Choosing the Right Gas Heater

To estimate what size gas heater you need to purchase for your swimming pool or spa there are a few concepts to understand.  

  1. BTU stand for British Thermal Unit
  2. It takes 1 BTU to raise one gallon of water 1 degree
  3. One gallon of water weighs 8.33 pounds


Using the following example here is how you would estimate the heater size you need to buy.

In this example we will use 20,000 gallons.

First Calculate how the gallons of water are in your pool
20‘ x 30’x 4.5’ average depth x 7.48gallons= 20,000 (Rounded Number)

Then calculate the weight of the water in your pool
20,000 X 8.33 pounds = 166,600 pounds of waters

Calculate the heat rise desired in the pool
85 degree desired temperature - Average Low temp daily 65 = 20 degree heat rise

Multiply weight of water times temperature desired for total BTU’s needed per hour
166,600 X 20 = 3,332,000 BTU’s per hour

Then you take the BTU’s needed per hour and divide by the numbers of hours desired for the pool to reach the desired temperature of 85 degrees
3,332,000 / 10 hours = 332,000 BTU


332,000 is how many BTUs the heater must have in order to heat this pool from 65 degrees up to 85 degrees in ten hours.  However, efficiencies in gas heaters are typically around 80 percent so you would need to buy a 400,000 BTU heater to meet the demand.