Two valve shower, no mixing valve. Out going water lines meet at a T' which goes down to the spicket with shower hose attached. I have put a T with a 4" capped pipe on both hot and cold side, (in between valves and the union T). I just did the hot water side (knocking side) as warranty work, the knock changed from initial turn on that would stop when half open or more to only at the last turn of shut off. Drained HW tank twice and all lines are secure.. wish I could post a picture. Can anyone share some insight as to cause and resolution?

It sounds like you installed an air chamber. 4" is way too short and the effectiveness of air chambers is debatable. Have you check for a loose/defective stem washer?

air chambers are useless and illegal.

Not illegal everywhere. San Francisco requires them.

Steve, are you talking air chambers(dead end pieces of copper) or hammer arrestors?.

Air chamber.

From the 2019 SF Plumbing Code -
609.10.2 Add the following section:

609.10.2 Air Chambers. Air chambers shall be installed on all hot and cold water outlets supplying quick-acting valves to provide water hammer relief. The size of each air chamber shall be 24 times the diameter of the fixture supply.

609.10.3 Add the following section:

609.10.3 Quick-Acting Valves. The following valves shall be considered quick-acting: flushometers, single-handle faucets, single-handle tub and tub/shower combination control valves, single-handle shower control valves, rapid response electric solenoid valves (such as used on ice-makers, clothes washers, etc.) and any other appliance or fixture that dispenses water in a quick open and close method in its operation.

Steve, could you post a picture of one?. Read this steve,

[www.siouxchief.com]



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They're just the standard old school air chamber. California eliminated them from the State Plumbing Code, adding arresters in place of the chambers, but SF amended the state code and added the chambers back into the SF code.

that's nice.. that give you 5 places to hit with screws when attaching a vanity base cabinet.

A nice place for bacterial growth and a haven for Legionnaires disease Steve. Back in trade school we installed a ball valve on a hot and cold water line with a foot of pipe above and a brass male cap at the end of the pipes. At the end of two weeks we closed the ball valves and un-screwed the cap's and found the chambers were filled to the top with water. Water absorbs air. air chambers are ineffective/illegal and have never worked. My Code book states no dead-end pipe longer than 6".advocating air chambers (telling them there's are to short)is very bad thing.

correct

air chambers have turned out to be waterlogged dead ends

however

the key words are 'turned out'

they were INTENDED to be periodically 'serviced' by draining the system COMPLETELY allowing the water to empty


we know how that turned out

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"Measure Twice & Cut Once" - Retired U.A. Local 1 & 638

Dead end pipe.
I never gave water arrestors much thought, figured they were just a glorified pipe stub. All the years of plumbing I've never had a knock so I've never installed either.
I guess I'll try installing two arrestors.
Out of curiosity what is the science behind an arrestor? I'm guessing baffles.


The bacteria chamber info is good to know.

Thank you to all who contributed to this thread!!!

the 'old days' NYC code defined 'dead end' as 24" or more


dead ends are one reason to use EVERY fixture for 5 minutes a month

==============================================

"Measure Twice & Cut Once" - Retired U.A. Local 1 & 638

Either a diaphragm or piston is inside the arrestor.

Maybe it's a good thing I can smell chlorine when I turn the faucet on. Not that I've ever cared or trusted tap water, now I have this mental image of an evil cocktail of bacteria and fluoride swimming around in my pipes.
I might never drink tap water again.

Very educational
Thanks again.



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