In message , IMM
writes

One version of the neataheat had a plug in relay. It sounds as if this has
failed. have a look it is quite large. Check it out. Some time you can
see it arcing. If it has failed go to Maplin and buy one the same. The
makers charge more for a standard common relay you can buy anywhere..


Yes, this version - the other version is the Netaheat electronic John

If the main gas valve is opening and staying on "reliably" for a couple
of minutes, then I doubt it's the relay. If the relay contacts were on
their way out, I would expect it to either work intermittently or just
fail. It sounds to me more like the overheat stat opening and shutting
down the boiler.

I don't have any experience regarding scaling up of cast iron heat
exchangers, but I would think it is likely that, for some reason, the
heat delivered to the heat exchanger is not being dissipated and so the
boiler overheat stat is shutting the boiler down
--
geoff