Noisy Pipe when I run any hot water tap !!

[Mezza]

I have a downstairs bath and when I run the Hot Water tap after about 20 secs a cold pipe running from downstairs into the loft makes alot of noise like its struggling to keep up with the amount of water but the pipe is fairly big & I'm only running the 1 tap.
When I run the cold water tap the noise is hardly anything at all.
The noise makes alot of noise through the whole place.
Any help is Appreciated !!

 

[bathjobby]

Without knowing what sort of noise and more about the system; !
Is the knocking noise mechanical i.e. pipe knocking or a noise caused by the water in the pipe i.e gurgling etc ?
I'm guessing the cold water pipe in question is the feed to your cold water storage tank, so as you run hot water the cold tank ball valve will open a short time later (20 secs) and the noise starts !? Whatever is going on in the cold feed above the level of your bath is significantly reduced when you run cold water, probably because that cold tap is fed from the same cold supply to the tank, hence opening it significantly reduces the pressure supplied to the tank. Check your ball valve in the cold water tank when open, is it bouncing i.e. bouncing open and closed causing a back pressure ? If so try turning your cold water pressure down at the stop cock, failing that, a new ball valve is only a few pounds. or is the pipework mechanically rattling i.e. is it a long run of pipe which isn't physically secured or is knocking against something else.

 

[felix]

My money's on the ball valve in the cold tank. You can get a very interesting form of positive feedback whereby waves in the tank make the ball bounce which sends spurts of water into the tank which makes waves which --- well, you get the idea! It's impressive to observe but not nice to the pipework or the eardrums.

Whatever the cause, I don't favour the idea of turning down the main stopcock. This will restrict the flow right round the house and cause all your cold taps to run slow every time somebody flushes the toilet. If there's a stop valve serving only the cold tank you could try that but it's a less than ideal fix. If you have a problem ball valve take Bathjobby's advice and change it.