Buying a Pool Heat Pump? Look Past the Kilowatts

If you want to heat a pool with electricity, nothing beats a heat pump for efficiency, and that comes down to physics rather than sales talk. The catch is that “heat pump” is a very broad label. At one end sits a basic on/off machine that hammers away at full power until it reaches its set point; at the other is a full-inverter unit that barely sips electricity once the water is warm.

Either one will warm your water. Only one will do it without a nasty surprise on the monthly municipal or Eskom bill. Below we look at the three questions that should drive your choice: how efficiently the unit turns electricity into heat, how quickly it will lift your pool’s temperature in real local conditions, and what you need to fit alongside it so that heat isn’t simply lost again overnight. The two Fairland ranges our branches carry make useful case studies, because they sit at different rungs of the efficiency ladder and comparing them shows exactly why the spec sheet matters.

How a heat pump warms water (it doesn’t generate the heat)

Unlike the element in a kettle, a pool heat pump doesn’t produce heat from electricity. It relocates heat that’s already around it. A fan draws outside air over a refrigerant-filled coil; the refrigerant soaks up warmth from that air; a compressor pressurises it so its temperature climbs; and a titanium heat exchanger then passes that heat into the pool water flowing through the unit. The air comes out the back a few degrees colder, and the water goes back to the pool a little warmer.

Why this is important: your electricity only drives the fan and the compressor. The heat itself was sitting in the air at no cost. That’s how a heat pump can deliver several kilowatts of warmth into your pool for each kilowatt it pulls from the socket, and that ratio is exactly what COP describes.

COP explained: the figure that predicts your running cost

COP is short for Coefficient of Performance, and the sum behind it is simple: heat delivered divided by electricity consumed. A unit with a COP of 5 puts 5 kW of heat into the water for each 1 kW of electricity it uses. An electric resistance heater, the kettle-element method, sits at a COP of 1: one kilowatt paid for, one kilowatt of heat received, and not a watt more.

That means the kilowatt figure printed on the carton tells you the unit’s size, while the COP tells you what it will cost to run. Picture two 13 kW heat pumps, one with a COP of 5 and one with a COP of 9. They’ll heat your water equally quickly, yet the second one gets there using only slightly more than half the electricity.

Reading the small print properly

That note under the figures isn’t there to be skipped, so here’s what it means in everyday terms. A heat pump’s COP isn’t a fixed number. It shifts with the weather and with the workload. Warm, humid air holds more heat to collect, so COP rises; cold, dry air holds less, so it falls. A frosty August morning on the Highveld offers a heat pump far slimmer pickings than a humid February afternoon in Durban, Ballito or Grand Baie.

Workload matters just as much. An inverter heat pump idling at 20% of capacity to keep an already-warm pool topped up is far more efficient than the very same unit at 100% trying to raise a cold pool by ten degrees. That’s the reason inverter manufacturers quote their best COP at part load. It isn’t sleight of hand, because part load is where these machines spend most of their running hours once the pool is warm. Still, the fair way to put it is this: the lowest COP of a full-inverter unit, working at full speed on a cool day, lands roughly where a basic on/off unit’s highest COP does.

Kilowatts decide how quickly your pool warms. COP decides what that warmth costs. You’re paying for both.

Three kinds of heat pump: on/off, inverter and full inverter

Pool heat pumps fall into three broad groups, and which group a unit belongs to tells you more than the name on the casing.

Most people imagine an on/off unit when someone mentions a pool heat pump, and that’s where the reputation for noise and power hunger comes from. If a neighbour’s heat pump has ever woken you at 7 am, odds are it was an on/off model. It’s also why you won’t find that type in the heat pump line-up we recommend.

What a heat pump can realistically deliver (and what it can’t)

This is the part that seldom comes up at the sales counter. A heat pump doesn’t guarantee a water temperature. What it gives you is a gain over ambient: a certain number of degrees above the average air temperature that it can maintain once the pool is warm. Ask for 40°C in the middle of July and the unit will run at full tilt, run up a big bill and still never reach it. That’s not a defect; it’s simply physics.

