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I'm having a bizarre problem. At least it's bizarre to me.

I'm repacking a HW faucet connected to our washing machine. To do so, I turned off the HW heater, and then the valve on top of the HW heater.

I then opened up the HW faucet on the sink in our downstairs and upstairs bathrooms to drain as much water as I can out of the loop. Works fine. No hot water coming out of any faucet now--including the one for the washer.

Then my son enters the bathroom and turns on the cold water. It sputters for a second but then comes on as you'd expect.

Now here's the weird part...suddenly, water comes sputtering out of the HW faucet for the washer!

Turn of the bathroom cold water, and water stops coming out of the HW faucet for the washer. Turn it back on for 10 seconds, and we get 10 seconds of water coming out of the washing machine faucet.

Now it's an experiment. We discover that any bathroom will cause this (As they are stacked, so likely all on the same wet wall). The kitchen faucet will not cause it.

What's going on here? Is there some natural siphoning thing going on somewhere in my system where cold water is being siphoned into the HW line? Or do I just have very messed up plumbing?

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The only time I ran into something similar it was due to a failed anti-sweat mixing valve on a toilet that allowed back-flow through the hot water pipes. That would also explain in your case why the kitchen sink doesn't seem to be effected by the issue.

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About the only way I can think of to test this theory (without opening walls) would be if you can cut your cold water off to the bathrooms without cutting the supply to the hot water heater. In the unlikely event that you can do that, your toilets would still be able to fill from the hot water line when flushed.

  • interesting! I'll head under the crawlspace tomorrow. – DA01 Aug 23 '14 at 4:53
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Many faucets do not have integral check valves and will allow water to pass from the hot into the cold and vice versa. Typical this isn't an issue as there is pressure on both sides.

How much water comes out of the washing machine faucet? If it's just a little sputtering, then it could simply be some water passing through the bathroom faucet back into the hot line and draining back out of the washing machine faucet.

  • Quite a bit...essentially as much cold water as I send down the drain. This sink has separate hot/cold valves, so that's the confusing part. – DA01 Aug 28 '14 at 3:27
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If your bathroom faucets are single lever, you are probably opening both the hot and cold valves, which is allowing cold water to pass into the hot supply side since both connect to the spout. If your kitchen sink has two separate valves, you'll get the same result by opening both the hot and cold valves at the same time.

  • What is actually happening in that case? How does opening the cold tap, send water through the hot tap elsewhere? (This is a sink with two separate taps) – DA01 Aug 28 '14 at 3:26
  • If the two taps have separate spouts, this theory wouldn't apply. If they share a single spout, both taps send water to a Y connection to the spout. When it gets there, it will take the path of least resistance, which is usually out the spout. If the other tap is open and there is no pressure in the line, some of the water will take that route. – TomG Aug 28 '14 at 11:33
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this problem is caused by a term called a cross conection by leaving tohe cold on isolating the hot. Water is flowing to the other fixture as the second fixture is tied into the supply line before the valve you turned off now having a 2 handle faucet opening only 1 tap namely the hot nothing turn off the hot turn on cold water flows open both taps and water will flow through and feed other fixtue water

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