Hot Water Cylinders for Heat Pumps

If you are moving from a combi boiler to a heat pump, the single biggest change inside your home is likely to be the appearance of a hot water cylinder. Unlike a combi, a heat pump does not provide hot water instantly, so a cylinder is needed to store water heated by the heat pump ready for when you need it¹. Most heat pump manufacturers require a cylinder precisely because it lets the machine run efficiently: heating water slowly over a longer period suits the way a heat pump works². Government guidance says the same thing: to provide hot water, heat pumps require a water storage cylinder, similar to older generations of gas boiler systems³.

For homes with a combi boiler today, this is a genuine change of layout. Which? advises that if you are replacing a combi boiler with a heat pump, you will probably need to install a hot water tank or cylinder⁴, and Citizens Advice tells combi households the same: you will need to install a hot water cylinder to heat and store your hot water⁵. The same applies to ground source systems, where a home without an existing tank needs a suitable space for a new storage cylinder⁶. The exception is a hybrid setup: where a heat pump is coupled with a combination boiler, there is no need to install a water cylinder for domestic hot water⁷.

Why a heat pump cannot deliver hot water on demand

A combi boiler heats water the moment a tap opens, which is why combi homes have no cylinder at all. A heat pump works differently. Its heat comes from a refrigerant cycle, a process that moves warmth rather than generating it in a burst¹³, and the heat it produces is generally at a lower temperature than a traditional heating system¹⁴. That lower temperature is a feature, not a flaw: it is what makes a heat pump cheap to run for space heating, and it is why heat pumps pair naturally with underfloor heating¹⁴. But it also means hot water cannot be produced in the explosive rush a combi manages.

So the cylinder does the waiting. Your installer will usually fit a hot water cylinder to store water that has been heated by the heat pump, ready for when you need it¹⁵. Energy Saving Trust describes the cylinder as the usual way of storing hot water for a standard heat pump⁸. The heat pump heats the stored water gradually, typically during cheaper or quieter periods, and the household draws on that store through the day.

The practical consequence for a combi household is space. Indoors, the home will require space for a hot water cylinder if you do not already have one⁹, and for a ground source system you need space for the heat pump itself and a hot water cylinder¹⁶. This is the trade a combi home makes: instant hot water with no storage, replaced by stored hot water that needs a cupboard. For a household thinking about energy independence, the cylinder is also a small buffer of its own: a full store of hot water is hot water you already own, rather than gas you are about to burn.

What is different about a heat pump cylinder

A cylinder designed for a heat pump is not the same animal as the old copper tank that sat beside a boiler. The starting point is temperature. Most heat pumps heat hot water to 50 to 55°C very efficiently, and while many heat pumps can provide hot water over 60°C consistently, doing so all the time costs efficiency⁹. A cylinder built for a heat pump is designed around those lower everyday temperatures, with a sterilisation cycle raising the temperature periodically when needed⁹.

Because the incoming heat is gentler, the heat exchange surface inside matters more. Where another low-temperature heat source is added to a home, guidance on solar water heating notes that a larger hot water cylinder is likely needed and this will take up extra room¹⁷. The same logic applies to heat pumps: more stored volume and more exchange surface compensate for lower flow temperatures. Our page on whether heat pumps need a larger hot water tank or coil covers this in detail.

Insulation is the other half of the story, because water stored at 50 to 55°C for hours must not leak its heat away. A cylinder jacket can reduce heat loss by up to 75%¹⁸, and if a cylinder has less than 80mm (3 inches) of insulation, adding more is worth considering¹⁹. A well-insulated cylinder is what turns stored hot water from a running cost into genuine independence: heat you have already paid for, kept until you want it.

Sizing: what the numbers actually say

Sizing a cylinder for a heat pump is part of sizing the whole system, and the official rules treat it seriously. Sizing heat pump systems correctly is vital to reduce the risk of inefficiency and high operating costs, according to the government's Future Homes and Buildings Standards response²⁰, and Approved Document L requires that heat pump systems have been sized appropriately²¹. The cylinder is sized alongside the heat pump, not picked off a shelf.

