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Ed K
Water Hammer After New Water Heater Installed
Ed K

I really hope that someone here can help me. After we had our hot water storage tank replaced, we get 1-2 bangs every time we turn off a water faucet, a toilet is finished refilling, or during parts of a washing machine cycle. It's especially bad with the 2nd floor toilet and the washer in the basement. I know that the bangs are coming from the tank area because the bangs disappeared when I shutoff the cold water feed to the tank.

My plumber also confirmed it and believes it to be the check valve, so he added two Watts 150A water hammer arrestors: one on the cold water pipe leading into the tank and one on the hot water pipe coming away from the tank. That hasn't helped. The plumber suggested changing the toilet fill valve to something slower, but what about the sink valves? Finally, he thinks we may have to install a larger arrestor at the main feed to the house, but I'm skeptical that it would work.

Any advice is very much appreciated.

Ed K
Re: Water Hammer After New Water Heater Installed
Ed K

Thanks for looking, JacktheShack. I'm expecting another plumber from the same company later this afternoon, so I'll post what happens.

The last plumber said the cause was quick-shutting water valves, like the toilets and the washing machine. At the time when he installed the second arrestor, the faucets weren't bad, but there was definitely a thump. Since he left, it's gotten worse for the faucets and now the dishwasher is also causing banging. I'm beginning to wonder if he knows what he's talking about, since he did install a circulating pump backwards and gave bad advice on keeping the old pump when it was corroded... and he's the foreman! :mad: He never did take a water pressure reading.

I did try to vent the water pipes by closing the main shutoff valve and opening all the water faucets in the house. I then closed them and re-opened the main shutoff. However, I think I closed the valve to the boiler system before I did this, so that didn't flush out the hot water storage tank.

I'll ask the plumber to empty the tank and to check the water pressure.

Re: Water Hammer After New Water Heater Installed


I wasn't sure I understood your 1st post, the equipment you have there (hot water heater, indirect HW heater, cold water storage tank/pump), & exactly what component was recently changed.

Do you have a cold water storage tank/pump for the incoming cold water supply from a well or city supply, & is this what is giving you the banging.

Do you have a stand-alone (gas-fired,oil-fired, electric) hot water heater, or what's known as an indirect HW heater that's been changed, or some other hot water tank setup connected to the boiler??

Your posts seem to indicate the thumping is coming mainly from the cold water piping or washing machines when a faucet or valve closes.

Sorry for the confusion.

Ed K
Re: Water Hammer After New Water Heater Installed
Ed K

We have an oil furnace with an indirect water heater. Hot water is stored inside a 50-gallon storage tank that recirculates the hot water through the furnace. We get our cold water from the city's tap. We live in Queens, NY, if that helps.

OK, the plumber's stumped. He says that there is still water hammer somewhere along the cold water line because there's a slight thump and vibration (hardly noticeable) even when the cold water feed to the heating system is shut off. However, he can't explain why the loud banging is happening within the heating system.

The big boss just came and thinks that the 2 arrestors are too small. He's getting an expansion tank. He also thinks the source is the check valve and is getting a spring-loaded soft-sealed check valve. I sure hope this works. Stay tuned....

Re: Water Hammer After New Water Heater Installed


Your latest post gives more clarification to things.

You have a boiler there (boilers heat & circulate hot water to heat the house; furnaces heat and circulate air).

The last part of your post indicates they probably got air into the heating distribution piping (radiators/baseboard) when they changed the indirect heater; some of this may have gotten back to the cold water supply via the 50 psi supply piping.

The air in the pipes can be removed by opening the small bleed valves with a screwdriver (starting with the upper floor radiators/baseboard) until all the air is out & water bleeds out.

Some radiators/convectors don't have bleeding valves on each convector & the system has to be bled using the purging station, that is located in the return piping near the boiler.

You should be aware that the heating system water & the domestic water supply are actually two separate systems.

The heating system water in the boiler & convectors is kept at 12 psi by a pressure reducing valve from the 50 psi cold water supply that reduces the 50 psi house water supply down to 12 psi; this prevents leaking convectors & less pressure buildup.

The piping inside the indirect water heater is also separated; the boiler water at 12 psi circulates its hot water thru the IWH usually by a copper coil to heat the 50 gallons of 50 psi tap water sitting inside the IWH.

Let us know how you make out with the plumbers & if we can be of further help.

If you want further info on your system you can Google some of the phrases used in these posts: "boiler distribution piping", "how a boiler works","purging station","pressure reducing valve", "expansion tank", etc.; there are often drawings associated with these sites.

The site below has additional info; click onto "Systems" at the top of the page, then "Hot Water", then scroll down to "Loop HW heating" and "Diverter HW heating."

Another resource at the top of the page is "Question", which will bring you to "The Wall".

The Oil Heat America site has additional topics about heating systems on the right sidebar at the site.


Ed K
Re: Water Hammer After New Water Heater Installed
Ed K

Wow! Thanks for all that info! Very helpful and now I understand my heating system a little better.

Well, it looks like the repairs worked. :D First, the plumber replaced the check valve with the soft-sealed spring-loaded one. When we tested a faucet, the banging noise was gone, but instead there was a squishing (swooshing?) noise. Progress. He then replaced one of the arrestors (on the cold water feed pipe) with the expansion tank, which I remember seeing on an episode of Ask This Old House (that made me feel better :) ). Tested again and I heard something I hadn't heard in over a month... silence!

The plumber told me that we still have a slight water hammer in the pipes, but it can be resolved by installing a DIY arrestor at the washing machine, like I saw on another episode of Ask This Old House. I'll get on it, but it's not too pressing since it doesn't make any noise and there's only a slight vibration in the washing machine's hoses. I'm just happy and relieved.

For the record, I knew that the heating system water is separate from the potable hot water. Just didn't know how to describe it. :o

Should I still purge the air as you described? Wouldn't the trapped air be pushed out through the radiators when I turn on the heat in the winter?

Re: Water Hammer After New Water Heater Installed

Glad to hear you're finally blessed with the "sounds of silence"
(author Paul Simon is from Queens, BTW).

Yes, it's always a good idea to bleed air out of a HW radiator/baseboard system; if there IS air in there it won't come out by itself.

The plumbers probably should have done it as part of their HWH installation.

It's an easy diy task if you have those tiny bleeder valves at the end of each radiator/baseboard in each room.

Take a small screwdriver & open each valve; if you see water come out, close immediately; if you see air, let all the air out & then close.

Scrunch a wad of paper toweling under the valve to catch any water.

Although it's not heating season yet, it won't hurt to turn on the heat for a few minutes after you do the bleeding to see if you hear any gurgling or other noises in the heat piping.

Repeat the bleeding process again if you do.

The supply water valve (pressure reducing valve) would have to be open to allow new water into the heating pipes to replace any air removed.

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