Boiler gets up to temperature very quickly then cuts out.

Hi
We have just moved and have inherited a 10 year old oil boiler and heating system. The whole thing needs power flushing but that's not the problem. It worked fine when first used but now the boiler fires up for both hot water and heating demand but once the radiators/hot water cylinder are up to about 45C it cuts out and then will not restart until the system has cooled right down. The room stat is set to 25 and I have checked that it is working. The pump is a little nosy but the doesn't seem to be any air in the system. The return from the expansion tank is working fine. Could it be a fault with the boilers thermostat? any other suggestions welcome.
Thanks
KA

You say your system needs a powerflush, then say "that's not the problem".
Where as I'd say say your issue is circulation which, if the pump is working, a powerflush would cure.

KDJA said:

The return from the expansion tank is working fine.

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I hope you don't mean that you can see water flowing into the feed and expansion tank?

KDJA said:

doesn't seem to be any air in the system.

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do you mean that you have bled all the radiators, water squirts out forcefully from all of them, but little or no air?

KDJA said:

The pump is a little nosy

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do you mean it makes a humming motor noise, or a running water noise? Is it surprisingly hot?

KDJA said:

The whole thing needs power flushing but that's not the problem.

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I bet it is!

Where do you measure the temperature from, to the radaitors and cylinder? Check the boiler temperature, at the same time.

Circulation problem, either by the pump struggling, restricted pipe flow, or blocked heat exchanger. All caused by sludge in the system .