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Old 12-04-2008, 02:15 PM
 
Location: Grosse Ile Michigan and Sometimes Orange County CA
15,820 posts, read 29,850,172 times
Reputation: 11702
Default Any scientist types know about heating water?

We live in a cold climate (Michigan). We have a pool that is enclosed in a sort of greenhouse made of carbonate so that we can use the pool all year. The pool water is covered by a floating "solar" blanket when not in use. The solar blanket seems to hold the heat in pretty well.

We use the pool mostly on weekends.

Our question is whether it would be cheaper to turn the pool heater down to about 50 degrees on Monday and then turn it back up to 80 on thursday morning for weekend swimming. It takes about 36 hours to heat the water up. It seems like it might be cheaper to maintin the heat than it would be to heat the pool back up regularly. I have asked several freinds who know womsethign about science, and the only answer that I got was "That is a really intersting question, I am sure someone out there knows the answer"

Someone? Are you out there?

I do not know wheter it matters, but the pool heater is 180,000 BTU gas. The pool is about 20,000 gallons. It is 18 feet wide, 40 feet long and about 5 feet deep int he center and a litle over 3.5 feet deep at the sides. We keep the pool at 81 degrees, but if we are having guests we sometimes turn it up to 90 for a few hours becuase it is really nice to swim in 90 degree water when it is 22 outside. The enclosure is not heated, but there is a fiureplace in there. We use the fireplace while swimming ot warm up the enclosure. It is pretty cold in there. (About 55 withthe solar blanket off, about 40 with the solar blanket on). The carbonate is single paned and about 1/4" thick.
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Old 12-04-2008, 04:24 PM
 
Location: South Dakota
732 posts, read 2,716,084 times
Reputation: 634
All I can tell you - and I'm no scientist - that after comparing equivalent therms of natural gas and adjusting for outdoor temperature, it's cheaper to leave myhouse's hot water heating system at a constant temperature than to step it up and down. Could be the same with a pool...but then again, that's an interesting question.
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Old 12-04-2008, 05:07 PM
 
Location: The beautiful Rogue Valley
4,072 posts, read 7,734,610 times
Reputation: 3224
How efficient is the pool heater? What is the power curve for it? Is it as efficient at half power as it is at full power? Will it even run at half power? Are the pipes that water circulates through insulated? With what?

How is the pool itself insulated? Not the cover, but what is between the pool and the earth?

The boiler for our radiant floors has two coils that we can switch on, because, with both coils on, the boiler is not very efficient at light load. So if we only need a little heating (short time of the day or relatively low delta T), we only use one coil.

So even if someone makes a quick little calculation, it'd be first-order and not accounting for the actual conditions you use it in. Your best bet is to try it two different ways over two weeks and watch your gas consumption, and you'd get a comparison for your actual circumstances. I'd lean toward it being more efficient to crank up the heat 36 hours before you want to use it, myself.

ETA: by "carbonate" do you mean polycarbonate? So you've got a greenhouse type structure with plastic panels instead of glass?

Last edited by PNW-type-gal; 12-04-2008 at 05:15 PM.. Reason: another question
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Old 12-04-2008, 05:41 PM
 
Location: Eastern Washington
8,747 posts, read 21,448,435 times
Reputation: 4679
The amount of heat transferred from the pool to the surroundings is given by the formula:

Q= U*A*ДТ, where ДТ stands for the (average effective) temperature difference, U is the heat transfer coefficient (essentially 1/R-value) and A is the area. To do a real calculation one would have to divide up the area into the underground parts of the pool, the surface of the pool transferring heat into the air above the pool (which would involve evaporation, although evaporation would be minimized by your bubble-sheet) and then from the air in the dome to the outside air.

Anyway, the point is, the greater the temperature difference between the pool water and the surroundings, the greater the heat transfer, in fact, to a first order approximation, they are directly proportional.

The only time it takes less energy to maintain a temperature would be in the case of an air-to-air heat pump, where you would maintain on the heat pump itself, but catch up using the resistance heater.

Every hour you let the pool cool off, is an hour with less heat transferred from the pool to the surroundings.

Of course you have the practical issue that if you let the pool cool off significantly, you will need time to heat it back up. But if you don't use it during the week, and are OK with it not really being available during the week, I'd say switch the heat off sometime on Sunday, and turn it on at the last minute on Thursday or Friday that has it warm enough to use the first time you'll want to use it.
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Old 12-04-2008, 06:35 PM
 
Location: Somewhere in northern Alabama
11,339 posts, read 25,111,450 times
Reputation: 13660
M3 Mitch has it dead-on.

The operative phrase is delta T. Maintaining a temperature level always requires more energy than allowing the temp to drop, and rebuilding it. The logic is simple. Consider if you left the pool cool for two weeks instead of one. Think your energy use would be less? Of course it would.

The only obvious change would be if you used a heat pump that had varying efficiencies, based on air temps.

If you wanted to reduce the pool heating bill, the most cost effective way would be to change from a simple evaporation prevention pool cover to something with a much higher R-value, like adding a folding closed cell foam cover over that.

