Boiler Cutting in/out

Hello board!

I have a open pumped system with a gloworm ultimate 30FF boiler. HW storage tank, no immersion, motorised T system, Cold storage in the loft with a feeder tank.

I've had a lot of problems - there's a thread called Temper Mental somewhere which basically ended in me getting the gas valve replaced. At the time, the guy said the boiler looked clean inside and the heat exchange looked good. He gave it a once over and it sounded a lot better after he'd been.

There is one problem remaining, that was there sporadically before and has continued.

On turning the heating system on, everything fires up, turns on etc. etc. and the rads get hot within a minute. The boiler, however, cuts out shortly after that. A minute or so later and it will fire up, then cut out again a few minutes after that.

The heating does work and it gets warm -which is a lot better than we had last year!!! But this obviously isn't right and good be improved.

Is this Kettling? Or is there an airlock somewhere that is stopping the pump from doing its job, and the boiler is cutting out as a safety measure?

The pump is working but it doesn't seem to make any difference whether it's on 1, 2 or 3.

Thanks for any input you can give!

Mr. Tickle.

everything fires up, turns on etc. etc. and the rads get hot within a minute. The boiler, however, cuts out shortly after that. A minute or so later and it will fire up, then cut out again a few minutes after that.

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That's correct. The boiler stays on until IT is hot - as determined by its own thermostat. The system takes out heat as fast as it can, determined by radiator size (may be impeded but pump but quick heat up of rads implies yours isn't.)
When the boiler cold a bit, it comes on, etc, until the room stat is happy.

It may well be on longer on HW because a cold tank of water is a good heat sink!

What makes you think there's something wrong?

Just a nagging doubt about your statement

"all the rads get hot within a minute"

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Please confirm this is a 30FF (30,000btu/hr) boiler.
How many rads have you got?

On a reasonably well matched system with fixed output (about 9 kW) I would expect it to burn continuously for about 20 minutes from cold before the boiler 'stat comes into play unless the water is not circulating properly, but if it's not circulating properly the rads wouldn't get hot.

Are you only feeling one rad close to the airing cupboard to judge whether the system is hot or not? Are there others which are not hot at all?
Does the boiler thermostat go all the way from OFF to MAX with no sign of the knob slipping? Set it midway for now.

As you have had your gas valve changed you should check your gas rate to confirm the heat input. As a rough guide you should burn about 35 cu ft/hr, so about 1 minute 45 secs per cubic foot at the meter with no other appliance on. If it is wildly different have a CORGI fitter check it as a matter of urgency.

A curious state of affairs.......hmm