Every heat pump needs to accommodate defrosting, which is a completely natural process. Your heat pump runs its outdoor coil about 8°C below the air temperature.
If the outdoor coil is below 0°C, it will freeze the water in the air, causing ice to build up. Defrosting is most frequent in wet, cold conditions – especially when it’s foggy and around 3°C. Paradoxically, when the temperature drops below -2°C, defrosting occurs less often because there’s less moisture in the air, meaning ice build-up happens more slowly.
A heat pump defrosts by reversing the system, pulling heat out of the water in the house. If there isn’t enough heat in the water, the unit struggles to defrost. If this becomes a persistent issue, the heat pump may not fully defrost, turning into an ice block over time. This problem is caused by poor heating system design and inadequate software. The key to effective defrosting is ensuring the unit receives sufficient heat. The hotter the water in the radiators, the quicker and more effective the defrosting process.
What actually happens during defrost?
The defrosting algorithm works roughly like this: The system checks whether it has been running for more than 47 minutes since the last defrost, whether the coil temperature is below 0°C and still falling, and whether the water temperature is either stabilising or slowly falling. If all these conditions are met, the defrost cycle starts.
At this point, the compressor either stops or slows down considerably, the outdoor fan halts, and the reversing valve switches the direction of the refrigerant. The once frozen coil now receives warmth, while the heat exchanger that was previously heating the house begins to cool. The outdoor fan stops to allow the coil to heat up quickly, and the refrigerant pressure rises as the compressor ramps up. This pressure difference is crucial because the valve doesn’t move mechanically; it’s controlled by pressure.
After a few minutes, the coil heats up, the ice melts and water drains from the unit onto the ground. The system monitors the coil temperature, and once it rises a few degrees above freezing, defrosting is complete. The valve switches back, and the unit resumes heating, with the outdoor fan restarting, the compressor ramping up, and the freezing process starting again.
Ensuring defrost efficiency
The water in the system never stops circulating. If the water temperature drops too low during defrosting, the unit struggles to resume normal heating. This happens because the system relies on a pressure differential to switch the valve back to heating mode and get the refrigerant moving.
To avoid problems, it’s vital that the water in the heating circuit isn’t too cold when coming out of defrost. The heat from the water is what creates the necessary pressure and temperature difference.
If you’ve ever encountered a unit that doesn’t fully defrost, it’s likely due to insufficient heat being available to defrost the coil within the required time, causing the system to get stuck in a loop. To prevent this, manufacturers recommend a minimum water volume in the system. A large buffer tank can help, as it ensures the water temperature doesn’t fall too much during defrost. However, this approach isn’t very imaginative – it’s essentially solving the problem with a bulky, unsightly tank. There has to be a smarter solution.
For example, we could send a signal from the heat pump to the heating system, making all zones and underfloor loops open up and activate their pumps. This would provide the necessary water volume without the need for a buffer tank – cheaper, simpler and more compact.
Some manufacturers install an electric heater in the heat pump to boost the water temperature during defrost. While this works, it’s expensive to run and reinforces the notion that heat pumps can’t function without auxiliary heaters.
Another option could be to create an artificial pressure differential. Instead of focusing on the water temperature, the cold coil pressure could be reduced by running the compressor at a higher speed, dropping the evaporator temperature (the outdoor coil). This method works but is energy-intensive, and it accelerates freezing, requiring more frequent defrosts.
More advanced solutions
One common solution in large commercial systems (around 30kW) is to split the refrigeration circuit into two. This allows one half to defrost while the other continues heating, avoiding the need to take heat from the water circulating through the building. Once one side finishes defrosting, the other side can start. Although this requires more hardware and clever control, it could easily be applied to domestic heat pumps, albeit at a higher cost.
There’s also a more elegant solution that, unfortunately, no one seems to implement. When a heat pump goes into defrost mode, the fan is stopped to allow the coil to heat up quickly – a process called head pressure control, which adjusts the pressure via the fan. We could apply a similar method with the water pump.
Once defrost is complete, the water pump could either stop or run at a very slow speed. This would allow the heat exchanger to heat up and pressurise quickly, allowing the unit to switch the valve and return to heating much faster. Modern heat pumps can already monitor flow rate, water temperature, and, in the better models, refrigerant pressure. The water pump could modulate to maintain pressure and ensure smooth operation.
But for now, this remains a dream. We continue to use buffer tanks as a ‘just in case’ measure, which feels like using a sledgehammer to crack a nut. There’s no allowance for system temperature, and many manufacturers recommend the same size buffer regardless of the heat pump’s capacity.
Final thoughts
Whatever system you have, I recommend monitoring the operation of your heat pump. Make sure it isn’t cycling too much and that the water doesn’t get too cold during defrost. If your system has the capability, you can check this in the app.
If not, consider adding monitoring equipment. I recommend Planet Devices, which provides all the data you need, and soon, it will offer insights into system volume and defrost performance as well.
