The joy of being a Heating Engineer

 

GC No. 47-311-18
In this article we will look at the Highflow 400’s components, the electrical sequence of operation, the hydraulic sequence of 
operation, and some hints and tips.

This information is intended to help you understand the operation of the boiler and help you resolve problems that may arise.

COMPONENTS

Control panel 
The control panel consists of:

• user-adjustable control thermostats for hot water and central heating temperature control
• two indicator LEDs
• mode selector switch for standard models; or the twin-channel programmer which replaces the selector switch
• the heat bank overheat thermostat (left-hand side) and the safety limit overheat thermostat (right-hand side) reset buttons are located on the underside of the control panel, with the thermostat bodies located behind the control panel.

The two facia LEDs indicate the following. 

Orange ‘mains/standby’ indicator
The orange LED will be illuminated unless:

1. The mains supply is off to the appliance.
2. The 3.15 amp fuse on the control PCB has failed.
3. The heat bank overheat thermostat has tripped.

Green ‘demand’ indicator 
On models with an earlier PCB fitted this will only be illuminated when the appliance burner is in operation, On models or 
appliances with later PCB fitted this will be illuminated when:

1. The selector switch or programmer is in the heating and hot water demand position, a hot water tap is turned on and the domestic hot water flow switch has operated.
2. The selector switch or programmer is in the heating and hot water position and all external controls are calling.
   Note: The demand indicator will not illuminate when the boiler is requesting pre-heat only.

TEMPRATURE AND CONTROL THERMOSTATS

Central heating temperature control thermostat (dry pocket)

The central heating (CH) thermostat is a liquid capillary type with its thermostat phial located in the top manifold phial pocket. The electrical connections of the thermostat (two white wires) are connected to pins 2 and 3 on plug X15 on the printed circuit board (PCB). The temperature range of this thermostat is customer adjustable from 62°to 82°C.

Domestic hot water temperature control thermostat (dry pocket)

The domestic hot water (DHW) thermostat is a liquid capillary type with the thermostat phial located in the heat bank phial pocket. The electrical connections of the thermostat (two pink wires) are connected to pins 1 and 3 on plug X2 on the PCB. The temperature range of this thermostat is customer adjustable from 55°C to 75°C.

Heat bank overheat thermostat (dry pocket)

The thermostat phial is located in the heat bank (same pocket as the DHW control thermostat phial) and the body of the thermostat is located behind the control panel (left-hand side). Its electrical connections (two red wires) are connected to 
pins 1 and 4 on plug X14 of the PCB. If the thermostat has tripped, the mains/ standby indicator will not be illuminated and the boiler will not function in any mode. The manual reset button for this thermostat protrudes through the lower panel of the control box (left-hand side). This thermostat will trip if the heat bank temperature exceeds approximately 90°C.

Note: If this thermostat trips it isolates power to all the boiler controls including the pump, so this will always make the safety limit thermostat trip.

Safety limit thermostat (dry pocket)

The safety limit thermostat phial is located on the top right-hand side of the main heat exchanger in the combustion chamber, and the body of the thermostat is located behind the control panel (right-hand side). The electrical connections (two orange wires) are connected to pins 1 and 2 on plug X8 on the PCB. If this thermostat operates or fails, the ignition sequence will stop, and the pump continues running with the standby and demand indicators illuminated. The manual reset button for this thermostat protrudes through the lower panel on the control box (right-hand side). This thermostat will trip if the gas to water heat exchanger temperature exceeds 101°C.

DHW high limit thermostat (dry pocket)

Located on the domestic hot water outlet pipe from the DHW plate heat exchanger, this thermal reset thermostat will operate if the domestic hot water temperature exceeds 78°C. The electrical connections (two black wires) are connected to pins 1 and 2 on plug X16 on the PCB. If the thermostat loses continuity through the two terminals during a hot water demand, the power to the air-pressure switch, fan and gas valve will be broken; hot water will still be given at the tap until the heat bank loses temperature. When the temperature at the thermostat falls below 78°C, continuity through the thermostat will be restored and the boiler will re-start its ignition sequence.

HYDRAULICS

Heat bank

The 60-litre insulated storage vessel houses:

• connections for the system pressure gauge (used in sealed systems only)
• expansion vessel flexible connection
• DHW thermostat and the heat bank overheat thermostat phials.

