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I was leaving town for 2 weeks and this freeze storm was about to blow through. I shut off water at the main and opened all bathroom and outdoor faucets to drain.

The hot water heater is in the attic. I left the gas hot water heater on. I didn’t shut off the cold supply valve because the main was off, and it’s not easy to get to in the attic. I left cold and hot taps open when I left the house, no water was coming out, but when I opened the hot faucet it did occasionally sigh for a few minutes. The temp dropped from 65 to 12 degrees Fahrenheit later that day.

I’ve read about siphoning and pressures. I felt like if I had both hot and cold faucets open then that should not allow a vacuum or an over pressure situation and so the tank should remain full of water.

I’m really stressing about this and not enjoying my vacation worrying that leaving the faucets open and not shutting the water in valve off was bad.

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I have seen water heaters siphon off water and it depends entirely on how the house is plumbed. If your cold water tees are all between the water heater inlet and the main shutoff, you'd be setting up siphon action if the main supply was shut off, you opened the hot water faucets, to break the vacuum at the top of the tank, and then open a cold water faucet below the tank. If you opened the cold water faucets first, you'd break that column of water and you wouldn't have any siphoning. The fact that you set all this up and no water was running out of the faucets after you shut off the main and opened the faucets would imply no siphoning would take place.

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  • Thanks. I did open hot and cold together in a sink but then opened only the cold outside which is the lowest faucet before shutting off the sink faucets and turning on only hot in the tub before leaving. I agree that with no water coming it shouldn’t have been siphoning, but I also left within 20 minutes so I didn’t know if it might be a slow siphon that gradually got bigger. Dec 23, 2022 at 15:49
  • If it did happen to lose water (from siphoning or evaporation after 17 days of being on (not pilot bit on)) then are there safeguards in the heater to turn off gas prior to it burning up? Dec 23, 2022 at 16:41
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The pipes to and from the attic heater are high risk of freezing. I would turn off the heater and drain the tank.

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