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  1. #1
    Join Date
    May 2017
    New Zealand
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    MHI Heat Pump (src71zea-s) inverter won't ramp up on heat.

    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.

    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

  2. #2
    Join Date
    Oct 2007
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    Re: MHI Heat Pump (src71zea-s) inverter won't ramp up on heat.

    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
    Last edited by install monkey; 14-05-2017 at 05:39 PM. Reason: added sensor resistances

  3. #3
    Join Date
    Oct 2001
    Nottingham UK
    Rep Power

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

    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.
    I'm back on the Pale

  4. #4
    Join Date
    Apr 2008
    Southampton, England
    Rep Power

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

    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?

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