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Heat Pump SCOPs – The Truth Might Not Be What You Think

 
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Air Source Heat Pumps (ASHPs)
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 David Stanford
(@david-stanford)
Active Member Contributor
Joined: 2 years ago
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Topic starter 03/12/2024 11:19 am   [#1909]
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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.



   
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Mars
 Mars
(@editor)
Illustrious Member Admin
Joined: 5 years ago
Posts: 4474
03/12/2024 11:26 am  

Some super interesting findings. Not surprised about heating DW to 45C being the most efficient way to go. That's what we set ours to.


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 A E Deans
(@A E Deans)
Joined: 1 year ago
Posts: 4
03/12/2024 11:38 am  

Thanks Mars I am getting 3.7 Scop with an oversized Mitsubishi 8.5 kw heat pump . Still
Trying different settings to try and improve it. Have installed open energy monitoring and
It’s under Dalkieth. Set hot water 47c . Does anyone know if switching down pump speed helps presently 25.61 l/m
Cheers Andrew Scotland


This post was modified 1 year ago by Mars

   
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JamesPa
 JamesPa
(@jamespa)
Illustrious Member Moderator
Joined: 3 years ago
Posts: 4634
03/12/2024 12:27 pm  

↑

Does anyone know if switching down pump speed helps presently 25.61 l/m

Pump speed needs to be sufficient to carry away the energy, any less wont do, any more is wasting energy.

The energy carried away is flow rate (l/s) * specific heat capacity of the transfer medium (water or glycol as appropriate) * deltaT at the heat pump (normally designed for 5). 

Assuming water and 5C DT thats 25.61/60*4200*5 =8.9kW, so just abut right for an 8.5kW pump!

If the heat pump is oversized then there may be the potential to reduce the water pump speed.  Some heat pumps modulate pump speed according to energy demand.  If yours does or can then thats an option.  Otherwise you could try turning it down on the water pump itself but the heat pump may complain.  The energy saving from turning down the water pump is small however so this isnt likely to make a massive difference.


This post was modified 1 year ago 3 times by JamesPa

4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.


   
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 Gary
(@gary)
Reputable Member Member
Joined: 3 years ago
Posts: 350
03/12/2024 1:04 pm  

I have tried my 8.5kw Ecodan at various pump speeds its currently at pump speed 2 that gives 14L/min, the recommended flow rate for 8.5 Ecodan is 15.4L/min, to achieve the optimum dT of 8, not 5 that you see quoted.

Using the mass flow calc James quoted you get to 8.6kw at dT8 so that makes sense as well.

The advantage of using a lower flow rate is that the Ecodan doesn't have its own internal pump so ours is in the airing cupboard so the slower the pump the less noise it makes too


This post was modified 1 year ago by Gary

   
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bontwoody
 bontwoody
(@bontwoody)
Noble Member Contributor
Joined: 4 years ago
Posts: 1009
03/12/2024 6:28 pm  

Two other things that greatly effect SCOP but havent been mentioned are the climate where you are living and what you deem to be a comfortable internal temperature.
Someone living in London who is happy with a 19C indoor tempearture will have a much better SCOP than someone with exactly the same system living in the North of Scotland who likes a 22C room.


This post was modified 1 year ago by Mars

House-3 bed partial stone bungalow, 5kW Samsung Gen 6 ASHP (Self install)
6.9 kWp of PV
5kWh DC coupled battery
Blog: https://thegreeningofrosecottage.weebly.com/
Heatpump Stats: http://heatpumpmonitor.org/system/view?id=60


   
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 Mike H
(@mike-h)
Reputable Member Member
Joined: 4 years ago
Posts: 200
05/12/2024 7:00 pm  

Posted by: @david-stanford
↑

Aim for 15+ l/kW System Volume: This seems to be a reasonable rule of thumb for optimising efficiency.

Can you explain the mechanism for this effect? Could it just be coincidental rather than causal? System volume may prevent short cycling, but there are other ways of achieving that. 


This post was modified 1 year ago by Mars

   
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 A E Deans
(@A E Deans)
Joined: 1 year ago
Posts: 4
06/12/2024 8:58 am  

Yes I have room temperature 20.5 and live in central belt . Perhaps I was overthinking scop it’s gone back to daily cop of 4.5 . Will try lower pump speed as heat pump comes off and on 2 or 3 times an hour
thanks for commenting


This post was modified 1 year ago by Mars

   
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 A E Deans
(@A E Deans)
Joined: 1 year ago
Posts: 4
06/12/2024 8:58 am  

Thanks I will try that and look at results


This post was modified 1 year ago by Mars

   
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DREI
 DREI
(@drei)
Estimable Member Contributor
Joined: 8 months ago
Posts: 97
03/09/2025 11:00 pm  

A heat pump is only as good as the desired room temperature. Bearing in mind that optimal room temperature for cognitive function sits between 21 and 24 degrees, with average around 22/23 degrees. 45 flow temperature relates to around 42.5 mean water temperature (MWT). Make sure to use that with your heat loss calculation, desired room temp and radiator sizes for best outcome.



   
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DREI
 DREI
(@drei)
Estimable Member Contributor
Joined: 8 months ago
Posts: 97
03/09/2025 11:06 pm  

You can even go as low as 40c flow temp, if you want a high SCOP, as long as your radiators are sized correctly and you use the correct desired room temp, nothing is impossible. I don't think you will find any radiators that can do 40c flow rate and room temp higher than 21c, unless going to silly 3 meters K3 rads or very very expensive multi chamber rads (£10k per radiator).



   
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 Old_Scientist
(@old_scientist)
Honorable Member Member
Joined: 2 years ago
Posts: 391
04/09/2025 9:52 am  

Posted by: @drei
↑

You can even go as low as 40c flow temp, if you want a high SCOP, as long as your radiators are sized correctly and you use the correct desired room temp, nothing is impossible. I don't think you will find any radiators that can do 40c flow rate and room temp higher than 21c, unless going to silly 3 meters K3 rads or very very expensive multi chamber rads (£10k per radiator).

We run a standard flow temp of around 32C all year round (certainly between 30-35C) and achieve a comfortable (for us) 20C room temp. Our radiators are all of standard K2 types. Other than on the very coldest of nights, even running a flow temp of 30C overnight puts out more heat than required as we like a setback of 18C for sleeping.

As long as your emitters are large enough to dissipate the required amount of heat at reduced flow temps, very low flow temps are possible. The limiting factor for us is actually dissipating the ~3.5kW of heat produced by the heat pump when running at minimum during milder weather, hence the required 32C flow temps. Any lower and the radiators cannot dissipate that 3.5kW output flowing from the heat pump.

 


Samsung 12kW gen6 ASHP with 50L volumiser and all new large radiators. 7.2kWp solar (south facing), Tesla PW3 (13.5kW)
Solar generation completely offsets ASHP usage annually. We no longer burn ~1600L of kerosene annually.


   
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