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Thread: Debate: Can we really heat pool to 90F in 40F weather?

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    Debate: Can we really heat pool to 90F in 40F weather?

    Spouse and I are debating whether it's possible to raise the pool temp by 50F (or more). Is the amount of heat a pool heater is capable of generating essentially endless? Or is too much heat lost into the cold air and ground surrounding the water, so (for example) you can reasonably expect to raise the temp by 20F, but no more?

    We are not debating whether this is too expensive to consider ... only whether it's actually possible. This is using a "standard" gas heater, not 10 gas heaters online together or some other fancy set-up. Spouse believes we could turn the pool into a gigantic hot tub if we left it running long enough, regardless of the weather. I want to know whether we could keep it open when the temp is below freezing, should we someday have a sauna and want a cold plunge.
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    cramar's Avatar
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    Re: Debate: Can we really heat pool to 90F in 40F weather?

    I have no clue...but I wonder if you'd need some serious insulation on top of the water.
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    Richard320's Avatar
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    Re: Debate: Can we really heat pool to 90F in 40F weather?

    Maybe you should http://askthephysicist.com/
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    mas985's Avatar
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    Re: Debate: Can we really heat pool to 90F in 40F weather?

    It depends on the size of the heater and if you have a pool cover. Without a pool cover, I would say it is nearly impossible but with a large heater and good cover, it might be possible.

    For your pool, a 400k heater can add 1.75 degrees per hour. A typical pool with a cover loses about 3.3 BTU/hr/sq-ft/dF so that would be about 0.5 degrees per hour (@ 90F) or an average net gain for 1.5 F per hour. So I think it is possible. It would take over 33 hours and cost a fortune (132 Therms).

    Without a cover, the heat loss is about twice as much so it would take the heater 40 hours at 160 therms to heat pool so it might even be possible that way too.
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    Re: Debate: Can we really heat pool to 90F in 40F weather?

    I live in Tucson where th "winters" are pretty mild. My neighbor is a part time resident. One year at Christmas he was visiting and the temp was in the high 40s maybe 50s peak daytime, he had left his heater on the spa on while visiting and the spill over into the pool had raised the pool to 90. This was without even a cover on the pool.

    The pool is right about 17,000 gallons and the heater is a 300k BTU unit. So I say it's possible, but pretty expensive. He told me after that his gas bill for that weekend was over $300.
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    Isaac-1's Avatar
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    Re: Debate: Can we really heat pool to 90F in 40F weather?

    Sure it can be done, years ago (early 1980's) I remember being on a ski trip to Colorado and the place we were staying had a heated semi indoor (at least covered) pool while there was snow on the ground. The thing I really remember about it is lots of clouds of "steam" in the air around the pool.

    Ike
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    JasonLion's Avatar
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    Re: Debate: Can we really heat pool to 90F in 40F weather?

    I have a 190,000 BTU gas heater and a 19,000 gallon pool. Without a solar cover the best I can do, with the heater running 24x7, is a 15 degree gain. Heat loss goes up rapidly as the water temperature increases. Even with a larger heater, smaller pool, and a solar cover I rather doubt that you will be able to get a 50 degree increase.
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    Re: Debate: Can we really heat pool to 90F in 40F weather?

    We have a mostly opaque electric safety cover. It's thinner than a bubble-type cover and insulates about half as well as a result. It roughly cuts heat loss about in half instead of by 75% or so if we had a more insulating cover.

    This winter, we did something we've never done before and that is try and keep the pool heated and open most of the time and our target pool temperature was 88F. Without heating or significant sunlight, pool water will attain roughly the average day/night temperature. In the past, our pool got to around 50F in most winter months, but the lowest was around 45F. So figure that we were heating our pool by around 40F.

    We have a 16,000 gallon pool and a 200,000 BTU/hr output gas heater (unfortunately it's inefficient at 80% so uses 250,000 BTU/hr when operating). We were able to maintain the pool temperature most of the time, but there were some particularly cold nights where the temperature dropped a few degrees in spite of the heater being on all the time. So perhaps the heater could not keep up with a 50F or so temperature difference in our particular situation. With a better more insulating cover or with a larger heater, it probably would have been able to keep up.

    Our marginal gas rate was around $1.25 per Therm and our monthly usage incrementally for the pool heating was around 10-14 Therms per day so $375 to $525 per month. Though high, it was less than I expected. We did this because my wife is pretty much fed up with the commercial indoor community center pool and likes our pool so much better, but clearly it's way more expensive to heat compared to the $85 or so monthly fee for use of the community center pool. It's also quite unpleasant outdoors in the cold so we are looking at tent-like cover options, but those also aren't cheap and a pain to install and remove.
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    Isaac-1's Avatar
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    Re: Debate: Can we really heat pool to 90F in 40F weather?

    Chem geek have you given any thoughts to an aquashield type roll off pool cover?

    Ike
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    Lershac's Avatar
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    Re: Debate: Can we really heat pool to 90F in 40F weather?

    I can attest to the fact it can be done. it WAS expensive, but we have heated the pool to 90 degF while it was in the upper 30s outside. No cover or anything.

    A major problem with doing this turned out to be evaporation. We lost over an inch of water.

    But it was pretty cool to be out there swimming while it was so cold... just that once.

    We do it with the hot tub regularly in the winter... thats the best, when there is FREEZING weather and steamy snuggle in the the 'tub.
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    Re: Debate: Can we really heat pool to 90F in 40F weather?

    Quote Originally Posted by Isaac-1
    Chem geek have you given any thoughts to an aquashield type roll off pool cover?
    Thanks for that reference. We can't have it be very tall, but their Low Aqua Shield model looks like it might work for us, though a telescopic design means that one end will still be covered in the summer. That's the trade-off of having something easily opened but still partially there vs. something harder to deal with but that can be completely removed. This looks like it's a $20-30K expense. Other options we've been looking at are the inflatable domes that run around $7K and that can be removed for the summer. We're more likely to go with that approach, though the domes are taller than we'd want.
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    Isaac-1's Avatar
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    Re: Debate: Can we really heat pool to 90F in 40F weather?

    Depending on the pool design the various track mounted enclosures can be rolled past the end of the pool for summer storage so they don't cover any of the pool. There are 2 or 3 major brands, Aqua Shield seems to be the most well known, sometimes they show up used on ebay at significant discounts, but those are often local pickup only, so there is question of luck of location, which may work better for you being in California.
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    Re: Debate: Can we really heat pool to 90F in 40F weather?

    This photo and this photo gives you an idea of the limited space we are dealing with. We could retract beyond the area one goes into the pool, but that blocks a lot of deck area. The fence where the roses are located has been replaced and is now taller, though still only about 5-1/2 feet from the ground. We'll look at various options to decide. Too bad no one offers something that can be helicopter'd (or craned) in, clamped, used for the winter, then unclamped and removed. Then again, the idea of a crane has me think of this video.
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    Re: Debate: Can we really heat pool to 90F in 40F weather?

    Thanks, everyone! We may try it in the late fall for a sauna party. (Have to get a sauna first, but that's in the works.) There is nothing better than baking yourself in the sauna and then jumping in a cool pool, if you're a Nordic-type like me.
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    Re: Debate: Can we really heat pool to 90F in 40F weather?

    A good friend of mine has had his in-ground pool open since early March, we live in northern Illinois and it's been a chilly spring. Air temps were down in the low 40's and he has kept his pool at 92-95f, no cover. He is around 17k gallons and I'm not sure about the size of his gas heater. It was pretty neat swimming when it was that cold out...but I couldn't imagine the heating bill
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