The published figures give a sense of the range. Standard assessment data for existing homes treats a normal hot water cylinder as 110 litres, with an indicative size of up to 130 litres¹². The same assessment table also carries a figure of 210 litres for larger dwellings¹²; the two figures describe different home sizes within the same table rather than contradicting each other. At the top end, one household documented in Which?'s research on heat pump adoption reported that they would have had to store a big tank of about 300 litres²².

What drives the number upwards is demand: more people, more baths, more simultaneous use. What pushes it downwards is the heat pump's ability to reheat the store gradually and efficiently over a longer period². Your installer should match the cylinder to your household's actual hot water pattern as part of the heat pump sizing process, and the figure they land on may sit anywhere across the range above. Undersizing shows up as cold showers at the wrong moment; oversizing shows up as heat lost from water nobody used.

How much room you need

The good news on space is that a modern cylinder is not the airing-cupboard monster of older memory. These cylinders can usually fit inside any cupboard that measures around 80 x 80cm⁸, and the same figure appears in Energy Saving Trust's guidance on solar assisted heat pumps: the cylinder can usually be fitted inside any cupboard that measures around 80cm x 80cm²³. For many homes, the old airing cupboard or a tall kitchen cupboard is enough.

What you are allowing for is not just the tank. Indoors, you need space for the heat pump's indoor hardware and a hot water cylinder¹⁶, plus the pipework connecting them and, on some systems, room for the controller. If the cylinder is larger than the one it replaces, because the heat pump needs more stored volume, the extra room has to be found¹⁷. And if the cylinder sits in a cupboard that also houses shelving, the cylinder's height and the clearance above it for pipework and valves both count.

For combi homes the question is starker, because there is no cylinder to replace: the space has to be created. That is one reason the survey stage matters. An installer assessing your home will identify where the cylinder goes, whether the floor can take the weight of a full tank, and how the pipework routes to the heat pump. Homes with genuinely no indoor space still have options, covered below, but the default answer for a standard heat pump is a cylinder indoors, in a cupboard of roughly 80 x 80cm⁸.

Buffer tank or no buffer tank

A buffer tank is a separate vessel in the heating circuit, and it is often confused with the hot water cylinder. It should not be. Official metering guidance from Ofgem states the distinction plainly:

Ofgem, Domestic RHI Guide to Metering²⁴

A buffer tank holds heating water that circulates between the heat pump and the radiators; a hot water cylinder holds the domestic hot water you actually use at taps and showers. They sit in different parts of the system, and on metered installations whether a meter is placed before or after a buffer tank can make a difference to the metering requirements²⁴.

Whether a buffer tank is needed at all is a design decision, not a standard fitting. Your installer, together with the recommendations of the manufacturer, will decide whether a buffer tank is appropriate and what size²⁵. Buffers exist to add volume to a small heating circuit, which can stop a heat pump from rapidly cycling on and off. But a buffer that is larger than needed adds volume the heat pump must heat without benefit, which works against efficiency. Some modern heat pumps need no buffer at all where the system volume and the pipework are adequate.

For the householder, the practical position is this: ask your installer whether the design includes a buffer, why, and where it goes. Our page on buffer tanks, volumisers and low-loss headers explains the design arguments. The hot water cylinder, by contrast, is not optional on a standard heat pump system: it is where your hot water comes from¹.

Types of hot water cylinder

Most heat pump installations in the UK use an unvented cylinder, which stores hot water at mains pressure and delivers it at mains pressure to your taps. The prevalence of this arrangement is visible even in the boiler-cover market, where service plans exist specifically covering a "Heat Pump and Unvented Cylinder Service" comprising an air-source heat pump, an unvented hot water cylinder and installation pipework for both, and a separate "Unvented Cylinder" plan covering the unvented hot water cylinder and installation pipework in a heat-pump system²⁶. Our page on whether heat pumps provide hot water at mains pressure covers what that means in practice.

Vented cylinders, fed from a loft tank, still exist in older homes, but the unvented cylinder is the modern default for heat pumps. A third arrangement is the integrated cylinder: many solar assisted heat pumps include an integrated hot water cylinder, which means you do not need a separate cylinder²³. Some models can also be connected to an existing cylinder, though you will need some space near the cylinder to install the compressor unit²³.

