Hissing from cold water pipe when hot taps opened

15 Jun 2020
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United Kingdom
ever since I had a new condesing boiler fiited, whenever I run the hot water tap in my kitchen or bathroom I start to hear hissing coming from a cold water pipe. This pipe runs from the attic, straight down through the floor of the airing cupboard and the noise sounds like it's in the pipe. A couple of minutes after the hot water tap is closed off the hissing stops and the water stops running through the pipe.
is this something to be concerned about? Or perhaps simply an excess of pressure due to the new boiler, but also why is the cold pipe hissing?
I have a hot water tank, so it's not the cold going to the boiler to heat up as it would in a combi boiler.

I also dont use too much hot water, so the tank is usually very low, could this also be a reason?
thanks in advance.
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If you draw off hot water it will be replaced by cold. If that's drawn from a cistern then the cistern will top up from the mains. Either way there is likely to be some noise in a cold water pipe.
Hi alec, thanks, makes sense.
If the hot tank was fuller, then it wouldnt need to fill up as much? Or does it fill up the amount that you've just used of hot?
Really sure the pipe never used to hiss before the new boiler was put in.... possibly because the hot water was on a lot more, whereas now I only boost it once a week or so.
If you have a vented system with a conventional hot cylinder fed from a cistern some way above it then the cylinder will always be full. So the amount of cold topped up equals the amount of hot water drawn out.
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Ok thanks a lot. I'm glad that the hissing is nothing to be worried about and is normal. Appreciate the help
the ball-cock in the loft tank (if such it is) often gets noisier as it wears with age. If the noise is noticable, have a look at it. it might benefit from replacing with new. Post a photo as there are various types, not all of them suitable.

Noise can also result from a valve that has been partially closed, forcing the water to squirt through it at higher pressure.
I'll have a look but it's only a 12month old cistern and ball, but could well be a valve, I'll have a look. Thanks for the tip

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