With winter on the way, now is the perfect time to bring up the subject of replacing an old heater.

Upgrading a heater is a great way to make a profit, while also helping a client lower their heating bills and making them a loyal customer in the future.

High efficiency heaters

Gas pool heaters are rated by BTU input/output (BTU stands for British thermal unit). Inputs/outputs range from 75,000 BTUs to 4,500,000 BTUs. Today’s swimming pool heaters start off at 85 to 90 percent efficiency. This means that when a heater is rated 85 percent efficient with a 400,000 BTU input, what comes out of the heater (output) or actual heating capability is 85 percent of the total BTUs — so in this example, only 340,000 BTUs come out of the heater.

Heaters that are seven to 10 years old started off at about 78 percent efficient. So using the same example above, with a heater input of 400,000 BTUs, the heater only produces an output of 280,000 BTUs. This assumes the heater is still operating at the same rate it did when it was first installed. However, most heaters start losing efficiency over time just from basic operation.

Costs of reduced efficiency

Let’s take a 20,000-gallon pool as an example of how these less-efficient heaters affect a facility’s costs. It is important to note that one BTU will raise one pound of water one degree Fahrenheit.

Now, let’s take a more in-depth look at our original example: a new 400,000 BTU heater that is 85 percent efficient — or, in other words, one that has a 340,000 BTU output. With that 340,000 BTU output, it will take 9.8 hours to raise that 20,000-gallon pool by 20 degrees (3,332,000 BTUs ÷ 340,000 = 9.8 hours).

As we all know, gas companies charge by the therm. So, to figure out how much this pool will cost to heat, it’s crucial to know that 100,000 BTUs = 1 therm. If we know that it takes 9.8 hours to heat a 20,000-gallon pool with a 20-degree temperature rise using a 400,000 BTU heater that is rated at 85 percent efficiency (i.e., a 340,000 BTU output), then it only takes a few steps to figure out the cost of the gas required to heat that pool with that heater:


[hours of heat up time] x [input BTUs] ÷ 100,000 x [cost of 1 therm]

9.8 x 400,000 = 3,920,000 total BTUs

3,920,000 ÷ 100,000 = 39.2 therms used

If the cost of 1 therm of gas is $1.20, then

39.2 x $1.20 = $47.04

So, it will cost $47.04 to raise the temperature of that 20,000-gallon pool by 20 degrees in that 9.8-hour period.

However, if we had a seven-year-old 400,000 BTU heater that was rated 78 percent efficient — but is now probably only operating at 65-percent efficiency — then the heater outputs only 260,000 BTUs:


3,332,000 BTUs required ÷ 260,000 = 12.8 hours

12.8 x 400,000 = 5,120,000 BTUs

5,120,000 BTUs ÷ 100,000 = 51.2 therms

51.2 x $1.20 = $61.44

This means that it costs the facility 30 percent more to heat the same body of water with the older heater.

A comparison with propane

Now let’s look at a similar example, where a propane heater is used to heat a pool of the same size:


1 gallon of propane = 92,000 BTUs

9.8 hours of run time x 400,000 BTUs input = 3,920,000 total BTUs required

3,920,000 ÷ 92,000 BTU = 42.61 gallons of propane

If the cost of 1 gallon of propane = $3.00, then

42.61 x $3.00 = $127.83

But let’s say the facility’s existing older heater takes 12.8 hours to increase the pool’s temperature by 20 degrees. In that case:


12.8 x 400,000 = 5,120,000 BTUs

5,120,000 ÷ 92,000 = 55.65 gallons

55.65 x $3.00 = $166.95

Again, it will cost the pool owner or facility 29 percent more to heat their pool with the older, less efficient heater.

Choosing a heater

This will catch the client’s attention, because it’ll enable them to better understand how their heater works. So be prepared to offer a heating unit that is appropriate for their pool. It’s important to realize that pool size does matter — as do factors such as whether the pool is indoor or outdoor.

To calculate an approximate heater size for a pool, just follow these steps:

1 Determine the desired swimming pool temperature (average is 82 degrees for competitive swimming).

2 Determine the average temperature for the coldest month of pool use (if it is an outdoor pool).

3 Subtract the average temperature for the coldest month from the desired pool temperature. This will give the temperature rise needed.

4 Calculate the pool surface area in square feet.

5 Use the following formula provided by the U.S. Department of Energy:

Pool area x temperature rise x 12 = the BTUs/hour output required.

To clarify, heaters are sized based on a 24-hour temperature rise.  So a heater with one million BTUs takes 24 hours to raise the pool temperature 15 degrees.

Being armed with all the facts makes it easier to approach a potential client about upgrading their heater to a newer, more efficient unit that will immediately reduce their energy bills. Whether the sale involves

approaching a current customer with a proposition to upgrade their heater, or trying to get a foot in the door with a prospective customer, selling a new heater upgrade will not only increase profit, but will also earn loyal accounts who are sure to be pleased with their lower monthly operating costs.