Knocking sound in central heating system

[Simon Elliott]

After a new boiler was installed, our CH system started knocking after it had been running for a while. Turning the thermostat on the hot water cylinder up higher would make the knocking stop for a while. The guys who did the installation tried turning the pump speed down, which helped a bit but didn't fully solve the problem.

The chap who came round to service the boiler suggested that it might be washer flutter in one of the thermostatic valves. Today being a cold day, the knocking started again. I went around the radiators with thermostatic valves, and found one which seemed noisy. Turning the valve slightly stopped the noise. Playing with the valve, it seemed to be quite clunky.

Is there anything I can do with this valve without draining the whole system?

[Harry Bloomfield]

Simon Elliott was thinking very hard :

Just a suggestion with no idea of its worth....

Could you try turning the valve down at the other end of the rad to reduce the flow a little - that might prevent the clunking.

[Simon Elliott]

Interesting idea! I'll give it a try when/if the problem recurs.

[bland]

An old chestnut reply is to ask if the valve is fitted the right way round for the water flow. Older TRV's will rattle if the flow is backward, these should have an arrow or indicator to show the correct flow direction. Some newer valves are bidirectional

bland

[gna03633]

Funny enough I'm having a similar problem.

I had 90% of my central heating system re-plumbed recently, the other

10% was replaced a couple of years ago. When the Boiler starts up I hear a tinkering from some of the valves, it something to do with the flow of water - turning the valve down reduces the noise. I haven't had some time to investigate.

[Blueyonder]

Yes - leave the valve fully on or fully off.

I have the same problem, since installing a new boiler. I suspect one or two valves were put in the wrong way around, but it did not cause a problem with the old pump. The new pump more powerful and so is causing the valve to knock. Next time the system is emptied, I'll turn the valves around, but in the meantime, leaving a few valves fully on is not too big a problem.

-- JJ

[Simon Elliott]

I think you've hit the nail on the head. I assume that the water is flowing from the hotter radiator pipe to the cooler one, and if so, it's fitted back to front. Looks like I'll need to wait till the next time I want to drain the system.

[Dave Plowman (News)]

Some TRVs can only be fitted on (usually) the flow side. They'll often have an arrow on them denoting direction of flow. Fitting them on the return can give the symptoms you describe. So check by seeing which pipe gets hot from cold first - this will be the flow.