Heating for Christmas - how long

JJ_Tex

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We are toying with the idea of heating the pool for christmas. I know the spa goes up about a degree a minute, but I have no idea about heating the pool. Is there a calculator or formula I can use to figure out when to start heating?

My pool is 13.5k gallons and we have a 400k btu natural gas heater. Current water temp is mid 40’s so we would want to heat it up by 40 degrees or so, with a forecast temp being around 60 degrees as the high and lows around 40.
 

jseyfert3

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I like this calculator. Note it’s purely the energy to heat the water, and doesn’t account for heat loss, so actual times will be longer. Water Heating Calculator for Time, Energy, and Power

I remember seeing someone up in northern US or Canada that would have a hot tub pool weekend, heating to like 96 in much colder ambient. Let me see if I can find that.
 

jseyfert3

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I can’t find the post. I know it was on here, a couple would once a year heat their pool really hot in the middle of winter, said it was expensive but cheaper than a vacation. Ah well.

Anyhow, note on the above calculator it’s assuming heat input to the water, so use 320,000 BTU/hr, not 400,000, or whatever the actual heat input is if you know your heater efficiency.
 
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HermanTX

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My pool is 13.5k gallons and we have a 400k btu natural gas heater. Current water temp is mid 40’s so we would want to heat it up by 40 degrees or so, with a forecast temp being around 60 degrees as the high and lows around 40.
I have the same heater and based on the specs, I interpret the chart to be about 12-15 hrs for a 40 degree temp. rise in a 14k gal pool. It states it would take 24hrs for a 24k gal pool is my baseline. Have fun and good luck.

Edit - I did some further research and I was using 80% efficiency (of the 400k BTU's) it may be closer to 14-16 hrs. Just note that it is harder to heat the deep end so if you have a bottom drain be sure to have that at least 50% open for suction.
 
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monoptn

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I have the same heater and based on the specs, I interpret the chart to be about 12-15 hrs for a 40 degree temp. rise in a 14k gal pool. It states it would take 24hrs for a 24k gal pool is my baseline. Have fun and good luck.

Edit - I did some further research and I was using 80% efficiency (of the 400k BTU's) it may be closer to 14-16 hrs. Just note that it is harder to heat the deep end so if you have a bottom drain be sure to have that at least 50% open for suction.
I've been thinking about doing the same.....my pool will warm about 3 degrees per hour in the spring. I too am curious about how much longer it would take with the ground cold and air temp in the 30-40 range. Keep us posted!
 

JJ_Tex

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Thanks. I also got 14 hours on the calculator, which was much shorter than I was expecting. Glad I posted on here as I was thinking 24 hours plus.

I also talked to my neighbor as he accidentally turned on his heater for 3 days last month and heated his entire pool up to 90 degrees before he noticed. He said he hardly noticed on his bill and the combined gas/electric bill was under $300 for the month (up $50 from the prior month, but their house heater also ran in November but not October). Seems pretty reasonable to me.
 

markayash

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It cost me about $20 In Natural gas to do it one year but ran solid for 2 days. I don’t have a cover and it was mid 40’s outside at night and 60 in day so 80 was about as hot as my heater ( 400k ) would heat my pool.
We did it for a high school band party and the kids had a blast
 
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jseyfert3

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Thanks. I also got 14 hours on the calculator, which was much shorter than I was expecting. Glad I posted on here as I was thinking 24 hours plus.
I need to emphasize that the calculator does not account for heat loss, which becomes larger the hotter the pool gets. @markayash did this in similar weather to your forecast and though they have a bigger pool than you it took them two days. At the very least I suspect it would take at least 24 hours. As I’m learning with slow cooking meat on my new pellet grill, you can easily keep it warm for a while after it’s done, but you can’t speed up the process and eat earlier if you start too late. Similar logic would apply for heating a pool.
 

markayash

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It was worth every penny, just put heater on kill mode 2 days before and enjoy it :)
Our forecast show possible snow and a low of 23 for Christmas which is cold for Atlanta. Snow would be super cool though :)
 

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JJ_Tex

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As I’m learning with slow cooking meat on my new pellet grill, you can easily keep it warm for a while after it’s done, but you can’t speed up the process and eat earlier if you start too late. Similar logic would apply for heating a pool.

Now that is an analogy I can understand.
Christmas #1 on the 25th, is just my mom coming over. Christmas #2 is the 26th & 27th with my niece and nephew. I'll just start the heater on the 25th to make sure we are set for Christmas #2, and any swimming or spa usage we get on Christmas #1 will be an added bonus.
 

JJ_Tex

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Just a quick update for my own notes, but they may be useful to anyone else:

At 9am the temp was 47 degrees and I turned on the heater. By 5:00pm the pool was 68 degrees, so 21 degrees in 8 hours (2.6 degrees per hour) and the air temp was in the low 60's.

