Is it cheaper to heat your pool all month or just for weekends

chem geek

TFP Expert
LifeTime Supporter
Mar 28, 2007
12,082
San Rafael, CA USA
#2
Re: How much to heat my pool - please double check this math?

It is always cheaper (uses less energy) to heat up a pool over a shorter period of time rather than trying to maintain a temperature. The reason is that the rate of heat loss is proportional to the temperature difference between the pool water and air so if you let the pool water temperature drop (i.e. don't try and maintain the temperature), then you will lose heat more slowly so that is less heat that needs to be recovered. Of course, if you plan to use the pool frequently, then you don't save very much letting it drop. As an example, I assume that the daily heat loss is equivalent to 6ºF with a 30ºF temperature difference between the pool at desired temperature and the lower day/night air temperature. This means that to maintain the pool temperature you'd need to replace 6ºF every day. If instead the pool was left to drop its temperature, then even if it dropped 30ºF eventually, that's 30ºF of temperature needed to get it back to where you want so is equivalent to 5 days of maintaining the temperature. I show the actual effect on a daily basis below where "T" is the desired temperature:

......... Letting It Drop ..... Maintaining
Temp . Loss . CumLoss . Loss . CumLoss . Savings
.. T ..... 6.0 ...... 6.0 ...... 6.0 ..... 6.0 ......... 0%
. T-6 ... 4.8 ..... 10.8 ..... 6.0 .... 12.0 ....... 10%
T-10.8 . 3.8 ..... 14.6 ..... 6.0 .... 18.0 ....... 19%
T-14.6 . 3.1 ..... 17.7 ..... 6.0 .... 24.0 ....... 26%
T-17.7 . 2.5 ..... 20.1 ..... 6.0 .... 30.0 ....... 33%
T-20.1 . 2.0 ..... 22.1 ..... 6.0 .... 36.0 ....... 39%

If the loss rate is lower, then maintaining at least for a short time isn't as much of a waste. I show below what happens with only 1ºF loss per day (again the energy units are in degrees so arbitrarily scaled just for comparison) and the same 30ºF temperature difference between air and water.

......... Letting It Drop ..... Maintaining
Temp . Loss . CumLoss . Loss . CumLoss . Savings
.. T ..... 1.00 .. 1.00 ..... 1.0 ..... 1.0 ........ 0.0%
T-1.00 . 0.97 . 1.97 ..... 1.0 ..... 2.0 ........ 1.5%
T-1.97 . 0.93 .. 2.90 ..... 1.0 ..... 3.0 ........ 3.3%
T-2.90 . 0.90 .. 3.80 ..... 1.0 ..... 4.0 ........ 5.0%
T-3.80 . 0.87 .. 4.68 ..... 1.0 ..... 5.0 ........ 6.4%
T-4.68 . 0.84 .. 5.52 ..... 1.0 ..... 6.0 ........ 8.0%

Of course you have to consider the fact that raising the temperature of the pool is not instantaneous so if your heater is only able to output somewhat more than the heat loss, you pretty much have no choice but to maintain the pool temperature because it would take too long to heat up. So in the first example if the heater wasn't able to heat much faster than the 6ºF loss per day, then you wouldn't want to not maintain the temperature.
 

pdomson

LifeTime Supporter
Oct 25, 2014
5
palm harbor,fl.
#3
Re: How much to heat my pool - please double check this math?

Thanks for the prompt reply. Allow me to give u the scenario. Mon. Thru fri. Grandkids not here. Sat. And sun. They swim in pristine 87 degree water.
Nanny and pop-pop use the 100 degree spa prn. Own a hayward 400,000 btu {not sure if it is input or output}. Thanks again.

14,500 gal below ground plaster pool,pentair intelliflo i5 control system,pentair easy clean de filter,tiger shark cleaner 1x/week. Screened enclosure.Chemistries ideal.
 
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chem geek

TFP Expert
LifeTime Supporter
Mar 28, 2007
12,082
San Rafael, CA USA
#5
Re: How much to heat my pool - please double check this math?

Thanks for the prompt reply. Allow me to give u the scenario. Mon. Thru fri. Grandkids not here. Sat. And sun. They swim in pristine 87 degree water.
Nanny and pop-pop use the 100 degree spa prn. Own a hayward 400,000 btu {not sure if it is input or output}. Thanks again.

14,500 gal below ground plaster pool,pentair intelliflo i5 control system,pentair easy clean de filter,tiger shark cleaner 1x/week. Screened enclosure.Chemistries ideal.
The Hayward 400,000 BTU/hr input gas heater probably has an efficiency of around 83-84% so let's figure an output of 335,000 BTU/hr output. 14,500 gallons is 120,785 pounds of water so one can expect a temperature rise (ignoring losses during that time) of 335,000/120,785 = 2.8ºF per hour.

I don't know your pool's rate of temperature loss. If I assume the 6ºF per day loss when the temperature is at 87ºF, then if you stopped heating on Sunday night then after 5 full days the water temperature would drop to around 67ºF. So you would need 20/2.8 = 7.1 hours minimum to heat the pool back up and actually more than that since there would be loss during that time so figure 8 hours minimum. So you should stop heating on Sunday night when the pool is no longer used and start heating it on Friday night so that it will be at the desired temperature on Saturday morning. You may need to experiment to see how much time is actually needed to heat the pool since there will be more loss at night so you might need to start earlier -- perhaps 12 hours before the grandkids arrive.

In the scenario I described, you would save nearly 40% on energy costs by letting the pool water temperature drop. However, this savings is very dependent on the amount of heat loss, so the rate of temperature drop, which I do not know. Nevertheless, you will always save energy by not trying to maintain heat in the pool during the week. Also, if you were to use a pool cover, you would cut down on losses substantially, by at least half, so would save a lot of energy. So to save the most, use a cover and do not heat the pool during the week.
 

zethacat

LifeTime Supporter
#6
Re: How much to heat my pool - please double check this math?

I don't know your pool's rate of temperature loss. If I assume the 6ºF per day loss when the temperature is at 87ºF, then if you stopped heating on Sunday night then after 5 full days the water temperature would drop to around 67ºF.
Doesn't this depend on what the low temp is? For example, if the overnight low is 75 how can the water temp drop to 67?
 

chem geek

TFP Expert
LifeTime Supporter
Mar 28, 2007
12,082
San Rafael, CA USA
#7
Yes it does depend on the air temp and I reused my assumption of a 30ºF temperature difference between the initial or goal water temperature and the average air temperature. Current forcecasts for Palm Harbor, FL show daytime highs from 54ºF to 77ºF and nighttime lows from 40ºF to 63ºF. The average of these is 58.5ºF which is 28.5ºF lower than the 87ºF initial/target temperature. So my assumption of 30ºF for the temperature difference is not that far off.

My assumption of a 6º 24-hour loss in temperature when starting at 87ºF is a guess based on my experience in my pool if there is no cover and there is a roughly 30ºF temperature difference. This also assumes little sun exposure since this sort of drop in my pool was in the winter when the sun was mostly low behind nearby trees (in the summer there's much more sun exposure).

However, regardless of the assumptions used, it is always more energy efficient to let the water cool off in between uses and not to continually heat the pool to maintain a temperature.