I recently replaced a portion of the 15mm mains inlet pipe work to our cold water storage tank.

The pipework went from the following copper/flexi-hose (15mm flexi with flexi connector to tank) setup:

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To the following 15mm copper only (with tap connector to tank) setup:

enter image description here

Since making this change the new section of pipe is producing a 'hissing sound' when it is filling the connected cold water storage tank. I assume it's the new section of pipe as when i place my hand over a part of this section the hissing is partially muffled.

I appreciate of course there might be other parts of the system producing the sound and it resonating in this area; but this is the only recent change so seems to be responsible in some way.

There are no obvious leaks in the area, or elsewhere, that might be causing the hissing. Also when the float valve on the connected water tank closes the hissing stops.

Also the new section of pipe work should not be any more restrictive to the water flow compared to the previous configuration. The previous flexi-hose was 15mm and looks to be 3/4", although not clear from the markings; and bore size not marked. Arguably this was more restrictive to the flow than the current copper only configuration.

One could say that the flexi-hose offered a "smoother" flow to the water based on it's more curved bend. Also the current copper pipe is rippled slightly on the top bend, on the inside of the pipe; also i fitted an isolation value to the start of the new copper pipe section.

New isolation value, ball valve, is full bore and wasn't there previously with the flexi in use.

Any ideas as to why the current copper configuration might be causing a hissing sound when the tank fills? Are there any possible solutions without replacing the pipes, or would, for example, i need to try replacing this section of pipe with 22mm copper pipe i.e. increase the flow capacity?

Over the course of a few weeks observation, i believe the hissing is actually water flow through the new section of pipe particularly through the top bend.

Previously, when the flexi-hose was in place, the rubber internals likely contained the noise of high velocity water passing through this pipe section.

I also understand copper wall thickness is now typically 0.7mm, on 15mm pipe i believe, whereas many years ago it used to be 1mm. This could further exacerbate the noise level depending on the pipe section.

My options seem to be, replace the section of pipe with thicker walled pipe and/or replace with 22mm to "spread the load". Or revert back to some kind of flexi-hose with rubber internals to dampen the noise.

Any further suggestions are welcome?

1 Answer 1


The source could be the restriction given by the ball valve.

However, just closing that ball valve a bit might drop the velocity sufficiently to quieten it down.

Was that ball valve there when the flexi was in use?

  • Great observation, the Pegler ball valve, at the water tank egress to supply upstairs bathroom(s), is also a new addition. It was however there for a week with the flexi in use, and no hissing was being produced. Commented Nov 23, 2021 at 14:45
  • @Worldly-Guest8115 no, not the one on the outlet of the tank, but the one you use to join the new copper to the supply.
    – Solar Mike
    Commented Nov 23, 2021 at 14:49
  • Ah sorry, the ball valve (isolation valve) was not there when the flexi was in use. It was just a straight compression connector when the flexi was in place. Commented Nov 23, 2021 at 14:53
  • Also the new ball valve (isolation valve) is full bore, so shouldn't be restricting any more than previously (less)? Commented Nov 23, 2021 at 15:33
  • Closing the ball valve makes a small difference, but other than closing the ball valve completely the hiss remains. So looks like the hiss could be emanating from the float valve inside the connected cold water tank. Is there a concept of low/high water pressure float valve to be considered? Commented Nov 23, 2021 at 17:59

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