Some people think that swimming pool electric heat pumps use the same amount of power any time they are running. In fact, some people think that heat pumps use less energy as the pool water temperature approaches the desired setpoint, thinking that it’s getting easier to heat the water. Nothing could be further from the truth.

We have explained “Delta T” in several previous articles. In any heat transfer system, the difference in temperature between two sides of a heat exchanger dictates the efficiency of heat transfer. A larger difference means more efficient heat transfer. Conversely, when the difference in temperature is reduced, more work (energy) is required to transfer heat.

This can be demonstrated through actual electrical consumption data from a heat pump. The better illustrate this, we can look at a client’s home where they frequently heat their spa each evening. Since this is a smaller body of water than a pool, and the desired temperature is much higher than that typical of pool heating, the effect is more pronounced. Take a look at this solar production an heat pump consumption graph.

The heat pump energy usage is in orange. You can see that the heat pump ran for about 2-1/2 hours to get the spa up to the desired temperature. Each bar is a 15-minute increment. If you look at the first and last 15-minute increment, you can see the difference in power required to heat the spa water.

When the heat pump started, the pool water was presumably in the low 70’s, which is typical for late February. It was drawing 5.5kW on average. By the time the heat pump reached the desired spa setpoint temperature, presumably around 100ºF, it was drawing 6.9kW. That is a full 25% more power required.

It gets harder to heat water as the Delta T, difference in temperature, between the two sides of the heat exchanger approach each other. More work (energy) is required. For example, it is much harder to heat water from 99ºF to 100ºF than it is to heat water from 79ºF to 80ºF. In a perfect system, it would require the same amount of energy to heat water by the same amount, 1ºF. However, you have to consider that we are using an air source heat pump. The ambient temperature stays the same, essentially, as the water to be heated continues to increase. That is where the efficiency starts to break down.

This is by no means a scientific explanation of thermodynamics. This is simply anecdotal proof that you pool heat pump will consume more energy if you set it to a higher desired temperature. This is similar to the air conditioning in your home. It’s much easier to reduce your home’s temperature from 80ºF to 79ºF than it is to reduce it from 73ºF to 72ºF. The cost rises exponentially as you set your thermostat lower and lower. The same thing applies to pool heating, except in reverse.

You can reduce your pool heating costs quite a lot if you set a reasonable desired temperature. And since we are dealing with Delta T here, the warmer it is outside, the less energy is needed to heat your pool.