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

If you're moving away from a combi boiler, one question comes up almost every time: does a heat pump need a hot water cylinder? For homeowners used to instant hot water on demand, bringing a cylinder back into the house can feel like a step in the wrong direction. But for the vast majority of air source heat pump systems, a cylinder isn't an optional extra it's a core part of how the system is designed to work.

The reassuring part is that most homes already have, or can easily find, a suitable spot for one. And in some cases, an existing cylinder may even do the job.

Why Heat Pumps Usually Need a Cylinder

A combi boiler heats water instantly, on demand, at high output. A heat pump works differently. It produces heat more gradually and runs most efficiently over longer periods at lower output, rather than firing up to full power the moment a tap is turned on.

That's where the cylinder comes in. It acts as a thermal store, building up a reserve of hot water so it's ready whenever you need it, without forcing the heat pump to chase sudden demand. This steadier way of working is a big part of why heat pumps can hit strong efficiency figures across the year, and it ties directly into how the system's flow temperature is set, since cylinder temperature and flow temperature are closely linked in a well-designed system.

Can a Heat Pump Run Without One?

Yes, but only in a specific scenario: if the heat pump is providing space heating alone, with no domestic hot water responsibility. This setup exists, but it's uncommon in UK residential properties.

If you want the heat pump to cover both heating and hot water which is what most homeowners want a cylinder is normally required. It's also worth knowing that the Boiler Upgrade Scheme (BUS) requires a heat pump to provide both space heating and domestic hot water to qualify, which in practice means a suitably specified cylinder needs to be part of the installation. This applies just as much to homes switching from oil or LPG under the recently increased £9,000 BUS grant as it does to those replacing a mains gas boiler.

If a combi boiler is what you're replacing, the cylinder is usually the single biggest visible change to the system but it's also the component that lets the heat pump produce hot water efficiently rather than working against its own design.

Can You Keep Your Existing Cylinder?

Sometimes, yes. If you already have an unvented cylinder in good working condition, it might be reusable but it shouldn't be assumed suitable without a proper check.

Cylinders designed for heat pumps typically differ from older, boiler-matched cylinders in a few important ways:

• A larger heat exchanger coil

• Better heat transfer at lower flow temperatures

• Lower pressure losses through the system

• Stronger recovery performance when running cooler

Older cylinders were often specified around a gas boiler's higher output and were never intended to work efficiently with a heat pump's lower-temperature heat. Fitting one anyway can quietly reduce the performance of the whole system, even if the heat pump itself is correctly sized a pattern similar to what shows up in cases where a heat pump ends up oversized or mismatched for the rest of the system around it.

Why Are Heat Pump Cylinders Built Differently?

Because a heat pump delivers heat more gently than a gas boiler, the cylinder needs a much larger internal coil to transfer that lower-grade heat into the stored water effectively. A bigger coil surface area means the cylinder can absorb heat without pushing the heat pump into unnecessarily high flow temperatures just to compensate.

Get this right and it shows up as:

• Faster hot water recovery after use

• Better overall system efficiency

• Lower running costs

• More consistent performance day to day

Will You Run Out of Hot Water?

Not if the system has been sized properly for your household. Cylinder size should reflect how many people live in the home, how many bathrooms are in regular use, and typical daily demand not a generic assumption based on house size alone.

As a rough guide:

• Smaller households often suit a 150–180 litre cylinder

• Larger families may need 210–300 litres or more

These numbers are only a starting point. A proper heat loss calculation and hot water demand assessment should always be carried out before a cylinder is specified, in the same way a heat pump itself should be sized against actual heat loss rather than floor area or rough estimates. Getting either the cylinder or the heat pump wrong at this stage tends to cause problems that only become obvious after the system is installed and in daily use.

Does a Cylinder Mean Slower Hot Water?

No. Once water is stored and heated inside the cylinder, it reaches your taps at the same speed as it would from a conventional boiler-fed system. The only real difference is in how the water is heated before it gets to that point, not how quickly it arrives once you turn the tap.

No Airing Cupboard? That's Usually Fine

A dedicated airing cupboard isn't essential. Modern cylinders are routinely installed in utility rooms, garages, plant rooms, larger cupboards, loft spaces (where structurally suitable), or purpose-built enclosures. Working out where the cylinder can go safely and practically is one of the first things a competent heat pump installer should assess, and it's a question worth raising early rather than after equipment has already been ordered.

This is particularly relevant in older properties, where available space and existing pipework layouts can shape the options more than they would in a newer build. Our article on whether heat pumps work well in older houses covers some of the wider design considerations that tend to come up in these homes.

Is a Hot Water Cylinder Worth It?

For most homeowners, yes. It's easy to see a cylinder as an extra piece of kit to accommodate, but it's what allows the heat pump to run efficiently while still delivering hot water reliably throughout the day. A correctly specified cylinder also pairs well with solar PV, battery storage and smart electricity tariffs, which makes it a sensible piece of future-proofing rather than just a compliance requirement for the heat pump itself.

Not Sure If Your Existing Cylinder Will Do?

It's common for homeowners to be told they need a brand-new cylinder without a clear explanation of why. Sometimes that's genuinely the right call but not always, and it's worth getting an independent view before accepting it at face value.

Our Pre-Installation Heat Pump Review gives you an independent assessment of your proposed system, including whether your existing cylinder could be reused, whether replacement is actually justified, and whether the wider system design is right for your home before you commit to any installation costs.