Home Heat Pump Guide

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

If you currently have a combi boiler, switching to a heat pump means you will almost certainly need a hot water cylinder. Unlike a combi boiler, which heats water on demand at high temperatures, a heat pump heats water more slowly and at lower temperatures — so it needs somewhere to store that hot water until you need it.

This is one of the most common surprises for homeowners considering a heat pump installation. The good news is that modern hot water cylinders are well-insulated, efficient, and available in sizes to suit most homes. This guide covers everything you need to know: whether you need one, what size to choose, how much it costs, and where it can go.

Why Heat Pumps Need a Hot Water Cylinder

A combi boiler heats water instantly to around 55-65°C when you turn on a tap. It does not need a storage tank because it produces hot water on demand at a very high rate.

A heat pump works differently. It heats water more gradually and operates most efficiently at lower flow temperatures — typically 45-55°C for domestic hot water. Because it cannot produce a burst of very hot water the way a gas boiler can, it needs to heat a large volume of water in advance and store it ready for use.

This is why a hot water cylinder is essential for the vast majority of heat pump installations. Without one, you would not have enough hot water for showers, baths, or running multiple taps.

What If You Already Have a Cylinder?

If you already have a traditional hot water cylinder — for example, if you have a system boiler or a regular (conventional) boiler — you may be able to keep it. However, there are some important considerations:

  • Size: Your existing cylinder may be too small for a heat pump system. Heat pumps work best with larger cylinders because they heat water more slowly.
  • Coil size: Heat pump cylinders need a larger internal coil (heat exchanger) than those designed for boilers. The larger coil compensates for the lower flow temperature from the heat pump.
  • Insulation: Older cylinders may have poor insulation, which wastes the energy your heat pump has worked to produce.
  • Condition: If your cylinder is more than 15 years old, replacing it makes sense regardless.

In practice, most installers recommend fitting a new cylinder specifically designed for heat pump use. The cost difference between reusing an old cylinder and fitting a proper heat pump cylinder is relatively small compared to the overall installation cost, and the performance benefit is significant.

What Size Hot Water Cylinder Do You Need?

The right cylinder size depends on how many people live in your home and how much hot water you use. Here are general guidelines:

  • 1-2 people: 150 litres
  • 2-3 people: 200 litres
  • 3-4 people: 250 litres
  • 4-5 people: 300 litres
  • 5+ people or high usage: 300-400 litres

The most common size for UK families is 200-250 litres. If in doubt, go slightly larger rather than smaller. A bigger cylinder does not cost significantly more to run — the heat pump only heats the water you use — but it does mean you are less likely to run out during peak usage times.

Why Heat Pump Cylinders Are Bigger Than Boiler Cylinders

With a boiler, a 120-litre cylinder is often sufficient because the boiler can quickly reheat water if it runs low. A heat pump takes longer to reheat, so a larger cylinder acts as a buffer, ensuring you always have enough hot water available.

Your installer will calculate the exact size based on your household's hot water demand as part of the commissioning process. This is not guesswork — MCS standards require a proper hot water demand calculation.

Types of Hot Water Cylinder for Heat Pumps

Unvented Cylinders

Unvented cylinders are sealed systems connected directly to the mains water supply. They deliver hot water at mains pressure, which means strong, consistent flow from showers and taps. They are the most popular choice for heat pump installations because:

  • No cold water tank needed in the loft
  • Better water pressure throughout the house
  • More hygienic — no open water storage
  • Can be installed almost anywhere in the home

The main requirement is that your incoming mains water pressure must be adequate — typically at least 1.5 bar. Most UK homes meet this requirement.

Vented Cylinders

Vented cylinders work with a cold water storage tank, usually in the loft. Water is gravity-fed, so pressure depends on the height difference between the tank and the taps. They are simpler and cheaper but generally deliver lower water pressure.

Vented cylinders are still used in some heat pump installations, particularly in older properties where the plumbing is already set up for a gravity system and the homeowner does not want to upgrade.

Heat Pump Specific Cylinders

Several manufacturers produce cylinders specifically designed for heat pumps. These feature:

  • Oversized coils: Up to twice the surface area of standard boiler cylinders, allowing efficient heat transfer at lower flow temperatures
  • Enhanced insulation: Thicker insulation to minimise standing heat losses
  • Dual coil options: A second coil for solar thermal panels if you have or plan to add them
  • Immersion heater boss: A connection point for a backup electric immersion heater

How Much Does a Hot Water Cylinder Cost?

The cost of a hot water cylinder for a heat pump depends on the type, size, and brand:

  • Standard unvented cylinder (200L): £500-£900
  • Heat pump specific cylinder (200L): £700-£1,200
  • Heat pump specific cylinder (300L): £900-£1,500
  • Premium brands (Mixergy, OSO, etc.): £1,200-£2,000

Installation labour adds £300-£600 on top, depending on the complexity of the pipework and whether the cylinder is replacing an existing one or being installed in a new location.

