Potterton EP3000 programmer HW and CH buttons not working

From the Horstmann HEATING PROGRAMMER REPLACEMENT AND WIRING GUIDE.

EDIT

I think the diagram you have, is showing the internal view of the programmer (simplified of course). In this image the blue rectangle represents the internals of the programmer, and the red lines represent the lines going to the boiler and power.

The lines with arrows are probably switches.

And if both HW and CH were on, it would look like this.

Because of this, 5 has to be linked to L to supply power to CH, like so.

But I'm guessing the new programmer supplies power to CH internally, so with the new programmer it should look like this.

Again, I could be wrong here as I've never worked on a system exactly like yours. After looking at the wiring diagram the arrows are either switches, or diodes. Though, diodes would look more like this ->|- .

Hope this helps.

Despite scouring the Internet, I didn't find the solution - the gas man came today and after describing the symptoms to him he identified it immediately:

• Faulty zone valve contact switch

Basically, the sequence is as follows:

• Timer turns on - sends current to thermostat
• Temperature is too low - thermostat relay sends current to zone valve
• Zone valve turns fully on - contact switch triggers to instruct boiler to fire up
• Timer turns off or temperature reaches threshold - current no longer passed to zone valve
• Zone valve turns off - But faulty return spring kept switch on instructing boiler to remain on

It turns out that these zone valves have a plastic casing, rather than a metal one, so the return spring hooks around a small plastic spur... which just snapped off.

The fix: popping in a small screw to hook the spring round

Callout charge: £90