Water Hammer


Old 03-31-03, 10:17 AM
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Water Hammer

We have a terrible sound coming from a water hammer in our house which only occurs when our pool's automatic fill line is engaged. The water line to the pool comes off the main water line and is completely external to the house. When the auto fill comes on, the pipes above our water heater (the pipes are in the plenum underneath the second story of our house) shake loudly and wakes us up in the middle of the night every couple of weeks.
There is a hammer suppressor right near the main line the comes up off of the tee that feeds the pool line and then there's also a water vent - about six inches of PVC coming straight up off the pool water line about 4 feet down from the tee. After the vent, the line goes underground all the way to the auto fill unit.
I have been able to reduce the amount of hammering by turning the water down to almost nothing, but we still have noise throughout the house when it comes on. Is there any way to reduce the hammer any more? Does it help to make the vent longer than six inches?
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Old 03-31-03, 06:12 PM
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Water Hammer

Matthew, I dont think you are having a problem with what is known as water hammer. Water hammer is caused by a moving body of water suddenly stopping in a pipe. Water is almost incompressible. Therefore, when a valve is closed quickly, a shock is sent through the system. This shock has been known to create pressures as high as 800 psi. The intensity of water hammer depends upon the volume of water, the velocity at which it is flowing and how suddenly the valve is closed. Self-closing and quick-compression faucets, valves and pumps cause nearly all water hammer. From your discription this is a continuos sound the entire time the fill valve is open. (I would wonder if you had a similar sound while running the tub) I have no clue as to the supply size to the refill valve, I would guess 3/4" or 1" supply. Most inground pools use a standard ballcock for the refill valve. If the tee for this valve is ahead of the house supply this may explain why you dont hear the noise while using indoor fixtures.
While the refill valve is open and calling for water it creates low pressure on the system behind the tee. a loose bib washers on any valve in the house or outside hose bibb will begin to vibrate violently. Though it will sound as if its coming from all over the house it will be more prevalent at the valve causing the problem. The first place I would check would be the outside hydrants. Make sure they are all closed tight. Activate the fill valve for the pool and if there is still a vibration go to each fixture to find the origin. As for the vent aka Vacuum Breaker, here in Texas, code says it must a minimum of twelve inches above the highest opening of the system it serves. If the fill valve is below the apron of your swimming pool and vacuum breaker is up six inches above the ground level it sounds like your fine.
I wish you luck and please post again to let us know how it went.
Old 04-05-03, 03:42 PM
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Richard, Thanks for the information - it makes a lot of sense. I attached a couple of pictures to show what I'm talking about since it's hard to describe. We only have the noise when the pool fill is on, and the noise is worst right above our water heater which is the closest appliance to the point where the pool line tees off the main. This first picture (Pool Fill Line) shows where the pool line comes off the main. There's a four way joint with the air hammer suppressor coming off the top. The pool line goes down and then there's a 6" vent line coming up vertically before it goes underground. The second picture shows the water heater where the noise comes from. The noise seems to be above the heater in the plenum, and I can't really feel the lines right above the heater shaking too badly. I don't know what that big green unit is, but your statement about looking for a leak makes me wonder if that could be the cause. I am trying to isolate where the leak could be coming from, but no luck so far.
(I guess I can only attach one file at a time - I'll put the other one in a separate post).
Old 04-05-03, 03:44 PM
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Here's the second picture.
Old 05-19-03, 03:50 PM
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We are starting to get some warmer weather now, and we are starting to have problems again with the water hammer in our house. I took your advice and tried to isolate the offending valve. It seems to the one on top of the water heater - there is an expansion tank, and then copper piping which leads to a valve and then up into the plenum between the 1st and 2nd stories. While the vibration was occurring (with the pool fill activated), I turned off the valve above the water heater and the vibration stopped. The sound doesn't seem to coming from the valve, but rather the piping within the plenum (or wherever it goes from there). My question is - what is that expansion tank used for and is it safe to try to replace that valve? It seems odd that there would be that much sound coming from a loose fitting within the valve, but I'm willing to try to replace it to see if it fixes the problem.
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