Heating water is an expensive proposition. A large portion of home energy usage goes to simply heating the water in the home hot water tank.

One British Thermal Unit (BTU) is the heat that will raise the temperature of one pound of water by one degree Fahrenheit. If the pool is 20,000 gallons and a gallon of water is roughly 8 pounds, then each degree of rise in temperature requires 160,000 BTUs. Your heater is a nominal 180,000 btu/hr heater. From 50 to 80 is 30 degrees, roughly 30 hours at near 100% efficiency, add in the 6 hours to heat the surrounds and take care of other inefficiencies, and it all works out.
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Old 12-04-2008, 09:08 PM
 
3,021 posts, read 16,034,321 times
Reputation: 2399
Default Yep, it theory leaving it off and then reheating should be the winner.

Delta T is the key, at least in theory. You should input less fuel over a week by not continuously maintaining temperature at setpoint. Letting it drift down and then reheating in one long cycle should use less fuel.

The wild card is the heater and its efficiencies at various loads. Maybe some other foobaa's in the system that are unnamed.

You can just try the simple test. Do it one way, one week, try the other the next. Compare the fuel consumption the best you can. That can be a bit skewed in the results based on outside temperatures, that is the other variable that might have some impact.

But in general any system that has the lower Delta T will have the lowest heat losses, cost the least to operate when figured over an extended time period. That will generally be true no matter the amount of insulation. It is all about heat loss. Higher the Delta T, the more the loss within any given system when considered as an absolute.
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Old 12-04-2008, 09:39 PM
 
Location: Visitation between Wal-Mart & Home Depot
8,310 posts, read 20,542,566 times
Reputation: 6630
I have no idea how to do the math to calculate how fast the pool would drop to 50 degrees (I didn't want to be an engineer because I didn't want to take differential equations) but, like Harry, I'm assuming 100% efficiency and coming up with 5,000,000 BTU to raise the temperature of 20,000 gallons of water 30 degrees. BTU value of natural gas is variable, but for calculation purposes most people use around 1000 to 1030 BTU's per standard cubic foot. Still assuming 100% efficiency, you would need about 4.86 MCF of gas to heat the pool 30 degrees. Assuming residential natural gas rates of around $11 (I have no idea what prices are like in Michigan this year) it would cost you about $50 to raise the temperature of the pool from 50 to 80 degrees. If it's taking you 36 hours to heat the pool you're in the vicinity of 75% efficiency and you're probably spending $70 to $80 over that span (dependent on the price of gas, of course).

Jeez, I must be exceptionally bored...

Last edited by jimboburnsy; 12-04-2008 at 09:47 PM..
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Old 12-04-2008, 09:47 PM
 
Location: 96820
796 posts, read 1,460,104 times
Reputation: 369
Quote:
Originally Posted by Coldjensens View Post
We live in a cold climate (Michigan). We have a pool that is enclosed in a sort of greenhouse made of carbonate so that we can use the pool all year. The pool water is covered by a floating "solar" blanket when not in use. The solar blanket seems to hold the heat in pretty well.

We use the pool mostly on weekends.

Our question is whether it would be cheaper to turn the pool heater down to about 50 degrees on Monday and then turn it back up to 80 on thursday morning for weekend swimming. It takes about 36 hours to heat the water up. It seems like it might be cheaper to maintin the heat than it would be to heat the pool back up regularly. I have asked several freinds who know womsethign about science, and the only answer that I got was "That is a really intersting question, I am sure someone out there knows the answer"

Someone? Are you out there?

I do not know wheter it matters, but the pool heater is 180,000 BTU gas. The pool is about 20,000 gallons. It is 18 feet wide, 40 feet long and about 5 feet deep int he center and a litle over 3.5 feet deep at the sides. We keep the pool at 81 degrees, but if we are having guests we sometimes turn it up to 90 for a few hours becuase it is really nice to swim in 90 degree water when it is 22 outside. The enclosure is not heated, but there is a fiureplace in there. We use the fireplace while swimming ot warm up the enclosure. It is pretty cold in there. (About 55 withthe solar blanket off, about 40 with the solar blanket on). The carbonate is single paned and about 1/4" thick.
That 'old saw' if you have to ask the price, you can not afford it.
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Old 12-04-2008, 10:12 PM
 
Location: Knoxville
3,256 posts, read 10,080,036 times
Reputation: 3191
This made my head hurt almost as much as those problems where a train leaves Los Angeles at 9am traveling at 50 mph with 47 cars, at the same time a train leaves Chicago traveling at 43 mph with 83 cars. At what time does the south side of a box car get painted with graffitti?

You also have to consider that the heater may or may not be performing at peak efficiency.
I like the try it one way a week, then the other method. Much easier on the head.
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Old 12-05-2008, 08:37 AM
 
878 posts, read 1,265,705 times
Reputation: 444
Buy a current sensor and hook it up to your heater. Try both cycles over a couple of weeks, see which is more efficient.

M3 Mitch has the right math, but the efficiency of your heater also matters as well. Your heater may be very efficient at heating water from 78' to 81' and very inefficient at heating water from 50' to 81'. If so, it is probably cheaper to run the heater throughout the week.
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