Heat Pump Running Constantly — Is It Normal?

This is one of the most common questions homeowners ask after having a heat pump installed. And it is a fair question, because heat pumps behave very differently from the gas boilers most people are used to.

The short answer is that heat pumps are designed to run for much longer stretches than a boiler but running non-stop, 24 hours a day, without ever switching off, is not normal. In most cases it indicates a system that is struggling rather than one working efficiently.

A boiler fires up, heats the water quickly to 70 or 80°C, and shuts off. A heat pump works at a much lower intensity over a longer period. On a cold January day in the UK, a well-configured air source heat pump might run for 10 to 14 hours. On a mild day in October, perhaps 4 to 6 hours. But there should always be periods where the system satisfies the thermostat and stops or at least modulates down to minimum output.

If your heat pump has been running solidly for days on end and the house is still not reaching temperature, something in the design or configuration needs attention.

The system may be too small for your home

This is the most fundamental cause, and unfortunately it is not uncommon. Heat pump sizing should be based on a proper room-by-room heat loss calculation not on your old boiler's output, not on floor area alone, and not on an installer's estimate from looking at the outside of the house.

A heat loss calculation accounts for wall construction, insulation levels, window area, ventilation rates, and the local design temperature (typically minus 2 to minus 3°C for most of England, colder in Scotland). The total heat loss figure tells you how many kilowatts the heat pump needs to deliver at peak demand.

To give a rough sense of scale: a well-insulated modern three-bedroom semi-detached house might have a heat loss of 5 to 6 kW. A typical 1930s semi with partial insulation could be 8 to 10 kW. A poorly insulated four-bedroom detached property might be 12 to 15 kW.

If a 5 kW heat pump has been installed in a home that actually needs 10 kW, no amount of tweaking will fix the problem. The system physically cannot deliver enough heat, so it runs at full capacity all day trying to compensate.

We reviewed a system last year in Essex a 1960s end-terrace with a 5 kW Mitsubishi Ecodan. The property's calculated heat loss was 9.2 kW. The heat pump had never been able to maintain 21°C in the living room during cold spells, and the backup immersion heater was activating several times a week. The homeowner's electricity bill for heating alone was over £200 per month in winter. The root cause was straightforward: the system was undersized by nearly half.

Your radiators may not be large enough

Even when the heat pump is correctly sized, the radiators need to be able to deliver that heat into the rooms at the lower water temperatures heat pumps operate at.

The key concept here is Delta T the difference between the average radiator temperature and the room temperature. Boilers run at a Delta T of around 50 (flowing at 75°C, room at 20°C). Heat pumps typically run at a Delta T of 20 to 25 (flowing at 40 to 45°C, room at 20°C).

At Delta T 20, a radiator produces roughly 40 per cent of its rated output. So a radiator rated at 2000 watts at boiler temperatures only delivers about 800 watts at heat pump temperatures.

If the radiators were not upsized when the heat pump was installed, some rooms will never reach temperature. The heat pump keeps running because the thermostat is never satisfied, even though the heat pump itself is working perfectly.

The usual fix is to upgrade radiators in the rooms that struggle most typically north-facing bedrooms, rooms with large glazed areas, or rooms at the end of long pipe runs. In many cases, upgrading two or three radiators is enough to transform system performance.

Flow temperature may be set too high

Sometimes we see systems where the flow temperature has been fixed at 55 or even 60°C. The installer has done this to compensate for undersized radiators or poor insulation — it gets the house warm, so the homeowner does not complain initially.

The problem is that efficiency drops sharply above 45°C. Every degree higher costs more electricity. A system running at 55°C might achieve a COP of 2.0 to 2.5, while the same system at 40°C could achieve a COP of 3.5 to 4.0. That is the difference between paying 10p and 6p per kilowatt-hour of heat.

When the flow temperature is locked this high, the system also tends to run constantly because it is working much harder than necessary. The correct approach is to address the underlying cause — whether that is radiator sizing or insulation rather than masking it with high temperatures.

Weather compensation may not be working

This links directly to our guide on weather compensation settings. When weather compensation is properly configured, the heat pump automatically reduces its flow temperature on mild days and only ramps up when it is genuinely cold outside.

When weather compensation is disabled or misconfigured, the system runs at the same intensity regardless of the weather. On a 12°C October day, it pushes 50°C water through radiators designed for minus 2°C conditions. The thermostat keeps cutting the system off and on, the heat pump cycles inefficiently, and it can feel like the system is running all the time even though it is constantly stopping and starting.

In one case we investigated, short cycling was caused entirely by a zoning design problem — multiple zones closing at once created dangerously low flow conditions. Our heat pump short cycling zoning design issue case study covers the full diagnosis and fix.

Correcting the weather compensation curve is often the single most effective change for a system that seems to run too much. It costs nothing and typically improves both comfort and efficiency within days.

For a full breakdown of how to check whether your curve is active and how to adjust it, see our dedicated guide on weather compensation settings for heat pumps. Getting this right is often the single most effective change — and it costs nothing to fix.

The heating circuit may not be balanced

If some radiators get too much flow and others get too little, the rooms with restricted flow stay cold. The room thermostat in those areas never reaches the set temperature, so the heat pump keeps running.

A quick diagnostic: check whether some rooms are always noticeably warmer than others, even with all TRVs set to the same position. If the living room is 23°C and the back bedroom is 17°C, the system almost certainly has a balancing issue. The fix is to partially close the lockshield valves on the radiators that are running hot, which redirects more flow to the ones that are running cold.

How to tell if your system genuinely has a problem

Ask yourself these questions: Does the heat pump run all day even when outdoor temperatures are above 8°C? Does the home never quite reach the thermostat set point, or only reaches it in some rooms? Does the backup electric heater activate regularly outside of deep winter? Are your electricity bills significantly higher than the estimates you were given before installation?

A real example of this is a system that kept running throughout summer due to a control wiring fault — see exactly what happened in our heat pump running in summer wiring fault case study.

If you answered yes to two or more, the system most likely has a design, sizing, or configuration issue that a technical review can identify.

If you answered yes to several of those questions, it is also worth reading our guide on how to tell if your heat pump was installed correctly — it walks through the most common signs of a poor installation and what they usually mean in practice.

What happens in a review

We look at your heat loss against the installed heat pump capacity, your radiator sizing against the required output at heat pump flow temperatures, your current weather compensation and flow temperature settings, and the balance of heat across different rooms. From there, we give you specific, prioritised recommendations starting with the changes that will make the biggest difference for the least cost.