Water hammer on low pressure system...

[MattChan]

I took the bath out today too get to some flooring underneath it. Now that it is reinstalled it's ok but when I turn the cold tap off I get savage water hammer in the pipes.

Its a 22mm run straight from the tank in the loft, all bends are pulled. Can't see what I have done but it didnt do it this morning before I started... Any ideas?

 

[holty]

try opening any valve you turned off fully e.g the mains stop valve.

 

[MattChan]

Mains stop valve was not turned off... All that was turned off was the 1/4 turn full bore lever valves on the tails to each bath tap!

 

[holty]

when you mean water hammer do you mean whining or loud clunks in pipework

 

[MattChan]

A savage loud bang in the pipe work.

 

[holty]

try changing the tap washer it may me jumping when closed off most commenly known on cermic ones when turned off quickly

 

[MattChan]

Cool cheers bud I will have a look.

 

[MattChan]

Well I swapped the hot and cold taps over but I still get the hammer noise when turning the cold tap off quickly regardless of which tap is fitted. The hot water however behaves perfectly with either tap connected to it.

Both hot water and cold water run parallel to each other clipped to the wall over about 4m to the airing cupboard. Then up through the airing cupboard where the hot connects to the cylinder and finally up through the loft over about another 4m where the cold attaches to the 50 gal cistern and the hot expansion pipe goes over the cistern.

Taps are 1/4 turn ceramic and the pipes are all 22mm copper which is clipped at 1m intervals and doesn't touch the wall or anything else at any other point.

Is there anything else I can look at or is it just a case of buying a shock arrestor and installing it?

 

[holty]

didnt realise you have 1/4 turn taps i thought you ment the isolation to the taps were 1/4 turn lever valves and thats it ,take it your colds. mains pressure and your hots gravity try the lever valve to the cold tap and close it slightly to restrict flow.but like i said ceramic and also anything that shuts off too quickly, are renound for this due to high velocity e.g flow.

 

[chrishutt]

It's very odd to get water shock or hammer on a low pressure system. It sounds like the feed tank is about 3 metres or so vertically above the bath, which is very low pressure. Are you absolutely sure that the bath cold tap is fed from this tank rather than the mains?

If it really is such low pressure, it might be worth checking what valves there are between the feed tank and bath tap. Valves should be gatevalves or ballvalves (¼ turn). If you have a "screwdown" type valve (as with normal stop taps) it might be causing the hammer. Fitting a shock arrestor should solve the problem, as might making sure the pipe is adequately clipped to the wall.

 

[MattChan]

Hi guys thanks for the replies.

I installed all of the pipe work earlier in the year so the bath is definitely fed from the tank in the loft on both hot and cold feeds. The tank is indeed around 2.5m above the taps on the bath and all the isolators are 1/4 turn full bore ball valves.

I think I will try adding a few more pipe clips and see what happens. Like I say I assumed every meter would be adequate but obviously not!

 

[chrishutt]

I think I will try adding a few more pipe clips and see what happens. Like I say I assumed every meter would be adequate but obviously not!

Every metre is better than most installers would do. No point in going over the top there. From what you say I guess the hammer must be caused by the sudden closing of the quarter turn bath tap.

If you restricted the flow slightly (partially closing a valve) it would reduce the water velocity at the tap and hence the shock on closure. Try it out and tell us what happens. It may be that the flexible tap connectors (if you have them) have a narrower bore than the 22mm pipe and so increase water velocity. A larger bore connection might help.

 

[MattChan]

I didnt see your reply so I haven't tried closing off one of the 1/4 turn valves yet. I have clipped the pipe work at 0.5m distances however and this made chuff all difference.

There are no flexible tap connectors fitted its all in 22 mm with 5 pulled bends and 1 elbow over the length of the pipe. I reinstalled the sink tap which is supplied by 2m of 15mm from a T off of the 22mm. Turning this off quick gives the same noise!

I'm starting to think my ears are just over sensitive!

 

[chrishutt]

I think you've made too good a job of the pipework (typical DIYer!). There's so little resistance to flow that the water manages to reach a such a high velocity that the sudden turning off of the tap causes a shock wave. Introduce some more resistance to flow, or a shock arrestor.

 

[MattChan]

Fitted a shock arrestor today and it seems to have sorted it!

I think I was being a little aggressive with the tap anyhow as I was literally snapping it shut as fast as I could. Measured the flow and its somewhere in the area or 12->14 litres per minute through each bath tap so there was a reasonable column of water being shunted to a stop!

Thanks for the advice Chris!