How to Choose the Right Radiators for Your Heat Pump System
How to Choose the Right Radiators for Your Heat Pump System
Choosing the right radiators is an important part of getting the best performance from an air source heat pump.
Heat pumps generally operate at lower water temperatures than traditional boilers. This means a radiator that worked well with a boiler may not necessarily provide enough heat when connected to a heat pump system.
The right radiator needs to provide enough heat for the room at the flow temperature the heat pump has been designed to operate at.
Why Do Heat Pumps Need Correctly Sized Radiators?
Traditional boiler systems commonly operate with relatively high water temperatures, allowing radiators to produce a large amount of heat from a smaller surface area.
Heat pumps are different.
They are generally most efficient when supplying water at lower temperatures. As the temperature of the water flowing through the radiator decreases, the amount of heat the radiator can release into the room also reduces.
This is why heat pump systems may require:
- Larger radiators
- Higher-output radiator models
- Additional radiators in some rooms
- Alternative emitters such as underfloor heating
The correct option depends on the heat loss and heating requirements of each individual room.
Energy Saving Trust also recommends using properly sized radiators or low-temperature underfloor heating when installing a heat pump.
Start With the Room Heat Loss
Before choosing a radiator, you need to know how much heat the room requires.
A room-by-room heat loss calculation considers how quickly heat escapes from different areas of the property.
Factors can include:
- Room size
- Wall construction
- Insulation levels
- Window and door sizes
- Glazing performance
- External walls
- Design outdoor temperature
- Ventilation and air leakage
The result is normally expressed in watts and tells you approximately how much heat the radiator needs to provide to maintain the required room temperature.
Simply choosing a radiator based on the physical size of the room can therefore lead to poor results.
Consider the Heat Pump Flow Temperature
Radiator output depends heavily on the temperatures at which the heating system operates.
Many radiator manufacturers provide output figures based on standard test conditions commonly associated with traditional heating systems.
A heat pump system may operate at considerably lower flow temperatures, so the radiator's actual heat output can be lower than the headline figure shown in a product specification.
When comparing radiators, check the manufacturer's output information and correction data for the temperature difference relevant to your system.
For example, a radiator capable of comfortably heating a room on a high-temperature boiler system may need to be replaced with a larger model when the property changes to a lower-temperature heat pump.
Bigger Radiators Can Help a Heat Pump Run at Lower Temperatures
Increasing the surface area of a radiator allows it to transfer more heat into the room at a lower water temperature.
This is why upgrading to larger or higher-output radiators can be beneficial when installing an air source heat pump.
Correct radiator sizing can help the system operate at a lower flow temperature, which can improve heat pump efficiency.
Do You Need to Replace Every Radiator?
Not necessarily.
Some existing radiators may already be large enough to provide the required room output at the proposed heat pump flow temperature.
Others may need to be upgraded.
This is particularly common in rooms with:
- High heat loss
- Large external wall areas
- Large windows
- Higher desired temperatures
- Smaller existing radiators
A room-by-room assessment is therefore much more useful than assuming every radiator in the property needs to be replaced.
Single, Double and Triple Panel Radiators
Radiators are available in different panel and convector configurations.
Increasing the number of panels and convector fins can increase the available heat output without dramatically increasing the radiator's width.
This can be useful where wall space is limited.
For example, a higher-output double or triple panel radiator may provide the required heat output in a location where fitting a much wider radiator would be impractical.
However, the correct radiator should always be chosen based on its calculated output at the system's intended operating temperatures rather than the number of panels alone.
Radiator Size Isn't the Only Consideration
When choosing radiators for a heat pump system, also think about:
- Available wall space
- Furniture positioning
- Pipework routes
- Radiator depth
- Window positions
- Required room temperature
- Radiator valves and controls
- Overall appearance
Radiators should ideally be planned early enough in the project to ensure suitable wall space and pipework positions are available.
What About Underfloor Heating?
Radiators are not the only option for a heat pump.
Water underfloor heating is particularly well suited to low-temperature heating because it uses a large floor area to distribute heat throughout the room.
Some properties therefore use a combination of both systems.
For example:
- Underfloor heating downstairs
- Radiators upstairs
A mixed system can work effectively when both the underfloor heating and radiators have been designed around the same overall heat pump system.
How Does an Air Source Heat Pump Work With Radiators?
An air source heat pump extracts heat energy from the outside air and transfers it into the property's heating system.
That heat can then be distributed through radiators, underfloor heating or other suitable heat emitters.
If you're still researching heat pumps and how they operate, our guide What Is an Air Source Heat Pump? explains the basics of the technology and how it provides heating and hot water.
Avoid Choosing Radiators Using Boiler Sizing Alone
One of the biggest mistakes when changing from a boiler to a heat pump is assuming the existing radiator specification will automatically remain suitable.
The original radiators may have been selected to operate at much higher water temperatures.
A heat pump design should instead consider:
- The room heat loss
- The proposed system flow temperature
- The radiator output at that temperature
- The overall heat pump performance
If the radiator cannot provide the required heat output at the chosen temperature, it may need to be increased in size or replaced with a higher-output model.
Use BPC Heat's Design Service
If you're planning an air source heat pump system, BPC Heat's heating design service can help determine the heating requirements of the property before equipment is ordered.
Send us your property plans and our design team can assess the project and help specify a complete system, including:
- Property heat loss requirements
- Air source heat pump selection
- Radiator sizing
- Underfloor heating
- Hot water cylinders
- Heating controls and zoning
Designing the radiators and heat pump together helps ensure each room has sufficient heating output while allowing the system to operate at suitable flow temperatures.
Choosing the Right Heat Pump Radiators
The best radiator for a heat pump system is not simply the biggest radiator that will fit on the wall.
The radiator needs to provide the required room heat output at the temperatures the heat pump has been designed to operate at.
Start with accurate room heat losses, consider the proposed flow temperature and check radiator performance data before selecting each emitter.
