What a pool heat pump does and why it works differently than a furnace
A pool heat pump moves heat from the air outside into your pool water instead of generating heat by burning fuel or running an electric coil. It works like a refrigerator in reverse: a refrigerator pulls warmth out of its box and dumps it outside; a heat pump pulls warmth from outside air and pushes it into your pool. This is why pool heat pumps are most efficient in warm climates and lose effectiveness when outdoor air drops below 50°F.
The machine runs on electricity, but it uses far less energy than a traditional electric pool heater because it is moving existing heat rather than creating it from scratch. On a 70°F day, there is still usable heat in the air—a heat pump extracts it. On a 40°F day, the same heat exists, but the pump has to work much harder to find and extract it, and many models straightforward shut down because the effort is no longer worth the result.
Key Takeaways
- A pool heat pump moves heat from outdoor air into your pool water using a refrigerant that cycles between liquid and gas states.
- The compressor is the engine of the system—it pressurizes the refrigerant so it can absorb heat from cold air and release that heat into warmer pool water.
- Pool heat pumps work most efficiently when outdoor air is between 50°F and 80°F; below 50°F, many models stop running because energy use exceeds benefit.
- The evaporator coil sits in the outdoor air and collects heat; the condenser coil sits in contact with pool water and releases that heat into the pool.
- A reversing valve allows some heat pump models to run in cooling mode during summer, pulling heat out of the pool to keep it from overheating.
The four main parts and what each one does
Every pool heat pump has four core components that work together in a loop. The compressor is the motor that drives the whole system—it pressurizes a refrigerant (a special liquid that boils at very low temperatures) so the refrigerant can absorb heat from cold air and release it at higher temperatures. Without the compressor running, nothing happens.
The evaporator coil sits outside in the air. Cold, low-pressure refrigerant flows through it. As outdoor air passes over the coil, heat from that air transfers into the refrigerant, causing the refrigerant to boil and turn into a gas. Even on a 50°F day, there is enough heat in the air to do this.
The condenser coil is where the heat gets delivered to your pool. The hot, pressurized refrigerant gas flows through this coil, which sits in contact with your pool water (or in a metal tank that pool water flows through). As the refrigerant cools down and condenses back into a liquid, it releases its heat into the water. This is where your pool actually gets warmed.
The expansion device (usually a small valve) sits between the condenser and evaporator. It reduces the pressure of the liquid refrigerant so it can flow back to the evaporator and start the cycle again. The whole loop repeats continuously while the pump is running.
How the refrigerant cycle moves heat from air into water
The refrigerant is the working fluid that carries heat around the loop. It is chosen because it boils at temperatures far below water's freezing point—typically between −40°F and 40°F depending on the type. This means even cold outdoor air contains enough heat to boil it.
Here is the cycle in order: The compressor sucks in low-pressure refrigerant gas from the evaporator and squeezes it, raising its pressure and temperature. This hot, pressurized gas flows into the condenser coil, where it touches pool water. The water is cooler than the compressed gas, so heat flows from the gas into the water. As the refrigerant loses heat, it condenses back into a liquid. This warm liquid then flows through the expansion device, which drops its pressure. The low-pressure liquid then enters the evaporator coil, where outdoor air is warmer than the liquid, so heat flows from the air into the refrigerant, boiling it back into a gas. The cycle repeats.
The key insight is that the compressor does not create heat—it creates the pressure difference that allows heat to flow from a colder place (the outdoor air) to a warmer place (the pool water). Without that pressure difference, heat would flow the wrong direction and nothing would happen.
Why outdoor air temperature matters so much
A pool heat pump is most efficient when the temperature difference between the outdoor air and your pool water is small. If your pool is 80°F and the air is 75°F, the heat pump barely has to work—heat naturally wants to flow from the warmer air into the slightly cooler pool, and the pump just assists that flow. The compressor runs at low power and uses little electricity.
When outdoor air drops to 50°F and your pool is still 80°F, the temperature difference is large. The compressor has to work much harder to extract heat from cold air and push it into warm water. The machine becomes less efficient—it uses more electricity to move the same amount of heat. Below 50°F, many heat pumps straightforward shut down automatically because the energy cost exceeds the heat gained.
