Machine-gun water hammer

[linty]

My water hammer sounds like a machine gun that bangs in the pipes at a frequency of about 3 or 4 times a second and the episode lasts about 20 seconds. It can be triggered by shutting off a faucet or sometimes it seems to start spontaneously. After an episode of hammering, nothing will cause it to happen again for at least 10 or 15 minutes. It bangs from one end of the house to the other so I can't pinpoint the cause. There are hammer arresters on the washing machine. Any ideas on why this is happening?

[kreemoweet]

Why? Perhaps a chattering automatic valve of some kind. You can test for this by shutting off suspected appliances one by one to see if the hammering stops.
Another big cause is unsecured pipes. That's a tough one, because you'd need to isolated the area and then open walls to fix it. Leaky toilets are very
likely the cause of hammer occuring when "nothing" is running.

[LLigetfa]

It could also be a bad PRV.

Are you sure you are not exaggerating it. That is between 60 and 80 bangs, which is quite a lot.

[jadnashua]

What is the water pressure in the house? A toilet fill valve that is chattering (possible with high pressure) could cause this. I'd turn the water off to the toilet(s) and see if the level in the tank drops, indicating a flapper valve leak. This can cause asynchronous partial fills. Also make sure that all of the shutoff valves are fully open. A loose washer in a shutoff can cause chattering, but only when there's water flow.

If you have a PRV and a failed expansion tank (or none), as already called out can cause noises. Some PRVs have a bypass valve that could make that noise, too, but it should normally never open unless you have a failed expansion tank (or none). Double-check that the T&P on the water heater is not discharging.

[dwindle]

Is there possibly a mixing valve somewhere?

[linty]

The expansion tank was replaced just a couple years ago so I don't think that's failed. Already replaced the PRV so ruled that out. Most of the shut-offs are ball valves. Don't know the house pressure since there's no gauge.

What is the water pressure in the house? A toilet fill valve that is chattering (possible with high pressure) could cause this. I'd turn the water off to the toilet(s) and see if the level in the tank drops, indicating a flapper valve leak. This can cause asynchronous partial fills. Also make sure that all of the shutoff valves are fully open. A loose washer in a shutoff can cause chattering, but only when there's water flow.

If you have a PRV and a failed expansion tank (or none), as already called out can cause noises. Some PRVs have a bypass valve that could make that noise, too, but it should normally never open unless you have a failed expansion tank (or none). Double-check that the T&P on the water heater is not discharging.

[linty]

The boiler has a tankless coil that goes to a mixing valve before the hot water gets to the rest of the house. Why do you ask?

Is there possibly a mixing valve somewhere?

[jadnashua]

Expansion tanks generally last more than a few years, but it's possible...knock on it...if it sounds a dull thud, it's toast. If it sort of rings, it should be okay.

[Terry]

Things that may require a hammer arrestor
Dishwasher
Clothes washer
Icemaker

Anything with a solenoid shutoff

If you have a refilling toilet tank, and a bad fill valve or wall shutoff
Make sure the wall shutoffs are open all the way. A floppy washer will hammer.

[linty]

Thanks for all your advice. Finally narrowed it down to the leaky tank flapper valve and the pulsating fill valve on my Toto Drake toilet.

The boiler has a tankless coil that goes to a mixing valve before the hot water gets to the rest of the house. Why do you ask?

Things that may require a hammer arrestor
Dishwasher
Clothes washer
Icemaker

Anything with a solenoid shutoff

If you have a refilling toilet tank, and a bad fill valve or wall shutoff
Make sure the wall shutoffs are open all the way. A floppy washer will hammer.