There’s a common belief that ASHPs might falter as the temperature drops. We had a chat with Vital Energi’s Elliott Sharpe (strategy and partnerships director), Dave Wilkinson (design director), Chris Green (engineering director), and Liam Grice (senior engineer), who advised how you can maximise a heat pump’s efficiency during cold weather, the best location for it, how you can prepare it for cold weather, and more.
Air source heat pumps are one of the most effective technologies for reducing carbon. They harness the natural heat energy in the air to warm up a space. They work by extracting heat from the outside air and transferring it into a building. The pump uses a working fluid (refrigerant) to absorb heat from the outdoor air, which is then compressed to increase its temperature. This refrigerant is then circulated through a heat exchanger to distribute this heat into a building.
So, if they absorb heat from the air, how are they affected when it’s cold outside? You might think that if the temperature outside falls below zero, the heat pump will stop working; however, even in cold air, there is still sufficient heat for the heat pump to absorb and convert into useful energy.
How efficient is an air source heat pump in cold weather?
When discussing the efficiency of an air source heat pump (ASHP), we often consider how many units of heat we get from an ASHP for each unit of electricity used. Because it gets energy from the surrounding air, we might get 3 units of heat for every 1 unit of electricity, so an efficiency of 300% in the summer months. As the outdoor temperature drops as we head towards winter, the efficiency of an ASHP does reduce. As this reduces, we could find a situation where the heat pump produces 1 unit of heat for every 1 unit of electricity, which is 100% efficient, this might not sound all that bad, but heat pumps can deliver much higher efficiencies than this when deployed correctly.
How do you maximise efficiency during winter?
Generally, the heat pump operates at its worst efficiency during cold weather when most heat is required. Careful design of the heat pump and its connected system is important to maximise efficiency. Understanding that heat pumps may have a reduced capacity at low temperatures is important in correctly sizing the heat pump to cope with this. If you buy a 300kW boiler, that boiler is 300kW all year round. That’s not quite the case for an ASHP, which might provide 300kW in a +10°C ambient temperature and only 150kW during the coldest days. The efficiency of heat pumps increases as their supply temperature reduces, so it’s beneficial to design heat emitters to operate effectively at lower temperatures, e.g., underfloor heating can work on a 45C flow. Including buffer vessels or thermal storage can also provide flexibility, lowering the heat pump’s heating cost. Optimisations of the heat pump defrost cycle are essential for achieving the best winter performance and should be carefully considered.
Is there anything you can do to prepare your heat pump for cold spells?
Absolutely! The main objective should be to ensure that the heat pump operates as it should. Proactive maintenance will assist with this, where the entire system should be inspected, refrigerant and oil levels checked, and any issues addressed. Consider scheduling maintenance before winter begins to ensure optimal performance.
The heat pump will enter a defrost cycle more often in cold ambient temperatures. The air source collector should be unrestricted from debris for optimum efficiency and performance. If you notice any excessive ice build-up on the air source collector, it might suggest that the defrost cycle may not function correctly.
Is there an optimum position or location where an ASHP will perform better during colder months?
ASHPs use fans to move ambient air over the collector. Any restrictions to the airflow will reduce performance. The air is cooled as it passes over the collector, so it is important to minimise air recirculation. Manufacturers’ guidance should be followed to ensure sufficient free space around the heat pump and consider any additions, such as acoustic panels and their impact on air circulation. Computational Fluid Dynamics (CFD) modelling is a useful investment to make during the design process to make sure the air source collector position is optimise
Are there specific refrigerants or technologies that can improve winter performance?
When choosing a heat pump, assessing its suitability for various operating conditions over its lifespan is crucial. Understand your load profile to determine peak performance needs. The system’s heat output and efficiency will decrease if the ambient temperature drops below the design threshold. Consider these factors carefully to select the optimal heat pump technology and refrigerant that precisely meets your requirements.
How can defrost cycles be optimised to reduce energy consumption and output limitations?
Typically, when ambient temperatures are below 7ᵒC, ASHPs must regularly complete a defrost cycle to remove frost, which forms on the coil surface as moisture from the air freezes. The process requires energy to melt the frost, and generally, the heating output is reduced during this time, so the process must be completed quickly and efficiently. This can be optimised as part of the commissioning and O&M activities, ensuring all temperature probes are fitted correctly and the settings are correct. Hence, the defrost process is no longer frequent and is longer than needed. It is a balance because not defrosting correctly causes severe performance issues. If all the frost has not melted and drained away, this will refreeze and eventually create ice, which blocks the coil. Manual intervention is often then needed to get the system running optimally again.
Are there any energy conservation measures that can be implemented during the winter months which will help a heat pump run more efficiently?
There is a term often used, which is ‘fabric first’. This means the first port of call for any project should be to try and reduce your energy demands before looking into any new technology. This fabric-first approach could improve the performance of your windows to reduce how much heat you lose. It could mean adding more insulation to walls and ceilings. All of these measures will require less energy for your site/building.
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