Glossary · Heat pumps, grants and regulations
What is a buffer tank?
A buffer tank is an insulated water store fitted between a heat pump and the radiators or underfloor heating. It adds system volume so the heat pump does not switch on and off too often, gives it water to draw heat from during defrost and can separate the heat pump circuit from the heating circuits.
Key points
- A buffer tank adds water volume to the heating system. It sits between the heat pump and the radiators or underfloor heating.
- Baxi lists its jobs as meeting minimum system volume, smoothing changes in demand, supporting defrost cycles and hydraulic separation.
- It is optional. Baxi says whether you need one depends on the system design, and some systems do not need one at all.
- Vaillant makes a purpose-built buffer that adds 30 litres to a slimline cylinder or 40 litres to a standard diameter one.
- The sizing and connection of a buffer tank are for the heat pump installer to decide from the manufacturer's instructions.
How a buffer tank works
A buffer tank gives a heating system a defined amount of extra water. In a heat pump installation it is positioned between the heat pump and the distribution system, such as radiators or underfloor heating.[1]Baxi, When do you need to use an ASHP buffer tank? That extra water can serve several purposes depending on how the system is designed.[1]Baxi, When do you need to use an ASHP buffer tank?
Baxi lists what a buffer tank can do.[1]Baxi, When do you need to use an ASHP buffer tank? It can provide system volume where the heating system does not hold enough water at all times. It can act as a hydraulic separator between the heat pump circuit and the heating circuits. It can smooth fluctuations in demand by adding thermal mass as zones open and close. It can support defrost cycles by giving the heat pump a volume of water to draw energy from. And it can help keep the right flow through the heat pump.[1]Baxi, When do you need to use an ASHP buffer tank?
Vaillant describes its purpose-built buffer in similar terms. It says the buffer limits on and off cycling of the compressor, acts as a hydraulic decoupler, always provides the required minimum circulation volume and stores extra heating capacity for peak times.[2]Vaillant, Purpose-built buffer
When one might be used
Baxi says a buffer tank is an optional accessory and that using one is a matter of system design.[1]Baxi, When do you need to use an ASHP buffer tank? It may be considered where:
- the system does not have enough water volume
- several heating zones could reduce volume or flow
- different circuits have different flow needs
- the heat pump and secondary circuits need hydraulic separation
- the minimum system volume is not available at all times, for example when thermostatic radiator valves close or whole zones shut off
The last point links to heating zones and the thermostatic radiator valve. A buffer is not the only answer. Daikin lists a buffer tank among the potential extras that can come with a heat pump installation, along with emitter upgrades and insulation improvements.[3]Daikin UK, Types of Heat Pumps: Which One Is Right for Your Home?
Example
Lauren’s house in Ilkley has a heat pump and four heating zones. On a mild afternoon three zones close and only a small bedroom circuit is calling for heat. The water in that small circuit is not enough for the heat pump to run steadily, so it would switch on and off. Her installer has designed in extra volume, which can be a buffer tank, so the heat pump has enough water at all times. The right arrangement depends on the model, so the installer follows the maker’s guidance.
What people get wrong
A frequent assumption is that a buffer tank is a standard part of every heat pump. Baxi describes it as optional and says it should not be specified automatically. It should be added when the design needs it.
That is Baxi’s position on its own heat pump range.[1]Baxi, When do you need to use an ASHP buffer tank?
A buffer is also not a hot water cylinder. A cylinder stores water for taps and showers, while a buffer holds water in the heating circuit. See unvented cylinder for the hot water side. Baxi says how a buffer is connected and controlled determines how well it performs, which is why layout and controls are a design task for a qualified installer.[1]Baxi, When do you need to use an ASHP buffer tank? Work on the heat pump refrigerant circuit needs a person with their own F gas qualification.[4]GOV.UK, Qualifications to work with F gas
The bottom line
A buffer tank adds water volume between a heat pump and the heating circuits. It can help with minimum system volume, cycling, defrost and hydraulic separation, but it is optional and depends on the system design and the manufacturer’s rules. Ask your installer why one is, or is not, in your quote.
Common questions
Does every heat pump need a buffer tank?
No. Baxi says a buffer tank is an optional accessory in an air source heat pump installation. For some installers it provides useful extra volume or helps manage circuits, and for others it is not necessary at all.
What does a buffer tank do during defrost?
It gives the heat pump a volume of hot water to draw energy from while it defrosts the outdoor unit. Baxi lists support for defrost cycles as one of the things a buffer tank can do.
Why would a system not have enough water volume?
Baxi says low volume can happen where multiple heating zones shut off, or where thermostatic radiator valves close so the minimum system volume is not available at all times.
Who decides if I need one?
Your heat pump installer, using the heat pump maker's installation instructions and a design for your home. Baxi says whether to use a buffer tank is down to system design, so there is no single rule for every home.
Sources
We cite the original source for every figure: government, regulators, standards bodies and manufacturers' own documents. Each one shows the date we last checked it.
- Baxi. When do you need to use an ASHP buffer tank?. Checked 11 November 2026. ↩
- Vaillant. Purpose-built buffer. Checked 11 November 2026. ↩
- Daikin UK. Types of Heat Pumps: Which One Is Right for Your Home?. Checked 11 November 2026. ↩
- GOV.UK. Qualifications to work with F gas. Checked 11 November 2026. ↩