For homes with no space for a cylinder at all, the alternatives are real but change the system. Energy Saving Trust lists a hybrid heat pump, a heat battery or an instantaneous water heater as options if you do not have space for a hot water cylinder⁸. A hybrid heat pump paired with a combi boiler needs no cylinder for domestic hot water⁷, and Energy Saving Trust's hybrid guidance confirms that with a combi boiler or instantaneous water heater you would not need to install a hot water cylinder alongside the heat pump²⁷. Heat batteries, covered on our heat batteries with heat pumps page, store heat in a compact unit rather than a tank of water.

Fitting, controls and qualified installers

Installing or replacing a hot water cylinder is not a DIY job. If you are installing a hot water cylinder from scratch, or replacing an old one, you will need to use a professional plumber or heating engineer²⁸. Beyond that, the installation of heat pumps and associated heating systems is work covered by building regulations and should only be undertaken by competent technicians who have undertaken appropriate training and assessment¹⁴.

The official rules set out what competence means. The person carrying out the work should be suitably competent, and the installer should install and commission the system to the standards set out in the approved document²¹. Heat pump installation work can be self-certified by a registered competent person or certified by a building control body²¹. The government also expects that the person carrying out the commissioning and testing is a member of an organisation which regularly assesses their competence and performance, including relevant competent person schemes, and names the registered schemes for renewable technologies including heat pumps: APHC, BESCA, Certsure, HETAS, NAPIT and OFTEC²⁰. Industry codes of practice reinforce this: the Flexi-Orb Heat Pump Code of Practice is built around high quality, professionally designed and compliant installations²⁹.

Controls matter as much as fitting. Approved Document L requires that the system has adequate controls²¹, and heat pump systems are typically controlled by the end user via weather compensation or internal temperature control, such as a room thermostat, together with a timer or programmer¹⁴. Our page on heat pump controls explains how these interact with hot water scheduling. On hybrid systems, installers need skills for both heat pumps and condensing gas boilers³⁰, so a hybrid household should expect a installer competent across both technologies.

Once installed, the system needs an annual check. Heat pumps should be serviced once a year, or as recommended by the installer¹¹, and Energy Saving Trust recommends an annual service for heat pumps¹⁰. Heat pump systems require very little maintenance but should still be checked periodically by a registered heat pump installer¹⁴. Our page on heat pump servicing and maintenance covers what a service includes.

Running temperatures and Legionella protection

Stored warm water is the one place in a heating system where bacteria can grow, and the rules around it are clear. If you have any kind of system with a hot water cylinder that stores water for later use, a minimum water temperature of 60°C applies to kill Legionella bacteria³¹. Heat pumps, which prefer to hold water at 50 to 55°C for efficiency⁹, handle this with a sterilisation cycle: your hot water system will include a sterilisation cycle that periodically heats the cylinder to above 60°C to prevent bacteria¹⁰.

The cycle is a standard, scheduled function rather than an emergency measure. Your heat pump usually has this function available either daily, weekly, or permanently³². Home Energy Scotland describes the same arrangement: installers will establish a sterilisation cycle to ensure all stored water heats over 60°C periodically to kill harmful bacteria like legionella³³. Energy Saving Trust notes that to reduce the risk of legionella bacteria in the hot water cylinder, the heat pump may run at a higher temperature from time to time¹¹.

What this means for running costs is a small, honest cost of storage. The everyday hot water temperature is 50 to 55°C because that is where the heat pump works most efficiently⁹; the sterilisation cycle briefly pushes above 60°C¹⁰, which costs a little more but is doing necessary work. Some heat pumps can hold over 60°C consistently⁹, but holding that temperature all the time would give up the efficiency that makes the system worth having. The sterilisation cycle is the compromise: efficient storage, with a periodic safeguard.

Two practical points follow. First, the schedule is usually configurable, daily, weekly or permanently³², so ask your installer how yours is set and what it costs to run. Second, the annual service¹⁰ should include checking that the sterilisation cycle is actually reaching its target temperature, because a cycle that is scheduled but not achieving above 60°C is not protecting the household. For a home aiming at energy independence, this is the one place where independence has a hard limit: stored hot water must be managed, not just collected.