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Thanks Freddo.

Replaced the SM5203 with an SZ2301 and we're all hunky dorey again. Thanks again for all the helpful advice. One point that troubles me somewhat, the SM5203 was power open/power close whereas the SZ2301 is spring close. System works fine but is it OK to swap like this or am I missing something...

Ah, OK. So there's a microswitch that closes when the valve is open?

****, I keep posting before I've really worked out what's going on. Bugger. So SM5203 actuators don't have relays in to provide the switched live for a boiler?

DP, don't trust them for checking a switched live connection because they lie. Mine's an S Plan. Current theory is that the CH valve actuator's relay is duff and the 'intermittency' is when the HW thermostat switches in providing the demand for the boiler. I've got the tools to find out...

Touche ;)

I dunno yet. I've got a box in the airing cupboard with a bunch of choc blocks, various old wire ends, sellotape and knots as strain relief. I've told the wife I'll leave it alone for an hour or two with L1/L2 linked whilst the house warms up ... and I can watch some football.

Picasso/Freddo - Super job. Boiler fires up fine with L1/L2 linked so it's not a boiler problem. Thanks very much. I'll be back when I've tracked down the root cause. Unfortunately, my system was upgraded to the current boiler by "professionals" in 2006 and the wiring is an utter pigs ear.

Just testing with a mains tester screw driver. That's a very interesting suggestion on the basis that it's now looking like an intermittent problem (it seems to fire up every now and then). Thanks, I'll give it a go.

Thanks for the advice. The problems I had five years ago were caused by a leak but it's as dry as a bone now. I probably will get a pro in if I can't make any progress myself this weekend. I've got a spare control PCB as it happens, bought for me by the last pro I had in to fix it when it was...

Thanks for the reply. The fault finding guide only covers fault codes and my boiler isn't showing any fault codes. I've fixed it myself when I had fault codes in the past - replaced the flame sensor wire that had lost its insulation and was shorting to the case - remade the connections to the...

Hello helpful people of DIYnot :) It's getting a bit colder so my heating is playing up. A ten year old Ideal Icos HE24 sits stubbornly showing status 0. The switched live (L2) has volts on it - indicating demand? - but it doesn't do anything. Am I right in thinking this indicates a boiler...