How Does a Pool Heat Pump Actually Work?
A pool heat pump works by pulling warmth out of the outside air and transferring it into your pool water, using a refrigerant, a compressor and a heat exchanger. It does not burn anything and it does not have a heating element. It moves heat that already exists in the air rather than creating heat from scratch, which is exactly why it costs a fraction of what gas or electric element heating costs to run. Think of it as an air conditioner running backwards, dumping heat into your pool instead of out your window.
Not sure what size you need? Try our pool heat pump sizing calculator. Enter your pool size and postcode and it shows the right kW, running costs and the models that fit.
The short version, in one paragraph
Cold refrigerant absorbs heat from the air, gets squeezed by a compressor until it is hot, passes that heat into your pool water through a titanium heat exchanger, then expands back to cold and starts again. That loop runs continuously while the unit is on. Your pool pump pushes water through the heat exchanger the whole time, so the water gains a small amount of heat on every pass and the whole pool creeps up over hours, not minutes.
The four parts that do the work
A pool heat pump has four working components, and every brand from 9kW to 35kW uses the same four. What separates a cheap unit from a good one is the quality and control of those parts, not the concept.
1. The fan and evaporator coil
The fan pulls outside air across a finned coil that contains very cold liquid refrigerant. The refrigerant inside that coil sits well below the air temperature, often around zero degrees, so even on a 12 degree winter morning the air is warm by comparison and gives up heat to it. That heat boils the refrigerant into a gas. This is the step people find counterintuitive: there is usable heat in cold air, and the heat pump is simply cold enough to take it.
2. The compressor
The compressor squeezes that cool refrigerant gas into a much smaller space, which raises its temperature sharply. Squeeze a gas and it gets hot. By the time it leaves the compressor the refrigerant is well above your pool temperature, often 70 degrees or more, which is what makes the next step possible. The compressor is the only part drawing serious electricity, and it is the part that decides how long the unit lasts.
3. The titanium heat exchanger
The heat exchanger is where the hot refrigerant hands its heat to your pool water without the two ever touching. Water from your pool pump flows through one side, hot refrigerant flows through the other, and heat crosses the titanium wall between them. Titanium is used because pool water is chlorinated or salted and will eat cheaper metals. If a heat exchanger is listed as anything other than titanium on a pool unit, walk away.
4. The expansion valve
The expansion valve drops the refrigerant pressure back down, which makes it very cold again, ready to absorb more heat from the air. Having given up its heat to the water, the refrigerant is now a cooler liquid under high pressure. The valve releases that pressure and the temperature falls with it. The refrigerant returns to the evaporator coil and the loop starts over.
The cycle, step by step
Here is the same loop as a table, in the order it happens. One full circuit takes seconds, and it repeats for as long as the heat pump is running.
| Step | Component | What happens to the refrigerant | Result |
|---|---|---|---|
| 1 | Fan and evaporator coil | Cold liquid absorbs heat from outside air and boils into gas | Heat captured from the air |
| 2 | Compressor | Gas is compressed and its temperature rises sharply | Refrigerant now hotter than the pool |
| 3 | Titanium heat exchanger | Hot gas gives up heat to pool water and condenses | Pool water warms on every pass |
| 4 | Expansion valve | Pressure drops and the refrigerant turns very cold | Ready to absorb heat again |
Why moving heat beats making heat
Moving heat is roughly five times cheaper than making it, and that single fact is the whole argument for heat pumps. An electric element heater converts one unit of electricity into one unit of heat, and that is the physical ceiling. A pool heat pump uses one unit of electricity to run a fan and a compressor, and that effort relocates five or six units of heat that were already sitting in the air. The ratio is called COP, or coefficient of performance. A COP of 6 means six units of heat delivered per unit of electricity drawn.
That is why a 20kW heat pump does not draw 20kW from your switchboard. A unit like the Midea M Series 20kW delivers around 20kW of heat into the water while drawing a fraction of that in electricity. The full cost comparison against gas and electric element heating is in our gas vs electric vs heat pump breakdown.
Why air temperature changes the numbers
Colder air means the heat pump works harder and its COP falls, because there is less heat available to move. Manufacturers quote COP at a specific air temperature, usually around 26 degrees, and the real figure on a 14 degree August morning in Melbourne is lower. This is not a fault, it is physics, and it is the reason a heat pump sized for a Brisbane summer will disappoint in a Hobart shoulder season.
