Mitsubishi Electric EHST20C-VM2C.UK Service Manual page 56

No. Fault symptom
22 Water discharges from 1.
expansion relief valve
- part of Inlet Control
2.
Group
(EH*T20*-MHCW only)
(sanitary circuit)
3.
4.
5.
23 Noisy water circulation Air in water circulation pump.
pump
24 Noise during hot water 1.
draw off typically
worse in the morning.
2.
25 Mechanical noise 1.
heard coming from the
cylinder unit.
2.
26 Water circulation pump Water circulation pump jam prevention mechanism
runs for a short time (routine) to inhibit the build-up of scale.
unexpectedly.
27 Milky/Cloudy water Oxygenated water
(Sanitary circuit)
28 Heating mode has The time of "Delay" set in "Economy settings
been on standby for for pump" is too short. (Go to "Service menu"
a long time (does → "Auxiliary settings" → "Economy settings for
not start operation pump").
smoothly.)
29 The cylinder unit The cylinder unit is designed to run in an operation
that was running in mode with a higher priority (i.e. DHW mode in this
the heating mode case) at power recovery.
before power failure
is running in the DHW
mode after power
recovery.
30 The energy monitor val- 1. Incorrect setting of the energy monitor
ue seems not correct.
2. Non-connectable type of external meter (local
supply) is connected.
3. External meter (local supply) failure
4. FTC board failure
OCH570
Possible cause
If continual – field supplied pressure
reducing valve not working.
If continual – expansion relief valve may be
damaged.
If intermittent – expansion vessel charge
may have reduced/bladder perished.
DHW tank may have subjected to backflow.
Unit has overheated – thermal controls have
failed.
Loose airing cupboard pipework.
Heaters switching on/off.
Heaters switching on/off.
3-way valve changing position between
DHW and heating mode.
Explanation - Solution
1. Check function of pressure reducing valve and replace if necessary.
2. Turn the handle on the expansion relief valve to check for foreign objects
inside. If the problem is not still solved, replace the expansion relief valve
with a new one.
3. Check gas-side pressure in expansion vessel.
Recharge to correct precharge pressure if necessary.
If bladder perished replace expansion vessel with a new one with
appropriate precharge.
4. Check pressure in DHW tank. If pressure in DHW tank is similar to that in
the incoming mains, cold water supply that merges with incoming mains
water supply could flow back to DHW tank. Investigate source of back-
feed and rectify error in pipework/fitting configuration. Adjust pressure in
cold supply.
5. Switch off power to the heat pump and immersion heaters. Leave water
running. 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 1 bar on primary circuit.
1. Install extra pipe fastening clips.
2. Normal operation no action necessary.
Normal operation no action necessary.
Normal operation no action necessary.
Water from any pressurised system will release oxygen bubbles when water is
running. The bubbles will settle out.
Increase the time of "Delay" in "Economy settings for pump" .
• Normal operation.
• After the DHW max. operation time has elapsed or the DHW max. temperature
has been reached, the DHW mode switches to the other mode (ex. Heating
mode).
1. Check the setting by following the procedure below.
(1) Check if the DIP switch is set as the table below.
Consumed electric energy
Electric energy meter
SW3-4
(Local supply)
OFF Without
ON With
(2) In the case external electric energy meter and/or heat meter is not used,
check if the setting for electric heater and water pump(s) input is correct by
referring to <Energy monitor setting> in section 9-4.
(3) In the case external electric energy meter and/or heat meter is used, check
if the unit of output pulse on external meter matches with the one set at the
main controller by referring to <Energy monitor setting> in section 9-4.
2. Check if the external meter (local supply) is connectable type by referring to
<Energy monitor setting>" in section 9-4.
3. Check if signal is sent to IN8 to IN10 properly. (Refer to section 6. WIRING
DIAGRAM)
Replace the external heat meter if defective.
4. Check the FTC control board.
• Check for faulty wiring.
• If no problem found with the wiring, the FTC control board is faulty. Replace
the board.
56
Delivered heat energy
Heat meter
SW3-8
(Local supply)
OFF Without
ON With