx 15-20C daytime min)

    As a side line, any ideas on location of web info on loco &/or ship oil
    I've found most of the info on 'domestic' ideas. But I'm working on an idea
    of using a scaled down version of a loco or ship oil burner (I thinking to
    burn waste oil)
    ......just can find info on them.

    Again many thanks for your prev great info.
  11. That was for an average stove, where a greater heat output would
    overheat an average room. Certainly you can use a larger stove and
    burn more wood to shorten the time. The usefulness of the example is
    in gauging the relative amounts of wood and energy required.
    Could work, if you are willing to expend the effort.
    That might be overkill. :) Besides, loco and ship burners were
    replaced in most cases by diesel engines or turbines, and for good
    reason. Unless you are into co-gen in a big way, having a diesel with
    a dozen 12 inch (or larger) diameter pistons sitting by your pool
    might be a tad loud.

    A way to get efficient pool heating might be to have a small water
    cooled diesel driving a heat pump. The combination of heat pump
    heated water, plus scavanging of the exhaust heat and coolent heat
    would make it about as efficient as you could get short of solar.

    I'd probably stay away from oil because of the cost, do as much with
    passive systems as practical, and supplement with a heat pump.
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