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Performance of Heat Pumps in Mild Weather

 
Air Source Heat Pumps (ASHPs)
Last Post by RobS 9 hours ago
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 Heatgeek
(@heatgeek)
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Topic starter 05/04/2025 12:41 pm  

For those that may be interested, I am attaching graphs of the performance of different heat pumps during milder weather as it gives an indication of their cycling behaviour.

1. 12kW Vallaint HP in Surrey, a top performer. This is characterised by a fixed high water flow which switches on and off in cycles. In this case the flow has been turned down by the user to 28 l/min to provide a maximum output <10kW. Minimum power is around 5kW with DT very low.

Graphs Vaillant 12kW

2. 10kW Vaillant with fixed flow set at maximum output. This exhibits similar cycling behaviour to the previous one with minimum power 5kW, although the target room temperature is not known. DT is again very low.

Graphs Vaillant 10kW

3. 5kW Samsung Gen 6 of @bontwoody. This exhibits more stable behaviour without much cycling. Flow has been modulated down by the HP from the maximum of 14 l/m to 7l/m providing minimum output of 2kW with DT around 3.3C. Instead of modulating the flow, the HP sets a fixed flow of 9 l/m, down from the max of 14 l/m, and modulates the DT which is contrary to the normal belief that it maintains a steady 5C DT and modulates the flow. It may be that the minimum flow is set above the 7 l/m flow switch cutoff so that it can provide a lower power output. Interestingly, the Samsung outperforms Vaillant on COP. Another characteristic of the Samsung is a high turn-on power which is sustained for a period before turning down to 2kW. This ramps up temperature faster but the downside consequence of this is seen in the next example.

Graphs Samsung 5kW

4. 8kW Samsung Gen 6 in Shropshire. The consequence of rapid cycling with the Samsung is seen here. At each initiation, there is high power output which merges and destroys COP. This shows the way not to do things. It is not known why the off cycle is so short, but the consequence is terrible.

Graphs Samsung 8kW

5. 10kW Viessmann Vitocal in Sheffield which is the best performing UK heat pump, largely due to the fact that room temperature is maintained at 18-19C, which flatters on COP. The unit cycles regularly but, unlike the Samsung it has a very short heat spike before settling on a minimum power of 3.8kW with a relatively low DT of 3.3C. The HP maintains a constant flow at 15 l/m, which is modulated down from the maximum of 28 l/min (5C DT).

Graphs Viessmann 10kW

6. 8kW Daikin Altherma in Farnborough. Like the Samsung, it has a short period of high power output before modulating flow down to 7 l/min from max of 24 l/m with a steady minimum power output of 1.6kW with DT 3.5 to 5C without cycling. This has the most stable performance with the lowest minimum power output. Counterintuitively, the COP does not reflect this performance but may be somehow due to the very high room temperature of 22-23C, even though this should not affect COP. I have not tried to analyse this. However, maybe this makes a case for a thermostat to maintain a reasonable room temperature which would improve heating bill, rather than an overheating clamp.

Graphs Daikin 8kW
This topic was modified 17 hours ago by Heatgeek
This topic was modified 16 hours ago by Heatgeek

Private individual. No affiliation with commercial "Heat Geeks" of same coincidental name.


   
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 Grantmethestrength
(@grantmethestrength)
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05/04/2025 3:09 pm  

This is quite interesting as my system cycles a lot and we have a flow rate of around 28l/min and this is on medium pump setting. We usually have a low DT, I have it set to constant night mode to stop it using stupid amounts of energy and this seems to be working. I wonder if I reduce the pump to it’s lowest setting it will help with the cycling or if I should ramp it up again and get higher flow rates. The learning curve is steep with ASHPs!

Kind Regards
Si
——————————————————————————
Grant Aerona3 13kW
13 x 435w Solar Panels
Solax 3.5kW Hybrid Inverter


   
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 Heatgeek
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Topic starter 05/04/2025 7:18 pm  

 

@grantmethestrength   Do you know what your DT was when you were heating at -2C outside? This will give an indication of what your peak heat load was. Grant are unusual for a heat pump manufacturer as they specify that the peak DT should be set at 8C as the flow is fixed. You can calculate your heat load with the following formula:

Power kW = Flow(l/m)/60 x DT x 4.2

For 13 kW power and 28 l/m, your max DT would be 6.6C. For 8C DT, the flow would be 23 l/m assuming that you can use all the 13kW power at -2C. If your heat pump is over-sized, then the flow should be even lower. Grant state that this is how a system should be set up to match the power output required. Bear in mind though that your minimum flow is 15 l/min. The reason for the 8C is because the flow is fixed and as days get warmer, the DT of the HP will decrease to ensure reduced power. The 8C avoids too low a DT at low power for example. IMHO Vaillant users should also adopt this policy but no one hears.

To understand the effects, look at the example of a Grant Aerona I have given which is not performing too well. At just 5C OAT, the DT is 1.6C with flow of 34 l/m. If you do the math you will see that the power output of 3.8kW matches. 1.6C is way too low. When cycling there are big power peaks and short operation times – not good for COP as the results show. So in this case the flow needs to come down to widen the DT. It looks over-sized. They are also not using weather compensation as the flow temperature is constant which makes matters even worse. Too much heat output on warm days causing short cycling.

I don’t know your circumstances but you can do the math. My gut feel tells me that you need to reduce the flow further as per the above calculations. Your DT at lowest outdoor temperature will give you the best indicator according to Grant recommendations. Besides the pump setting, you can also adjust flow on the flow meter. Hope this helps.

Graphs Grant Aerona 3

 

grant-aerona-r32-installer-uk-doc-0136-rev-2-4-february-2024.pdf
This post was modified 10 hours ago 2 times by Heatgeek

Private individual. No affiliation with commercial "Heat Geeks" of same coincidental name.


   
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 RobS
(@robs)
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05/04/2025 8:29 pm  

Posted by: @heatgeek
↑

3. 5kW Samsung Gen 6 of @bontwoody.

...

Interestingly, the Samsung outperforms Vaillant on COP.

As both Vaillants do DHW but the Samsung doesn't, you'd expect the daily mean COP of the Vaillants to be less. Also the first Vaillant does a defrost cycle demonstrating that the environmental conditions weren't the same. 

An example of constant running with a low dT without a very low (7-9 lpm) flow rate:

2025 04 05 18.43.47 emoncms.org 5fbe3aa8 dc4d 4353 89e7 f6b068082965 1

This is a Mitsubishi BTW. 

 


   
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