Boiler not Firing for CH, only DHW - 3-Port Valve Prob?

[t78]

Boiler: Stelrad WRS 450
Room Thermostat: Honeywell
Control Unit: Honeywell ST6400C
3-Port Valve: Myson MPE322

Hi guys,

The hot water appears to be working ok. However, when the heating is meant to be on, the boiler does not fire.
I investigated the 3-port valve in the loft - when I moved the slider to heating manually, the boiler fired up and (thankfully) the place warmed up. So is the problem therefore with the 3-port valve or have I missed something? Should the slider move itself under normal operation?

The heating has stayed on for a few hours and is still working - will the slider move back of it's own accord and will the problem reoccur when that happens?

Are there any other checks I need to do? Is it something I can fix myself?

Thanks for any help!
Tris.

 

[gas4you]

Soundslike a 3 port valve fault if you are sure the programmer and room stat are working ok.

Could be as simple as replacing the valve head, if you have one that can be replaced. If you do have one, remove the head and see if the spindle can move freely.