Water hammer from toilet cistern

[MM]

My low-level cistern has a button to press to flush the loo. If I simply press the button I get water hammer noise. If I press it initially very gently, so as to release some water into the bowl, and then press fully down, I DON'T get water hammer.

So where is the *actual* water hammer noise emanating from? I've read before that water hammer is something to do with high pressure and valves, but I don't get how this only started recently (over the past couple of months).

This is the cistern internal 'gubbins':

[Bob Minchin]

The hammer will be coming from the fill valve. I expect pressing the flush gently start the refill slowly rather than quickly when doing a 'fast' flush. Maybe changing the diaphragm in the valve will shut it up. Or fit a hammer suppressor in the pipe nearby.

Bob

[MM]

Looking at the top picture at

do you mean the white component in the right corner (closest to tiles) on which the ball valve arm is mounted? It has a cap of some kind with "teeth" around the circumference. Presumably this can be unscrewed?

I could, but this hammering only started fairly recently, so something must have happened to cause it, e.g. something or other worn after five years' usage.

MM

[Bob Minchin]

Yes & yes. turn off the water first! note which way round the diaphragm is fitted.

Only bother to do this if the diaphragm change does not fix it.

Bob

[MM]

I've had the lid off the top of the cistern and just realised that the water hammer occurs AS SOON AS I depress the float arm, even by only a tiny movement. Does that confirm your "diaphragm" diagnosis?

Thanks, BTW!

MM

[The Medway Handyman]

Its the other bit with the blue float.

Is the water inlet plumbed in copper or is there a SS braided flexible?

[Spamlet]

Seems to me your initial problem was waves/ripples joggling the float. It may well go away if you lower the float a little so the water level pushes it up more firmly.

S

[MM]

Plumbed in copper. With an inline shut-off valve (screwdriver slot) almost directly beneath the cistern.

MM

[Fredxx]

How is the adjustment on the screw thread holding the assembly away from the side? If there's any movement, I could see that this may cause hammer from interaction of the float arm and assembly.

[Matty F]

If the mains water pressure has increased recently that could cause a problem. I have fitted a pressure reducng valve in my main pipe and that has fixed all my plumbing problems. Rubber diaphrams can cause hammer while fibre diaphrams won't.

[NT]

[The Medway Handyman]

Try turning that down a bit.

[MM]

I did a few weeks ago. Made an initial difference, but now the hammering is back. However, I'm sure the diagnosis of the diaphragm being at fault is correct, since when I (lid off) depress the float arm ~~~very~~~ slightly, the hammering is immediate and one can almost see the plastic boss on which it is mounted vibrating a bit.

Trouble is, I have scoured the internet (Screwfix, Plumb Center etc) and have so far failed to find any replacement diaphragm. Maybe the whole unit will have to be replaced, which would be no big deal since a bottom-fitting one is only £3.99 at Screwfix.

MM

[NT]

any plumber's merchant should have a little rubber disc for it. Replacement discs dont necssarily need to be of identical design.

NT

[Matty F]

Rubber disks cause water hammer. Fibre disks don'r.

[MM]

Doesn't the disk have a small hole in it to equalise pressure? That's what I've read on one of the suppliers' sites.

MM

[John]

Not sure you can get fibre ones for the type he illustrated with a photo. I know you can use fibre on the old ball valves - but modern diaphragm ones need the flexible walls of rubber. (surely)