Mitsubishi Electric EHST17D-VM2D Service Manual page 58

No. Fault symptom
14 Heating system does Heating system operates depending on the heating
not reach the set load to prevent low-load heating system from the
lower temperature. frequent switching (ON/OFF) of the compressor.
15 In 2-zone tempera- 1.
ture control, only
Zone2 does not
reach the set tem-
2.
perature.
3.
4.
5.
16 When a PUHZ-FRP The outdoor unit is set to have operation of the
outdoor unit is con- indoor unit of air conditioner take precedence over
nected, DHW or Heat- that of the cylinder unit, and in the main remote
ing operation cannot controller settings "Electric heater (Heating)" or
run. "Electric heater (DHW)" is turned off.
17 When a PUHZ-FRP When the outdoor unit is set to have cooling
outdoor unit is con- operation of the indoor unit of air conditioner take
nected and is in heat precedence over that of the cylinder unit, the out-
recovery operation,
door unit controls the frequency of the compres-
the set temperature sor according to the load of air conditioner. The
is not reached. DHW and heating run according to that frequency.
18 After DHW operation At the end of the DHW mode operation the 3-way
room temperature valve diverts hot water away from the DHW circuit
rises slightly. into space heating circuit.
This is done to prevent the cylinder unit compo-
nents from overheating.
The amount of hot water directed into the space
heating circuit varies according to the type of the
system and of the pipe run between the plate heat
exchanger and the cylinder unit.
19 The room tempera- 3-way valve failure
ture rises during DHW
operation.
20 Water discharges 1. If continual – pressure relief valve could bite
from pressure relief foreign objects and the valve seat may be
valve. damaged.
(Primary circuit)
2. If intermittent – expansion vessel charge may
have reduced/bladder perished.
21 Water discharges 1. If continual – field supplied pressure reducing
from pressure relief valve not working.
valve.
2. If continual – pressure relief valve could bite
(Sanitary circuit)
foreign objects and the valve seat may be
damaged.
3. If intermittent – expansion vessel charge may
have reduced/bladder perished.
4. DHW tank may have subjected to backflow.
22 Water discharges 1. If continual – field supplied pressure reducing
from temperature valve not working.
and pressure relief
2. If continual – temperature and pressure relief
valve
valve could bite foreign objects and the valve
( EHPT20X-MHEDW only)
seat may be damaged.
(Sanitary circuit)
3. If intermittent – expansion vessel charge may
have reduced/bladder perished.
4. DHW tank may have subjected to backflow.
5. Unit has overheated – thermal controls have
failed.
OCH714A
Possible cause
When Zone1 and Zone2 are both in heating
mode, the hot water temperature in Zone2
does not exceed that in Zone1.
Faulty wiring of motorized mixing valve
Faulty installation of motorized mixing valve
Incorrect setting of Running time
Motorized mixing valve failure
Explanation - Solution
Normal operation, no action necessary.
1. Normal action no action necessary.
2. Refer to installation manual, "5.3 Wiring for 2-zone temperature control".
3. Check for correct installation. (Refer to the manual included with each motor-
ized mixing valve.)
4. Check for correct setting of Running time.
5. Inspect the mixing valve. (Refer to the manual included with each motorized
mixing valve.)
Turn ON Electric heater (Heating) or Electric heater (DHW) using the main re-
mote controller.
Normal operation no action necessary.
If Air-to-Water system is given priority in operation, comp Hz can be regulated
depending on the load of DHW or Heating. For more details, refer to the PUHZ-
FRP installation manual.
Normal operation no action necessary.
Check the 3-way valve.
(i) Manually override 3-way valve using the main remote controller. (Refer to
<Manual operation> in "9-4. Service menu".) If the 3-way valve does not
function, go to (ii) below.
(ii) Replace 3-way valve coil. If the 3-way valve coil is replaced but the 3-way
valve does not function go to (iii) below.
(iii) Replace 3-way valve. (Refer to "11. DISASSEMBLY PROCEDURE".)
1. Turn the handle on the pressure relief valve several turns. If leakage persists,
replace the pressure relief valve with a new one.
2. Check pressure in expansion vessel.
Recharge to 1 bar if necessary.
If bladder perished replace expansion vessel with a new one.
1. Check function of pressure reducing valve and replace if necessary.
2. Turn the handle on the pressure relief valve several turns. If leakage persists,
replace the pressure 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
pre-charge.
4. Check the 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 wa-
ter supply could flow back to DHW tank. Investigate source of back-feed and
rectify error in pipework/fitting configuration. Adjust pressure in cold supply.
1. Check function of pressure reducing valve and replace if necessary.
2. Turn the handle on the temperature and pressure relief valve several turns.
If leakage persists, replace the temperature and pressure 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
pre-charge.
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.
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