Potterton Central Heating -where to check next - your advice pse

[Mike]

Background: Central Heating uses Potterton Suprima 80 ( 2 yr old) ; Honeywell timer ( Stc6400)NEW: All rads with stats + hot water tank separate thermo; Wired to Y plan. The fault: Heating started to come on 2 or 3 hrs after timer programm, then failed to ignite boiler. Hot water will operate fine with or without the Heating switched on.

Heating now will operate only when Hot Water timer is also on, otherwise boiler fails to light .

Advice sought is this aboiler or Electrical fault?

Any 1st steps to check? Your help sought Mike

[Set Square]

I'd put my money on it being an almost failed motor in the actuator part of the 3-port valve. If the motor *almost* works, it can take it a *very* long time to move the valve from the HW to the CH position - and it has to be most of the way there before it switches the boiler and pump on, unless there is also a HW demand - in which case the boiler is switched directly, and not via the actuator. Have a look at the Y-Plan wiring diagram in

and you'll hopefully see what I mean.

It's *just* possible that the wet part of the valve has partially seized - which could cause the same symptoms - but far more likely that it's the actuator. With most makes of valve you can replace the actuator without having to disturb the wet part. You can even replace just the motor if you're happy with soldering.

[Mike]

Please forgive my next Question: Where would the 3 way value be located? close to the pump my guess ans yes happy to solder items. Many thnaks for your advice Mike

[Set Square]

Trace the flow pipe from the boiler to the hot water cylinder in the airing cupboard. At some point there will be a pump. Somewhere *after* the pump the HW and CH circuits separate. This is where the valve will be. It may well be in the airing cupboard - but it might not be. It will look something like

- but it may not be blue.

[Ed Sirett]

They also supply wire-nuts (spit) to make the connection. There is also quite enough room to use a choc strip and sparky-tape.

[Ed Sirett]

Invariably it's near the HW cylinder, although there is no absolute need for this. How do you know you have Y-plan if you haven't found the valve?

[Mike]

Yes the wiring and 3 way valve are below a panel under the H water tank, a replacement part sourced and fitted 25 mins later Bliss Heating back on

Thanks again for all your help Best Wishes to all on DIY Mike