Heat Pump Not Heating the House — Causes & Fixes
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Heat pumps are brilliant for cutting energy bills and carbon footprint, but they work very differently from the gas boilers most UK homes have relied on for decades. When a heat pump is installed—especially as a retrofit—homeowners sometimes find the house doesn't warm up as quickly or reach the same cosy temperature they're used to, even though the unit appears to be running normally. This can be alarming, but it's often down to settings, sizing, or commissioning rather than a faulty unit. The good news is that most causes are straightforward to diagnose and fix. This guide walks you through the logical checks you can do yourself, and flags when you'll need a qualified MCS-certified heat pump engineer to step in—which is particularly important because refrigerant work and fault diagnosis require specialist training and certification.
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Most likely cause & what to check
Check the flow temperature setting — heat pumps run at much lower flow temperatures than gas boilers (typically 35–50°C vs 60–80°C). If your system was previously heated by a gas boiler, the controls may have been set too low for your radiators. A correctly commissioned heat pump should still heat the home, but it takes longer and runs for more hours per day than a boiler would.
Check the weather compensation curve — most modern heat pumps (Mitsubishi Ecodan, Daikin Altherma, Vaillant aroTHERM, etc.) use weather compensation to automatically adjust flow temperature based on outdoor temperature. If the curve is set too conservatively, the house will underperform on very cold days. An MCS-certified heat pump engineer can recalibrate this.
Check the room thermostat or controller — the heat pump will not run if the thermostat target temperature is already met. In mild weather this is normal. If the house is genuinely cold and the stat is still calling for heat, move to the next step.
Check radiators are correct size: heat pumps working at 45°C flow temperature produce significantly less heat per radiator than a boiler at 70°C. If radiators were not upsized when the heat pump was installed, they may be undersized for the heat load. An MCS engineer or heating designer can do a heat loss calculation to confirm.
Check the heat pump is running: go outside and listen — the outdoor unit fan should be running when the heat pump is in heating mode. If the unit is completely silent and not running, there may be a fault code on the indoor controller. Note the code and refer to the manufacturer's manual.
If the system has a buffer tank or cylinder, check these are reaching the correct temperature. If the buffer tank is cold, the heat pump may be locked out on a fault. An MCS-certified engineer should diagnose any fault codes — many heat pump faults require specialist knowledge and F-Gas certification.
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Frequently asked questions
Why is my heat pump not heating the house as well as my old boiler did?
Heat pumps run at much lower water temperatures (typically 35–50°C) compared to gas boilers (60–80°C), which means they deliver heat more slowly and continuously rather than in quick bursts. If your radiators weren't upsized during installation, they may not be large enough to transfer sufficient heat at these lower temperatures. This is normal and fixable—an MCS engineer can assess your heat loss and advise on radiator upgrades if needed.
Should my heat pump run constantly, or only when needed like a boiler?
Heat pumps run for longer periods and more frequently than boilers, because they're moving heat rather than generating it rapidly through combustion. This is completely normal and actually more efficient. If your system seems to run excessively without reaching temperature, that suggests an undersizing or settings issue that needs professional diagnosis.
What's weather compensation and why does it matter?
Weather compensation automatically adjusts the heat pump's flow temperature based on how cold it is outside, so it runs efficiently in mild weather and works harder on freezing days. If the curve is set too conservatively, your home will feel chilly during cold spells. Your installer should have commissioned this correctly, but if performance drops in winter, ask an MCS engineer to review the settings.
How do I know if my heat pump is actually faulty or just undersized?
Check that the outdoor unit fan is running when you expect heating (listen outside), and look for any fault codes on your indoor controller. If the unit is silent and cold, or displays a code, it needs professional diagnosis—many faults lock out the system safely and require an F-Gas certified engineer. If the unit is running smoothly but just heating slowly, it's more likely a commissioning or sizing issue.