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mb0709 08-25-2012 06:27 PM

Warm Water out of Cold Water Outlet
My house has two water heaters. One for one side of the house, the other for the other side of the house.

I was washing my car today in the driveway, at the west side of the house, and noticed towards the end of the wash that the water coming out of the outdoor faucet was WARM. I made sure this wasn't due to sun heating the hose or anything - the copper pipe was warm at the point it entered the side of the house.

Went inside and checked the hot and cold water lines to the west side hot water heater. Both were warm.

Ran some hot water in the laundry room, which is right next to this west side hot water heater for a while, and again went back to the water heater to check the temperature of the cold and hot lines. This time, the hot line was hot, and the cold line was definitely cold.

We've been noticing that even when we have our taps on full cold, the water doesn't really feel cold. And, after finishing washing my car, there were white deposits on the paint and glass indicating that the water had come from the hot water heater.

I looked into one-way valves and thermal expansion issues for hot water heaters, and didn't really find anything conclusive. One way valves usually involve dealing with preventing backwash into the city lines only. But in this case, the water heater is backwashing hot water into my cold lines.

Do I install a one-way valve on the cold water intake to this hot water heater? Do I install a thermal expansion tank on this hot water heater? How do I prevent the hot water from backwashing into the cold water line?

Thanks for your help!

wassermeister 08-25-2012 10:23 PM

We are talking about this topic in another thread. You must have a faucet, washing machine, shower/bathtub with a defective mixing valve. Cold and hot water lines meet and mix water to your desired water temperature. When you stop running the application (faucet, washing machine, etc.) the mixing valve stays open (defective)....when you are using cold water anywhere the hot water enters the cold water lines through that open valve.....

Your hot water heater is fine. It is a problem somewhere where hot and cold water mixes.

joecaption 08-25-2012 10:44 PM

One other thing it could be is if you live in the south in the sumer time the cold water is never really cold because the pipes are run so shallow the ground warms them up.

wassermeister 08-25-2012 11:51 PM


Originally Posted by joecaption (Post 996496)
One other thing it could be is if you live in the south in the sumer time the cold water is never really cold because the pipes are run so shallow the ground warms them up.

Yep, you mentioned this in the other thread. I was not aware of this possible reason. Living in the North. :whistling2:

mb0709 08-26-2012 01:03 AM

Good information, all.

That narrows it down to faucets that don't have individual outlets for hot and cold, right? There are a couple of each kind on this side of the house.

What's the best way to figure out which mixing valve is bad? Can you throw me a link to the other thread that you mentioned?


oh'mike 08-26-2012 05:52 AM

Close the shut off valves under the sinks ---one at a time---to the single handled fixtures----shower mixers can not be shut down easily---

Close the washing machine hot line also---rare but not unheard of that the cross over could come from there.

gregzoll 08-26-2012 08:01 AM

How warm has it been where you live? With the high temps, no rain in a lot of areas, the ground has not been able to stay cool enough to keep potable water, etc cool enough.

mb0709 08-26-2012 08:34 AM

I'll try shutting things down one by one and see how that goes. I'm on the West Coast. We had a heat wave two weeks ago, but I doubt there's enough ground heat to keep the pipes as warm as this water is.

mb0709 08-26-2012 08:39 AM

Wouldn't this work: don't shut off any hot water valves; run water at the outside faucet until it starts feeling warm again; at each single-handled fixture, check the cold water supply line to see if it's warm/hot. Shouldn't all of them be cold? If there's a warm one, wouldn't that indicate the problem fixture more efficiently than running the water after shutting down each warm supply line to each single-handled fixture?


Alan 08-26-2012 12:59 PM

The crossover would have to be close to the hose bibb in question, because the hot water is taking the path of least resistance, which is somehow less than the path the cold water has to take to refill the heater.


I also like the idea above about running around and feeling all of the supply lines.

I wonder if maybe you're having a problem with a main valve, or some kind of flow problem.... cold water supply should keep up enough to prevent this from happening. :huh::huh::huh::eek:

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