Here’s why. The warmer your pool is compared with the surrounding air, the faster it sheds heat. A 28°C pool on a 20°C day loses heat at a steady rate. That same pool on a 5°C night loses it several times faster, because the temperature difference it’s leaking across is more than three times larger. Eventually the heat the unit adds and the heat the pool loses even out, and that balance point is where your water temperature settles. Anything that widens the gap (cold air, wind, no blanket, a unit that’s too small) drags the balance point lower. Anything that narrows it (a blanket, shelter from wind, a larger or more efficient unit, warmer weather) lifts it.

So rather than asking “how warm will this make my pool?”, the better question is “how many degrees above the outside air can it sustain, and does it keep doing that in winter?” With a correctly sized inverter unit and a blanket on overnight, the answer for most of South Africa falls between 8 and 12°C above the average daily air temperature. During a Johannesburg summer, when the average is roughly 20°C, that gives you a lovely 28 to 30°C pool. In a Johannesburg July, when the average drops to about 10°C, the same setup keeps the water in the low twenties: fine for the brave, but not a heated spa. A unit that can’t run in cold air, or that has only one speed and can’t work efficiently for long stretches, loses much more of its gain as winter arrives.

A heat pump adds degrees on top of the weather. It can’t switch the weather off.

That’s why the comparison table further down includes a column for sustainable gain over ambient, and why that column deserves more of your attention than the kilowatt rating. It also explains why units that keep working in sub-zero air and can throttle right down hang on to their gain through winter, while conventional units let it slip.

Warm-up time: let’s do the maths

Everyone wants to know how quickly the water will warm up, and straight answers are rare, so let’s run the numbers. Warming water is entirely predictable: raising 1,000 litres by 1°C takes about 1.16 kWh of heat. Everything else follows from that one figure.

Two practical lessons come out of those numbers. The first is that a heat pump isn’t instant heat. It’s a steady, long-haul appliance that performs best running for many hours at low speed to maintain a temperature rather than racing to hit one. Give the first warm-up a few days, then let the inverter drop into maintenance mode, where its efficiency really pays off.

The second lesson matters even more: the biggest single influence on how quickly your pool warms and how much it costs to keep it warm isn’t the heat pump. It’s whether you cover the pool.

The solar blanket: the other half of the system

Here’s the part of pool heating people would rather not hear. Most of a pool’s heat escapes through the water’s surface, chiefly by evaporation, and every litre that evaporates carries a sizeable amount of heat away with it. According to the US Department of Energy, evaporation is by far the biggest cause of heat loss from a pool, and a pool cover can lower heating costs by 50% to 70%.¹

Now apply that to the sums above. A heat pump adds roughly 5°C a day to an uncovered pool, then watches much of that drift away overnight. With a solar blanket on, the gains stay put. The same unit reaches temperature in about half the time and then holds it while barely turning over, which is exactly the low-speed, high-COP mode where an inverter heat pump proves its worth.

• It keeps the heat you’ve paid for in the water. A 500 micron blanket on the pool overnight sharply reduces evaporation, which would otherwise swallow most of your heat pump’s effort.
• It collects free heat in the daytime. A quality solar blanket soaks up sunshine and transfers that warmth into the pool, often adding several degrees before the heat pump even starts.
• It can mean a smaller heat pump. A covered pool has a far lower heat load, which can be the difference between needing the next size up and getting away without it.
• It trims running costs from both sides. With less heat to replace, the compressor runs fewer hours, and the hours it does run are at lower speed and higher COP.
• It saves water and chlorine as well. Less evaporation means less topping up, and less sun on the water means chlorine burns off more slowly.

Put simply: install a heat pump without a solar blanket and the running cost will disappoint you, and the heat pump won’t be to blame. That’s why we suggest the two together every time. Our recommended cover is the TriTherm 500 micron solar blanket, and adding a roll-up station from the solar blankets and roll-up stations range turns “I really should cover the pool” into a half-minute routine you’ll actually stick to every evening.

Fairland against the rest: a side-by-side comparison

We recommend Fairland because, on the specs that count, nothing else at a similar price comes near it. That’s a strong statement, so here’s the evidence next to two typical alternatives on the South African market: a premium full-inverter model from a big international brand that goes head to head with the InverX20, and a popular conventional heat pump from a familiar local air-conditioning brand, the sort that sells in large numbers at about the same price as a Comfortline. All figures come from each manufacturer’s own published specifications, measured at the same standard 26°C to 27°C air and water conditions.

