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Heat Pump Not Heating The House
The heat pump is running but the house never gets to temperature, or it struggles as soon as it turns cold.
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What's happening?
A heat pump does not work like a boiler and cannot be run like one. It moves heat rather than making it, and it does so efficiently by delivering water at a much lower flow temperature — often around 35 to 50°C rather than the 70°C-plus a boiler uses. That only works if the emitters and the fabric were sized for it: radiators large enough, or underfloor heating, plus a reasonable level of insulation, and a control strategy that runs the system steadily rather than in short blasts. Most complaints that a heat pump cannot heat a house turn out to be one of three things: emitters undersized for the low flow temperature, a weather compensation curve set wrong, or the system being scheduled off and on as though it were a gas boiler.
What usually causes it?
- Radiators sized for a boiler's flow temperature and never upgraded for lower-temperature operation
- Weather compensation or the heating curve set too low for the property
- The system scheduled in short bursts rather than run steadily, so it never reaches temperature
- An outdoor unit iced up, sited where airflow is restricted, or defrosting too often
- Low refrigerant charge or a compressor fault reducing output
- A buffer tank, low-loss header or volumiser plumbed or controlled incorrectly, mixing flow and return
- Insulation and draught-proofing left as they were, so the heat loss exceeds the unit's output
Is it dangerous?
Low. Nothing here is a safety hazard. The practical risk is cost: a heat pump forced to run at a high flow temperature to compensate for undersized radiators uses far more electricity than it should, so an underperforming installation shows up on the bill before it shows up as discomfort.
Can I fix it myself?
Change how you run it before you change anything physical. Set it to run continuously at a steady setpoint rather than timed on and off — that is the single most common self-inflicted heat pump problem. Open all thermostatic radiator valves fully so the system is not fighting them. Clear leaves, snow and planting away from the outdoor unit and make sure there is free air on all sides. Check the outdoor unit is not iced solid; a short frost pattern is normal during a defrost cycle, a solid block is not. Do not adjust the heating curve or refrigerant side yourself.
When should you call a heating engineer?
Call a heating engineer with heat pump experience if the house still does not reach temperature when the system runs steadily, if the outdoor unit ices up and does not clear, or if electricity use is far above what you were told to expect. Where the installation was done under the Microgeneration Certification Scheme, the installer has obligations under that scheme and its consumer code, so go back to them first with the performance estimate you were given.
What will the heating engineer actually do?
- Recalculate the room-by-room heat loss and compare it with the emitter output at the design flow temperature
- Check and reset the weather compensation curve and the control strategy for continuous operation
- Measure flow and return temperatures and flow rate against the manufacturer's design figures
- Inspect the outdoor unit for airflow restriction, defrost operation and refrigerant performance
- Check buffer, volumiser and low-loss header pipework for flow and return mixing
- Rebalance the system, upgrade undersized emitters where needed, and recommission with recorded settings
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