Boiler Cutting Out

This normally happens when the boiler and pump are turned off by a room stat - and the residual heat in the boiler's heat exchanger continues to heat the water to the point where the overheat stat trips.

Boilers which are susceptible to this usually control the pump directly - and have an over-run stat which continues to run the pump for a while after the boiler stops firing - in order to dissipate the residual heat.

Clearly, the water must have an available circulation path during this over-run period - often requiring a by-pass loop, depending on the exact setup.

The first thing to check is whether your boiler has a pump over-run stat and, if so, to make sure that it is wired up correctly.

If you still have problems, come back with further details of your system setup. Does it look like one of the "plans" described in

In message , Neal Harwood writes

The impeller of the pump hasn't broken off the shaft has it ?

or as happened to a friend of mine, the blades had all fallen off the impeller (and were nowhere to be found).

I don't think the system would run for between 1/5 and 1/4 of a week if it had!

I think it's something to do with sets of circumstances which occur now and again where the pump can't effectively overrun. Maybe it's not wired to do so, or maybe there isn't a by-pass and it gets caught out when all the TRVs are shut.

In message , Set Square writes

You have a point there - I'll try to read threads better in future

Thanks for the replies so far I believe the pump is fine, because it DOES work most of the time, just cuts out occasionally. It has also only started doing this recently. It used to happen VERY occasionally (2 times per year), but recently it is much more frequent.

This 'cut-out due to residual heat' sounds very feasible I will try and work out the circuit type (thanks for the link) and get back to you

Neal

Having just read some old (c1998) posts (I Love GOOGLE Groups), someone else suggested problems with an airlock in the CH system We HAVE had to drain the system a couple of times over the last year (to move some pipes and rads in the basement). Is it possible I have an airlock of some sort? Where would it be? Would it cause my symptoms? And of course, HOW do I get rid of it?

I doubt if it's that. An air lock after re-filling can prevent any primary circlation (I've had one!) but once you've got circulation going, any residual air will usually park itself somewhere where it doesn't do any major harm - probably in one of the radiators. I don't think an air lock is likely to cause an intermittent circulation failure.

Of course, I could be wrong . . .