@jamespa Thank you, just need to get it fixed!

@ecoste From the graphs I wonder why its heating water DHW to 55C from midnight, is that when you want it super hot? Should take around 45 minutes to heat the 200L tank. A 10kW heat pump uses max 2.5kW/h if flat out  I think there are back up heaters being energised, but hard to tell without seeing the set up

@dgclimatecontrol I am on an Octopus Intelligent Go Tariff, so I tend to heat my water when that starts and then heat the house for the remaining duration of the low cost time. There is very little temperature drop (2C max) in the water by next morning when I need it.

There is an immersion heater but I am not aware of any other back up heaters, where would they be located?

@ecoste Okay, back up heaters should only be on the HWC but depends if thats two 3kW or one. Also will depend on the controller settings. Assuming no other cylinder as some have heaters in them.

@dgclimatecontrol I only have 1 cylinder and I can confirm that the Ecodan is set to standard, which I understand means that the heat pump alone heats the water

Posted by: @ecoste

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@dgclimatecontrol I am on an Octopus Intelligent Go Tariff, so I tend to heat my water when that starts and then heat the house for the remaining duration of the low cost time. There is very little temperature drop (2C max) in the water by next morning when I need it.

There is an immersion heater but I am not aware of any other back up heaters, where would they be located?

I have the same 10kW R290 heat pump as you, installed in June, and currently being run via Havenwise on the same Intelligent Go Tariff.  I see similar power draws to yours by the heat pump, but for limited periods of time and with an overall COP of around 3.0 at present.  For the last week, which has included the first sustained cold weather, Havenwise shows a CoP of 2.3 for DHW and 2.9 for heating.  MelCloud shows that over the last 7 days, the pump has spent 29.1% of its time doing heating, 3.7% doing DHW, 63.2% of its time in stop mode, 3.6% in Freeze Stat (which I believe kicks in below 5C OAT) and 0.3% of time running a fortnightly Legionella cycle.  To date, I've equated the relatively low CoP figure with the fact that it runs harder for shorter periods of time, hence spends a lot of the time sitting idle (and the experience living in the house is that the heat provided is constant and comfortable in all rooms, so it's doing exactly as I'd hoped from that perspective).

Having just done some analysis on the costs over the last month, I've calculated the cost of the heat pump energy draw as £8.39 for DHW and £34.03 for Heating from 28th Sep to 27th Oct, which compares to a gas bill of £117 for the same period last year, so I'm getting the expected overall cost efficiencies from the system, despite the reported CoP not being as high as many people report from their systems.

The peak power draw, as with your system, occurs when the once daily DHW cycle kicks in during the cheap rate period and, like you, I see usage spikes at times showing upto 5kW power draw for brief periods attributable to the heat pump, and usually as it starts either a heating or DHW cycle.  Total energy consumption for the past week, per Havenwise reporting from the MELCloud data, has been 21.6kWh for DHW and 134.2kWh for heating, neither of which looks excessive to me compared to circa 400kWh of gas consumption on last year's gas bill for the same period (based on a monthly consumption of just under 1,600kWh on that bill).

Ultimately, I'm more interested in cost savings than SCoP chasing, and I've attributed the low CoP scores (admittedly nowhere near those you have calculated on your system) with short periods of harder running that Havenwise has chosen to use as the optimal mode of operation, and this seems to be being delivered so far.

It might be worth you signing up to Havenwise for their free 30-day trial, to see whether this gives you any additional insight on performance.  There's no obligation to continue after this but the app will provide you with a weekly summary of CoP for both DHW and Heating during this time, using the MELCloud data.

Happy to provide more info on my stats, if this will assist.

@sheriff-fatman So your electricity bill for the month has increased by about £42 but gas will be £117 less, good result if accurate and as you say its about lower bills and keeping warm, the cop is irrelevent and maybe inaccurate.

@sheriff-fatman Thank you. Like you I am not chasing a particular COP, I am using COP to demonstrate the poor efficiency and hence the high cost. I have tried Havenwise but I found that their control reduced the COP and did not work well with the 12kWHr of battery storage that I have. The "maximum demand from the heat pump"  approach tended to fully drain my batteries in just over a couple of hours.

