Getting the size right is crucial for air source heat pump performance – too small or too large and efficiency, comfort, and longevity suffer. Jamie Ansell of EDF Heat Pumps explains why detailed design matters.
Air source heat pumps are an increasingly popular heating solution for new and existing homes, offering energy efficiency, lower carbon emissions, and long-term cost savings. However, to realise their full potential, one key factor must be carefully addressed at the outset – correct sizing. An improperly sized heat pump can compromise both the performance and efficiency of a system, leading to higher running costs, reduced comfort, and a shorter system lifespan.
When an ASHP is undersized for the property, it will struggle to meet the heating demands during colder weather. The unit may run continuously at maximum capacity, consuming more electricity than necessary and potentially failing to maintain a comfortable indoor temperature. This not only drives up energy bills but also increases mechanical wear, reducing the expected service life of the system.
On the other hand, an oversized heat pump can be just as problematic. Rather than operating efficiently, an oversized unit will frequently “cycle” in spring and autumn switching on and off in quick succession as it rapidly overshoots the desired indoor temperature. Short cycling leads to unnecessary strain on system components, diminished efficiency, and inconsistent indoor comfort. Poorly zoned central heating systems can make this scenario worse!
The foundation of a correctly sized ASHP lies in a detailed heat loss calculation. This process analyses factors such as insulation levels, window specifications, geographical location, air tightness, and overall floor area to determine the heating requirement of the property. Here at EDF heat pumps, we calculate each room’s heat loss giving us an overall heat loss figure for the building and from that we can determine the correct emitter sizes for each room. Relying on simple rules of thumb or oversimplified estimates can lead to significant errors. A professional heat loss assessment ensures that the selected system is matched perfectly to the building’s needs, enabling the heat pump to operate at its optimal efficiency range.
Another important consideration for homeowners and installers is how to operate an air source heat pump efficiently. Unlike traditional fossil fuel boilers that are typically run intermittently, ASHPs perform best when operated continuously. Running the system 24/7 at low flow temperatures allows it to maintain steady indoor conditions without the need for high energy inputs. This “low and slow” approach not only maximises system efficiency but also enhances occupant comfort by eliminating significant temperature fluctuations throughout the day and night.
Correctly sizing and operating an air source heat pump is therefore essential to unlocking its full performance potential. For self-builders, renovators, and anyone upgrading their heating system, investing time in a proper design and specification process will pay dividends in long-term comfort, energy savings, and system reliability.
Jamie Ansell is training manager at EDF Heat Pumps