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Thread: Pool heating question. I must be missing something...

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    Pool heating question. I must be missing something...

    Based on what I've read here, this is what I've come up with. What am I missing?

    1 btu 1 hour 1 lb 1 degree

    5000 gals
    8 lbs

    40000 lbs H2O
    5 degrees
    200000 BTU Needed

    400000 BTU Avail
    0.5 Time to heat (hr)

    0.45 per therm
    100000 BTU per Therm
    2 Therm to heat
    0.9 cost to heat
    Bob - Palm Beach by San Juan Pools. approx 5000 gals., Pentair 320 cartridge filter (all new guts installed by me), Goldline SWG, 'New to me' Kreepy Krauly Sand Shark, Intermec 104 Timer Test kit: TF-100 w/Speed Stir

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    Re: Pool heating question. I must be missing something...

    Quote Originally Posted by pabeader View Post
    Based on what I've read here, this is what I've come up with. What am I missing?

    1 btu 1 hour 1 lb 1 degree

    5000 gals
    8 lbs

    40000 lbs H2O
    5 degrees
    200000 BTU Needed

    400000 BTU Avail
    0.5 Time to heat (hr)

    0.45 per therm
    100000 BTU per Therm
    2 Therm to heat
    0.9 cost to heat
    I don't see anything missing.
    (Although your first line includes 1hr which is meaningless)
    Water weighs almost 8.4 lbs per gallon and heaters aren't 100 percent efficient.

    It takes about 1/2 hour and costs about 90 cents to heat your 5,000 gal pool 5 degrees.

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    Re: Pool heating question. I must be missing something...

    So why do folks see such large changes in their bills? Like 200 or 300 dollars extra. Once it's up to temp, set your temp differential to 1 or 2 degrees and it's only like 20 cents to keep the pool up to temp.

    I believe the 1 hour is the most important thing and it's left out of almost all discussions. That's why I included it.
    Bob - Palm Beach by San Juan Pools. approx 5000 gals., Pentair 320 cartridge filter (all new guts installed by me), Goldline SWG, 'New to me' Kreepy Krauly Sand Shark, Intermec 104 Timer Test kit: TF-100 w/Speed Stir

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    Re: Pool heating question. I must be missing something...

    You assume your heater generates 400,000 BTUs but it probably generates only about 320,000-340,000 of usable BTUs even though you have to pay for the full 400,000. In addition, on different locations and under various circumstances heat loss can occur more rapidly for some people than others (which means it is more costly to maintain). Further, many people need to raise the heat much more than five degrees ( some people might need to raise it 15 degrees) and I suspect most people who claim the heat is expensive have pools much larger than 5000 gallons.
    1200 Sq Ft 41,000 Gal. Gunite, JANDY SVRS VS pump, JANDY JXI 400,000 BTU, Polaris 280 w/ 3/4 BP, Rockwall and Grotto (w/6 jets and blower), 2 JANDY Lam Jets, 2 LED Bubblers, 5 Ledlights, AQUALINK P16 RS, 90% Quartz, SWF 185 GMP BP, SWF 125 BP, Levelor K1100 Autofill, AP Digital SWG, 2 skim and 5 returns, and mostly 3 inch piping

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    Re: Pool heating question. I must be missing something...

    A BTU is the amount of energy needed to raise 1 lb of water 1 deg. Time is of no relevance in this formula up to this point. So, lets look at a real life example. My pool holds 45,000 lbs of water. My heater is rated at 100,000 BTU input and 80,000 of output. This puts my heater at 80% efficiency. This is about where most pool heaters come in at and is also something most forget. This is also the hourly burn rating of the heater. It does not burn 100,000 BTU's of gas instantly or constantly but, rather over an hours time it consumes 100,000 BTU worth of fuel (nat gas)

    So, at 80,000 output (the other 20,000 goes up the chimney) and 45,000 lbs of water, I can raise my pool temp 1.77 deg (calculated). Real world will be a bit different as ambient air temp, wind and sun exposure will all contribute to an increase or decrease in actual temp change. This summer, my effective gas cost per therm varied by month from $.18 - $.25/therm. With that, when I went out on a Sat morning and turned the pool heater on for 5 hours, I consumed about 5 therms of fuel for a cost of $1.25 (.25/therm) and was able to bring the pool temp up with the help of the sun, about 2 Deg/hr. Now, my schedule and time commitments during the week do not allow me to use the pool much during the week, so I don't bother heating it then. My wife and I do use it on occasion on the weekends, so that is when I heat it. I may also do the same on Sunday mornings if I feel the water is a bit too cool for us so figure anywhere from 5-15 hours of run time on the weekends. I also use a solar cover on the pool at night as the night time temps around here can sometimes drop into the upper 50's but, mostly are in the mid 60's. This is important as Greater differences in temp will contribute to greater evaporation/heat loss. All that considered, my weekly heating costs for my pool are ~$5.00

    Next, my friends pool...He keeps his 100,000 lbs of water at 85-90 deg all the time. He also refuses to use a solar cover at night as his wife hates not only the look of it but, also having to take it off and put it on. Since he keeps his pool heater "on" all the time it cycles off and on all day/night long. This contributes to a higher energy bill in both electric cost to run the pump full time as well as an increase in gas costs. We figured his heater runs about a total of 10 hours/day for a cost, roughly, of $5.00/day (250,000 BTU heater ~ $.50/hr of fuel)So a monthly cost of $150 is not unrealistic in his case.