The phial pocket in the heat bank, which contains the thermostat phials, is a serviceable part and can be renewed as a separate item.

The 60-litre heat bank is heated to the temperature set by the customer on the domestic hot water control thermostat on the facia (55°C-75°C). When a hot water draw-off is opened, this store of water is pumped around the main heat exchanger, through the water-to-water heat exchanger and back to the heat bank. When the heat bank’s temperature drops below its pre-set temperature, the burner will fire on full gas pressure with voltage to all three coils on the gas valve (main, pilot and high/low solenoid coils).

With a hot water draw-off open, a 20°C temperature drop can be expected across the water-to-water heat exchanger – witha flow rate of approximately 18 litres/min. At lower flow rates the temperature differential will be less. When the hot water draw-off has finished, the burner will continue until the temperature in the heat bank satisfies the domestic hot water thermostat preset temperature. The burner will then extinguish and the pump will overrun.

Note: On some appliances a dip tube was added to the heat bank. This was in the form of a small copper extension piece which fitted into the back connection of the heat bank. This tube extended into the heat bank to eliminate any air trapped at the top of the heat bank from being drawn into the primary circuit. It was introduced to eliminate any main heat exchanger noise/lockout problems.

Pump

A three-speed Grundfos UPS 15-60 is used, set at its maximum setting. It is secured to the bottom manifold using ‘O’-ring sealed couplings held in place by wire U-shaped clips. A non- return valve is connected to the top connection of the pump, which can be pressed open to help removing air when filling the boiler. The pump head can be replaced using a standard Grundfos UPS 15-60 pump, with the speed set to number 3. The voltage to the pump is 230V AC, and the pump coil will have a resistance of approx 180 Ohms.

Note: If the pump body is removed for replacement etc.. it is possible to refit the body incorrectly with the non-return valve at the bottom. In this position when the boiler is refilled and re-commissioned, it will fire on heating demand but the primary water will not leave the boiler to the radiators: hence no central heating. This is due to negative pump pressure at the diverter valve’s weep tube (see ‘Diverter valve’).

Diverter valve

Secured to the bottom manifold by four fixing screws, the spring-loaded valve rests in the hot water position. In a central heating demand the solenoid is energised (230V AC), the servo valve arm will lift to close the drain from the diaphragm chamber, and pump pressure through the 6mm tube connecting the pump and diverter valve will force the spring-loaded valve to close off the hot water port from the heat bank. At the end of the heating demand, the solenoid will de-energise, the servo valve arm will move to allow system water to move freely past the servo valve without causing any pressure on the diaphragm, thus allowing the spring-loaded valve to close off the return port from the radiator circuit. No service kit is available for this valve.

Note: The diverter valve will open to the heating port completely only if the solenoid is energised and the 6mm tube from the pump to the diverter valve is clear and not restricting pump pressure to the diaphragm (see diagram).

Circuit filter

A primary circuit filter was introduced on products manufactured from 2000. This is to collect primary debris to stop problems related to blocked heat exchangers and so on. This filter is located in the top manifold around the primary side of the central heating thermostat phial pocket housing. This filter can also be added to boilers produced before this date.

Internal adjustable bypass

Located in the top manifold, the bypass is only in use when the appliance is in a heating demand.

Non-return valve

Housed in the heating return pipe isolator, this plastic non-return valve (NRV) prevents the radiators being heated when the boiler is in a hot water demand (reverse circulation). Take care when replacing the NRV. Ensure that it is located correctly in the boiler side of the isolation valve, with the spring on the NRV visible (see illustration 6). This is a serviceable part and can be obtained as a separate item.

Domestic hot water flow switch

This is located on the cold mains inlet close to the top manifold. It is important that the paddle-arm switch is correctly aligned, with the arrow on the front end of the switch body pointing directly downwards. A thimble-shaped wire mesh filter is fitted in the cold mains inlet connection, and a flow regulator is fitted to the outlet connection of the valve (this should be discarded if the domestic system is cistern-fed). When a hot water tap is opened, the switch contacts close (rests in the open circuit position) and makes continuity through the switch (230V AC on pins 1 and 3 on plug X1 on the PCB).

Fan

The fan is mounted on the flue hood inside the combustion chamber and is held in position by two securing screws and a rear 
slide in a locating bracket. The 230V AC single-speed fan has a coil resistance of approx 60 Ohms. It has two silicone rubber tubes (positive and negative) that interconnect to the air- pressure switch. For ease of access when cleaning the fan blades and venturi, remove the fan-mounting plate from the flue hood of the appliance, and then remove the fan.

