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When I was replacing the hot water heater and the shut off valve I found this issue. Once I turned on my main water I kept the shut off valve closed to the input so I could ensure there was no leak from the replacement. I noticed during this time that the tank was still filling through the output pipe. I turned on the valve for the input pipe and it allowed water to come through (like it should). I did some trouble shooting to make sure it wasn't crossed in installation. So where is the water coming from on the output side? Why is it also filling the tank? Do I have a bad mixer at a shower or sink that is causing a loop?

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Did you drain your lines while the tank was disconnected? –  The Evil Greebo Sep 18 '13 at 19:17
    
It doesn't matter which way the water comes in. It won't hurt the tank. It can reduce the efficiency if it was plumbed backwards permanently. But initially filling it backwards accidentally won't cause any problems. –  longneck Sep 19 '13 at 1:49
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2 Answers 2

This is just a guess, but you're probably only seeing existing hot water in your house's pipes flow back to the water heater, since there's none of the resistance that the full tank of water can provide.

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it only flows out when i've turned the main water back on... i know that it isn't good for the hot water tank to recieve water from that direction.. should i see if it stops at any point by feeding it outside so that i don't ruin the new hot water tank? or put a one way there to stop any water from going into the tank? –  user15169 Sep 18 '13 at 20:29
    
It probably is a function of the air in the tank blocking backflow until the main is opened. –  bib Sep 18 '13 at 21:51
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Are all the faucets shut off? If one of them is open, the cold would flow back through the 'hot' pipes to the heater.

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