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Mitsu PUHZ-SW120YHA + EHSC-VM20Uk - Performance

Air Source Heat Pumps (ASHPs)

Last Post by JamesPa 2 months ago

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DavidAlgarve

(@davidalgarve)

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Posts: 219

Topic starter 30/12/2025 10:23 am

I would appreciate any comments on the performance of my ASHP as demonstrated by the following recent graph from Melpump.

On the one hand, I am particularly struck by how sensitive the control is to minute OAT changes.

On the other hand, I wish the heat output could be steadier for inside heat demand although Havenwise are critical of the granularity of the Mitsu PAR 60 (+/- 0.5C) and this may be part of the problem.

The location of the PAR 60 has proved to be critical, but in general we now have acceptable temperatures upstairs and down and N vs S.

COP is another matter and daily figures at these ambients (5-10C night time) are between 2.6 and 2.9 and perhaps I could expect higher.

I will have to decide if I can do better with straight weather compensation or adaptive but the PAR 60 has been disappointing in its performance and application.

342sq m "Upside down" house in Algarve. Portugal
Mitsubishi PUHZ-120YUK 12kW ASHP
12 Solar Panels Growatt Inverter
2 x Growatt 7.5kW Batteries
Fronius EV Charger
Kia e- Niro 64kW

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JamesPa

(@jamespa)

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Joined: 3 years ago

Posts: 4388

30/12/2025 1:06 pm

@davidalgarve

Fwiw my house is more stable with pure WC than it is with any room influence , with the exception of the extreme ends of the season where solar gain can dominate. That's not entirely surprising given the fact that the house takes 12hrs plus to react to changes, so any iat based control system is compromised from the outset due to this delay. With the boiler, which didn't feature WC and therefore only had iat based control, stability was much worse.

You seem to be running at a pretty high ft and a pretty high deltaT flow/return despite mild oat. That's going to compromise cop quite considerably. Are your emitters large enough and have you got a buffer tank/LLH. How and where are you measuring COP?

This post was modified 2 months 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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DavidAlgarve

(@davidalgarve)

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Joined: 2 years ago

Posts: 219

Topic starter 30/12/2025 3:57 pm

@jamespa Thanks for your comments James.

The answer to your question is not quite straightforward. We live mainly on the 1st floor which is either South facing or gets some sun early or late and the emitters in these rooms (Roca alumnium) are probably adequate.

The downstairs rooms were converted when we moved in 20 years ago and consist of a family room and master bedroom/ en suite.

These rooms are North facing and the emitters are probably not big enough, but I could not contemplate changing them at this stage and we don't need them as warm as our main upstairs living room

It has been necessary to locate the PAR 60 in the coldest room (our bedroom!) where it is chasing 23C but never getting much above 19C.

It is subjective but I think we had less difficulty heating downstairs last year with weather compensation and I see that I achieved a COP of 3.3 in December 2024 even with a buffer tank! However, I am now heating up a large DHW tank at some time in the night as you can see on the graph.

342sq m "Upside down" house in Algarve. Portugal
Mitsubishi PUHZ-120YUK 12kW ASHP
12 Solar Panels Growatt Inverter
2 x Growatt 7.5kW Batteries
Fronius EV Charger
Kia e- Niro 64kW

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JamesPa

(@jamespa)

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Joined: 3 years ago

Posts: 4388

30/12/2025 4:16 pm

Posted by: @davidalgarve

↑

@jamespa Thanks for your comments James.

The answer to your question is not quite straightforward. We live mainly on the 1st floor which is either South facing or gets some sun early or late and the emitters in these rooms (Roca alumnium) are probably adequate.

The downstairs rooms were converted when we moved in 20 years ago and consist of a family room and master bedroom/ en suite.

These rooms are North facing and the emitters are probably not big enough, but I could not contemplate changing them at this stage and we don't need them as warm as our main upstairs living room

It has been necessary to locate the PAR 60 in the coldest room (our bedroom!) where it is chasing 23C but never getting much above 19C.

It is subjective but I think we had less difficulty heating downstairs last year with weather compensation and I see that I achieved a COP of 3.3 in December 2024 even with a buffer tank! However, I am now heating up a large DHW tank at some time in the night as you can see on the graph.

All understood.

Obviously its a bit house dependent but many people do find that pure weather compensation is the way to go both for comfort and cost so, if you do come to this conclusion, don't be surprised.

The manufacturers of smart controls have been misleading us for decade, certainly in the UK, into micro-zoning in time and space. For many all it does is reduce comfort whilst increasing costs or at best not materially reducing them. Even with my old gas boiler dropping all the fancy controls and just operating it 24x7 at the lowest flow temperature it could saved about 10% and improved the comfort of the house!

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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