Thumping noise when pump for back boiler is activated

It doesn't happen all the time, but maybe 70% of the time.

Previously, we had turned on the pump at the first sign of clanging pipes - the radiators were all heating thoroughly, no problem but often we get this rhythmic, pulsing, thumping noise seemingly coming from the roofspace. It can be loud enough and pretty irritating.

Is this maybe just caused by an asthmatic pump (it's old)?

As an experiment, we have recently been waiting longer - pipes clanging constantly (not just for a few seconds) - and not turning on the pump until there is an ominous rumble from the roofspace. Probably not a good idea, but this approach seems to minimise the thumping noise.

Any ideas anyone?

the banging of pipes indicates that the water in the pipes has turned to steam and the noise you hear in the attic is water/steam being released into tank. I would recommond you wire your pump through a pipe stat, so the pump will run automically at a preset temperture.

DavyJones said:

I would recommond you wire your pump through a pipe stat

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Thanks - but can you explain this as though you would to a child

...and give me an idea of cost

What about the thumping though? - as distinct from the clanging pipes - it's definitely a different thing.

the thumping is caused by the expansion of the boiling hot water. Its under a lot of pressure in a restricted space, think pressure cooker.
As for the thermostat its about 20 euro or less, the wiring is easy enough, the most basic way of explaining it is cut the cable between pump and socket/ fused spur and run it through the stat, L N E will be marked on stat. attach the pipe stat to the higher of the two pipes (flow) with the cable tie supplied and set wheel to desired temperture. when the water hits that temperture the stat will turn the pump on.

Thanks again - but I really think the 'thumping' is something else other than merely boiling water related.

One night, I left the pump on all night by mistake - even though the fire had died, next morning, fire out and radiators stone cold, the thumping noise was still to be heard.

But anyway, the thermostat sounds like a good idea.

Strange one, so the pump is running when you hear the thumping?

Caveat said:

, thumping noise seemingly coming from the roofspace. It can be loud enough and pretty irritating.

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Firstly check your attic for “Flowers in the Attic” see ISBN 0-671-41124-1 for further information.

While your up there check the small black tank which feeds the boiler and rads for the fire. It should be half full of water and perhaps when you hear the banging go up and look and see if there is water filling the tank from the pipe which is bent over the top of it.