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Why do I have 3 copper pipes in my gas water heater? It seems two are cold and one is hot. All videos I see online show only two pipes; one for cold coming in, one for hot going out.

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A picture, or a few, would help immensely. You'll have to make links as a new user and someone will edit and embed them for you. –  Jason Aug 12 '13 at 19:33
    
Can you provide the make and model of your water heater? –  pdd Aug 12 '13 at 19:40
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The third might be connected to the pressure/temp relief valve, in which case it should be going to a drain. This is good, because if that valve discharges significantly and doesn't get to a drain, it can cause a flood. –  gregmac Aug 12 '13 at 19:43
    
gregmac is likely right, only other possibility I can come up with is a recirculation pump, but those tend to be attached to the cold water pipe. –  BMitch Aug 12 '13 at 21:25
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1 Answer

Depending on the specific system installation and type of tank it could be any of these:

  1. Cold in, hot out, and overflow: Standard water heater.

  2. Cold in, hot out, recirculate in. This configuration has diverse uses:

    • typical for a solar water heating booster.
    • In heavy use applications, it could be for an external hot water reservoir.
    • In long lines applications, this could be for recirculating hot water so that a faucet 400 feet away has hot water available just a few feet away. Many motels and hotels are plumbed this way.
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