Mitsubishi Electric EHST20C-VM6HA Service Manual page 51

No. Fault symptom
Heating system does
1 1. Prohibit, schedule timer or holiday mode selected.
not get up to set tem-
. Check settings and change as appropriate.
3. The room in which the temperature sensor is located is at
perature.
4. Heat pump not working.
5. Booster heater cut out has triggered.
6. Booster heater breaker (ECB1) has tripped.
7. The booster heater thermal cut-out has operated and can
8. Incorrectly sized heat emitter.
9. 3-way valve fault
10. Battery problem *wireless control only
At the end of the DHW mode operation the 3-way valve diverts hot
13 After DHW operation
water away from the cylinder into space heating circuit.
room temperature
This is done to prevent the cylinder unit components from overheating.
rises a little.
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.
3-way valve fault
14 The room temperature
rises during DHW
operation.
Water discharges from 1. If continual – pressure relief valve seat may be damaged.
15
. If intermittent – expansion vessel charge may have
pressure relief valve.
(Primary circuit)
Water discharges from 1. If continual – field supplied pressure reducing valve not working.
16
pressure relief valve.
(Sanitary circuit)
. If continual – pressure relief valve seat may be damaged.
3. If intermittent – expansion vessel charge may have
4. Unit is being back pressurised.
Water discharges 1. If continual – field supplied pressure reducing valve not working.
17
from temperature and
. If continual – pressure relief valve seat may be damaged.
pressure relief valve
(EHPT20X-VM2HA
3. If intermittent – expansion vessel charge may have
only) (Sanitary circuit)
4. Unit is being back pressurised.
5. Unit has overheated – thermal controls have failed.
18 Noisy pump Air in pump.
Noise during hot water Loose airing cupboard pipework.
19
draw off typically Heaters switching on/off.
worse in the morning.
0
Mechanical noise 1.
heard coming from the
.
cylinder unit.
Pump jam prevention mechanism to inhibit the build up of scale.
1 Pump runs for a short
time for no reason.
Milky/Cloudy water Oxygenated water

(Sanitary circuit)
Possible cause
a different temperature to the rest of the house.
not be reset using the manual reset button.
reduced/bladder perished.
reduced/bladder perished.
reduced/bladder perished.
Heaters switching on/off.
3-way valve changing position between DHW and heating mode.
1. Check settings and change as appropriate.
. Check the battery power and replace if flat.
3. Reposition the temperature sensor to a more suitable room.
4. Check heat pump – consult outdoor unit service manual.
5. Check booster heater thermostat and press reset button if
safe.
Reset button is covered with white rubber cap see component
parts diagram section 10-6. for position.
6. Check the cause of the trip and reset if safe.
7. Check the resistance of the thermal cut out, if 0 then the
connection is broken and the booster heater will have to be
replaced.
Please contact your Mitsubishi Electric dealer.
8. Check the heat emitter surface area is adequate
Increase size if necessary.
9. Check plumbing/wiring to 3-way valve.
(i) Operate 3-way valve manually using the main remote con-
troller. (Refer to <Manual operation> in 9.2).
(ii) Replace 3-way valve coil.
(iii) Replace 3-way valve. (Refer to Procedure 6 in "11. DISAS-
SEMBLY PROCEDURE."
10. Check the battery power and replace it flat.
Normal operation no action necessary.
Check the 3-way valve.
(i) Operate 3-way valve manually using the main remote
controller. (Refer to <Manual operation> in 9.2.)
(ii) Replace 3-way valve coil.
(iii) Replace 3-way valve.
(Refer to Procedure 6 in "11. DISASSEMBLY
PROCEDURE.")
1. Remove cartridge – check seat and renew if necessary.
. Check pressure in expansion vessel.
Recharge to 1 bar if necessary.
If bladder perished replace vessel.
1. Check function of pressure reducing valve and replace if
necessary.
. Remove cartridge – check seat and renew if necessary.
3. Check pressure in expansion vessel.
Recharge to correct precharge pressure if necessary.
If bladder perished replace vessel.
4. With cylinder cold, check pressure in cylinder. If this is the
same as the incoming mains pressure then you are getting
back feed. Install a balanced cold supply.
1. Check function of pressure reducing valve and replace if
necessary.
. Remove cartridge – check seat and renew if necessary.
3. Check pressure in expansion vessel.
Recharge to correct precharge pressure if necessary.
If bladder perished replace vessel.
4. With cylinder cold, check pressure in cylinder. If this is the
same as the incoming mains pressure then you are getting
back feed. Install a balanced cold supply.
5. Switch off power to the heat pump and immersion heaters.
Leave water supply on.
Wait until discharge stops. Isolate water supply and replace if
faulty.
Use manual and automatic air vents to remove air from system.
Top up water if necessary to achieve 1bar on primary circuit.
Install extra pipe fastening clips.
Normal operation no action necessary.
Normal operation no action necessary.
Water from any pressurised system will release oxygen bubbles
when flowing. The bubbles will settle out.
51
Explanation - Solution