Service And Maintenance; Troubleshooting - Mitsubishi Electric EHST20C- VM2C Operation Manual

Ehst20 series; ehpt20 series; erst20 series; ehsc series; ehsd series; ehpx series; ersc series; ersd series
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Service and Maintenance


The following table is to be used as a guide to possible problems. It is not exhaustive and all problems should be investigated by the installer or another competent person.
Users should not attempt to repair the system themselves.
At no time should the system be operating with the safety devices by-passed or plugged.
Fault symptom
Cold water at taps
(systems with DHW tank)
Heating system does not get up
to set temperature.
The cooling system does not
cool down to the set tempera-
ture. (ONLY for ERSC(D) and
ERST20C(D) models)
After DHW operation room tem-
perature rises a little.
Heating emitter is hot in the DHW
mode. (The room temperature
Schedule function inhibits the
system from operating but the
outdoor unit operates.
Pump runs without reason for
short time.
Mechanical noise heard coming
from cylinder unit
Noisy pipework
Water discharges from one of the
relief valves
Small amounts of water drip from
one of the relief valves.
An error code appears in the
main remote controller display.
<Power failure>
All setting will be saved for 1 week with no power, after 1 week Date/Time ONLY will be saved.
Maintenance to the cylinder unit and hydrobox should be carried out annually by a competent person only. Users should not try to service or replace parts of the cylinder unit
or hydrobox themselves. Failure to observe this instruction could result in injury to the user, damage to the unit and the product warranty becoming invalid.
In addition to annual servicing it is necessary to replace or inspect some parts after a certain period of system operation. Please see tables below for detailed instructions.
Replacement and inspection of parts should always be done by a competent person with relevant training and qualifications.
Parts which require regular replacement
Pressure relief valve (PRV)
Air vent (Auto/Manual)
Drain cock (Primary/Sanitary circuit)
Inlet control group (ICG)*
Possible cause
Scheduled control off period
All hot water from DHW tank used
Heat pump or electric heaters not working
Prohibit, schedule or holiday mode selected
Incorrectly sized radiators
The room in which the temperature sensor is located is at
a different temperature to the rest of the house.
Battery problem *wireless control only
When the water in the circulation circuit is unduly hot,
Cooling mode starts with a delay for the protection of the
outdoor unit.
When the outdoor ambient temperature is significantly low,
Cooling mode does not start running to avoid freezing of
the water pipes.
At the end of the DHW mode operation the 3-way valve
diverts hot water away from the DHW tank into space
heating circuit. This is done to prevent the cylinder unit
components from overheating. The amount of hot water
directed into the space heating circuit is dependent on the
type of system and the pipe run between the plate heat
exchanger and the cylinder unit.
The 3-way valve may have foreign objects in it, or hot wa-
ter may flow to the heating side due to malfunctions.
Freeze stat. function is active.
Pump jam prevention mechanism to inhibit the build up of
Heaters switching on/off
3-way valve changing position between DHW and heating
Air trapped in the system
Loose pipework
The system has overheated or overpressurised
Dirt may be preventing a tight seal in the valve
The indoor or outdoor unit is reporting an abnormal condi-
replace every
Possible failures
6 years
Water leakage
Check schedule settings and change if necessary.
Ensure DHW mode is operating and wait for DHW tank to re-heat.
Contact installer.
Check settings and change as appropriate.
Contact installer.
Reposition the temperature sensor to a more suitable room.
Check the battery power and replace if flat.
Normal operation
If the freeze stat. function is not necessary, contact installer to change the
Normal operation no action necessary.
Contact installer.
Normal operation no action necessary.
Normal operation no action necessary.
Normal operation no action required.
Normal operation no action necessary.
Try bleeding radiators (if present) If the symptoms persist contact installer.
Contact installer.
Switch off power to the heat pump and any immersion heaters then con-
tact installer.
Twist the valve cap in the direction indicted until a click is heard. This will
release a small amount of water flushing dirt from the valve. Be very care-
ful the water released will be hot. Should the valve continue to drip contact
installer as the rubber seal may be damaged and need replacing.
Make a note of the error code number and contact installer.
Parts which require regular inspection
Check every
Immersion heater
2 years
Water circulation pump
20,000 hrs (3 years)
Parts which must not be reused when servicing
* O-ring
* Gasket
note: always replace the gasket for pump with a new one at each regular
maintenance (every 20,000 hours of use or every 3 years).
Possible failures
Earth leakage causing
circuit breaker to activate
(Heater is always OFF)
Water circulation pump

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