Heat Pump Hot Water Cylinder Sizing

The short answer

A heat pump needs a bigger cylinder than a gas boiler did, and one with a much bigger coil. For most UK households:

A common cross-check is 35–45 litres per occupant plus an allowance for a second bathroom. Your installer sizes formally under MCS 011; these bands are for sanity-checking the quote.

The number that matters at least as much is the coil surface area. A cylinder built for a gas boiler typically has a coil of 0.8–1.5m². A heat pump cylinder needs at least 1.5m² and usually 2.0 to 3.0m² for systems up to about 12kW. Fit a heat pump to an old boiler cylinder and it will work badly no matter how many litres it holds.

Why a heat pump needs a bigger coil

A gas boiler pushes water round the coil at 70–80°C. A heat pump runs at 50–55°C for hot water, because pushing higher wrecks its efficiency. Less temperature difference across the coil means less heat crosses it per minute.

The only way to get the same heat into the tank at a lower temperature is more surface area. That is why heat pump cylinders are physically taller and why swapping the heat source without swapping the cylinder is the most common expensive mistake in a retrofit. Symptoms are a reheat that takes half a day, a heat pump that never seems to switch off, and a bill nobody expected.

How long it takes to reheat

Reheat time is arithmetic, and worth doing yourself before you accept a design:

Hours ≈ (litres × temperature rise in °C × 4.19) ÷ (coil kW × 3600)

Take a 210-litre cylinder going from 10°C to 50°C, a 40-degree rise. That is 210 × 40 × 4.19 = 35,196 kJ, which is 9.8kWh of heat. Through a coil transferring 9kW, that is about 65 minutes. Through a coil managing only 3kW, because it is an old boiler cylinder, it is over three hours.

A well-matched heat pump cylinder should reheat fully in roughly 45 to 90 minutes. If a quote implies longer, the coil is the thing to question.

What a tank of hot water costs

The 210-litre example above needs 9.8kWh of heat. Heat pumps are less efficient making hot water than heating rooms, because of the higher flow temperature, so assume a hot water efficiency around 2.5 rather than the 4.0 you might see for space heating:

• 9.8kWh of heat ÷ 2.5 = 3.9kWh of electricity
• 3.9 × 26.11p = about £1.02 for a full tank from cold

In practice you rarely start from cold, so a daily reheat costs less than that. Shifting the reheat into a cheap overnight window drops it further, which is the single easiest saving available on a heat pump system.

Legionella and the immersion

Because heat pumps store water at 50–55°C rather than 60°C+, every heat pump cylinder runs a periodic pasteurisation cycle, typically weekly, lifting the whole tank above 60°C to kill off legionella. Most systems do this with the built-in immersion heater on a timer, and it is set up at commissioning.

Two practical points. First, that cycle uses direct electric at full price, so schedule it inside a cheap window if you have one. Second, do not disable it to save money. It is a health measure, not a comfort setting.

Getting the size right, not the biggest

Oversizing looks like the safe error and is not free. A bigger tank has more surface area losing heat to the airing cupboard every hour of every day, and it takes longer to bring up to temperature. Undersizing gives you a household that runs out mid-shower and starts using the immersion as a top-up, which is the expensive way to heat water.

• Count simultaneous demand, not people. Two bathrooms used back to back at 7am is the design case, not the total occupancy.
• Ask about the standing loss figure. Every cylinder has one, in kWh per day, and a well-insulated modern unit is meaningfully better than a cheap one.
• Check it physically fits. Heat pump cylinders are taller for the same volume. Measure the cupboard, including getting it through the loft hatch or up the stairs.
• Confirm the coil area in writing. Litres are on every quote; coil area often is not. Ask for it.