A simple question but there is no simple answer.
Gas pool heaters typically range from 100,000 BTU/hour to 400,000 BTU/hour and all will heat your pool. Larger heaters will warm a pool faster and are slightly more energy efficient but, of course, they cost more.
The main alternative electric heat pumps are much less powerful 100,000 – 130,000 BTU/hour and cost more but they are very energy efficient and therefore cheaper to run.
Below is a table giving some guidance based on the capacity of the pool or the surface area but this is very just a general guide. Much will depend on:
How you will use your pool heater
Will you want your heater to heat up your pool as quickly as possible at intervals or will you keep it heated all the time during the season. If the former you want the highest capacity heater you can afford so that the pool is ready for use quickly when you need it. If the latter you need a lower capacity energy efficient heater which keeps the pool ready for use at the lowest cost.
The location of your pool
Wind is the big enemy here. It will cause more evaporation which causes the pool to lose heat. So if your pool is in an exposed position your pool heater will have to work harder. You also need to consider the lowest air temperature in the months you want to heat the pool and therefore the temperature raise required.
Your use of a pool cover
Use of a pool cover when the pool is not in use will dramatically reduce heat loss and therefore the demand on your pool heater.
Below is a simple table to give an indication of the size of gas heater that will be appropriate. If you want to undertake a more detailed calculation have a look at this page.
|Size Heater||Gallons in Pool||Sq. Ft. Surface Area of Pool|
|100 – 200 BTU heaters||1,000 gals to 10,000 gals||up to 300 sq ft|
|200 – 300 BTU heaters||10,000 gals to 20,000 gals||up to 500 sq ft|
|300 – 400 BTU heaters||20,000 gals to 40,000 gals||up to 800 sq ft|
|400 BTU heaters||40,000 gals to 80,000 gals||up to 1200 sq ft|
In general it is best to work out what you think you need then get something slightly larger as, apart from the higher purchase price, there is no harm in having a more powerful heater than you need.
If you want to heat your pool quickly you should go for a gas heater. However, before doing so you should look at the running costs compared to a pool heat pump as it may be that using a heat pump to keep the pool heated for longer periods would be more cost effective.
One of the major manufacturers Raypak has an interesting program for sizing a heat pump and showing the savings you will make. Click Here to visit this program,