Hi all. I’m trying to get the data together for Passivhaus certification. I’ve got the datasheet for the air to air minisplit in my house (attached here). There are two different sets of COP data for heating, one for ‘Average’ season and one for ‘Warmer’ season. At the same outdoor and indoor temps the Warmer season one gives a worse COP, not by much for outdoor temps of 7°C and 12°C, but the difference is massive at 2°C (COP of 5.3 for the average season, 3.4 for the warmer season).
Does anyone know what these two seasons refer to, and why there would be such a big difference in COP between them?
Addenendum: The heating capacities are also very different between the two different seasons, at the same temperatures. The warmer season has higher heating capacities. Strangely, the heating capacity goes up as the outdoor temperature goes down.
‘Average’, ‘Warmer’ and (potentially) ‘Colder’ climates relate to the nominal test conditions defined in EN 14825, the European standard which covers seasonal performance testing of Air-to-Air heat pumps.
The full text of EN 14825 is problematic to access unless you’re willing to pay (although some libraries might have copies). However, there are open-access documents which provide guidance on testing heat pumps in accordance with this standard; I reckon this one from the Danish Energy Agency (in English) is pretty good:
It’s evident the Mitsubishi doc you shared was prepared in accordance with the same methodology.
Basically they’ve used historical temperature profiles from three European cities:
Hi David. That’s really helpful, thanks. I can see why the different climates would have different SCOPs, but why would they result in different COPs and heating capacities at the same source and sink temperatures?
Then, when they measure the COP at different temperatures, they’re taking account of these different Heating Capacities and (generally) running the units at part-load. That’s described in this part of the Danish guidelines:
It seems reasonable to me that the measured COP would be higher when the same unit is moving less than half as much heat at the same temperatures - especially compared with running at maximum capacity (which I think is why the differences are less pronounced at other temperatures).
If it’s able to modulate down to the lower output without cycling then it’s not going to incur the COP penalty of starting and stopping cycles.
I suspect defrosting is the bigger effect: we know that 2C can be a problematic temperature for defrosting, and we know that units which are working ‘flat out’ tend to frost up much more than those running at part load.
@dMb thanks for taking the time to explain that to me in so much detail. Really appreciate it. I think I could have got there in the end but it would have taken me ages. Shout if I can ever return the favour!
@Ringi My experience of running the unit is that it is able to modulate down quite well. I can’t monitor heat output but in terms of electricity input the maximum is about 1400 W and it can go as low as about 210 W before it starts cycling. Below that the cycling is a real problem, but I’m hoping I can sort that out with Home Assistant and some coding.
I think this unit can modulate lower than mine, down to about 100 W apparently. And maybe better at hitting the right temperatures too (mine was very inaccurate until I dug the thermister out of the casing). More expensive though. MSZ-LN_R32_APRIL_2023.pdf (533.9 KB)
You’re most welcome Es. I learned a lot from all the info you shared during your Passivhaus retrofit project so I reckon we’re even - and I’m sure this topic will come up again so it’s useful that we have the information recorded for posterity.
I’m planning on installing an Air-to-Air unit in a new, detached workshop currently being built adjacent to my house so it’s good to hear real-world experience of Air-to-Air systems to complement the wealth of Air-to-Water experience from other users.
- and I’m sure this topic will come up again so it’s useful that we have the information recorded for posterity.