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Removed the water heater in a house we are renovating. I capped the copper line running into the water heater, but didn't see a need to cap the hot water line coming out, figuring it was not under pressure.

I noticed when I run the cold faucet in the bathroom water shoots out of the hot water line at the water heater. Why is this?

  • Sounds like the same situation as this question - although the OP there never replied to confirm my suspicions. – brhans Mar 13 '17 at 12:52
  • Back-feed... Mixer faucet. – Tyson Mar 13 '17 at 12:55
  • Yes that other question does sound the same. There is only one faucet currently in this house and it has two handles. I guess closing the hot valve under the sink will resolve the issue? – What-About-Bob Mar 13 '17 at 12:57
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    Hot toilet water? I don't think so. – What-About-Bob Mar 13 '17 at 15:08
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    Yes, closing the hot water supply valve for the faucet should fix it, assuming there aren't other fixtures with the same problem. I would expect most pressure-balanced shower fixtures to have the same issue if used, but you may be more able to avoid using those. – Shimon Rura Mar 13 '17 at 15:53
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My sink has a hot faucet on the left side of the sink. It also has a cold faucet on the right side of the sink. If you want warm water, you have to mix it in the basin.

If every faucet in your house is this way, then your problem is impossible.

However, I suspect you have a valve newer than 1930, with a single spigot. Which means there is a chamber before the spigot where the hot and cold water merge and/or mix. At the risk of stating the obvious, this means the cold and hot valves are in communication.

Water can flow from the cold pipe system, through the cold valve, into the chamber, through the hot valve, and into the hot pipe system. Yes, it's supposed to flow out the spigot "instead", but realistically it will do both. Pressurized water will seek every path at once, like electric currrent.

Normally, the higher pressure in the hot-water pipe helps seat the valve. But here, pressure in the chamber is higher. No telling what'll happen.

This is more likely to be a problem on more modern valves like a Delta or one of those horrid temperature-control valves found in hotel showers. Or how about a washing machine? Do you have any users who leave the shower valves at their preferred temperature and shut it off at the showerhead? (I assume someone on the planet does this.) Any of these can cause this behavior. In a perfect world, every faucet valve would include an anti-backflow check valve. They don't.

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