How long to heat a pool??????

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Feb 20, 2016
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Dallas
Having a party on Jan 1. Anticipated water temp will be 41 degrees..... I am thinking about heating the Pool to about 83. How many hours (20K Pool) should I plan on to reach the temperature? Expected air temp will be 25 degrees
 
With the cover, maybe somewhere between 30 and 50 hours. Just a wild guess to help you with the question because it's one of those unanswerable things. The ground around the pool is probably the same temperature as the water, so there's a lot of mass absorbing the heat.
 
An 83% efficient 400,000 btu/hr will add 332,000 btu per hour to the water. That's about 2 degrees Fahrenheit per hour. For a 42 degree rise, that's 21 hours with no heat loss. So, that's the fastest possible time.

With heat losses, you can expect the heat up time to be closer to about 38 hours.

Heat losses depend on multiple factors. The difference between air temperature and water temperature is a big factor.

Initially, the losses will be low because the water temperature and air temperature difference is small.

By the time you get close to 83 Fahrenheit, your heat loss could be up to about 200,000 btu/hr.

Your initial net heat gain will be about 1.53 degrees per hour and it will drop to about 0.9 degrees per hour by the time you get to 83 Fahrenheit.

Using a good cover will help you get closer to 21 hours than 38 hours.
 
Average NG price in TX is about $2/therm. Your 400kBTU/hour heater will burn 4 therms/hr. So about $8/hr to run the heater. Assuming JamesW’s worst-case scenario of 38 hours, you’re looking at spending ~$300 to heat the pool....
 
JamesW said:
An 83% efficient 400,000 btu/hr will add 332,000 btu per hour to the water. That's about 2 degrees Fahrenheit per hour. For a 42 degree rise, that's 21 hours with no heat loss. So, that's the fastest possible time.

With heat losses, you can expect the heat up time to be closer to about 38 hours.

Heat losses depend on multiple factors. The difference between air temperature and water temperature is a big factor.

Initially, the losses will be low because the water temperature and air temperature difference is small.

By the time you get close to 83 Fahrenheit, your heat loss could be up to about 200,000 btu/hr.

Your initial net heat gain will be about 1.53 degrees per hour and it will drop to about 0.9 degrees per hour by the time you get to 83 Fahrenheit.

Using a good cover will help you get closer to 21 hours than 38 hours.
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I haven't seen a way to "Like" a post on this forum, but I just love these kinds of mathematical explanations!
 
Dodger, here's the rough formula James is using:

A=#Gallons x 8.34 (the weight of a gallon on water)= #BTU required to get 1 degree an hour

Then B=#BTU heater x Efficiency (eg approx 80%) = capacity of btus avail per hour

A/B gets you the optimum degrees-per-hour in your pool.
 

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The heater rated output x efficiency gets you the heat gain rate.

The other factor is the heat loss rate. That's much more complicated. You lose heat to the ground by conduction, to the air by convection and by evaporation.

The difference between water temperature and air temperature is a big factor. As you begin heating, the heat begins to go into the ground and air.

The warmer the water gets, the faster heat is lost.

So, while you're gaining 2 degrees per hour, you're losing some as well.

The 21 hours is a theoretical minimum assuming no losses.

38 hours accounts for estimated heat loss. But it's an estimate at best. There are lots of variables that make the heat loss tricky to estimate accurately.
 
I think one also has to consider wear & tear on the heater. Running a gas heater for 38 hours straight with it sucking in sub-zero air into the blower/combustor is a recipe for heat exchanger disaster. Incomplete combustion of NG from cold air can lead to the formation of very corrosive condensates on the heat exchanger. So you could wind up shortening the life of the heater considerably unless it happens to be in an indoor pool equipment room.
 
I just read another thread on this. Would be good to read if you're interested I'm doing it later. In the thread he said they heat their pool to "spa" temps several times a year, including below freezing. He said they normally heated it to around 96. Of course now I can't find the thread. He had a whole big list of instructions on how to make it cheaper, how to to figure out what it'll cost ahead of time, how long it'll take, etc. He'd usually run the heater for like 24 hrs or longer, as long as 48 when opening the pool the first time that year..

Of course now I can't find the thread!
 
I think one also has to consider wear & tear on the heater. Running a gas heater for 38 hours straight with it sucking in sub-zero air into the blower/combustor is a recipe for heat exchanger disaster. Incomplete combustion of NG from cold air can lead to the formation of very corrosive condensates on the heat exchanger. So you could wind up shortening the life of the heater considerably unless it happens to be in an indoor pool equipment room.
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Lol..and possibly even indoors, as my recent heat exchanger story may or may not attest ;)

Jsey, that thread is by someone who also has an indoor heater - here's the thread:
FAQ: Cheaply Keeping a Pool Hot -- 90F,95F,100F -- How we ran till Jan 11th in Canada
 
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