No heating - MA1 actuator not moving - valve not seized - HW fine but interrupted if UP2 HW switched on
I have the following setup:
- Glow-worm 30R regular boiler
- Hot water cylinder with cylinder thermostat
- British Gas UP2 programmer (Invensys)
- Honeywell T6360B1028 mechanical room thermostat
- Drayton MA1 3-port motorised valve
The problem:
System was fine before summer. No work was done. Now, central heating isn’t working. With heating ON at the UP2 and the room thermostat turned up to 30°C, the MA1 actuator remains at W (hot water position). No movement, motor noise, pump activity or boiler ignition
Hot water works, but behaves strangely:
- If heating is already ON at the UP2, the boiler won’t initially ignite for HW if it is switched from off to on, even with the cylinder thermostat calling.
- Switching heating OFF allows the boiler to ignite and heat the cylinder for HW on.
- Once the boiler has ignited for hot water, I can toggle heating ON/OFF without interrupting it.
What I’ve checked:
- Replaced the MA1 actuator head with a second-hand unit — identical symptoms.
- Tested both actuator heads detached from the brass valve body but electrically connected — neither moves, ruling out a seized valve spindle as the cause of no movement.
- Honeywell room thermostat clicks normally.
- Continuity between thermostat terminals 1 and 3 is present only when the thermostat is turned up (tested with wiring still connected).
- UP2 heating and hot water indicator lights operate normally.
- Confirmed UP2 terminal 4 (CH ON) has a red wire connected.
- System worked perfectly last winter. No wiring or controls have been changed over summer.
Current suspicion:
Possibly a failed UP2 heating output, a wiring/connection fault, or missing heating demand reaching the MA1 white wire.
I have a multimeter but haven’t yet measured the live control outputs. And it's only CAT2 up to 300V, apparently I need a CAT 3?
Questions:
Does this sound like a faulty UP2 programmer or something else in the Y-plan control circuit? Could the unusual hot water behaviour be related to the missing heating demand?
Any advice on narrowing down the fault would be appreciated before I start replacing more parts or just call an electrician/plumber.