Heat Pump Hot Water: Legionella, Cylinder Sizing and Temperature Settings

Hot water is often the most technically complex part of a heat pump installation, and it is also where things most commonly go wrong. Getting cylinder sizing, temperature settings, and legionella prevention right is critical for both comfort and safety. Many homeowners find that hot water works differently with a heat pump than with a gas boiler — here is everything you need to understand before and after installation.

Why heat pumps need a dedicated hot water cylinder

Most combi gas boilers heat water on demand — no cylinder needed. Heat pumps cannot do this: they produce hot water at low flow rates over extended periods, which requires a storage cylinder to accumulate enough volume for household use. A heat pump without a cylinder would take 30–60 minutes to fill a bath; with a correctly sized cylinder, hot water is available immediately on demand.

If your home already has a hot water cylinder (any system boiler or regular boiler installation), this can sometimes be reused with a heat pump — provided the cylinder is unvented or can be adapted, is large enough for the household, and has an appropriate coil surface area for the heat pump's lower temperature output. In practice, many cylinders are replaced as part of the heat pump installation to ensure correct specification.

Cylinder sizing: how much do you need?

Undersizing is a common installer mistake. A 150-litre cylinder for a family of four means running out of hot water after two showers. The heat pump will struggle to reheat a depleted cylinder quickly enough for morning demand. When specifying your installation, push for at least 200 litres for a three-person household, and 250 litres if there are four or more regular occupants.

Target water temperature: the 55°C rule

Heat pumps are most efficient when producing hot water at moderate temperatures. However, hot water stored below 60°C can harbour Legionella bacteria — a serious health risk. The standard approach is to store the cylinder at 55°C continuously, with a periodic legionella pasteurisation cycle to 60°C or above.

Running the cylinder at 55°C represents a compromise: it is high enough to prevent legionella growth at normal cylinder temperatures, but lower than the 60–65°C target used with boilers. Some heat pumps struggle to achieve 55°C efficiently — particularly at very low outside temperatures — and will use the backup immersion heater to reach this temperature. This is normal and necessary for hygiene, but it increases electricity consumption.

Legionella pasteurisation cycles

Even with a 55°C storage temperature, best practice is to run a scheduled legionella pasteurisation cycle that heats the entire cylinder to 60°C or above (typically 65°C is the target) at least once per week. Most heat pump controllers include a programmable legionella cycle — usually a Sunday morning or similar off-peak time when electricity is cheapest.

During a pasteurisation cycle, the heat pump runs at maximum output and the immersion heater element assists to reach 65°C. This typically uses 3–6 kWh of additional electricity. At 24.5p/kWh, the cost is approximately 73p–£1.47 per pasteurisation cycle — around £38–76 per year. This cost should be factored into running cost comparisons.

Heat pump cylinders vs standard cylinders

A standard vented copper cylinder is not ideal for heat pump use. A heat pump cylinder (also called a thermal store or dedicated heat pump cylinder) has a larger coil surface area in the lower section of the cylinder, allowing the heat pump to transfer heat effectively at lower flow temperatures (45–55°C rather than the 70–80°C used by boiler coils). Using a standard cylinder with a small coil forces the heat pump to run at a higher flow temperature to achieve the same heat transfer rate, reducing efficiency.

Reputable heat pump installers will specify a cylinder that is matched to the heat pump's output. If an installer is proposing to reuse your existing boiler cylinder, ask them to confirm the coil surface area is adequate for the heat pump model being installed — the manufacturer's specifications will state the minimum coil size required.

Smart controls for hot water

Modern heat pump controllers allow you to schedule hot water heating times to align with cheap electricity periods. If you are on a smart time-of-use tariff like Octopus Cosy or Agile, programming the cylinder to heat during cheap hours (and using the thermal mass of 250 litres of hot water to carry you through expensive peak periods) can significantly reduce the cost of hot water production. The same principle that makes EVs charge overnight applies to heat pump hot water cylinders.

Some controllers also allow "smart boost" — a manual command that triggers an immediate hot water heat cycle when you know you will need extra hot water (before a party, or after heavy use). Familiarising yourself with these controls at commissioning is time well spent; most installers will demonstrate them as part of the handover process.