Forum Title: Reverse water hammer after boiler install.
Hey guys wondering if anyone can give some advice. I recently completed a new boiler install (wm97+ with companion water heater) and in the process relocated the boiler which involved repiping domestic water lines. Now after the install Im getting a reverse water hammer when toilets are flushed and when the washing machine turns on. Also getting hammer when I turn on outside house bib. The reverse hammer consists of only one loud bang when during turn on not valve closing. If I turn on basement sink hot water and flush toilet no bang, if I turn valves down on toilets on washer almost to off no bang. I have air chambers installed at the washer and a shock arrestor where the bang is loudest in domestic cold pipe but neither seem to help at all. My question is do you think adding an expansion tank to cold water domestic would help? Would I benefit from installing a vacuum breaker? Static pressure at outside house bib is only 50psi. Everytime I turn on washer or flush toilet following the reverse water hammer I get a single drop of water from the vent of my back flow preventer on boiler fill line. Any ideas? Thanks a bunch in advance.
Category: Plumber Post By: ALBERTO BARBER (Macon, GA), 12/09/2016

I am not familiar with this system, But I pretty good at trouble shooting things. I found a manual for the boiler but not for the tank (YET) I review the video a few times. let me see if I am right about a couple things. The boiler is a closed loop for heating purposes that has make up water. The domestic water does not actualy pass through the boiler but passes through a heat exchanger (AQUALOGIC) As I was following you in the video you pointed out a check valve on the cold domestic supply. That looks like a swing check valve with flow going down. If so it's when you turn on cold water it could be slapping it up, which is kind of odd. turning on hot faucet before you open cold is creating water flow in it's intended direction before you open cold therefore not banging. OR something like that. vertical flow going down you need a SPRING CHECK valve. replace it or take it out

- MABEL MORAN (Blaine, MN), 09/04/2017

Here is a video of situation. Again thank you in advance.

- DONALD HERNANDEZ (Sacramento, CA), 09/14/2017

This system was a mess for years. Also had a swing check in the wrong application. Swing will hang open. reverse water flow can bang it closed.

- JACOB GREENE (Alameda, CA), 09/16/2017

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