The Consequences of Low Flow Rates in an Air Source Heat Pump
by Energie Solutions | May 2, 2025 | Air Source Heat Pumps, Heat Pumps, Renewable Energy, Technical Guides
Air Source Heat Pumps (ASHPs) are an increasingly popular solution for heating and cooling homes due to their energy efficiency and environmental benefits. However, for an ASHP to operate effectively, maintaining the correct flow rate is essential. A low flow rate can lead to a variety of issues that impact the system’s efficiency, performance, and longevity. In this blog, we explore the key consequences of low flow rates in an air source heat pump and why addressing them is crucial.
Increased Energy Consumption
When an ASHP operates with a low flow rate, it must work harder to maintain the desired output temperature. This increased workload leads to higher energy consumption and subsequently raises electricity bills. Homeowners may notice a significant spike in energy costs due to the pump running for longer periods to compensate for the inefficiencies caused by the low flow rate.
Cold House in Winter
Poor flow rates significantly and adversely affect the heatpumps unit to deliver the heat capacity that it is rated at. This means in cold weather the unit could be undersized and unable to maintain the desired temperatures in the house.
Increased Repair Costs
Poor flow rates result in the heatpump working harder for longer to deliver the heat output. This means the heart of the heat pump, the compressor, is running harder and longer to keep up with the heat demand. Which in the long-term leads to premature, costly replacement of the compressor or, worse still, the complete unit. Regular maintenance and ensuring proper flow rates can significantly extend the lifespan of an ASHP.
System Shutdowns
Heatpumps are sophisticated pieces of equipment with a number of temperatures and pressures are being monitored to ensure efficient operation of the unit. Poor flow rates can lead to system outages with common fault codes of E911, H7, P1, U1, P8.
Risk of System Freezing
During colder months, a low flow rate can cause the evaporator coil to freeze due to inadequate heat exchange during the defrost cycle. When this happens, the ASHP may enter defrost mode more frequently or ultimately not be able to full defrost the unit, further reducing its efficiency and ability to heat the property.
Shortened System Lifespan
Continuous operation under low flow conditions places additional stress on the compressor and other internal components. Over time, this can accelerate wear and tear, leading to premature system failure. Regular maintenance and ensuring proper flow rates can significantly extend the lifespan of an ASHP.
How to Prevent Low Flow Rate Issues
To avoid the negative consequences of low flow rates, regular maintenance is crucial. Homeowners and installers should:
- Ensure the system is installed to manufacturers required flow rate on installation – as a rule of thumb, the correct flow rate is usually 3x the rating of the heatpump. E.g a 5kw heatpump needs a flow rate of 15lpm
- Regularly check and clean filters to ensure unobstructed airflow
- Inspect and maintain pumps, valves, and pipes to prevent blockages
- Ensure the system is correctly sized for the home’s heating and cooling needs
- Monitor system performance and address any warning signs early
By maintaining an optimal flow rate, air source heat pumps can continue to operate efficiently, saving energy costs and ensuring long-term performance.
Conclusion
Low flow rates in an air source heat pump can lead to a range of efficiency, performance, and reliability issues. From increased energy consumption to premature system failure, the consequences can be costly and inconvenient. Regular maintenance and proactive monitoring are key to preventing these problems and ensuring that an ASHP operates at its best. Investing in proper system care will help homeowners maximise their heat pump’s efficiency, longevity, and comfort benefits.
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