Yesterday I went to replace the flapper in an upstairs toilet. That went smoothly.
Then I came downstairs to replace the ballcock in a toilet there. When I reconnected the water line and turned it on, there was a thunderous water hammer and naturally I turned it off for fear the house would explode. The hose is pretty new. And it's got nothing to do with the ballcock! I know this because I disconnected the hose and aimed it in a pail and turned the water on without it being connected to anything. It hammers even trying to fill the pail. I'm assuming there is an air bubble somewhere in the works because of shutting off the upstairs toilet?
Any suggestions on how to fix this?

"Water hammer" happens when you abruptly shut off the water. You have at least 40 pounds of pressure pushing the water flow. The weight of the water hitting a 90 degree bend causes the sound that you hear. We put in air chambers to cushion the hammer much like the shocks on your car.
Back in the 60's we were required to to put air chambers on all bath lavatories and kitchen sinks. Over time water and condensate built up in them and they must be recharged. First turn off the house at the main house shut off valve. Then open hot and cold faucets at the farthest bathroom to prevent air lock.(This is important.) You will find under each fixture little brass or chrome valves. These are called angle stops. With a small pail held under the angle stop to catch the run off look under the handle of the stop and you will see a nut with the stem running through it. This is called the bonnet packing nut. Put your wrench on it and back it off counterclockwise. Now open the stop and remove the stem and washer assembly, The stop will began to drain. When it has quit draining reassemble the stop and move on to the next one. Do this on each one of your angle stops. There will be six of them in a two bath home. When you are done, close off the faucets you have opened, turn on the water to the house, and give yourself a pat on the back.
You have just recharged your air chambers and saved yourself a hefty service call bill. Hope this helps you out and thank you for rating my reply. TOM

Thanks I will give this a try.
I'm curious though. Would the house really explode if I just opened the valve all the way? What would happen?

Thanks I will give this a try.
I'm curious though. Would the house really explode if I just opened the valve all the way? What would happen?

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I'm curious though. Would the house really explode

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No! Water hammer doesn't work that way.

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if I just opened the valve all the way? What would happen?

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You would simply be allowing more volume to enter the fixture. Recharge then air chambers and you'll be just fine, Good luck, Tom

OK well I just spent the last hour or two doing that and unfortunately it didn't fix the problem.

Then turn the angle stop a quarter turn counterclockwise until the noise stops. Good luck, Tom

Hi all:

If opening the shutoff fully (you did open the shut off completely right?), or shutting it in increments doesn't work then you will need to disassemble the shutoff and see if the washer has loosensed and is literally "flapping" when water fills the toilet.

If the washer moves, lifts from a corner, etc. then replace the washer and you should be up and running quietly soon enough!

Mark

It's only opened very little before the racket starts. If I turn it a quarter turn it will be off/closed.

If by shutoff, you mean angle stop, then no it was only opened very little before the racket starts. I can get a trickle out of it.
And also there is racket whether it is connected to the toilet.

Hi BB

Open the angle valve all the way... nothing will break... ;)

You may get some chatter, but you still want to open the valve all the way, OK?

Let the tank fill and then flush. Then tell us what happens when the tank refills. If no "water hammer" then you are all set. If you get "water hammer" then the washer at the angle valve needs to be replaced.

Mark