    Bigger pools = bigger heaters = bigger bills
    Unknown make 18' above ground (bought used in 1999) Sparco sand filter. Hayward 100,000 BTu heater. 2 speed pump

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    Re: Pool heating question. I must be missing something...

    I think I'm finally starting to understand why this whole pool heating discussion drives me nuts. There are parts of the conversation that are left out or assumed to be known. Like, "It does not burn 100,000 BTU's of gas instantly or constantly but, rather over an hours time it consumes 100,000 BTU worth of fuel (nat gas)" Seems so obvious when you put it like that, but to an EE, that concept isn't obvious at all.
    So that's why you said the 1 hour wasn't important; because it's assumed. But to me it was the most important thing, simply because it's never mentioned and I thought it was being forgotten somehow.

    The other thing that was tripping me up is the idea that you want the water to move relatively fast rather than slow through a heating system. That's because it's basically a closed loop. So, over time, X amount of BTUs is being added to the whole mass of water. The water that's in the pool retains that heat and recirculates back to get even more added to it. This continues to build, over time, and you get your temp rise.

    So if I don't let the water temp drop over night, by using a good solar cover, My heating needs should be minimal. Do you keep the heater on while the pool is covered? Seems like that would be a good idea.
    Bob - Palm Beach by San Juan Pools. approx 5000 gals., Pentair 320 cartridge filter (all new guts installed by me), Goldline SWG, 'New to me' Kreepy Krauly Sand Shark, Intermec 104 Timer Test kit: TF-100 w/Speed Stir

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    Re: Pool heating question. I must be missing something...

    Quote Originally Posted by pabeader View Post
    I think I'm finally starting to understand why this whole pool heating discussion drives me nuts. There are parts of the conversation that are left out or assumed to be known. Like, "It does not burn 100,000 BTU's of gas instantly or constantly but, rather over an hours time it consumes 100,000 BTU worth of fuel (nat gas)" Seems so obvious when you put it like that, but to an EE, that concept isn't obvious at all.
    So that's why you said the 1 hour wasn't important; because it's assumed. But to me it was the most important thing, simply because it's never mentioned and I thought it was being forgotten somehow.
    Correct, you don't need time as the amount of energy to raise the 1 lb 1 deg is not time dependent, but rather amount dependent. That amount is measured in BTU's. The only time that Time comes into play is to calculate heater size. A 100,000 BTU heater will consume 1/2 the amount of fuel as a 200,000 BTU heater. However, the 200,000 BTU heater will heat the same volume of water twice as fast.

    Quote Originally Posted by pabeader View Post
    The other thing that was tripping me up is the idea that you want the water to move relatively fast rather than slow through a heating system. That's because it's basically a closed loop. So, over time, X amount of BTUs is being added to the whole mass of water. The water that's in the pool retains that heat and recirculates back to get even more added to it. This continues to build, over time, and you get your temp rise.
    The reason you want to move water thru a heater quickly has to do with the rate of heat transfer. The bigger the differential the greater the heat transfer. Basically what you are doing is trying to keep the heater as cool as possible which will effectively pull more of the heat out of it and into the pool where you want it. It took me a quite a while to get my head wrapped around this when I was messing with some home made heaters (solar and gas) When I was trying the solar method, I had a coil of black pipe laying on the concrete patio. At full volume thru the pipe, I felt no temp change. I then throttled down the flow until I felt heat. In fact, I could slow it down so much that I could not hold my hand under the flow as it was too hot. Of course the flo was so low I did not think it would heat the pool at all. When I was talking to an engineer friend of mine he got me to understand it a little better. He said that the same amount of heat was going into the water based on the amount of sun and the surface area of the pipe, etc. However, as the water temp got closer to the pipe temp the heat transfer was slowing down. He then had me open up the throttle on the water flow to full and instead of using my hand to check the temp differetial, to use an accurate digital thermometer (borrowed from work) I noted that the water temp differential was around .8-1 deg. he then had me fill two buckets of a known volume and time them at the two different throttle settings. With the temp differential and the fill time for the buckets he was able to calculate that at full volume the heat transfer to the pool was almost double what the slow volume was.

    Quote Originally Posted by pabeader View Post
    So if I don't let the water temp drop over night, by using a good solar cover, My heating needs should be minimal. Do you keep the heater on while the pool is covered? Seems like that would be a good idea.
    I run the heater with the cover on when not swimming and the heater will sometimes be on when I am swimming (cover off).

    Example:

    If you want your pool to be 85 Deg when you swim, you somehow have to get it to that temp. A heater is the "best" way to accomplish this. If you allow the pool to cool down over night to, say, 65 deg, you will have to burn X amount of fuel/hour to get it back up to 85 deg the next day. If you use a cover and the pool only drops down to 75 Deg over night, then you will still use X amount of fuel/hr to get it back up to 85 Deg. The difference here will be the run time for the task to be accomplished. At 75 Deg, you will have roughly half the run time as you would at 65 Deg.
    Unknown make 18' above ground (bought used in 1999) Sparco sand filter. Hayward 100,000 BTu heater. 2 speed pump

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    Re: Pool heating question. I must be missing something...

    Thanks for the discussion. I'm going to let it wander around in my head some more.
    Bob - Palm Beach by San Juan Pools. approx 5000 gals., Pentair 320 cartridge filter (all new guts installed by me), Goldline SWG, 'New to me' Kreepy Krauly Sand Shark, Intermec 104 Timer Test kit: TF-100 w/Speed Stir

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