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I have erratic hot water from my tap in the bathroom sink, when I turn on the hot water it runs for about 30 seconds and then bangs and the water stops. After a few seconds the water returns with a force and we have a constant banging like a machine gun, from the tap.

What could cause this, and how can I fix it?

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First thing: check all of your shutoffs and ensure they are fully open –  Steven Mar 19 '12 at 13:17

2 Answers 2

You've got a bad valve causing the water to stop. You may be able to only replace the cartridge/valve inside the faucet to solve the issue, but keep in mind that this could also be the shutoff under the sink. As user663387 says, the banging sound is water hammer, but I suspect that's an effect rather than the cause.

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This is a well known effect called "water hammer". It happens at the end of long runs.

http://en.wikipedia.org/wiki/Water_hammer

When you open the valve you've got a long tube of water that starts accelerating. That tube of moving water has momentum and you can have problems either when you turn the tap off again, and the bolus of water impacts against the blockage, or when the water passes a certain critical speed. At certain speeds going around turns in the pipe can cause the effect you're seeing.

You want to install a water hammer arrestor.

They cost from $10-$40 depending on size, etc. and can be found at home improvement stores, plumbing supply stores, and online, e.g.

http://www.grainger.com/Grainger/water-hammer-arrestors/fittings/plumbing/ecatalog/N-a8x

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Water hammer would explain the banging when the water stops, but not why the water is stopping in the first place. –  BMitch Mar 21 '12 at 17:13

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