Do I Need a Hot Water Cylinder With a Heat Pump?

A hot water cylinder is often the part of a heat pump installation that causes the most concern. It needs space, it may replace a compact combi boiler, and it can affect where pipework runs. So, do I need a hot water cylinder with a heat pump? In most homes, yes. A properly specified cylinder is normally essential to delivering reliable hot water from an air source heat pump.

The reason is straightforward. A heat pump warms water gradually and efficiently, rather than producing a large volume of very hot water instantly. The cylinder stores that heat until your household needs it for showers, baths, taps and washing up. It is not an optional extra added for convenience – it is usually central to how the system works.

In this article:

Why Heat Pumps Usually Need A Cylinder

A gas combi boiler can heat mains water on demand through a powerful burner. When you turn on a hot tap, the boiler fires and transfers heat quickly enough to supply the flow, within its limits.

An air source heat pump works differently. It is designed to run at lower water temperatures and over longer periods. That approach is what makes it efficient, but it is not suited to instant, high-output hot water production. A cylinder allows the heat pump to heat a stored volume of water when demand is lower, then make that stored hot water available when the family needs it.

Most heat pump cylinders are indirect cylinders. Instead of the heat pump water mixing with the domestic hot water that comes from your taps, it passes through a large internal coil and transfers its heat across. The cylinder needs a coil designed for heat pump operation. A conventional boiler cylinder can sometimes be retained, but only if its coil is large enough and in good condition.

This is one reason we assess the existing equipment rather than assuming it can be reused. A cylinder that performs acceptably with a gas boiler may recover too slowly with a heat pump if the coil has insufficient surface area.

Is There Ever A Heat Pump Without A Hot Water Cylinder?

There are exceptions, although they are less common in domestic projects. A heat pump may serve space heating only, while another appliance provides hot water. For example, a property could retain a separate gas water heater for a period, or have a distinct hot water arrangement in a larger building.

Some heat pumps come with a cylinder built into an indoor unit. This can look more like a tall fridge-freezer than a separate tank, which helps where a utility room is tight. It is still a cylinder – simply an integrated one.

Instantaneous electric water heaters are another possible route, but they can be expensive to run and place a significant demand on the electrical supply. They are rarely the best answer for a whole family home that is moving to a heat pump.

For most households replacing a boiler, oil system or LPG system, planning for a dedicated hot water cylinder is the sensible starting point.

Choosing The Right Cylinder Size

Bigger is not automatically better. An oversized cylinder takes more room and has slightly greater standing heat losses, while an undersized one can leave the last person in the morning shower with lukewarm water.

As a broad guide, many family homes need a cylinder in the region of 180 to 250 litres. Household size, bathing habits and the number of bathrooms matter more than floor area alone. A couple who mainly take short showers may be well served by a smaller cylinder than a family with teenagers, a large bath and two showers used one after another.

The incoming mains pressure and flow rate also matter. An unvented cylinder can provide strong shower performance, but it cannot create more water flow than the mains supply provides. We check this during a survey, particularly in older properties or rural locations where supply conditions can vary.

Cylinder sizing should also account for the heat pump’s hot water output and the time available for reheating. A suitable system should give the household enough stored water for normal peak use, then recover predictably before the next busy period. It should not rely on the immersion heater as the regular solution to a shortfall.

The Cylinder Is Not The Same As A Buffer Tank

The terms are often muddled, but they perform different jobs. A hot water cylinder stores domestic hot water for taps, baths and showers. A buffer tank, volumiser or low-loss header sits within the heating circuit and contains heating water for radiators or underfloor heating.

Some heat pump systems benefit from a buffer or volumiser to maintain minimum system water volume, support defrost cycles or allow the heat pump to run steadily when heating zones close down. Others can be designed without one. It depends on the emitter system, controls, pipework layout and heat pump manufacturer requirements.

A buffer tank does not provide shower water, and a domestic hot water cylinder does not automatically solve a heating-system volume issue. Both should be specified as part of the full design, not added as standard items without a reason.

What Temperature Will The Cylinder Run At?

Heat pumps usually heat cylinders to around 45C to 55C during normal operation. That is sufficient for everyday use, especially where a thermostatic mixing valve blends hot water down to a safe outlet temperature.

The system will also periodically carry out a higher-temperature hygiene cycle, often using the immersion heater if needed, to manage legionella risk. The exact control strategy depends on the cylinder, heat pump and household usage. It should be set up and explained at commissioning, rather than left as a mystery in the controller menu.

A well-designed system balances comfort, hygiene and efficiency. Setting the cylinder unnecessarily hot all the time can increase running costs and reduce the effective volume available after mixing, so it is not simply a case of turning every temperature to maximum.

Finding Space For A Cylinder

The practical challenge is usually space. A modern unvented cylinder is commonly installed in a utility room, airing cupboard, garage or a purpose-built cupboard. It needs service access, suitable drainage provision for safety valves and sensible pipe routes back to the heat pump and bathrooms.

In some homes, replacing a combi boiler means reclaiming an airing cupboard that was converted to storage years ago. In others, a garage makes the most sense. A garage installation can work well, but the cylinder and associated pipework need appropriate insulation, and the location must be protected from freezing.

We also look at the wider indoor layout early in the process. The cylinder, hot water controls, circulation pump, expansion vessel and electrical components need to be accessible after installation. Hiding everything behind fixed joinery may look tidy on day one but makes servicing harder for the next decade.

Can An Existing Cylinder Be Kept?

Sometimes, yes. Retaining a sound, compatible cylinder can reduce disruption and material costs. We would check its age, insulation, condition, coil size, safety equipment and whether it meets the requirements for the proposed heat pump.

Many older cylinders have small coils intended for a high-temperature boiler. They can heat water eventually, but recovery may be slow and the heat pump may have to work at a less efficient flow temperature. In that situation, replacing the cylinder is generally the better long-term decision.

If a property has solar PV, a replacement cylinder can also be selected with provision for an immersion diverter. This can use surplus solar generation to heat water when it would otherwise be exported. It is a useful addition in the right circumstances, but it should complement the heat pump’s controls rather than conflict with them.

Design Matters More Than The Tank Alone

The cylinder is only one part of a good hot water system. Its performance depends on correct heat-loss calculations, the heat pump size, pipework, controls, shower demand, water pressure and how the household actually lives.

That is why we ask practical questions during a survey: how many people live there, whether anyone fills a large bath, when showers are normally taken and whether the property has multiple bathrooms used at the same time. The answers are more useful than choosing a cylinder size from a generic online chart.

For homeowners considering a heat pump, allowing space for a properly sized cylinder is usually the right decision. It gives the system the stored energy it needs to provide dependable hot water without asking the heat pump to behave like a combi boiler. A careful design turns that extra cupboard space into a predictable supply of hot water every day.

Frequently Asked Questions

From cold, typically somewhere between 45 minutes and two hours, depending on the cylinder size and the heat pump’s output. In everyday use the cylinder is rarely fully cold, so most top-ups are shorter. We set the schedule so the main reheat happens before your busiest times.

Yes. An electric shower heats mains water itself and doesn’t draw from the cylinder, so it can stay. Some households keep one as a backup or for a second bathroom, but bear in mind it uses a lot of electricity while it’s running.

Yes. An unvented cylinder should be checked once a year, including its safety valves, temperature relief and expansion vessel. We can do this at the same visit as an annual heat pump service.

Sometimes, but it’s rarely ideal. A full 200-litre cylinder weighs well over 200kg, so the joists need checking, and it has to be protected from freezing and easy to reach for servicing. We’d normally look at an airing cupboard, utility room or garage first.

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