Most quality units keep working down to about 5 to 10 degrees air temperature before output falls away meaningfully. Below that they still run, just slowly and expensively. If you want a long season in a cold climate, size up rather than expect a small unit to cope. Our pool heat pump sizing guide covers how much to add for climate and season length.
Fixed speed versus inverter, and what changes inside
An inverter heat pump varies its compressor speed instead of switching the compressor fully on and off. A fixed speed unit runs flat out until the water hits target temperature, shuts down, drifts, then slams back on at full power. An inverter unit ramps down to a low idle and holds temperature there, which uses less electricity, puts less stress on the compressor and makes far less noise.
That difference matters most on long seasons, where the unit spends most of its life holding temperature rather than climbing to it. The Guardian Platinum inverter range and the Guardian Lux-V full DC inverter both work this way. A fixed speed unit like the Guardian Eco Pro 20kW costs less upfront and is the sensible pick for a summer only season.
What the heat pump needs from the rest of your setup
A pool heat pump only heats while your pool pump is pushing water through it, so the two have to run together. That is the most common reason a new owner thinks their heat pump is broken. If the filtration pump is on a four hour timer, the heat pump gets four hours a day to work, and the pool will never reach temperature.
It also needs clear air around it. The unit dumps chilled air after stripping the heat out, and if that cold air gets recirculated back through the fan the whole thing loses efficiency. Do not box it in, do not tuck it under a low deck, and keep the discharge side clear. Top discharge units are easier to place tight against a wall for this reason.
What a pool heat pump will not do
A pool heat pump will not heat your pool quickly, and no model will. Expect 24 to 72 hours to bring a cold pool up to swimming temperature from scratch, depending on pool volume, unit size and air temperature. After that it holds temperature easily. Anyone promising a same day warm up from cold is selling you something.
It also will not overcome an uncovered pool losing heat overnight. Evaporation strips more heat off an open pool surface than most people believe, and a cover is the single cheapest upgrade to any heating setup. Many units also reverse the cycle to cool a pool in peak summer, which is genuinely useful in Perth and Darwin, and every model in our pool and spa heat pump range lists whether it does.
Frequently asked questions
How does a pool heat pump work in simple terms?
A pool heat pump pulls warmth out of the outside air using a cold refrigerant, compresses that refrigerant to make it hot, then passes the heat into your pool water through a titanium heat exchanger. It moves existing heat rather than generating it, which is why it uses far less electricity than an element heater.
Does a pool heat pump work in cold weather in Australia?
Yes, down to around 5 to 10 degrees air temperature for most quality units, though efficiency drops as the air gets colder. There is still usable heat in cold air, but less of it, so the unit runs longer for the same result. In cold southern climates, size up rather than expect a small unit to keep pace.
How long does a pool heat pump take to heat a pool?
Bringing a cold pool up to swimming temperature from scratch takes 24 to 72 hours for a correctly sized unit. Holding that temperature afterwards is much easier and much cheaper. A pool cover roughly halves the time and the cost of both.
Does the pool pump need to run for the heat pump to work?
Yes, always. The heat pump only transfers heat into water that is flowing through its heat exchanger, so it does nothing while the filtration pump is off. Set your filtration pump to run for the hours you want heating, or the pool will never reach temperature.
Is a pool heat pump the same as an air conditioner?
It uses the same refrigeration cycle, running in the opposite direction for a different purpose. An air conditioner takes heat out of your house and dumps it outside. A pool heat pump takes heat out of the outside air and dumps it into your pool water.
Why is my pool heat pump blowing cold air?
Cold air out the top is normal and means the unit is working properly. The heat pump has just stripped the heat out of that air and sent it into your pool water, so what comes out the fan is several degrees colder than what went in. Warm discharge air would be the actual warning sign.
Still working out which unit suits your pool
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Chasing the quietest option? The SensaHeat TDX 14kW is built for tight suburban blocks. Heating a spa instead of a pool? Look at the SpaNet SV 9kW integrated heat pump. Or start with our Midea 20kW guide and browse the full Heat Pumps collection.
This guide is general information for Australian pool owners and is not a substitute for advice on your specific pool and site. Pool heat pumps are electrical appliances and must be connected by a licensed electrician. Confirm final sizing and placement with us or your pool professional before you buy.