Unfortunately out Christmas plans had to change, so I turned off the heater, but hoping to heat it in a few weeks when we re-try Christmas.
 

monoptn

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Just a quick update for my own notes, but they may be useful to anyone else:

At 9am the temp was 47 degrees and I turned on the heater. By 5:00pm the pool was 68 degrees, so 21 degrees in 8 hours (2.6 degrees per hour) and the air temp was in the low 60's.

Unfortunately out Christmas plans had to change, so I turned off the heater, but hoping to heat it in a few weeks when we re-try Christmas.
How fast does it heat under more normal circumstances? Mine normally heats about 3 degrees per hour, but I figured it would be a bit slower with the ground and air so cold. I ended up not heating mine, as the high temp on Christmas Eve was low 30s and windy. My 15 year old did a polar plunge that night with the water temp of 42 degrees though! He had a look of horror on his face.
 

markayash

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How fast does it heat under more normal circumstances? Mine normally heats about 3 degrees per hour, but I figured it would be a bit slower with the ground and air so cold. I ended up not heating mine, as the high temp on Christmas Eve was low 30s and windy. My 15 year old did a polar plunge that night with the water temp of 42 degrees though! He had a look of horror on his face.
Crazy kid
They do that at Hilton head every year and a friend ask me if I was going..My response " are you on drugs" :)
 

jseyfert3

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My 15 year old did a polar plunge that night with the water temp of 42 degrees though! He had a look of horror on his face.
A few weeks back two of my sisters were visiting my wife and I. We were in the hot tub, and they got the idea they’d jump in the pool, which if I recall was 38 degrees. They got out REALLY quick. :ROFLMAO:

It’s now got at least an inch of ice, so no more of that. Probably a few inches probably, been cold recently.
 
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Candurin

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The day they filled our pool with water from the truck it was 54 degree water. I was committed to going in. Threw my wetsuit (5 m) on and it was COLD.

Then while winterizing, PB dropped the return line plug into the deep end. I volunteered to grab my dry suit (with thermoelectric undergarments!) and go retrieve it. Water was 41 degrees and stupid cold (and I’m an ice diver).

I can’t imagine just jumping in with nothing but my swimsuit. Ouch.
 
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setsailsoon

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The answer to the original post is fairly easy to calculate. Here's how to do it:

An important factor for this question is evaporation as our fantastic new TFP guide points out. High wind speed and dry air can be even more important than the temp rise required. This is one of the reasons a cover of any type will help. I found some good articles when researching this topic last winter. Here's one of them.

Also interesting is the dt component of the heat transfer coefficient. The overall heat transfer coefficient (U) is calculated as BTU/sq ft per degree of temp difference. Also the heat transfer rate (q) is calculated as U*A* dt. So at colder water temp the heat transfer rate is actually higher (assuming flame temp is about the same after the heater is running for a while.

The last thing to keep in mind is the relationship of multiple barriers to heat transfer when examining a heat transfer system. For our pool heaters there are several:
  • First, working from the inside of the tubes there's the cold pool water transferring heat from the hot tube to the water. This is a pretty good heat transfer situation and usually has a U1 of 100-120.
  • Then there's the transfer from the inside of the tube to the outside. This is extremely high. Can't remember how high is typical but we'll call it U2 =800
  • But here's the kicker the hot air side from the burner to the outside of the tube is pretty bad because the gas side has a very low density compared to a liquid. So U3 of 25 at most and can be much lower if the tube fins are corroded or dirty. Can be even close to 12 o so.
  • Now the really bad part. The overall heat transfer rate of the entire system is calculated as
    • U(overall) = 1/U1+1/U2+1/U3
    • So when you work the math out, the heat transfer coefficient of the worst transfer in the system limits everything
This is about all I recall on this topic from 40+ years ago but I bet If @JamesW @jseyfert3 @JoyfulNoise @MyAZPool go to town on this they'll be able to calculate the heat up time within a few minutes.

It'll probably also get this thread banished to the the Deep End pretty quickly.

Chris
 

jseyfert3

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Just because I can do some doesn’t mean I want to do it! :ROFLMAO:

The day they filled our pool with water from the truck it was 54 degree water. I was committed to going in. Threw my wetsuit (5 m) on and it was COLD.

Then while winterizing, PB dropped the return line plug into the deep end. I volunteered to grab my dry suit (with thermoelectric undergarments!) and go retrieve it. Water was 41 degrees and stupid cold (and I’m an ice diver).

I can’t imagine just jumping in with nothing but my swimsuit. Ouch.
Our pool was 54 when we filled it. I think it was warmer when we first got in, but I wore my 3 mm full wetsuit and was still cold. For reference, I wore that diving in Grenada and the water was 77 and I was perfectly comfortable, so I suspect we got in when the water was in the 60’s

My hand was going numb pulling leaves off the filter intake at 38, there was no way I was jumping in that cold water!
 
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