In most cases, the cylinder cost is included in your overall heat pump installation quote. It is also covered by the BUS grant, so you should not have to pay for it separately.

Where Does the Cylinder Go?

Finding space for a hot water cylinder is one of the biggest concerns for homeowners switching from a combi boiler. Here are the most common locations:

Airing Cupboard

If you have an existing airing cupboard, this is the obvious choice. A 200-litre cylinder is typically around 1,200mm tall and 550mm in diameter — it fits comfortably in most standard airing cupboards.

Utility Room or Garage

If you do not have an airing cupboard, a utility room, large kitchen cupboard, or garage can work well. The cylinder needs to be in a frost-free location if placed in a garage.

Under the Stairs

The space under the stairs is often underused and can accommodate a slimline or compact cylinder. Some manufacturers offer shorter, wider cylinders designed specifically for low-height spaces.

Bedroom Cupboard

Modern cylinders are very well insulated and virtually silent, so installing one in a built-in wardrobe or bedroom cupboard is entirely feasible. The gentle warmth can even help air clothes.

Loft Space

Possible but generally not recommended due to access issues and frost risk. If the loft is well-insulated and boarded, it can work, but servicing becomes more difficult.

Legionella Protection and Heating Cycles

Legionella bacteria can grow in water stored between 20°C and 45°C. Because heat pumps operate at lower temperatures than boilers, legionella protection is an important consideration.

Most heat pump systems address this by running a periodic pasteurisation cycle — heating the cylinder to 60°C once a week using either the heat pump itself (which can do this but less efficiently) or a built-in immersion heater. This kills any bacteria that may have built up.

Your installer will programme this cycle during commissioning. It typically runs overnight when electricity is cheapest, and the energy cost is minimal — perhaps £1-£2 per month.

Smart Cylinders and Energy Optimisation

Some modern cylinders include smart features that can further improve efficiency:

  • Mixergy: Heats water from the top down, so you get usable hot water faster and waste less energy heating the full tank
  • Integrated sensors: Monitor water temperature at multiple levels, allowing the heat pump controller to optimise heating schedules
  • Time-of-use tariff integration: Heat water when electricity is cheapest (e.g., overnight on an Economy 7 or Octopus Agile tariff)

Pairing a smart cylinder with a smart thermostat and time-of-use tariff can significantly reduce your running costs.

Common Concerns About Hot Water Cylinders

Will I Run Out of Hot Water?

With the correct size cylinder, running out of hot water is rare. A 200-litre cylinder heated to 50°C provides enough hot water for two to three consecutive showers. The heat pump will also be reheating the cylinder continuously, so it recovers as you use water.

Will It Take Up Too Much Space?

A 200-litre cylinder is about the size of a large suitcase stood upright. Slimline models are available for tight spaces. Many homeowners are surprised at how little room it actually takes.

Is the Water Safe to Drink?

Water from an unvented cylinder is mains-fed and suitable for drinking, just like water from a combi boiler. Water from a vented system (with a loft tank) is generally not considered potable.

Installation Considerations

When your installer plans the cylinder installation, they will consider:

  • Pipework routes: The cylinder needs to connect to the heat pump, the cold water supply, and the hot water distribution pipes
  • Structural support: A full 200-litre cylinder weighs around 220kg — the floor must be strong enough to support it
  • Expansion vessel: Unvented cylinders need an expansion vessel (or have one built in) to accommodate water expansion when heated
  • Discharge pipe: Safety valves require a discharge pipe routed to an external drain
  • Access for servicing: Leave enough clearance around the cylinder for annual maintenance

All of these are standard considerations that any MCS-certified installer will handle as part of the installation process.

Frequently Asked Questions

Can I use a heat pump without a hot water cylinder?

In theory, some heat pumps can provide limited instant hot water, but in practice, virtually all UK installations require a cylinder. Without one, you will struggle to get adequate hot water for showers and baths.

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

Typically 2-4 hours to fully heat a 200-litre cylinder from cold. In normal daily use, the heat pump tops up the cylinder regularly, so you rarely start from completely cold water.

Do I need an immersion heater as well?

Most heat pump cylinders include an immersion heater connection as standard. It serves as backup and runs the weekly legionella cycle. It is not needed for day-to-day hot water production.

What happens during a power cut?

The hot water stored in the cylinder remains hot for many hours thanks to the insulation. A well-insulated cylinder loses only about 1-2°C per hour, so you will still have hot water for some time.

Can I keep my existing copper cylinder?

Possibly, but it depends on its size, coil size, and condition. Most installers recommend a purpose-built heat pump cylinder for best performance. The cost difference is modest relative to the overall installation.

Is a hot water cylinder covered by the BUS grant?

Yes. The BUS grant covers the complete heat pump system, including the hot water cylinder, pipework, and controls.

Need help choosing the right cylinder for your heat pump? Get free quotes from MCS-certified installers who will assess your hot water needs and recommend the right size. Check our full cost guide for realistic installation prices, or use the suitability checker to see if a heat pump suits your home.