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Comparison

Hot tub heat pump vs the built-in heater

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In short

The built-in heater in an inflatable tub's pump unit turns electricity straight into heat, so 1 kWh in gives about 1 kWh of heat. A heat pump moves heat from the outside air into the water, so it can deliver more heat than the electricity it uses, though its output falls as the air gets colder. A heat pump can cut kWh, but it costs more to buy and needs space and suitable fittings.

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Heat pumps are often mentioned when people look at hot tub running costs. Many hard shell tubs and swim spas offer them, and add-on units are sold for inflatable tubs too. To judge whether one makes sense, it helps to understand how a heat pump differs from the electric heater already inside your tub's pump unit. This article explains both in plain terms, with energy in kWh rather than pounds, so you can apply your own unit rate.

How the built-in heater works

Inflatable tubs heat water with an electric element inside the pump unit. On most 13 A models it is rated at roughly 2 kW. Electric resistance heating turns almost all the electricity into heat in the water, so the energy used is close to the heat delivered.

Our heat-up calculator uses that fact: litres x 4.186 x temperature rise / 3600 gives the minimum kWh. For example, 800 L from 10 °C to 38 °C needs about 26 kWh before any heat loss.

01020304004812162024hours on a 2.2 kW heater°C600 L: about 9 h1,000 L: about 15 h1,000 L, cover off (slower)
Minimum heating from 10 °C to 38 °C on 2.2 kW. With the cover off, heat is lost and the climb is slower.

How a heat pump works

A heat pump uses a compressor and refrigerant to take heat from the outside air and pass it into the water. Because it moves heat rather than making it, it can deliver more heat than the electricity it uses. The ratio is called the coefficient of performance, or COP, and makers quote it at set air and water temperatures.

COP falls as the air gets colder, and many heat pumps have a minimum working air temperature. In a cold UK winter they may work less efficiently or switch to a backup heater. Check the stated working range before buying.

Side by side

PointBuilt-in electric heaterAir source heat pump
Heat per kWh usedAbout 1 kWhMore than 1 kWh, varies with air temperature
Cold weatherOutput steadyEfficiency falls as air cools
IncludedYesBought and fitted separately
SpaceInside the pump unitA separate outdoor unit with airflow
NoiseQuietA fan and compressor, like a small outdoor unit
ConnectionBuilt inHoses or pipework to the tub's circuit
Built-in heater and heat pump compared

Fitting a heat pump to an inflatable tub

Add-on heat pumps for inflatables connect into the water circuit with hoses and adapters. Not every tub has suitable connections, and some makers say an add-on voids the warranty, so check your tub's manual and the heat pump maker's compatibility list first.

A heat pump also needs its own power supply. Some run from a 13 A plug; others need a dedicated circuit. Do not run the tub and a heat pump from one extension lead.

Note:Hot tubs combine water and mains electricity. Use a weatherproof outdoor socket protected by an RCD, follow the maker's manual, and have any new circuit or socket fitted by a qualified electrician.

When does a heat pump make sense?

If you use the tub only now and then, better insulation usually gives a quicker win. See how to insulate an inflatable hot tub and running costs in the UK.

  • You run the tub year round and keep it hot all the time.
  • Your tub is large, so the heater runs for many hours.
  • You have room for an outdoor unit with clear airflow, away from neighbours' windows.
  • Your tub's maker allows add-on heating, or you are buying a tub designed for one.

Before you buy

Look up your tub's water volume and heater rating, estimate how many hours a week the heater runs, and work out kWh. Then compare with the heat pump's stated output and COP at temperatures you will actually see. Ask the seller about frost protection and what happens below its minimum air temperature.

If you are choosing a new tub with winter use in mind, insulated walls and a good cover matter as much as the heating method. Our winter tub guide focuses on those features.

See tubs suited to winter →

Frequently asked questions

Does a heat pump heat a hot tub faster? Not necessarily. It depends on the heat pump's output in kW at the current air temperature compared with the built-in heater.

Will a heat pump work in a UK winter? Many do, but efficiency falls in cold air and some stop below a set temperature. Check the working range.

Can I use a heat pump with a Lay-Z-Spa? Some add-on units are sold for inflatable tubs. Check Bestway's guidance and the heat pump's compatibility before connecting one.

Is a heat pump worth it for occasional use? Usually less so. Insulation and keeping the cover on tend to give better value when the tub is used now and then.

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