Do You Need a New Hot Water Cylinder for a Heat Pump?

Short answer: almost certainly yes. If you currently have a combi boiler, you will definitely need a new cylinder. If you already have a cylinder, it will probably need replacing with a heat pump-compatible model. The reasons come down to physics — heat pumps work at lower temperatures than boilers, and they need a different type of cylinder to deliver hot water efficiently.

This guide explains when you need a new cylinder, when your existing one might work, what the differences are, and how cylinder choice affects your overall heat pump installation.

Why Heat Pumps Need a Different Cylinder

A gas boiler heats water to 60-80°C and pushes it through a relatively small coil inside the cylinder. The high temperature means heat transfers quickly through even a small coil surface area. A heat pump heats water to just 45-55°C — sometimes as low as 35°C for space heating. At these lower temperatures, you need a much larger coil surface area to transfer the same amount of heat in a reasonable time.

A standard boiler cylinder's coil is simply too small for a heat pump. Using one would mean agonisingly slow hot water recovery times and poor efficiency. Heat pump-specific cylinders solve this with oversized coils — often twice the surface area of a boiler cylinder's coil.

Your Situation: What You Will Need

Key Features of a Heat Pump Cylinder

• Oversized coil: 2-3m² surface area versus 1-1.5m² in a standard cylinder
• High insulation: 50mm+ of foam insulation to minimise standby heat loss
• Correct sizing: 150-300 litres depending on household size (see our cylinder sizing guide)
• Immersion heater backup: For emergency use and periodic legionella pasteurisation cycles
• Unvented (mains pressure): The most common choice for heat pump installations, providing strong water pressure at every tap

Finding Space for a Cylinder

If you are coming from a combi boiler, finding space for a cylinder is often the biggest challenge. A typical 200-litre cylinder needs approximately 55cm x 55cm of floor space and 160cm of height. Common locations:

Your installer will assess possible locations during the survey. In some homes, a bedroom cupboard or a corner of the bathroom works when other options are not available.

What It Costs

A heat pump-compatible cylinder typically costs £800-£1,500 for the unit, plus £300-£600 for installation. This is included in your overall heat pump installation quote and covered by the £7,500 BUS grant.

If you are also considering solar panels, a twin-coil cylinder can harness both heat pump and solar thermal energy, maximising renewable hot water production.

Frequently Asked Questions

Can I use my existing cylinder with a heat pump?

Sometimes, but only if it has a large enough internal coil designed for heat pump temperatures (35-55°C). Most older cylinders have smaller coils designed for boiler temperatures. Your installer will assess this during the survey.

Do I need a cylinder if I have a combi boiler?

Yes. If you are switching from a combi boiler (which heats water on demand), you will need a cylinder added as part of the installation. This is one of the biggest practical changes when moving from a combi to a heat pump.

How long does it take to heat a cylinder with a heat pump?

A 200-litre cylinder typically takes 1-2 hours to reheat from cold using a heat pump, compared to 30-45 minutes with a boiler. Most homeowners schedule heating overnight or during off-peak electricity periods.