# How long to heat pool?

#### Cantabrigienne

##### Active member
New pool owner and we are wanting to warm up the pool for some thanksgiving swimming (hosting a bunch of family and thought that might be fun). Any idea how long it will take to heat up (will it even get up to temp when the nights are dipping down to freezing?)

(Edit to add: The pool is pretty large - 20x40 with a deep end + integrated spa and two tanning shelves. We have a gas heater 399K BTU. Water temp is 53 and we’d prob want to raise it 30 degrees.)

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#### ajw22

Gold Supporter
TFP Guide
How many BTUs are your heater to heat how many gallons of water from what temperature to what temperature?

1 BTU will heat 1 lb of of water (8.3lbs/gallon) 1 degree an hour.

#### Cantabrigienne

##### Active member
Ok, it looks like the heater is 399K BTU, and the pool is maybe 35000 gallons? Let’s say we way to raise the temp 30 degrees.

So 35000x8.3 = 290,500 lbs

290500 lbs / 399000 degrees/hr x 30 degrees = 22 hours?

I’m not sure if I did that right. Also not sure how that accounts for heat lost to the cold air…

#### Donldson

TFP Expert
Your units might be a bit off, but the math is all correct. That number is "in a vacuum" (as in not literally in a vacuum, but doesn't account for any outside elements) so heat loss to the ground or atmosphere is not accounted for.

The ground will suck up some heat but eventually that will slow as the ground near the pool warms up. The worst heat loss will be to the air. If you have a cover that will largely reduce this loss, but if you don't have the pool covered then as you get the water warmer it will take more and more to heat it as more and more is lost to evaporation.

Also you don't mention if this is a propane or NG heater. If NG then no problem, except for the bill of course . If propane then keep in mind it's going to burn about 17 pounds of propane an hour so you'll want to make sure you don't run your tank dry during a holiday weekend.

#### Cantabrigienne

##### Active member
Ok, yeah it’s NG. Just trying to figure out how far ahead to turn the heater on so it’s ready for thanksgiving…

#### ajw22

Gold Supporter
TFP Guide
Ok, it looks like the heater is 399K BTU, and the pool is maybe 35000 gallons? Let’s say we way to raise the temp 30 degrees.

So 35000x8.3 = 290,500 lbs

290500 lbs / 399000 degrees/hr x 30 degrees = 22 hours?

I’m not sure if I did that right. Also not sure how that accounts for heat lost to the cold air…

A NG heater is about 80% efficient. So a 400K NG heater will put about 320K BTUs into the water.

So your best case temperature rise is about 0.9 degrees/hour. or 27 hours.

Then as was pointed out above you will lose heat to the air and ground during the 27+ hours you are trying to heat the water.

What will the air temperature be during the heating period? I have found when the air temperature is more then 10-15 degrees below the water temperature I can't heat my uncovered pool more. The thermal loss to the air is as much as the BTUs the heater is adding.

I would start your heater about 35 hours before swim time an see what temperature the water gets to. Once at the desired temp let the heater maintain it there.

#### JamesW

TFP Expert
35,000 gallons x 8.34 = 291,900 lb water.

399,000 btu/hr x 82% = 327,180 btu/hr.

399,000 btu/hr. x 84% = 335,160 btu/hr.

327,180 btu/hr. ÷ 291,900 lb = 1.12 degrees per hour = 26.8 hours to gain 30 degrees with no losses.

335,160 btu/hr. ÷291,900 lb = 1.14 degrees per hour = 26.3 hours to gain 30 degrees with no losses.

What model is the heater?

#### HeyEng

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Silver Supporter
You have a few days, so you can give it a "test run" for a few hours to see how fast it heats.

The math says that I should get about 2.7 degrees an hour. Realistically, when it's 45 degrees (or lower) it's about 2 degrees until it gets up to about 80 degrees (water temp) and then it accelerates to about 3 -3.5 degrees an hour. Of course there are a LOT of variables for ANY heating, so a test run is the best way to get a more general idea.

*I would also remind you that NG costs have accelerated significantly over the last 12 months. I don't know the rates in AR but I can tell you that a year ago, I could run my heater for about 24 hours and *only* spend about \$31 (gas charge only) but using the cost this month (through the end of Nov) would be \$74...more than double from a year ago.

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#### Cantabrigienne

##### Active member
Thanks so much for all the replies!!! This is so helpful. I do have one more question - what temp do y’all this is best for swimming in the cold??? (It’s going to be quite cold - high of 49 and low of 29, so I’ve no idea if the pool will even get up to a good temp or not…)

#### ajw22

Gold Supporter
TFP Guide
what temp do y’all this is best for swimming in the cold??? (It’s going to be quite cold - high of 49 and low of 29, so I’ve no idea if the pool will even get up to a good temp or not…)

I like a swimming temp in the mid 80's but it depends on your swimmers. Some have no problem with water in the 70's. I am comfortable in water that my wife complains is too cold.

Crank up the heater and see what temp you can get the pool to and see who swims for how long.

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#### jseyfert3

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Bronze Supporter
TFP Guide
Thanks so much for all the replies!!! This is so helpful. I do have one more question - what temp do y’all this is best for swimming in the cold??? (It’s going to be quite cold - high of 49 and low of 29, so I’ve no idea if the pool will even get up to a good temp or not…)
I’ve not been in a pool at those ambient air temps but based on my hot tub I probably wouldn’t be comfortable without it being in the 90’s at those air temps.

There was someone here who would regularly do one weekend in winter cranking their pool up to the upper 90’s and having a party with ambient temps around freezing. But I tried searching once and wasn’t able to find that thread again when this topic came up before.

KJB1

#### JamesW

TFP Expert
what temp do y’all this is best for swimming in the cold???
Here is the official* guideline for water temperature vs. Air temperature.

Air..............Water.

0................100

10................98

20................96

30................92

40................86

50................84

60................82

70................81

80...............80

90..............76

100...........74

*The international, global, world consortium on recreational water temperatures for residential swimming pools.**

**A top secret organization that I made up.

All documents are top secret and cannot be released or read by anyone, anywhere at any time for any reason.

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#### PoolGate

Gold Supporter
TFP Guide
Do you have a solar cover for the pool? If not it'll take you 2 full days+ to get up to 90 with nighttime running hopefully maintaining what you gain during the day but I wouldn't expect any gains at night.

#### chiefwej

TFP Guide
Without a cover, I predict the OP will end up spending hundreds of dollars and still not reach an acceptable swimming temp.

#### revitup

Gold Supporter
Bronze Supporter
L
Here is the official* guideline for water temperature vs. Air temperature.

Air..............Water.

0................100

10................98

20................96

30................92

40................86

50................84

60................82

70................81

80...............80

90..............76

100...........74

*The international, global, world consortium on recreational water temperatures for residential swimming pools.**

**A top secret organization that I made up.

All documents are top secret and cannot be released or read by anyone, anywhere at any time for any reason.
Dont’t forget to apply the ‘Age Factor’ . For every year over 60, add two degrees.

#### Cantabrigienne

##### Active member
Quick update! We turned the heater on Monday mid-day, we reached 93 degrees by Thursday morning. Thursday was COLD (the high actually only got up to 40) but heater kept up! Pool was 93 and integrated spa was 102. Everyone swam and it felt very comfortable! I was a little worried 93 would feel too warm but it was nice.

#### HeyEng

Gold Supporter
Silver Supporter
Now, time to get a part time job to pay the gas bill!

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