Symptoms:
When CH demand turns on in morning ( CH was at 16 degs), the Pressure differential needed to activate ‘Pump On’ signal was insufficient and so failed to maintain ‘Pump On’ signal to the electronics, causing an F75 error. This appeared as a pump failure but was not actually so.
Adding more water increasing the CH pressure improved the Pump On signal but eventually the differential pressure signal becomes insufficient. This failure mode is caused by the pressure sensor having a clogged inlet such that fast pressure changes have reduced signal output. The signal output device is a simple potentiometer so small pressure changes are attenuated, especially if the diaphragm or return spring has fricton internally. The pressure sensor was replaced and a magnetic filter was later added to the system.
To Empty Boiler:
Symptoms: No error code on display but hourglass symbol on.
If not enough heating flow then S7 pump overrun hourglass pictue is present flashing and boiler cycles on about 2 mins and off for 6 mins stoppings radiators from heating properly. Overrun keeps the boiler off until pump flow cools heat exchanger. Lowering the CH temperature to about 48 degrees stops the overrun cycling as a temporary measure and keeps the radiators on at a lower temperature.
If the diverter valve is partially stuck open then the boiler heat exchanger is not cooled by any CH demand and radiators will be warm even if CH demand is off.
Test the diverter valve operation by observing the valve operation. When making a CH demand by increasing the room thermostat setting then lowering the setting under running conditions, the steel vave pin should move in and out unless stuck or the driver motor has failed.
The diverter cartridge valve is operated by the stepper motor.
There is a top view hole to see the valve pin. This should move in or out depending on CH or DHW demand. Tried this but no movement seen. The pin end can be disconnected from the motor to facilitate changing the valve cartridge or motor. The motor is fitted with a platic outer sleeve screw which can be undone ante clockwise. Have released motor and re fitted without disconnecting drive, applied a little oil on the pin but did not decouple the motor drive. There was small movement possible of the valve pin.
The motor drive has a fitting slot and side hole which allows the pin ball end to locate from the side, so the motor & drive can be taken out and checked. The motor connector is 6 pins in two rows, low voltage. Measure resistance across 1-3 and 4-6 to check windings. Motor is bi directional. Replacement cartridge valve available ebay £10 and motors at £18 so both these were ordered as a potential replacement. When switching on the boiler you can hear the stepper motor buzzing as it resets and boots up.
On removing the motor, fragments of plastic bits were discovered in the diverter body cavity. The valve pin was now accessable and found to be movable so just changing the motor fixed the problem. Life of duff motor about 10 years with plastic motor shaft, later motors use steel shaft.
Change Motor:
This is the easy option since the boiler need not be drained so simply detatch the connector and uncrew the outer plastic securing ring and ease the motor shaft from the valve pin. Fit in reverse order.
Change Diverter Valve Cartridge:
1). Power off, remove front cover, fold down display panel and Drain boiler .
2). Unscrew stepper motor and detach from valve center pin.
3) Attach a jubilee clip device with inner protrusions to the outer valve body & compress cartridge tabs inwards.
4) Pull out valve cartridge observing orientation, clean inside valve body
5) Apply silicon grease/oil to the replacement cartridge O rings, sliding pin and diverter body.
6) Align locking tabs to snap into valve body and push cartridge home ensuring tabs are locked in.
7) Refit stepper motor locating pin into motor drive socket.
8) Close open drain valves replace drained water from filler loop.
9) Power up and check water pressure then do a Purge.
Motor: 90141150701 4103791?
Factory 124-3 2125 A08 20% ED UC53N04A2GZ28 150 ohm
Pump has air extracter which can remove air in the system when Purge cycle is activated. P17
Loosen air purge cap to allow venting first. On Boiler display Press TL & TR buttons together, Press TR button for Menu point = 00.
Select 17 using minus/ plus buttons then TR to access P programs.
Select P.00 for heating circuit purge, press: one x TR button to start purging.
TL button cancels any changes to exit the purge program.