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Old 07-24-2010, 12:41 PM   #1
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water heater knock!! HELP!!

ok so about a week ago at midnight a knocking sound started in our home, it was the loudest in the master bedroom which shares a wall with the garage. I went out to the garage and the sound was coming from the water heater. When i placed my hand on the cold water coming into the heater i could feel the knock along with hearing it. when i shut the water off the sound did not stop but sounded more distant. the sound comes and goes and does not correlate with any water running. i have since flushed the tank several times, changed the anode tube, and replace and the nipples going into the tank, place flex tubing and installed a water hammer arrestor on the in line. the knocking still persists!! please help i have had 2 plumbing companys tell me 3 different things. first that is water hammer which didnt make sense to me sense there is no water flowing while it hammers, but i put an arrestor in anyways. second that if the nipples were corroded then they could cause the sound, so i replace the galvanized with copper nipples. and last that the anode tube could be vibrating so put a new one in! still screwed here PLEASE HELP!


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Old 07-24-2010, 03:04 PM   #2
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I've heard this too, but it's usually in the winter when the incoming water is very cold.
I would bet the sound correlates with the tank coming on to reheat. Have you tested the temp of the water after this knocking? Maybe the t-stat is allowing it to get too hot.


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Old 07-24-2010, 03:32 PM   #3
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i have thought this also but i look through and the burner is not lit when the knocking starts or continues, plus if anything i live in the desert and having cold water coming in is definitly not an issue! plus i have had the problem with the t stat set at like luke warm and scalding hot as well...
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Old 07-24-2010, 04:35 PM   #4
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Turn the temperature dial slowly until the heater kicks on. Does it knock at that moment? Wait 45 seconds and then turn the dial slowly the other way until it kicks off. Wait 45 seconds and try again just once. Then put the dial back where it was. I suggest never putting the dial in the "very hot" range for household use.

I could imagine a layer of sediment at the bottom of the tank causing a small amount of water under the sediment to boil and bubble through the layer of sediment every now and then causing a knock. Except I would have thought that draining the tank would have gotten most if not all the sediment out.
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Old 07-25-2010, 05:16 PM   #5
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i have tried this too with no luck, and when i drain the water it is clear so i dont believe it si sediment. anything else i could try please let me know
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Old 07-26-2010, 05:46 AM   #6
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It's caused by a heavy deposit of calcium at the bottom of the tank. Air gets trapped below the sediment when the burner runs. You need to drain the tank completely and then pour a gallon or so of vinegar in and let it sit several hours before draining, flushing and re-filling.
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Old 07-26-2010, 09:36 AM   #7
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I agree with draining the tank completely and flushing with vinegar. You may want to look at this site http://www.waterheaterrescue.com/ .

Look into adding a new drain tube and ball valve for a more complete drain. This will allow larger bits to flow out and not get stuck. I had chunks as big as a dime coming out of what I thought was a clean tank when I did the conversion.
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Old 07-26-2010, 07:00 PM   #8
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I've cleaned out commercial heaters(they have a bung hole cover) and had to chisle out chunks with a mallet and chisel--that kind of thing is not going to come out of a 3/4 inch hole.

Not unusual to pull out a 5 gallon bucket full from an 80 gallon heater.
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Old 07-26-2010, 07:11 PM   #9
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Mineral deposits made mine do that. I scooped lots out with a long handled spoon. It fixed it but probably wasn't worth the effort as the tank sprung a leak a year later. Don't know if this is always the case.


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