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Knocking Sound Near Water Heater
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My water heater (AO Smith – GAS 75 GAL) is about 14 months old and my house is probably about the same. I hear a knocking sound close by the water heater closet which is accessible from the outside. This knocking sound happens randomly but I noticed more often or whenever someone is using hot water.
Sometimes it's a fast knocking sound (twice every second) sometimes it's slow, like one knock every two seconds. Sometimes it's even slower like one knock every 15 seconds or 30 seconds. More often, it starts fast then slows down. I can't seem to find where it's actually coming from but it seems from the inside of the water heater. I've also flushed my tank twice now and not much build-up that came out either. My hot water pipes are all insulated and I notice this only happens during the cold December weather in Los Angeles, which is not really that cold compared to East Coast Weather - about 40s to 50s here in LA. PLEASE HELP, it's driving me crazy...
Is this a water heater problem or a plumbing problem?
 
Posts: 13 | Registered: 17 October 2005Reply With QuoteReport This Post
Picture of LA Marlowe
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It is possible. The heat trap nipple will look identical to a regular iron pipe nipple from the outside. Knocking can be a bear to track down, even for a professional plumber. If you google "water heater knocking" you will see a host of possible causes and solutions ranging from sediment, or air, to a tight spot in the piping or a baffle in the flue pipe. I wish you luck!
 
Posts: 273 | Location: VA, AL, GA | Registered: 23 October 2007Reply With QuoteReport This Post
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Perhaps its sand or silt at the bottom of the hot water heater. This causes air to be generated within the tank to bubble when heated. What happens is like when you boil water on the stove. If you place a few grains of sand on the bottom of the pot you will see air bubbles forming out of them like magic. Your hot water heater collects sand and silt on the bottom which causes this to happen as well. The side of the hot water heater has a drain valve. Its very reason is to flush out the bottom of the tank. In fact if you live in a area in which you have fire plugs in the street, you see them flush them once and a while. This is to flush out any sand an silt that gets into the water system. When they do this they disturb the water and the silt ends up in your pipes.
Check with the papers that came with the hot water heater, they recommend that you flush this tank once a year.
 
Posts: 1933 | Location: New Jersey | Registered: 31 January 2006Reply With QuoteReport This Post
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Thank you LLC and LA Marlowe. Yes, I have flushed this tank already (twice) and no difference and Marlowe is right, there are several different reasons for this knocking sound. However, I do believe that it has something to do with Metal Fatigue because I notice that it only happens during the cold weather season. I guess if I can't solve it now, I'll just have to wait until it gets a little warmer again and see if it disappears.
Last night when I was out and checking though, I really felt that sound/vibration emulating from the pipe attached to the hot water tank where the hot water comes out to supply the house.
 
Posts: 13 | Registered: 17 October 2005Reply With QuoteReport This Post
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Check the outlet pipe route to see if a pipe hanger or strap has the pipe bound up. As the pipe expands, it may be rubbing against a joist or a hole through a wall plate.
 
Posts: 245 | Location: Annville, PA | Registered: 03 July 2006Reply With QuoteReport This Post
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CCF -

Did you ever find a reason for this? I have the same water heater and have been encountering exactly the same issue!
 
Posts: 1 | Registered: 24 May 2010Reply With QuoteReport This Post
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Knocking sounds in water heaters are caused by sand and sediment particals on the bottom of the heater. Very common condition caused by owner not flushing the heater yearly. It is also a sign that the heater is near its last days and should be changed. Also be carefull not to keep the water to hot as this can make the noise worse.
Only way to fix this is to replace the heater with a new one and be sure to flush heater at least once a year.
 
Posts: 1933 | Location: New Jersey | Registered: 31 January 2006Reply With QuoteReport This Post
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