Worcester greenstar r1 intermittent firing

[Kiwi_graham]

Hoping someone can help me solve an ongoing mystery of why my boiler sometimes fires up and sometimes doesn't...

Worcester greenstar r1 boiler with an towerchron qe2 timer..it used to work fine, and respond reliably to the programmer but now only sometimes....why???!!!

Having read various other threads I can confirm
...that my thermostat will click when the CH timer light is on.
...the timer lights come on and off at the correct times according to the programmer
...the valve will return to W only when the timer is switched off and on at the wall fuse
...the valve switch does not stop in the middle M when both CH and HW lights are on, instead it moves straight across from W to H and stays there no matter what is done to the timer. Only resetting at the wall, seems to take it back to W.

Any clues as to how I can identify the problem would be very much appreciated!
Thanks

 

[stem]

The motorised valve, is what actually controls the boiler. The programmer and thermostat control the motorised valve and when the valve moves to the heating only position, it operates a small microswitch that fires up the boiler. It is fairly common for this switch to become worn and operate intermittently.

It can be tested by someone who has a basic understanding of electric circuits and knows how to use a multimeter safely. You would be looking for 230V on the orange wire from the motorised valve. If when the fault is present, and the valve has moved fully across to the heating only position (ie not the mid point) there is not 230V on the orange wire then it is faulty. The test should be done with the hot water off, because the hot water has a separate way of controlling the boiler.

The other descriptions regarding the operation of your valve are what I would expect. The valve will return to the hot water only position when power is removed. it's pulled there by a spring. And it won't stop in the middle unless heating and hot water are both on and their thermostats actually calling for heat.