Hot water pipes bang when shutting off faucets.

rlabonte

My hot water pipes bang every time the hot water shuts off. Cold water is fine. It sound like it is coming from around the furnace hot water tank area. I have a system 2000 which circulates water to the tank through a heat exchanger. Any advise would be appreciated.

JUGHNE

First suspect is an expansion tank on the domestic hot water system.

Pictures would help, of the boiler, indirect and piping would be good.

Larry Weingarten

Hi @rlabonte , Also, it would be good to know what the static water pressure is. High pressure makes systems more prone to having water hammer.

Yours, Larry

rlabonte

Water pressure is 68#, I put a new expansion tank in.  I thought of the water pressure but the cold water doesn’t bang.

JUGHNE

Is there a check valve on the CW supply to the WH?

Where was the exp tank you changed?
What pressure was it set at before installing?

rlabonte

It is on the inlet to hot water tank, same place it’s been for 14 years, I set pressure to match city pressure - 68#’s

rlabonte

There is a back flow preventer at meter

dopey27177

Is this condition new?
Does this happen in one faucet or appliance?
It is possible that a faucet has a loose washer or cartridge in it.

Jake

Intplm.

Sounds like a faucet issue. Is it all of the hot water or isolated to just one faucet? What shuts off when you hear the water hammer? Is it the laundry? You will need to find out what is shutting off that is causing it to hammer. When you do that you can then add a water hammer arrestor to that item.

rlabonte

I have a water arrestor at my washing machine.  Every faucet does it so I’m confident it’s not the faucets, very frustrating.

JUGHNE

If your expansion tank is on the CW to the WH tank and there are no other check valves in the system then that tank should be a shock absorber.

Some tanks have a check valve built into the cold inlet to prevent gravity flow back.

Your arrestor at the washing machine may have been the only cushion you had.
They can lose that capacity to hold air and may be water logged. Might be an easy change out. FWIW

HVACNUT

Was the old domestic extrol bad?

rlabonte

There are no check valves in the hot water tank according to energy genetics, its just a storage tank. I only put the arrestors in because of the banging and they never worked. Nothing has changed over the years as far as set up. I guess i will just have to listen to the banging. Old tank was an extrol, new one is the same.

rlabonte

Interesting find. I figured i would empty the hot water tank to see if there is anything on the inlet. I couldn't really see anything except some water. Not sure why there is a water level. what i did find while i was emptying the tank was if i shut the drain while it was draining there was a bang at the tank, this is while the inlet and outlet were closed. When my tank calls for heat a circulator pump runs to heat it up, while the pump is running there is no bang when shutting off the water, only bangs when pump is off.

rlabonte

Here are a few pics of my system, hard to get good pics. The first pic is just my expansion tank before the heater, the discharge from the tank goes to a tee just below the circ pump and goes into tank as water is used. The circ pump will start when the hot water temp is low. It circulates through a plate heat exchanger that the boiler water goes through to heat it up then back to the top of the tank.

Zman

68# of pressure is pretty high. Do you have a PRV? If not it may climb higher at times.

rlabonte

I do not have a prv, if it is the pressure then why doesn't the cold water bang? This is why I'm confused.

HVACNUT

Where is the extrol installed in relation to the cold water inlet at the tank? From the pic, it looks like it goes the opposite way. 
The circulator does have a check valve so extrol location is important.
Maybe back flush the HX. Couldn't hurt.

dopey27177

A shot in the dark.

Since the banging happens in the hot water faucets try moving the expansion tank to the hot water discharge side of the tank. That is where the pressure is slightly greater than the cold water.

Jake

rlabonte

the tank is in the right place. If you look at the second pic you will see a tee below the circ pump, that is where the cold water goes in. When using hot water the cold goes into the bottom of the tank, when it calls to be heated up the circ pump starts and pulls the water from the bottom of the tank then through the heat exchanger back to the cold side of the tank. I have had this setup for 16 years and no issues until a few months ago

HVACNUT

Was the original extrol bad?
Is that HX set up to be back flushed if necessary?
I wonder if having the flow check in the 006 stuck open would cause your issue. I'm just throwing darts because Occam's Razor isn't working on this one.

rlabonte

Apparently the old expansion tank was good, since changing it out didn't help. the installers didn't put any connections to back flush the heat exchanger. I would have to install something. Not sure that would even help, if it is scaled up it is 16 years worth. I did notice that the line coming out of the heat exchanger going to the hot water tank seems to be where the banging is coming from. I have a valve on the hot water line, if i turn the hot water on and close the valve next to the furnace then i can put my hand on the pipe and it bangs quite a bit. So I'm not sure if build up will make this thing bang. No banging while circ pump is running.