Pool Question..Indoor Heater

by Art
(Ontario, Canada)

My name is Art and I'm new at this pool heating. Sorry if this sounds crazy...lol.

Just purchased a house with indoor / inground pool. Can only use during the summer month from April to mid September in Ontario Canada.

Pool size I'm thinking is between 18 to 20 thousand litres....does that make sense??? The pool is kidney shape.

I'm 5' 9" and the deepest end goes to my neck.....so what I'm asking is what size of pool heater work for my pool. I've been to many pool places and they are telling all different size of BTU's that I need. So right now I just don't trust anybody and want to make sure that I'm spending my money the right way and not the wrong way.

I sure hope you can help. If you can't or I shouldn't be sending you this I do apologise. Please see pool pictures.

Thanks for your time


Thanks for the question Art

I have a great page about pool heaters that you may want to look over. It has other links at the bottom of different kinds of heaters.

Electric Pool Heater..Inground Pool Heater..Above Ground

Having an indoor pool you probably want either an electric or gas heater.

An 18,000 to 20,000 litre pool is about about 5,000 U.S. gallons, so that's what I'll be using.

It looks like the pool might be a little bigger than 5,000 gallons.

You need 4 measurements:

Length X Width X Average Depth X 7.5 = Total Gallons

By the looks of the pool from the picture it seems to be 16 X 14, so we'll go from there.

Average depth is the shallow end + the deep end divided by 2. For easy math we'll do an even 6 feet for the deep end and 3 feet for the shallow end.

6 + 3 = 9. The average depth is 4.5 feet.

Now we have our measurements:

16 X 14 X 4.5 X 7.5 = 7,560. Let's round it up to 8,000 gallons. It's better to go a bit higher than underestimate.

Remember there are many factors to consider when purchasing a pool heater. Some of them are:

Size of the pool

Square foot area


Heat loss

Evaporation (This accounts for about 70% of heat loss) so having a pool cover would be beneficial

Average air temperature

Desired pool temperature


Night changes

For an indoor pool you'll need air exchanges which
means you need to account for fresh air coming in and exhaust going out which will cool the pool down faster. If you're drawing in outside air, especially in Canada where it gets pretty cold, then take that into consideration as well.

If you don't have a good air exchange your combined chlorine will rise and you'll need to shock the pool more often. I'm the pool operator at our YMCA (80,000 gallon heavily used indoor pool) so I know about this.

Being that I don't know some of these details, this will be a very rough estimate.

You have an 8,000 gallon pool. Then you need to know the BTU per hour it takes to keep your pool comfortable. Most people like it between 78 - 82 degrees F. That's about 26 degrees C.

Here's the calculation, and again, this is a rough estimate.

1st - The average pool water temperature that's comfortable - say 80F which is 26C.

2nd - Average temperature of the area surrounding the pool during the coldest month you intend to use it. Let's say that's 70F, or 21C

3rd - Subtract the coldest temp, 70F, from the comfortable pool temp. 80F. That give you ten. The result is the number of degrees your heater will need to heat the pool in order to make it comfortable.

4th - Surface area of the pool. Yours is 224 feet. 16 X 14.

5th - Multiply the pool's surface area (224) by the number your pool heater need to heat the pool (10) and then multiply that by 12. This will give you the BTUs it takes to heat your pool, so:

224 X 10 X 12 = 26,880 BTUs

I would think a 100,000 BTU pool heater would be more than sufficient for your needs. Look at the efficiency rating. Many will say 90% efficient so you're actually getting 90,000 BTUs.

Then consider the rating 3 - 5 years down the road. Will it still be running at 90%?

A smaller 50,000 BTU may work as well. You always want to go a little higher so the pool heater doesn't need to work as hard to keep heating the pool. You can look at this one:

RAYPAK Versa B055BEN Natural Gas Pool Heater - 55k Btu

Hope this helps. I know it's alot of info. and calculations but I wanted to give you as much as possible to make a good decision.

Good luck


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