I currently have a COP of 1.3 for water heating across the 2 months since installation. I checked the initial power draw for water heating yesterday, it was 6.5kW. 

Do you have a problem with the Mitsubishi Energy Data updates, mine are very intermittent, sometimes 3 days between updates and sometimes one per day, but never more than one per day?

Thank you for the offer of more stats, I'll come back to you. I have a visit from my installer tomorrow.

For the benefit of the discussion I am going to repeat the opening post:

Posted by: @ecoste

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I have just had a Mitsubishi Ecodan R290 10 kW heat pump system installed. The system performance is excellent in most aspects, the house is warm, the temperatures are evenly distributed. An internal temperature of 22C at a flow temp of 35C and an external temp of 7C. Flow rate is 18 litres per minute and the delta T is 3C. However, under these circumstances the COP is 1.4. 

There is no buffer tank. 

It takes about 45 minutes to heat the 180 litre hot water tank to 50C, the COP for this is 1.2.

Does anyone have any suggestions why the performance is so poor. 

@ecoste 

Whatever the comments on cost etc, if the above is true and provided you are not energising a backup heater you have a problem, and one so serious it can only get worse.  Whys do I say that it must get worse?  Because so much energy is being used which should not be used.  This must do damage somewhere, probably to the compressor.  From your figures the excess input energy is 1.5-2kW, its getting dissipated somewhere and wherever its getting dissipated its unlikely to be doing any good.

So provided you are tolerably certain of the numbers, get it fixed as a matter of urgency.

All that said I do find the numbers very difficult to believe so I remain cautious about them.  However you have produced evidence that is difficult, on the face of it, to argue with.

@jamespa I agree, I have the installer visiting tomorrow. Unfortunately Mitsubishi will not engage with me and insist that they deal with the installer. It seems unfair for the installer to have to spend time when it seems highly probable that it is a heat pump problem. If you have any ideas how I could get Mitsubishi's attention I would be pleased to hear them.

Posted by: @dgclimatecontrol

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@sheriff-fatman So your electricity bill for the month has increased by about £42 but gas will be £117 less, good result if accurate and as you say its about lower bills and keeping warm, the cop is irrelevent and maybe inaccurate.

Yes.  I've analysed everything and reconciled it to the Octopus invoices (or the daily cost and usage estimates for the days awaiting invoicing) so the total cost analysis is accurate.  The only 'guesswork' in the analysis is the split of the usage figures.  I can get accurate car charging data for the Zappi, and MELCloud reports daily energy usage for the heat pump, split between heating, DHW and cooling (not applicable for me).  The Havenwise app is reliant on this data for its analysis too, but the assumption is that it is accurate in reporting energy consumption.  The balance is then attributed to 'general' consumption which, interestingly, is still the highest element of the overall usage and cost.  The sense check I can apply is to see if the 'general' column usage is consistent pre and post heat-pump installation and, on the limited data I've analysed so far, the two are within 1kWh per day of each other, taken over a full month, so there's no evidence of any usage 'bleed' falling into the general stats.

The car charging is always done on the cheap overnight rate, so is allocated the correct 7p/kWh cost (plus VAT).  The other usage is all assumed to be at the same blended rate achieved from peak and off-peak use, plus PV generation through the day.  This has averaged out to 9.35p/kWh over the last month, but is closer to 11p/kWh over the last fortnight as the weather has got colder.

I need to track it over a full heating season, and I expect the consumption to increase further as temperatures drop, but the gas costs from 28/10/24 to 27/03/25 totalled just under £900, so averaged around £180 per month in that time, so there's considerable scope to make savings.

Posted by: @ecoste

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@jamespa I agree, I have the installer visiting tomorrow. Unfortunately Mitsubishi will not engage with me and insist that they deal with the installer. It seems unfair for the installer to have to spend time when it seems highly probable that it is a heat pump problem. If you have any ideas how I could get Mitsubishi's attention I would be pleased to hear them.

I dont but in fairness to Mitsubishi you contract is with your installer so it is his job to sort it and refer if necessary.