This is why pool heat pumps are popular in Florida, California, and the Southwest, where winter air stays above 50°F. In colder climates, they work well only during spring, summer, and early fall. Some owners in cold regions use a heat pump as a supplement to a gas heater: run the heat pump on mild days to save money, and switch to gas on cold days when the heat pump would be inefficient.
The difference between single-speed and variable-speed models
A single-speed heat pump runs at full power whenever it is on, or it is off. The compressor either spins at maximum RPM or does not spin at all. This means the pump heats your pool quickly but cycles on and off frequently, which uses more energy overall and creates more wear on the compressor.
A variable-speed heat pump adjusts the compressor speed based on how much heat your pool needs. On a warm day when the pool is close to your target temperature, the compressor slows down and uses less electricity. On a cool day, it speeds up. This reduces energy use by 20 to 40 percent compared to single-speed models, but variable-speed pumps cost more upfront.
For most residential pools, a variable-speed model pays for itself within a few years through lower electricity bills, especially if you run the pump frequently or live in a mild climate where the pump runs most of the year.
How the reversing valve lets some models cool your pool
Many pool heat pumps include a reversing valve, a switch that reverses the direction of refrigerant flow. In heating mode, the evaporator coil is outside and the condenser coil is in the pool. In cooling mode, the roles flip: the coil in the pool becomes the evaporator (pulling heat out of the water), and the outdoor coil becomes the condenser (dumping that heat into the air).
This feature is useful in hot climates where pool water can overheat in summer. Instead of letting the pool sit at 95°F, you can run the heat pump in reverse to cool it back down to 82°F. The reversing valve is not standard on all models—check the specifications before buying if cooling is important to you.
Common problems and what causes them
The most frequent issue is frost buildup on the evaporator coil in cold or humid weather. When outdoor air is damp and near 40°F, moisture can condense on the cold coil and freeze. Most heat pumps have a defrost cycle that reverses the refrigerant flow temporarily to warm the coil and melt the ice, but this reduces heating output while it runs. If defrost cycles happen constantly, the pool heats very slowly.
A second common problem is low refrigerant charge. If the system develops a small leak, refrigerant escapes and the pump cannot extract as much heat from the air. The compressor works harder but delivers less warmth to the pool. This requires a service technician to find the leak, repair it, and refill the refrigerant.
Compressor failure is the most expensive repair. The compressor can fail if the system runs out of refrigerant (metal-on-metal friction without the lubricant in the refrigerant), if electrical power surges damage the motor, or straightforward from age. Compressor replacement often costs $1,500 to $3,000 and may not be worth it on an older unit.
Frequently Asked Questions
Can a pool heat pump work in winter?
Most pool heat pumps stop working below 50°F outdoor air temperature because the compressor cannot extract enough heat to justify the electricity use. Some cold-climate models are designed to work down to 35°F, but they are expensive and still very inefficient. If you want to heat your pool in winter in a cold climate, a gas heater or electric resistance heater is more practical.
How long does it take a heat pump to warm a pool?
Heating time depends on the pump size, pool volume, and outdoor temperature. A properly sized heat pump typically raises pool temperature by 1 to 3 degrees per day in mild weather. Warming a 15,000-gallon pool from 65°F to 80°F might take 5 to 10 days. Larger pools or colder outdoor air take longer.
Does a heat pump use a lot of electricity?
Heat pumps use far less electricity than electric resistance heaters but more than gas heaters. A typical pool heat pump uses 3,000 to 6,000 watts while running. Running it 8 hours a day costs roughly $30 to $60 per month in electricity, depending on local rates. A gas heater costs more in fuel but uses less electricity.
What size heat pump do I need for my pool?
Heat pump sizing is measured in BTU (British Thermal Units) per hour. A rough guide is 5,000 to 10,000 BTU per 10,000 gallons of pool volume, depending on climate and how fast you want to heat. A professional installer can calculate the right size based on your pool volume, desired temperature rise, and local weather patterns.
Can I leave a heat pump running all the time?
Yes, heat pumps are designed to run continuously during the heating season. Many owners run them 24/7 in spring and fall to maintain pool temperature. Running them constantly is more efficient than cycling them on and off, especially with variable-speed models. Just turn it off when outdoor air drops below 50°F or when you do not need heating.