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After a minute or so of hot water running in my kitchen the pipes start to knock, and water doesn't come out in a stream. It is like there is air getting in.

What is happening, and how can I fix it?

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If you let it keep running for a while does it stop, or keep happening? – The Evil Greebo Nov 20 '12 at 19:10
Never let it keep running. It's loud and I'm concerned something will break – CaffGeek Nov 20 '12 at 19:28
Let it run for a couple minutes, no change. The only thing that stops it is turning cold water in too – CaffGeek Nov 20 '12 at 20:00
A similar question was posted on Yahoo Answers. You may find it helpful: answers.yahoo.com/question/index?qid=20070903193516AAOtM0F – Hammo Nov 20 '12 at 22:12

You have a valve failing somewhere in your plumbing system, the trick is to figure out which one. If this only happens with your sink, then I would start there.

If your sink has a separate sprayer on a hose, then it's possible the diverter valve is flapping, trying to send water to the sprayer. If that's the case, it could be as easy as a blockage in the aerator of your faucet, which is often easy to remove and clean.

If that doesn't solve your issue and it's isolated to the sink, then I would try replacing the valve/cartridge in your sink fixture, or you could replace the entire fixture.

Beyond that, you'll need to work on a process of elimination to figure out which valve is causing the problem. There are likely shutoff valves under the sink, another at the hot water tank, and another for the house.

One last device that I've seen cause pipes to bang is a failing pressure reducing valve (PRV) that is bell shaped and found just after the main shutoff in some homes. This seems less likely in your scenario, since the last time I heard one cause pipes to bang was after I turned off the water and the pipes would rattle as the PRV allowed pressure to rise past the designed threshold.

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