Gain over ambient is how far above the average daily air temperature a unit can keep the pool once it’s warm, assuming a solar blanket goes on overnight and the unit is matched to the pool size. It reflects practical experience from installations around the country rather than a laboratory measurement. Pools that are uncovered, windswept or fitted with an undersized unit will see less.

Start with the first row. At peak efficiency the InverX20 isn’t marginally ahead: it’s a third more efficient than the best rival full inverter with a published figure we could find, and over three times as efficient as the conventional units most buyers end up with. In cold weather it’s in a class of its own, carrying on heating in air ten degrees colder than its nearest competitor can handle. If low running costs and year-round swimming are why you’re buying a heat pump, this is the benchmark. It’s also the only model in the table that keeps its full gain over ambient through a Highveld winter instead of tailing off as temperatures drop.

Next, compare the Comfortline with its real price rival. Competitor B represents what most South African pools end up with: a dependable, conventional on/off heat pump from a well-known name, sold through air-conditioning and hardware outlets for roughly what a Comfortline costs. With a single speed, its COP of about 6 is as good as it ever gets. The Comfortline’s peak of 9 is 50% higher, and since it can throttle down to 20% output, it spends most of its running time in the low-speed, high-efficiency band that a single-speed unit simply can’t reach. Factor in a spiral titanium exchanger with 30% more surface area, operation up to seven times quieter, and the same build quality as the InverX20, and we’d argue the Comfortline is the best heat pump available at its price, and one of very few genuine inverters in that bracket at all.

Within its price bracket, nothing beats the Comfortline. Across every bracket, nothing beats the InverX20.

Meet the two Fairland ranges

There are two Fairland ranges in our line-up because there are two kinds of buyer. One wants a sensible, efficient heat pump for an ordinary family pool. The other wants the lowest possible running cost and the quietest possible operation, and is happy to spend more at the start to get them.

Comfortline or InverX20?

For the majority of pools in the majority of regions, the Comfortline is the one to buy. It’s a genuine inverter with a strong COP, it runs quietly, and its six sizes cover everything from a small plunge pool to a very large family pool. The InverX20 costs noticeably more up front, and it earns that premium back in two cases: when you plan to keep heating through the coldest months of a Gauteng or Free State winter, where its sub-zero operating range and better cold-weather efficiency really count, or when the unit will stand close to a bedroom window or a neighbour’s boundary wall and quiet running is worth paying for. If neither applies to you, spend the difference on a blanket and roll-up station instead, and you’ll swim in warmer water for less.

Five habits for a happier heat pump

Even the best heat pump depends on how it’s installed and how you use it. These points are what separate satisfied owners from frustrated ones.

1. Let it breathe

   The unit warms your pool by extracting heat from the air flowing through it, so it needs plenty of airflow. Keep the space in front of the fan outlet and around the intake clear, never enclose it, and make sure it can’t draw its own cold exhaust back in. A heat pump squeezed into a corner under a lean-to is fighting with one hand tied behind its back.

2. Plumb in a bypass

   The heat pump is connected after your filter on the return line, with a bypass that lets you control how much water passes through it and isolate it for servicing or draining for winter. Too much flow is just as unhelpful as too little; your installer will set the bypass so the exchanger gets the flow rate the unit was designed for.

3. Run the filter pump with it, and for longer

   A heat pump heats only while water is moving through it, so your filter pump must be on whenever the heat pump is. Inverter models prefer long hours at low speed over short blasts at full power. If your pump runs on a timer, lengthen the schedule. A variable-speed pool pump makes this easy and inexpensive.

4. Choose a realistic set point and stick with it

   Each degree above 28°C costs disproportionately more to maintain, because the gap between water and air grows and the pool sheds heat faster. Decide on the temperature you genuinely enjoy swimming in, set it once and avoid creeping it upwards. The app makes it simple to lower it a few degrees while you’re away.

5. Cover the pool every single night

   If you remember only one thing from this guide, let it be this. The blanket decides whether your heat pump quietly holds temperature on a trickle of electricity or battles the night sky and loses. With a roll-up station, covering the pool takes less time than brushing your teeth.

Heat pump questions we hear at the counter