1. Incorrect Thermostat Settings
Check your controller is set to heating mode (not cooling or ventilation only). Ensure the target temperature is set higher than the current room temperature. Some controllers revert to previous settings after a power cut.
2. Low Flow Temperature Setting
If the system flow temperature is set too low, rooms may not reach the desired temperature. For a cold day in the UK, a flow temperature of 40–50°C is typically required for standard radiators. Check and adjust via your controller.
3. Low Refrigerant Charge
A slow refrigerant leak reduces the system’s ability to transfer heat. Symptoms include:
- The outdoor unit running continuously with little heating effect
- Ice forming on the outdoor unit’s evaporator
- The system displaying a low-pressure error code
This requires an F-Gas certified engineer.
4. Dirty or Blocked Air Filter
If your indoor unit has an air filter, a blocked filter severely restricts airflow across the heat exchanger, dramatically reducing heating output. Check and clean the filter according to the manufacturer’s instructions.
5. Outdoor Unit Iced Up
If the outdoor unit is frozen solid (not just light frost), airflow across the evaporator is blocked and heating output drops to near zero. Do not run the system — call an engineer to identify the cause of the excessive icing.
6. Heating System Airlock
An airlock in the heating pipework can prevent hot water from circulating to radiators. Bleeding your radiators may resolve the issue. Check that the circulation pump is running.
7. Faulty Temperature Sensor
A failed ambient temperature sensor or flow temperature sensor can cause the control system to incorrectly modulate the compressor output. This typically generates an error code.
8. Undersized System
If a heat pump was incorrectly sized during installation (too small for the property’s heat loss), it may struggle to maintain comfort temperatures on cold days. This is a design issue requiring assessment by a qualified engineer.
Sources: MCS standards and Electrical Safety First.
