Service And Maintenance; Basic Troubleshooting For Cylinder Unit - Mitsubishi Electric EHST20-Serie Installationshandbuch

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Service and Maintenance

The indoor cylinder unit is UNVENTED. Under UK law* the system must be
serviced once a year by a qualified individual. Servicing and maintenance of the
outdoor unit should only be done by a Mitsubishi Electric trained technician with
relevant qualifications and experience. Any electrical work should be done by a
tradesperson with the appropriate electrical qualifications. Any maintenance or

Basic troubleshooting for Cylinder Unit

Fault symptom Possible cause
Cold water at tap Direct – Booster heater cut out has triggered.
Direct – Booster heater breaker (ECB1) has tripped.
Direct – The booster heater thermal cut-out has operated and can not be re-
set using the Manual reset button.
Direct – Immersion heater cut out has triggered.
Direct – Immersion heater breaker (ECB2) has tripped.
Indirect – 3-way valve fault
Indirect – Heat pump not working.
All hot water used.
Water discharges from expan- If continual – locally supplied pressure reducing valve not working.
sion relief valve
If continual – expansion relief valve seat may be damaged.
If intermittent – expansion vessel charge may have reduced/bladder perished. Check pressure in expansion vessel. Recharge to  bar if
Unit is being back pressurised.
Water discharges from temper- Unit has overheated – thermal controls have failed.
ature and pressure relief valve
(EHPT20X-VM2HA only)
Milky/Cloudy water Oxygenated water
No hot water flow
Cold main off.
Strainer blocked .
Noise during hot water draw off Loose airing cupboard pipework.
typically worse in the morning
Noisy pump Air in pump.
Pump runs for a short time for Pump jam prevention mechanism to inhibit the build up of scale.
no reason.
Hot or warm water from cold If tap runs cold after a minute or so the pipe is picking up heat from heating
tap pipe work.
Heating system does not get Prohibit, schedule timer or holiday mode selected.
up to set temperature. Incorrectly sized radiators
The room in which the temperature sensor is located is at a different tempera-
ture to the rest of the house.
Battery problem *wireless control only
Mechanical noise heard com- Heaters switching on/off
ing from the cylinder unit. 3-way valve changing position between DHW and heating mode.
After DHW operation room At the end of the DHW mode operation the 3-way valve diverts hot water
temperature rises a little. away from the cylinder 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.
Duration is set to excessively short time at "Economy setting for pump". (Go
Heating mode has been on
standby for a long time (does to "Service menu" → "Auxiliary setting" → "Economy setting for pump").
not start operation smoothly.)
The 3-way valve may have foreign objects in it, or hot water may flow to the
Heating emitter is hot in the
DHW mode. (The room tem-
heating side due to malfunctions.
perature rises.)
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'DIY' fixes done by a non-accredited person could invalidate the Warranty and/or
result in damage to the cylinder unit and injury to the person.
* Building regulations – England & Wales Part G3, Scotland P3, Northern Ireland
P5. If outside of the UK please refer to local building regulations regarding un-
vented hot water storage.
Solution
Check booster heater thermostat and press reset button
if safe. Reset button is covered with white rubber cap see
component parts diagram page 6 for position.
Check the cause of the trip and reset if safe.
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.
Check immersion heater thermostat and press reset button,
located on immersion heater boss, if safe. If the heater has
been operated with no water inside it may have failed, so
please replace it with a new one.
Check the cause of the trip and reset if safe.
Check plumbing/wiring to 3-way valve.
Check heat pump – consult outdoor unit service manual.
Ensure DHW mode is operating and wait for tank to re-
heat.
Check function of pressure reducing valve and replace if
necessary.
Remove cartridge – check seat and renew if necessary.
necessary. If bladder perished replace vessel.
With cylinder cold check pressure in cylinder. If this is the
same as the incoming mains pressure then you are getting
backfeed. Install a balanced cold supply.
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.
Water from any pressurised system will release oxygen
bubbles when flowing. The bubbles will settle out.
Check and open stop cock.
Isolate water supply and clean strainer.
Install extra pipe fastening clips.
Use manual and automatic air vents to remove air from
system. Top up water if necessary to achieve bar on pri-
mary circuit.
Normal operation no action necessary.
Insulate/re-route pipe work.
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 no action required.
Normal operation no action necessary.
Normal operation no action necessary.
Set "Economy setting for pump" longer.
Manually override the 3-way valve several times to check
whether the problem will be solved. (Refer to the service
manual.) If the problem persists replace the 3-way valve
coil with a new one. If still no change replace the entire
3-way valve with a new one.