Hi all, I’m trying to understand why my Mitsubishi Ecodan system has such a low COP. The system was installed in 2017 when the house was built but is new to me since moving in a few months ago. It heats the house well, but reported efficiency is poor, and energy usage seems fairly high. Although it has been serviced in the past, I have found many issues including blocked filter, and faulty 3-way valve. I would greatly appreciate any input on whether the behaviour I am seeing is normal or if performance can be improved.

Heat Pump Model: PUHZ-W85VHA2-BS
Controller Model: FTC5 version 15.00
Tank: Ecodan 250l pre-plumed cylinder with low loss header
Emitters: UFH downstairs, Radiators upstairs
Thermostats - All thermostats, TRVs and valves open.
HW Setpoint: 47 degrees
House Type: Well insulated modern 3 bed house built in 2017 (design temps not known)
Pump Setup: 3x pumps (Primary loop to HP, Heating system pump on other side of LLH, DHW pump on other side of DHW heat exchanger.

What I’m Observing

• COP for heating is typically ~1.5-2, which seems low for 10°C outdoor temps.
• COP for hot water is (47°C setpoint, 250 L cylinder) is typically ~1.3, also lower than expected.
• Flow temperature constantly oscillates even with a stable target
• At low - medium load, compressor frequency cycles repeatedly between ~45 Hz and ~70 Hz, rather than settling at a stable low modulation.
• At high load the compressor will run at a stable frequency (70 - 90hz)
• No active error codes.
• Heating Flow rate (according to FTC controller):17l/m
• DHW flow rate: 14l/m

• Difference between return temp of UFH and Radiators: UFH return temp is approx 2-2.5 degrees less than RAD return temp.

What I’ve Tried So Far
• Cleaned the outdoor strainer (was blocked resulting in a flow rate of 6l/m)
• Flushed the system with central heating cleaner.
• Cleaned magnetic filter regularly
• Replaced the 3 way valve (which resolved leakage of hot water to radiators during DHW cycles)
• Verified flow through UFH loops via manifold flow gauges.
• Opened all radiators fully to increase system load - didn’t solve hunting.
• Adjusted radiator lockshield valves to attempt to balance radiators - didn’t resolve issue
• Reduced primary pump speed to discourage short-circuiting through the LLH - no real change observed
• Tested thermistors in outdoor unit are reporting accurately. As far as I could tell the measured resistance matched the expected values given in the Databook.
• Run the system for short periods in the morning (according to time of use tariff), and run the system for long periods (24 hrs +) to let things stabilise.

Running Examples:
I have been monitoring the system via the melcloud and melpump apps:

Medium load - (Outdoor temp 7-9 degrees)
- From 0520 - 0820 flow temp is below target temp (35 degrees) and compressor frequency is fairly stable while load is high and system aims for target.
- From 0820 - 1000 the flow temp seems to match the target temp. Flow (and delta t) stabilise and frequency appears to modulate down in steps.
- Rest of day. Compressor and flow temp fluctuating.
- Energy Use during this time (according to smart meter app): 35-40KWH (1.25 - 1KWH per hr)
- COP Heating: 2.41
- COP DHW: 1.11

Looking at a close up between 1600 and 1720

High Load - Outdoor temp ~-2
Observations
- Steady frequency of 92Hz
- Fairly steady increase in Flow and return temps, but Flow can be seen to fluctuate up and down despite constant compressor load.

What I’m Trying to Understand
• Is the constant hunting between 45–70 Hz, and fluctuating flow temp normal for this generation of Ecodan, or should I expect a steady low-frequency modulation.
• Could the unsteady patterns simply just be an issue with the unit not being able to modulate lower for the given flow/return/room temps, or is there potentially a deeper issue with the unit (e.g EEV, low refrigerant etc).

Any comments or advice would be greatly appreciated.

Thanks!