Is my central heating timer goosed?

[blueST]

We've got a normal gas central heating system with a Honeywell timer programmer the same as this http://www.honeywelluk.com/products/Time-Controls/...

I think it may be faulty, but maybe someone can advise. The heating comes on an off with the set programme as it should, i.e. on in the morning, off during the day and on again at night.

The problems start if you try to override the programme, by pressing the button to the right of the slider switch. This should turn the heating on if it's currently off, or turn it off if it's currently on. If I want the heating on and I press the button, the light comes on to show it's has activated and I can (sometimes) here the zone valves motoring about, but the boiler doesn't fire up and the pump doesn't activate.

I've tried a hard reset, still no joy.

Would you say this is likely to be the programmer? If it is, I'll just swap it like for like. Or do I need someone to look into it more deeply?

[blueST]

The thermostat is set to 22 and the room temp was showing as about 16. So it should be firing up.

I can here a relay click when I press the button. Sometimes I hear a zone valve move too. Strange.

[blueST]

Double post, ignore.

[blueST]

I did a direct swap of the programmer, like for like, over the weekend, and I've still got the same problem. Time to stop clutching at straws, hit Rated People and get a man in.

[blueST]

Turns out it was one of the zone valves. It would function fine most of the time, but every now and again it wouldn't move through it's whole travel, and so not hit the contacter to trigger the boiler. I guess it must have also stuck fully open at some point, explaining why we couldn't turn the heating off on one occasion.