A heat pump underperforming at service - diagnosis from operating data
A 10 kW ASHP installed two years ago is reported as 'expensive to run' and 'never warm enough'. At a service visit the outdoor temperature is 6 °C. The unit is running continuously. Measured data: flow 38 °C, return 32 °C, outdoor coil is partly frosted on one side, the defrost cycle appears to run every 25 minutes and takes 8 minutes. The cylinder thermostat is set to 60 °C and the heat pump is asked to produce DHW at 55 °C. The underfloor manifold shows 35 °C flow and 30 °C return. The customer has set the room thermostat to 23 °C. Filters in the primary circuit are clean. The unit's COP at these conditions should be around 3.5.
1. Using the data, identify the likely causes of poor performance and rank them.
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Flow 38 / return 32 gives a 6 K drop, which is reasonable, but the absolute flow temperature is higher than needed for underfloor (35 °C at the manifold). The gap between unit flow (38) and manifold flow (35) suggests mixing or bypass losses.
Ranked causes:
- Defrost frequency. Every 25 minutes with an 8-minute defrost is excessive - roughly a third of run time in defrost. Partial frosting on one side suggests uneven airflow or a refrigerant distribution/charge issue. This alone will crush average COP well below 3.5.
- DHW at 55 °C. Asking the heat pump to make DHW at 55 °C forces high condensing temperature and low COP. The cylinder stat at 60 °C means the unit is working near its limit. Consider a legionella cycle once a week rather than continuous high temp, and a larger coil cylinder.
- Room stat at 23 °C with underfloor - high setpoint increases run time and cost; explain comfort and setback.
- Mixing/bypass between unit and manifold wastes energy.
Confirm defrost issue with refrigerant pressures, coil sensor readings, and fan operation; check for blocked coil or faulty defrost sensor.
2. What service and compliance actions would you take and record?
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Service actions: investigate the defrost fault - check outdoor coil sensor, defrost termination sensor, fan speed and coil cleanliness; verify refrigerant charge and look for a partial blockage causing uneven frosting. Check the primary strainer and any bypass. Review DHW strategy with the customer and reset the cylinder stat to a sensible value with a scheduled legionella cycle. Reduce the room setpoint and explain weather compensation.
Compliance/recording: this is a refrigerant-containing system, so record pressures, temperatures, and any refrigerant handling on the service record; if refrigerant is added, log the amount and reason. Confirm the installation still meets building regulations and MCS/manufacturer requirements, and that the customer receives an updated service record and performance report. Note the measured flow/return and defrost data as evidence of the diagnosis so the follow-up visit can compare.