Our house faced its first real test this year. We had a heat pump installed in our home back in February. The weather finally turned very cold. We had snow and temperatures dropped below freezing. This was the perfect chance to see how the system performs under pressure. I watched our data closely and made a few small changes. Here is a full report on how our heat pump handled deep winter.

How Well Did the Heat Pump Perform?

Our house is a good test subject. It is a medium to large detached house in the UK. The total floor space is about 220 to 230 square meters. We are exposed to the weather with the coast nearby and open farmland around us. This means it can get quite cold.

The heat pump is a Vaillant aroTHERM plus, a 7 kilowatt system. It heats most of our home with standard radiators. We also have underfloor heating in the kitchen. The system was designed to work efficiently down to an outside temperature of -3°C.

Here are the key performance figures since it was installed:

• Total Heat Generated: 9.819 MWh (Megawatt-hours)
• Total Electricity Used: 2.377 MWh
• Average Seasonal Performance Factor (S.P.F.): 4.1

An S.P.F. of 4.1 is excellent. It means that for every 1 unit of electricity used, the pump provides over 4 units of heat. This is much more efficient than a standard gas boiler.

Real-World Results in Sub-Zero Cold

The real test came when the temperature fell below freezing. For the first time since installation, we saw temperatures as low as -4°C to -5°C. This was colder than the system’s design point.

The efficiency did drop, as expected. On a very cold day, the real-time Coefficient of Performance (COP) fell to around 2.38 to 2.43. This is normal physics for a heat pump. Colder outside air means it must work harder to extract heat.

Even with this lower efficiency, the system was still cost-effective. A COP of 2.4 means it is 2.4 times more efficient than a 100% efficient electric heater. When compared to a gas boiler, it can still be cheaper to run depending on the price of gas versus electricity.

Most importantly, the house stayed warm. We maintained a comfortable temperature of 20–21°C in every room. The system used a steady 3 to 4 kilowatts of power to achieve this. In freezing weather, that is a very good result.

Key Tweaks for Better Efficiency

I learned that you can improve your system’s performance with small adjustments. My main goal was to increase the Delta T. This is the temperature difference between the water leaving the heat pump (flow) and the water returning to it (return).

Aim for a Delta T of about 5°C. This shows your radiators are giving off heat efficiently. If the flow and return temperatures are too close, your system is just moving hot water around without heating your home well.

I achieved a good Delta T by balancing my radiators. I adjusted the valves on each radiator so heat was distributed evenly. Using monitoring software helped me see the impact in real-time.

Looking at other systems, many have a Delta T of only 3°C or 4°C. My installer confirmed this is common. I believe my tweak made our system more effective. It ensures we heat the house, not just the pipes.

Running a Heat Pump with Solar Panels and Batteries

Our heat pump is part of a larger home energy system. We also have solar panels and a home battery (a Tesla Powerwall). This combo helps manage running costs.

We use a smart energy tariff called Octopus Agile. Electricity prices change every half hour. The cheap periods are usually late at night. We use a third-party app called netzero to control our battery. It charges the battery to 100% when electricity is cheapest.

During the day, we use the stored solar or battery power to run the house. This includes the heat pump. On a sunny winter day, we can recharge the battery from the solar panels. This greatly cuts our dependence on the grid.

Energy Source	Percentage of Our Total Use (Since Feb)
Solar Generation	35%
Energy from the Grid	36%
Energy from the Battery	29%

The battery is set to “self-powered” mode during expensive peak periods (like 4-7 PM). It will not draw from the grid until the battery runs low or the cheap night rates begin. We also fix our solar export rate at 15p per unit. This setup is designed to use the energy we generate, not sell it.

Conclusion: Our Heat Pump Passed the Winter Test

This cold snap was the proof we needed. Our 7kW heat pump kept a large, detached house warm during sub-zero temperatures. The efficiency dropped in the extreme cold, but that is normal. The system still performed above its design expectations.

The takeaway is that a well-installed and finely tuned heat pump works. Pairing it with solar panels and a battery makes it even more economical. The key is monitoring your system and making small adjustments. Focus on getting a good flow-return temperature difference (Delta T). This ensures your home gets the heat.

For anyone considering a heat pump, my experience is positive. Even in a cold UK winter, it provided consistent, comfortable heat. It did so while being more efficient than traditional heating. With smart energy tariffs and a bit of tuning, it can be a great choice for a modern home.

Ready to share your experience? How is your heat pump performing this winter? Let me know in the comments what tweaks you’ve made to your system.