Heat Pump Hot Water: Everything You Need to Know

If you're switching from a combi boiler, the biggest practical change with a heat pump is hot water. You'll need a hot water cylinder — bigger than you might expect — and a few things work differently. Here's what to plan for.

Why You Need a Cylinder

A gas combi boiler heats water on demand — you turn the tap and it fires up. A heat pump works differently. It heats water more slowly (at lower temperatures) and stores it in an insulated cylinder ready for use. This is the same principle as an old-fashioned hot water tank, but modern unvented cylinders are far more efficient and deliver mains-pressure hot water.

The cylinder is larger than a traditional one because the heat pump takes longer to reheat it. A gas boiler can heat a cylinder in 20–30 minutes. A heat pump takes 1–2 hours. The solution: store more hot water so you don't run out.

Cylinder Sizing

If you have multiple bathrooms or a large household that takes several showers in succession, err on the larger side. Running out of hot water is the main complaint from heat pump owners who chose too small a cylinder.

Where to Put the Cylinder

The cylinder needs to go somewhere accessible and reasonably close to the heat pump's indoor unit. Common locations:

• Airing cupboard: The traditional choice. A 200L cylinder fits in most airing cupboards, though a 300L may need a taller space. The residual warmth from the cylinder keeps linen warm — a genuine benefit.
• Utility room: Ideal if you have one. Easy access for maintenance and plenty of space.
• Garage: Works if insulated and not too far from bathrooms (longer pipe runs mean slower hot water delivery).
• Under stairs: Some 200L slimline cylinders fit under stairs, though access can be tight.

If you're switching from a combi boiler and don't currently have a cylinder, finding space is the biggest practical challenge. Discuss this with your installer early — it's a common concern and there are creative solutions for most homes.

Cylinder Costs

The cylinder is usually included in your heat pump installation quote. If quoted separately, budget £500–£1,500 depending on size and specification. Look for a cylinder with a large coil surface area — this lets the heat pump transfer heat more efficiently at lower flow temperatures.

The Legionella Pasteurisation Cycle

Legionella bacteria can multiply in water stored between 20°C and 45°C. Since a heat pump typically stores hot water at 45–55°C (lower than a gas boiler's 60–65°C), there is a theoretical risk that needs managing.

The solution is a weekly pasteurisation cycle: the cylinder is heated to at least 60°C once a week to kill any bacteria. In most heat pump systems, this is handled by:

• The heat pump itself: Some models can boost output to 60°C+ for the pasteurisation cycle, though this reduces efficiency temporarily
• An immersion heater: A 3kW electric element in the top of the cylinder heats the water to 60°C+ once weekly. This uses about 3–5 kWh of electricity per cycle (roughly 75p–£1.25)

Most heat pump controllers automate this — you set the day and time, and it runs without you thinking about it. The cost is minimal: roughly £50–£65 per year for the weekly boost.

Immersion Heater Backup

Every heat pump hot water cylinder should have an immersion heater (electric heating element) as backup. It serves three purposes:

• Legionella pasteurisation cycle (weekly)
• Backup heating if the heat pump needs servicing or temporarily fails
• Boost function for periods of unusually high demand (guests visiting, etc.)

The immersion heater is not your primary heat source — it's insurance. If you find yourself relying on it regularly, your heat pump or cylinder may be undersized.

Hot Water Temperature

A heat pump typically stores water at 45–55°C. This is perfectly adequate for showers and baths — you mix it with cold water at the tap anyway. It's lower than the 60–65°C most gas boilers deliver, but in practice you won't notice the difference.

The weekly legionella cycle brings the temperature to 60°C+, and the immersion heater can boost to 65°C+ if needed. For daily use, 48–52°C is the sweet spot that balances comfort with heat pump efficiency.

Combining with Solar Panels

If you have solar panels, they can heat your hot water essentially for free during sunny months. A solar diverter sends excess solar electricity to the immersion heater, heating the cylinder during the day when the heat pump doesn't need to run. In summer, your hot water can be 100% solar-heated.

Some cylinders come with a dedicated solar coil — a second heat exchanger connected to solar thermal panels. This is an older technology but still effective if you already have solar thermal installed.

Frequently Asked Questions