What Size Heater Do You Need?


Before you rush off madly spending money, it is a good idea to figure out what heating capacity you need for your swimming pool. Following the instructions following on this page you will be able to calculate the volume of a pool and work out a rough minimum heating capacity for your pool. Going for a larger capacity heater will result in a shorter heating time and less wear and tear on the heater. The energy consumption will be little changed so don’t opt for a smaller heater to save dollars – it doesn’t work that way. A larger heater overall costs little more to run than a smaller one.

So into the nasty mathematical stuff. A BTU or British Thermal Unit is the standard for measuring heating capacity and most pool heaters will be described in terms of their BTU output. What we will do first is work out the number of BTUs you will need to increase the water temperature of your pool by 1oF in one day. From that we can easily work out the rest. So some things to work out.



Pool Volume in Gallons


If you already know the capacity of your pool that’s great, if you don’t it’s not hard to get a rough idea. Since pools are not all the same shape we will have to work things a little differently for the different shapes. If you can contact the pool builder, they should be able to give you an accurate volume for your pool and this is the best way to go.


Rectangular Indoor Pool


Rectangular or square – pool length (in feet) x pool width (in feet) (this gives surface area of pool) x average depth (in feet) x 7.5

The average depth will be close enough if you add the shallowest depth to the deepest depth then divide by 2. The 7.5 gives us the answer in gallons instead of cubic feet (1 cubic foot = 7.48 gallons US).

So, for example,  your pool is 20′ x 40′ and is 4′ in the shallow end and 6′ in the deep end.

You have an average depth of 5′ (4 + 6 = 10 divided by 2 = 5).

So 20 x 40 (= surface area of 800 sq ft) x 5 x 7.5 = 30,000 gallons.


Round – half pool width x half pool width x 3.14 (this gives surface area) x  average depth x 7.5

For example, your pool is 30′ wide, 4′ at shallowest and 6′ at deepest.

Your average depth is 5′ (4 + 6 = 10 divided by 2 = 5)

So you have 15 x 15 x 3.14 (gives 706 sq feet area) x 5 x 7.5 = 26,493 gallons (we will round these figures so 26,500 gallons is good).


Oval – length x width x average depth (giving area) x 7.5 x 0.9

The 0.9 will help give the approximate volume – we don’t need to be exact.

Your pool is 30′ long, 15′ wide, 3′ at shallowest point and 6′ at deepest.

Average depth is 3 + 6 = 9 divided by 2 = 4.5′

So we get to 30 x 15 (x 0.9 = 405 sq ft surface area) x 4.5 x 7.5 x 0.9 = 13,669 gallons (13,700).


Irregular Shaped Swimming Pool

Kidney shape – pretty close to an oval pool – width x length (x 0.95 = surface area) x average depth x 7.5 x 0.95

For example, you have a width (edge to edge) of 15′ and length of 30′.

You have a shallow of 4′ and a deepest point at 6′ so average depth is 4 + 6 = 10 divided by 2 = 5′

Then we go 15 x 30 ( x 0.95 = 427 sq ft area) x 5 x 7.5 x 0.95 = 16,031 gallons.


Irregular shapes – sometimes you will be able to come to a reasonable estimate by looking at the shape and breaking it down into a few shapes joined together and then work out a volume for each section and total them up. The best way would be to approach the builder of your pool as they should have a record and be able to inform you of the volume.



Heating Your Pool


Now you have the volume of your pool we can get to the nitty gritty of how much heating capacity you will need. There are two heating conditions you will need to be aware of  –  heating from cold and maintaining the desired temperature.


Heating from cold – this is the initial heating phase when you will be bringing your pool from cold to your desired temperature. If you will be using your pool continuously this will not need to happen often as you will reach your temperature and then maintain that temperature. If you will be using your pool occasionally then this will be done more often – gas heaters are usually better for this. Now back to the maths class!


Fistly, you need to know 1 BTU is the heat energy required to raise 1 pound of water by 1oF and there are 8.33 pounds of water in 1 gallon.


So   Volume x 8.33 = total pounds of water then divide by 24 to give BTUs per hour required to heat your pool 1 degree F in a day.


For example,    30,000 x 8.33 = 249,900 then divide by 24 = 10,412 BTUs per hour for a 1 degree rise in 24 hours.


