A Guide to Choosing the Right Size Heat Pump Tank
A Guide to Choosing the Right Size Heat Pump Tank
Choosing the right size heat pump tank is an important part of designing an efficient air source heat pump system. Too small, and your household may run short of hot water during busy periods. Too large, and you could be heating and storing more water than you actually need.
The correct size depends on more than the size of your property. Household occupancy, the number of bathrooms, how much hot water you use and the performance of the heat pump all need to be considered.
What Is a Heat Pump Tank?
When people refer to a heat pump tank, they are often talking about the domestic hot water cylinder used alongside an air source heat pump.
Unlike a combi boiler that can provide hot water on demand, a standard heat pump typically heats water which is then stored in a cylinder ready for use.
Heat pump systems can also include buffer tanks, but these perform a different role within the heating circuit. Buffer tank sizing should be considered separately as part of the overall heating system design.
You can explore BPC Heat's range of heating tanks and hot water cylinders, including options designed for modern air source heat pump systems.
Why Does Heat Pump Tank Size Matter?
The hot water cylinder needs to store enough water to meet the normal demand of the household.
If the cylinder is undersized, you may find that hot water runs low when several people shower or use hot water within a short period.
However, choosing the biggest cylinder available is not necessarily better. A larger tank:
- Takes up more space
- Contains a greater volume of water to heat
- May be unnecessary for smaller households
- Needs to be correctly matched with the rest of the heat pump system
The aim is to select enough storage capacity for everyday hot water demand while ensuring the cylinder and heat pump work effectively together.
Consider How Many People Live in the Property
Household occupancy is one of the first things to consider.
More occupants will normally mean greater daily hot water use, particularly when several people shower in the morning or evening.
Industry guidance such as the MCS Domestic Hot Water Cylinder Selection Guide can be used when assessing cylinder requirements. It considers factors such as household size, bathrooms and heat pump output when selecting an appropriate capacity.
Rather than choosing a cylinder based only on a general litres-per-person figure, the actual usage of the household should also be considered.
Think About Bathrooms and Hot Water Use
Two households with the same number of occupants can have very different hot water requirements.
Consider:
- How many bathrooms and ensuites there are
- Whether the property mainly uses showers or baths
- Whether several showers are likely to be used one after another
- The flow rate of showers
- How frequently baths are used
- When demand is highest during the day
A home with several bathrooms, a large bath or high-flow showers may require greater storage capacity than another property with the same number of occupants.
This is why cylinder sizing should form part of the wider heat pump design rather than being selected from property size alone.
Heat Pump Output and Recovery Time Matter Too
Tank capacity is only part of the equation.
Recovery time refers to how long the system takes to heat the cylinder again after hot water has been used.
The heat pump output, cylinder heat exchanger and water temperatures can all affect recovery performance.
A properly selected cylinder should therefore be compatible with the heat pump supplying it. Heat pump cylinders typically use heat exchangers designed specifically to work effectively with the lower water temperatures associated with heat pump systems.
This is also why replacing a boiler with a heat pump does not automatically mean that an existing hot water cylinder will be suitable.
Don't Choose a Cylinder Based on Floor Area Alone
The square metre size of a property can be useful when assessing space-heating requirements, but it does not tell you how much domestic hot water the household needs.
For example, two similarly sized homes may have completely different requirements if one has two occupants and a single shower while the other has a larger family, multiple bathrooms and regular bath use.
The cylinder should be selected using the expected hot water demand, alongside the heat pump and wider heating system requirements.
Make Sure You Have Enough Space
The physical size of the cylinder also needs to be considered during the design stage.
Before choosing a model, check:
- Available cupboard or plant-room space
- Cylinder height and diameter
- Access for installation
- Pipework requirements
- Clearances required by the manufacturer
- Space for associated heating components
Planning the location before equipment is ordered can prevent problems later in the installation.
Tank Sizing Is Part of the Complete Heat Pump Design
The cylinder should not be selected in isolation.
A properly designed heat pump system needs to consider the relationship between:
- Property heat loss
- Heat pump output
- Domestic hot water demand
- Cylinder size
- Cylinder heat exchanger performance
- Radiators or underfloor heating
- Flow temperatures
- Controls and zoning
Our Air Source Heat Pump Design Guide explains why equipment selection should follow proper system design rather than relying on basic assumptions or property size alone.
Get Help Choosing the Right Heat Pump Tank
If you are planning a new air source heat pump system and are unsure which cylinder size or heat pump you require, BPC Heat's heating design service can help.
Send us your property plans and our design team can assess the project and help specify a complete heating solution around the property.
Depending on the project, this can include:
- Air source heat pump selection
- Heat loss requirements
- Hot water cylinder sizing
- Radiators
- Underfloor heating
- Controls and zoning
Designing these elements together helps ensure that the heat pump, cylinder and heating system are properly matched rather than selecting each component separately.
Choosing the Right Heat Pump Tank
There isn't one tank size that is right for every home.
The best choice depends on how much hot water your household actually uses, how quickly that water needs to be recovered and how the cylinder works with the selected heat pump.
Consider occupancy, bathrooms, shower and bath use, available space and heat pump performance before making your choice.
Explore BPC Heat's range of heating tanks, or send us your plans for a tailored heating design if you need help selecting the right equipment for your project.