Air-pressure switch

The 230V AC three-wire switch is located in the control box, secured by two fixing screws. It has a positive tube connected to connection marked ‘H’ and a negative tube connected to connection marked ‘L’. The electrical connections are brown wire to C terminal, white wire to the N/C (Normally Closed) terminal and grey wire to the (Normally Open) N/O terminal.

Gas valve

This is a Honeywell VR4601Q softlite valve with three separate 230V AC solenoids:

• pilot solenoid
• main solenoid
• high-to-low solenoid.

The valve is non-modulating, with fully open or fully closed operation of the solenoids. The pilot and main solenoids are energised in central heating mode, allowing the burner to fire on a lower burner pressure. This pressure can be adjusted and 
allows the operative to range-rate the appliance to give the required heating output pressure. When in hot water mode the high-to-low solenoidis also energised, giving full burner pressure to satisfy the hot water demand.

The technical data for the gas valve is:

• main solenoid voltage = 230V AC
• high/low modureg solenoid = 230V AC
• pilot solenoid voltage = 230V AC
• main solenoid coil resistance = 5.2k Ohms
• high/low modureg solenoid coil = 5.5k Ohms
• pilot solenoid coil resistance = 1.5k Ohms.

Combustion chamber

The front and side insulation linings are easily removed, but the gas-to-water heat exchanger needs to be removed to take out or replace the rear insulation lining. The eight-bladed main burner slides onto the injector and is held by a bracket at the front. When refitting the burner after cleaning, take care to ensure the blades are located horizontally. The pilot burner assembly is attached to the main burner by a screw-secured bracket, and the combined spark and flame detection electrode should have a spark gap setting of 4mm-5mm.

The four-pass copper gas-to-water heat exchanger, which is sandwiched between the combustion chamber and flue hood, has the auto-air vent connected to the flow pipe on the left-hand side and the safety limit thermostat phial in a pocket on the right-hand side.

PCB

The main control and the ignition PCBs are located inside the control box and accessed by removing its front panel. The main control PCB is protected by a 3.15 amp quick-blow fuse, and houses the relays highlighted in the electrical sequence of operation.

Note: All connections on the PCBs are mains voltage. There are two versions of the main control PCB:

• The earlier PCB had a separate three-wire pump overrun thermostat.
• In the later PCB the pump overrun is built into its electronics to give a preset five-minute pump overrun after any demand.
• The later PCB also altered the sequence of operation of the boiler the ‘demand’ LED on the earlier PCBs would illuminate only when the burner was actually fired. On the later PCB the ‘demand’ LED illuminates as long as there is a hot water or heating demand, but will not illuminate whenthe appliance is operating to satisfy the Pre-heat.

Note: The later version control PCB is a direct replacement for the earlier version PCB. When replacing the earlier version with the later version the external pump overrun thermostat is removed/disconnected.

PRODUCTION CHANGES

Pump overrun thermostat

Early production boilers had a pump overrun thermostat fitted to the right- hand side of the main heat exchanger. If a replacement PCB is required for this early production model, the pump overrun thermostat will become obsolete and can be 
removed from the boiler (the new PCB has a five-minute pump overrun built into the board).

Expansion vessel connecting pipe

Because of problems with the flexible pipe kinking, the connection position, which was originally located at the top of the heat bank has been moved to behind the pump.

Primary filter

This was added at production in 2000 and fitted in the top manifold around the primary thermostat phial pocket.

PCB

This is the updated main control PCB with inbuilt pump overrun.

Dip tube

This tube can be added to the heat bank back connection, and it extends the original connection further into the heat bank to 
eliminate primary air/noise problems. When fitting the dip tube on older bailers, remove the ‘O’-ring behind the flared end. If the ‘O’-ring is not removed the pipe will not sit in the socket fully, making refitting of the 
upper pipe difficult. Seal the dip tube in with silicone if the ‘O’-ring is removed.

