Mitsubishi Electric City Multi PQHY-P72 Service Handbook page 272

Phenomena
2 Although heating operation
starts with the normal remote
controller display, the capacity
is not enough.
Cause
4. Insulation failure of the refrigerant
piping
5. Long piping length
Excessively long piping on the
high pressure side causes pres-
sure loss leading to increase in
the high pressure.
6. Piping size is not proper (thin)
7. Clogging by foreign object
8. The indoor unit inlet temperature
is excessively high.(exceeding
28°C [82°F])
9. Insufficient refrigerant amount
Protection works and compressor
frequency does not rise due to low
discharge temperature
Refrigerant recovery operation is
likely to start.
10.Compressor failure (same as in
case of cooling)
11.BC controller LEV1 and LEV3
failure
Refrigerant flow in the indoor units
is retarded because the failure of
BC controller LEV1 and LEV3 fail-
ure is causing the loss of indoor
unit pressure differential. (PQRY
only)
12.BC controller TH11,TH12,
TH15,TH16, HPS1, and HPS3
sensor failure, faulty wiring
BC controller LEV1 and LEV3 are
not controlled properly. (PQRY
only)
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[ IX Troubleshooting ]
Check method and remedy
Confirm that the characteristic of capacity
drop due to piping length.
-> Change the pipe
Check the temperature difference between
in front of and behind the place where the
foreign object is clogging the pipe (up-
stream side and downstream side). Difficult
to confirm if the object is clogged inside the
extended piping. Run the unit in cooling
mode, and find the foreign object using the
same method as that for cooling operation.
-> Remove the foreign object.
Check the inlet air temperature and for short
cycling. Change the environment where the
indoor unit is used.
Refer to 2 - 1. (Compressor frequency does
not rise sufficiently.)
Refer to the page on refrigerant amount ad-
justment
Check the discharge temperature.
Refer to the section "Troubleshooting the
LEV."
(9. [4] -6- ).
Pressure sensor and thermistor check
Wiring check