potty potterton

I have a potterton profile running a gravity-fed central heating system and a domestic (secondary) hot-water cylinder installed in the loft. I've noticed that the boiler still fires-up (although infrequently) when the mechanical timer is set to the TIMED position but during the OFF period when the pump is not running. The upshot of this is that ocassionally the boiler shuts down because the overheat thermostat pops out under the boiler . This tends to happen
during a longish OFF period. There is no cyliner / room thermostat or 3-port valve fitted - just an old-fashioned clunky mechanical clock.

Is the boiler faulty ? - why should it continue to fire-up when there is no external signal to tell it that we need heat !. BTW. The domestic water gets very very hot when the clock is set to heating/water, presumably because there is no cylinder thermostat/3-port valve fitted so the cylinder is therefore not by-passed. Is this excessive temperature contributing to the overheat problem ?.

help potty-me!

It seems you are getting a electrical feed which is either by-passing the clock or the clock is faulty. It can be a problem on digital or mech programmers. The clock may have a switch or screw in the back to deactivate some of the functions. If your going to look isolate the power for safety.

When its doing it see if any power is present at the boiler terminal strip,if yes you have an electical fault if not you have a sticking gas valve which is letting gas through it when it should not . And needs changing ASAP as it is dangerous to leave it in that condition

Oh, thanks a lot for your advice lads. I will attach a multi-metre on the supply cable from the clock to the boliler. Actually, I noticed last night that when the it's in this condition and I turn the thermostat up on the boiler, it also fires-up !.

Can I still buy one of these old mechanical clocks as a straight replacement ?, or would it be better to consider an electronic room stat, or a room stat in conjunction with a 3-port vale for economy reasons ?

just switch the power off and see if it still lights

Eureka !. The clock is on a fused spur, and I forgot to switch this on a few weeks ago after flushing the CH - I'm certain the boiler didn't fire-up with the clock isolated.

I guess this confirms it's a dodgy clock ?.

Thanks a lot.

less potty-me!