POINTS TO REMEMBER

• The preheated 60-litre water store (heat bank) is only used to supplement the primarycircuit. When a call for hot water is made, the pre-heated water is pumped directly through to the plate heat exchanger via the main heat exchanger to instantly heat the secondary domestic hot water. This large capacity heat store also allows prolonged hot water at a higher flow rate than is usually obtained by a conventional combination boiler.
• All connections and components are 230V AC!
• This boiler is not a modulating boiler: all internal thermostats are open/close thermostats, not thermistors!
• The main burner pressures are rated according to the low-to-off solenoid setting (range- rated for heating output), and the high-to-low solenoid setting, which gives maximum boiler output to satisfy hot water demands.

HYDRAULIC OPERATION

Heating operation

In a demand for central heating the pumped primary water flows from the top connection of the gas-to-water heat exchanger, into the top manifold, through the primary water filter, over the central heating thermostat phial pocket, and then out through the flow pipe isolation valve to the radiator circuit. The return water from the radiator circuit passes through the non-return valve (located in the return isolation valve) and into the energised diverter valve; pump pressure through the small weep tube on the diverter valve will hydraulically hold the valve in the heating position. This water then 
passes through the pump and back into the gas-to- water heat exchanger. The temperature of this water is controlled by the central heating thermostat on the control panel. 
Note: The pre-heated water stored in the heat bank is not used in a central heating demand.

Hot water operation

In a hot water demand the pumped primary water takes the same path from the gas- to-water heat exchanger through the primary filter in the top manifold, but now passes through the water-to- water plate heat exchanger and into the heat bank. From the heat bank, the primary water continues through the diverter valve (in its rest position), and back through the pump to the gas-to-water heat exchanger. Because the water in the heat bank will in normal operation be stored at a temperature of 55°C-75°C, the pre-heated water in the circuit will allow the plate heat exchangers’ temperature to rise rapidly. The water in this circuit is controlled by the domestic hot water thermostat on the control panel (the thermostat phial is located in the heat bank phial pocket). In normal operation expect a 20°C temperature drop across the plate heat exchanger during a hot water demand. At a very low flow rate the water temperature could be as high as the tank temperature. 
Note: The boiler will give maximum gas until the domestic hot water thermostat is satisfied. Any sludge in the primary hot water circuit may lead to the main heat exchanger overheat thermostat tripping. If this occurs, primary heat may pass to the radiator circuit in a hot water demand or whilst in pre-heat.

SEQUENCE OF OPERATION

GC No. 47-311-18 
Note: This sequence of operation is for the Worcester High-flow 400 RSF with the later version PCB fitted (preset five-minute pump overrun operated by the PCB).

At rest

1. Permanent live at appliance X11 pin 4.
2. 230V AC through 3.15 amp internal fuse to X14 pin 4.
3. Continuity through heat bank overheat thermostat (manually re-settable at approximately 90°C). 230V AC at X14 pin 4 and back at X14 pin 1 (two red wires).
4. Standby LED illuminated.

Note: In the following three sections, RLA, RLB and RLC are internal relays on the PCB.

Domestic hot water preheat

Note: Demand LED does not illuminate on preheat demand.

1. Programmer/facia selectorswitch set to hot water. 230V AC atX12, pin 3.
2. Continuity through the hot water control thermostat – user adjustable on the facia 55°C-75°C. 230V AC at X2 pin 1 and back at X2 pin 3 (two pink wires).
3. RLA is energised, giving 230V AC to the pump; pump runs.
   Note: If the pump does not run, the appliance will continue with its sequence. The boiler may fire and go to overheat because of lack of circulation.
4. RLB is de-energised which removes any power at the diverter (from any heating demand): the diverter will return to its rest position (water demand).
5. Relay RLC operates.
6. Hot water limit thermostat (78°C) is checked for continuity 230V AC at X16 pin 2 and back at X16 pin 1 (two black wires).
7. 230V AC through relay RLCto the high/low modureg on the gas valve (main solenoid has not yet operated).
8. 230V AC through relay RLC to the safety limit thermostat.
9. Safety limit thermostat (manually re-settable at 101°C) is checked for continuity. 230V AC X8 pin 1 and back at X8 pin 2 (two orange wires).
10. 230V AC into ignition PCB. Connection CM pin L.
11. Air-pressure switch is checked for continuity at Common to N/Closed.
    Note: If the APS is not at rest, the appliance wilt not continue its ignition sequence.
12. Fan runs.
13. Air-pressure switch operates, giving continuity at Common to N/Open.
    Note: If the APS does not operate, the appliance will not continue its sequence, the fan will continue running.
14. 230V AC is sent to the pilot solenoid, which opens, supplying pilot gas.
15. Spark at the electrodes, which ignites the pilot gas.
16. Pilot lights and rectification takes place.
    Note: If the pilot fails to light or rectification fails the boiler will continue to spark, no main burner/no lockout.
17. Ignition ceases.
18. 230V AC is sent to the main gas valve solenoid, the solenoid opens and the main burner cross-lights off the now established pilot.
    Note: On a hot water demand the high-to-low modureg operator is energised and the burner will light on its maximum preset burner pressure (see 7 above).
19. Preheat is now under the control of the hot water control thermostat (user adjustable between 55°C-75°C on the appliance facia).
20. Preheat is satisfied.
21. Burner and pilot extinguish, fan stops.
22. Pump overruns for five minutes (diverter is in the rest position/hot water).

