When it comes to ASHP efficiency, there’s often a lot of discussion and misunderstanding. SCOP (Seasonal Coefficient of Performance) is a key measure of efficiency, but what does it really tell us, and how can we interpret it? After analysing data from the top 20 heat pumps on heatpumpmonitor.org – all with SCOPs above 4.0 over the past 365 days – I’ve uncovered some insights that might surprise you.

What the Data Reveals (At Time of Writing)

1. Air Source Dominates: Of the top 20 heat pumps, 19 are air source, and only one is ground source.
2. System Volume is Often Overlooked: Surprisingly, 75% of users don’t calculate or record their system volume. For those that do, systems with 15 litres per kW of peak capacity or more perform best. The top-performing system has 16 l/kW. However, going far beyond 15 l/kW offers diminishing returns, and there isn’t enough data to confirm its benefits.
3. Manufacturer Diversity: The top-performing models come from a range of manufacturers, including Viessmann, Nibe, Vaillant, Grant, Samsung, Mitsubishi and Acond. This highlights that system design is often more critical than the brand.
4. Refrigerants Don’t Make a Big Difference: Among the top six ASHPs, there is an even split between those using R290 and R32 refrigerants, suggesting no significant efficiency advantage to either.
5. Single-Zone Systems Rule: Every system achieving a SCOP above 4.0 is a single-zone setup. Multi-zone systems seem to struggle to match this level of efficiency.
6. Flow Temperatures are Key: Systems with a maximum flow temperature of 45°C or lower dominate the top-performing list. Higher flow temperatures tend to drag down efficiency.
7. Modest DHW Settings Win: The top six ASHPs have domestic hot water (DHW) set to an average temperature of 45°C, reinforcing the importance of keeping DHW temperatures modest for better efficiency.
8. Building Efficiency is Less Relevant: Interestingly, building efficiency (measured in W/m2) varies wildly among the top systems, from 19 to 76 W/m2. This suggests that SCOP is influenced more by system design than the property’s insulation or efficiency level.
9. Supplementary Technologies Enhance Results: Over half of the top 20 systems either don’t heat DHW or use supplementary technologies such as hybrid boilers, photovoltaic (PV) panels or solar thermal systems to improve performance.

Notable Features in Top Systems

• Buffer Vessels: These are present in many high-performing systems.
• 10mm Microbore Pipework: Often considered suboptimal, this pipework appears in some top-performing setups, proving that careful system design can overcome perceived limitations.
• Thermostatic Radiator Valves (TRVs): Despite some reservations, TRVs are used in many systems, likely in specific zones like bedrooms.

Key Takeaways

From this analysis, a few clear lessons emerge:

• Aim for 15+ l/kW System Volume: This seems to be a reasonable rule of thumb for optimising efficiency.
• Focus on System Design: The manufacturer matters less than a well-designed, balanced and commissioned system.
• Refrigerant Choice Isn’t Critical: Both R290 and R32 perform equally well in top systems.
• Stick to Single-Zone Systems: Multi-zone setups don’t achieve the same efficiency levels.
• TRVs Are Acceptable: While they may reduce efficiency slightly, TRVs can work well if carefully implemented, likely in specific areas like bedrooms.
• Keep Flow Temperatures Below 45°C: Lower flow temperatures are crucial for high SCOPs.
• Lower DHW Temperatures: Reducing DHW temperatures has a substantial positive impact on system efficiency.
• Don’t Stress About Building Age or Efficiency: A correctly sized heat pump can deliver excellent results in properties with varying levels of efficiency.
• Supplementary Technologies Help: PV, solar thermal, and hybrid systems can boost overall system performance.