MHI Heat Pump (src71zea-s) inverter won't ramp up on heat.

[sheerluck]

Hi, I have a Mitsubishi Heavy Industries Split 7.1 Kw heat pump (src71zea-s) which is mis-behaving. It's about 8 years old. I've had the installer out to look at it but after trying to add more refrigerant and much head scratching he left promising to research and return - a few months ago.

Having read several other threads on this forum I have not got any further but figured a question to experts was worth a nudge.

Symtoms
Heat pump seems to run fine with no faults - except inverter does not ramp up in heat mode. Outside unit fan speeds up after start delay but compressor sounds like it is running though never speeds up. Inside unit goes through it's normal starting functions but similarly, fan doesn't speed up from low.

This operation is the same when High power mode is selected.

In cooling mode, the system behaves perfectly with the compressor ramping up and cooling very effectively.

Diagnostic/Service Mode
The unit has never stopped abnormally. When pushed into service mode, there is no stop error code indicated by flashing run/timer LEDs.


I would welcome any suggestions of sensors to check or advice on where to next.

Many thanks

[install monkey]

chec resistance of sensors- mainly discharge sensor on outdoor unit- and maybe indoor coil sensor- indoor coil sensor only detects freeze prevent in cooling mode- but controls speed of indoor fan in heating(coil should be over 30deg for indoor fan to increase speed)- check resistance
discharge sensor should be
20deg is 62kohm
30deg is 40kohm
40deg is 26kohm
suspect a thermistor is out of range - but not open or shorted
indoor coil sensor should be 5kohm at 25deg and 4kohm at 30 deg 10kohm at 10 deg

[frank]

I would agree with the indoor coil sensor being at fault.

To prevent cold draughts in heating mode, the indoor fan spins at low revs while the compressor stays at low Hz (38 - 40) until the indoor coil sensor picks up the rise in temperature. Once this increase is temperature is achieved, the indoor fan then ramps up as does the compressor.

Your symptoms of low indoor fan speed and the compressor holding at low Hz is typical when the indoor coil sensor is out of calibration/faulty.

[Tayters]

To what the others have posted, is the reversing valve sliding across properly. Long shot but might be getting stuck half way in heating so discharge pressure is low, that's why refrigerant got added?

[sheerluck]

Thank you all for your generous advice. I have eventually managed to get time to test thermistor resistances: both indoor ambient and coil appear fine as do outdoor discharge pipe, coil and air.

Is the reversing valve mentioned the EEV or the 4-way coil valve - any suggestions on best way to test?

Many thanks

[Tayters]

Reversing valve is the 4 way valve. Solenoid is normally around 1.4k ohms. Can measure it or just get power to it and pull the coil off the stem, see if there is any magnetic force there. Internals of the valve best tested with some magnetics. Tried it myself and worked a treat, here's a link that explains it better. https://www.youtube.com/watch?v=bv2yu6WPtiU Cheers, Andy.

[sheerluck]

Thanks for all your help folks! At long last I have managed to get some daytime back on this (lucky I have a fire for backup heating). Checked all of the above and found no issues (didnt get magnets test to work on the 4 way valve but solenoid checked out).

My heat pump symptoms have now developed further and now happening on heating and cooling - questioning my own judgement that cooling was ok before.

Heating
Fan starts as normal but compressor shudders to a varying degree as it repeatedly tries to start.
After a few minutes, the outdoor unit fan stops and the compressor stops trying to start while the indoor unit fan stops with vents part open. Indoor run light stays on.
After about 2-3 minutes, the outdoor unit fan resumes along with the compressor shuddering start attempts. Indoor run light flashes continuously ~1 sec period.
After another 2 minutes, the indoor run light stays on while the outdoor unit fan continues to run with the compressor shuddering start attempts.
After a further few minutes, system stops completely and displays error code 22: (run / timer flash 2 times repeatedly)
- outdoor unit error
- Input is 2A or lower (PWM 90% or higher)
- Compressor wiring is disconnected / Outdoor unit’s circuit board is faulty
- When PWM calculation results of 90% and an input current lower than the set value continue for 3 minutes or longer.

Cooling
Fan starts as normal but compressor shudders to a varying degree as it repeatedly tries to start.
After a few minutes, the outdoor unit fan stops and the compressor stops trying to start while the indoor unit fan continues with vents open. Indoor Run light remains on. No further changes observed after 10 mins.

Would a faulty outdoor circuit board explain the shuddering of the compressor? I presume it would be the power pcb (PWB2) at fault? Is there anything else I could check before trying to order a new on of these?

Thanks again