Hot water demand

1. Hot water demand at tap.
2. Hot water flow switch (paddle type) operates, giving continuity. 230V AC at X1 pin 3 and X1 pin 1 (blue and brown wires).
3. Demand LED illuminates.
4. 230V AC through hot water control thermostat (user adjustable on the facia 55°C- 75°C) X2 pin 1 and X2 pin 3 (two pink wires).
5. Relay RLA is energised giving 230V AC to the pump, pump runs.
   Note: If the pump does not run the appliance will continue with its sequence. The boiler may fire and go to overheat, due to the lack of circulation.
6. Relay RLB is de-energised which removes any power at the diverter (from any heating demand); the diverter will return to its rest position (water demand),
7. Relay RLC operates.
8. Hot water limit thermostat (78X) is checked for continuity, 230VACatX16 pin 2, and back at X16 pin 1 (two black wires).
9. 230V AC through relay RLC to high/low modureg on gas valve (main solenoid has not yet operated).
10. 230V AC through relay RLC to the safety limit thermostat.
11. Safety limit thermostat (manually re-settable at 101X) is checked for continuity, 230V AC at X8 pin 1 and back at X8 pin 2 (two orange wires).
12. 230V AC into ignition PCB, connection CN1 pin L.
13. Air-pressure switch is checked for continuity at Common to N/Closed.
    Note: If APS is not at rest, the appliance will not continue its sequence.
14. Fan runs.
15. Air-pressure switch operates, giving continuity at Common to N/Open.
    Note: If APS does not operate, the appliance will not continue its sequence, the fan will continue running.
16. 230V AC sent to pilot solenoid, which opens, supplying pilot gas.
17. Spark at the electrodes, which ignites the pilot gas.
18. Pilot lights and rectification takes place.
    Note: If the pilot fails to light or rectification fails the electrode will continue to spark, no main burner/no lockout.
19. Ignition ceases.
20. 230V AC is sent to the main gas valve solenoid, the solenoid opens and the main burner cross-lights off the now established pilot.
    Note: On a hot water demand the high-to-low modureg operator is also energised and the burner will light on its maximum preset burner pressure (see 9 above).
21. Hot water outlet temperature is now under the control of the hot water control thermostat (customer adjustable on the appliance facia between 55°-75°C).
22. End of demand, water flow switch goes open circuit, demand LED extinguishes.
    Note: Boiler may continue operating for a period until preheat is satisfied.
23. Burner and pilot extinguish, fan slops.
24. Voltage removed from the diverter valve: this returns to its rest position (hot water) under spring pressure.
25. Five-minute pump overrun.