Now you need to measure the temperature of your pool. This you will subtract from your desired temperature (usually around 80-84oF) to find the rise in temperature required. Suppose your pool is at 60oF and you want it up to 80oF then 80 – 60 = 20oF  rise.


Now multiply BTUs for a 1oF rise by the rise required to get BTUs per hour.


For the 30,000 (20′ x 40′ pool) gallon example:  10,412 x 20 = 208,240 BTUs per hour.


Another example, if your pool is 15,000 gallons: 15,000 x 8.33 = 124,950 divide by 24 = 5,206

Pool is at 55oF and required temp is 82oF so rise is 82 – 55 = 27.

BTUs per hour required are: 27 x 5,206 = 140,562 BTUs.


It is a good idea to add an extra 15-20% to the heating capacity you arrive at to make up for inefficiencies in the system. Remember, the BTU figure we have arrived at here is to achieve the temperature rise over one day. If you are going to do it in two days the per hour requirement is halved and if you want to get there in half a day then it is doubled. Also remeber your pool will be losing some of the heat it has gained due to natural losses such as evaporation so the desired temperature may take a bit longer. A pool cover will help stop this heat loss by about 50-80% so the benefits of a pool cover are obvious.



Maintaining desired temperature – you will require about 10 BTU per hour per sqaure foot of pool surface for each degree your pool temperature is above the air temperature. If we look at our 20′ x 40′ pool we have a surface area of 800 sq feet. If the air temperature is 60oF and you wish to keep your pool at 80oF then we have 80 – 60 = 20 x area of 800 = 16,000 x BTU per sq foot of 10 = 160,000 BTU per hour. With the addition of a pool cover this will drop dramatically.


Take the oval pool above for a second example. We have a surface area of 405 sq feet so with a temperature 20oF above the unheated temperature we have 405 x 10 x 20 = 81,000 BTU per hour. Again, drop this figure 50-80% with the addition of a pool cover.


And that is pretty well it. When all is said and done, a bigger capacity heater is always better than a lower capacity one. The energy cost difference is slight but wear and tear is significant. For a pool which is not used often and will be heated from cold rather than maintained at a constant temperature, gas heaters are the best choice. Use the figures you obtained through your calculations from above as a minimum rating for a heater.


Pick A Heater


Looking at heaters, if we take the 20′ x 40′ pool from above then we need to find a heater which is capable of  at least 210,000 BTU per hour. Remember that a higher capacity heater will take less time to do the same job and not use a great deal more fuel. To maintain 80oF the heater would be running almost full time where a heater around 400,000 BTU per hour would run for about 30 minutes per hour.



Looking at gas heaters, a 400,000 BTU unit would cost about $2,000 to $2,500 depending on which brand you chose. One thing to be wary of is how the manufacturer states the BTU capacity of the heater – some use input rather than output capacity. If the input capacity is quoted then you must look at the efficiency rating. Say the heater is rated at 80% efficiency then the output capacity is about 80% of the input capacity. For example, a heater is rated at 400,000 BTU input and 80% efficiency then multiply 400,000 by 0.8 to give an output capacity of  320,000 BTU per hour.



With heat pumps, the normal maximum output is about 130,000 BTU per hour and the cost is between $3,000 and $4,000 for the unit. The big advantage with heat pumps is they cost about a third to a half of the cost of a gas heater to operate. Some heat pumps are set up like reverse cycle airconditioners – that is, they can even cool your pool on those days when you jump in and get hotter! These models generally cost about $1,000 more than the plain heating models. A heat pump with capacity of 130,000 BTU/hour will take about 48 hours to heat the 30,000 gallon (20′  x 40′) pool in the examples above and would need a cover to be able to maintain the temperature.



Solar pool heaters are a different kettle of fish. Some manufacturers will quote a BTU per hour rating but there are too many variables to find an accurate result. Most solar pool systems will recommend about 50-80% of the pools surface area in solar panel area. So the 20′ x 40′ pool with 800 sq feet will need about 400-640 sq feet of solar panel. If the panels are 4′ x 10′ this means about 10 to 16 panels. Solar systems are best suited to extending your swimming season rather than trying to heat a cold pool to a comfortable temperature. If you want to spend some money to save some money, a combination of solar and either gas or heat pump would give a good year round result.