Heating demand

1. Time switch and facia selector calling for heat; 230VACatX12 pin 4.
2. 230V AC at PCB room stat connection: out at X13 pin 3, back in at X13 pin 2 (or linked).
3. Demand LED illuminates.
4. RLA is energised giving 230V AC to the pump, pump runs.
   Note: If the pump does not run. the appliance will continuewith its sequence. The boiler may fire and go to overheat, due to the lack of circulation.
5. 230V AC is sent via the closed contacts of relay RLB and gives 230V AC to the diverter valve solenoid. This voltage, combined with pump pressure at the capillary tube, causes the valve to open, allowing primary temperature to the radiators.
6. 230V AC from RLB is also sent to the heating control thermostat (user adjustable on the facia 62°-82°).
7. Demand from the heating control thermostat, 230V AC at X15 pin 2 and back on X15 pin 3.
8. 230V AC through the closed contacts of relay RLC (relay at rest) through the safety limit thermostat (manually re-settable at 101°C, 230V AC X8 pin 1 and back at X8 pin 2 (two orange wires).
9. 230V AC into ignition PCB, connection CN1 pin L.
10. Air-pressure switch is checked for continuity at Common to N/Closed.
    Note: If APS is not at rest, the appliance will not continue its sequence.
11. Fan runs.
12. Air-pressure switch operates, giving continuity at Common to N/Open.
    Note: If APS does not operate, the appliance will not continue its sequence, the fan will continue running.
13. 230V AC sent to pilot solenoid which opens, supplying pilot gas.
14. Spark at the electrodes, which ignites the pilot gas.
15. Pilot lights and rectification takes place.
    Note: If the pilot fails to light or rectification fails the electrode will continue to spark, no main burner/no lockout.
16. Ignition ceases.
17. 230V AC is sent to the main gas valve solenoid, the solenoid opens and the main burner cross-lights off the now established pilot.
    Note: On heating demand, voltage is required only at the main on/off solenoid. This on/ off gas pressure is adjustable, which enables us to range- rate the appliance to give the required heating output.
18. Heating temperature is now under the control of the facia heating control thermostat (62°C-82°C).
19. Once the boiler’s heating control thermostat is satisfied, the fan, burner and pilot shut down; the demand LED remains illuminated, with the pump and diverter still energised. The boiler re-fires when the control thermostat recalls for heat. 
20. End of demand.
21. Voltage is removed from the diverter valve, which then returns to its rest position (hot water) under spring pressure. 
22. The demand LED extinguishes; burner, pilot and fan shut down.
23. Pump overruns for five minutes (diverter in rest position/hot water).

HINTS AND TIPS

(appliance fitted with later PCB) 

Fault: Tripping of overheat thermostats.

Cause: Usually due to circulation problems, sludge or scale. 

Solutions:

1. Check the pump and pump over-run operation. (Pump can stick on initial firing of the demand period.)
2. Check all the thermostats are operating within their temperature range.
3. Check the temperature differential across main gas-to-water heat exchanger; expect 11°C.
4. Check the temperature differential across plate heat exchanger with water demand calling. Expect a 20 C differential, with a flow rate of 18 litres per minute at the tap.

Fault: Hot water satisfactory, no central heating.

Note: All external heating demand checks are confirmed (power at pin 2 on X13 on the PCB).

Cause (1): If the hot water flow switch sticks ‘On’, the boiler will stay in hot water mode, causing the appliance not to switch over to heating. 

Solution (1): Turn off the heating demand. With no water demand at the taps the demand LED should not be illuminated. If it stays illuminated check the water flow switch. You can disconnect connection X1 at the PCB to prove.

Cause (2): Pump body refitted incorrectly. 

Solution (2): If the pump body is fitted the wrong way around it will give negative pressure to the diverter valve, which will 
not allow the diverter valve diaphragm to operate. To confirm it is correct check the NRV on the pump body is located towards 
the top right of the pump.

Cause (3): Diverter not operating. 

Solution (3): Check for 230V AC at the diverter valve solenoid. If satisfactory then drain/clear the diverter weep tube at the 
diverter and at the pump connections. If these are satisfactory replace the diverter.

Cause (4): Pilot ignites but no burner pressure. 
Solution (4): Check the off-to-low solenoid gas pressure setting; if it is set too low the burner will not light on heating. The boiler will operate satisfactorily on hot water due to it lighting on its higher gas pressure. Reset the pressures as required.

Fault: No ignition on heating or hot water.

Cause: Gas valve pilot solenoid passing, causing the pilot light to stay alight. 

Solution: Check to see if the pilot light has extinguished. If the pilot stays alight after a demand this will cause the PCB to freeze. To prove, turn off the gas at the appliance gas isolation valve then re-establish, boiler should then operate until its next off/on period. Replace gas valve as required.

Fault: Radiators heating with water demand only.

Cause (1): Reverse circulation caused by a faulty non-return valve. Located in the return isolation valve. 

Solution (1): Isolate the return isolation valve to prove. Clean/replace NRV as required.

Cause (2): Restriction or blockage in the primary circuit feeding the plate heat exchanger may cause excess pump pressure to operate the diaphragm in the diverter valve, even with no power to its solenoid! 

Solution (2): Drain, clean out top manifold and primary filter (where fitted). Check/clear plate heat exchanger and bottom manifold. Recheck the temperatures around the primary circuits.