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Thread: Hot Water in Cold Water Supply

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    DIY Junior Member jrcnet's Avatar
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    Default Hot Water in Cold Water Supply

    Here is a description of my problem. I have a two story house with two water heaters. The water heaters are tied together. One water heater is on the master suite side of the house. The other is on the opposite side of the house serving the kitchen/laundry room. The upstairs bathroom has two sinks and a shower that pull hot water mainly from the kitchen side of the house. My problem is that when the washing machine is on (filling up with water), the cold water side of the fixture that is closest to the water heater is hot. It is not just lukewarm, it is steaming hot. I have isolated all of my single lever faucets and shower heads to rule out the possibility of cross-linked valve failure. I have also turned off the hot water to the washing machine to rule it out as well.

    I closed the valves and shut down the water heater above the kitchen, and cannot re-create the problem with just the one water heater from the opposite side of the house. After that, we opened the valves to the water heater without turning the heat back on. When I turned the washing machine back on (cold only), I can feel the cold water entering the hot water heater pipes. Then, I had my wife turn the sink faucet on (cold). I could feel the flow reverse in the hot water heater. Then cold supply line turned hot. It seems that the washing machine is creating a loss of pressure on the cold water supply which is causing the hot water to backflow out of the hot water heater.

    Anyone have any experience with an issue like this? I am thinking of installing a check valve on the cold water supply to the hot water heater to prevent water from backflowing out. Any issues with this?

    Comment/Suggestions Please!!

  2. #2
    In the trades Dana's Avatar
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    Sinks & showers with mixing valves (as opposed to two separate faucet handles) can leak across and misbehave when there is a pressure difference across them. When running only the remote HW heater the leak may be slow enough that there's too much volume, and the hot water never fully arrives, even if it's happening. The head-resistance of the longer distribution line would lower that flow rate too.

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    DIY Senior Member Runs with bison's Avatar
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    Quote Originally Posted by jrcnet View Post
    It seems that the washing machine is creating a loss of pressure on the cold water supply which is causing the hot water to backflow out of the hot water heater.

    Anyone have any experience with an issue like this? I am thinking of installing a check valve on the cold water supply to the hot water heater to prevent water from backflowing out. Any issues with this?
    It sounds like you've done an excellent job of testing and diagnosing the problem. A check valve sounds like the proper solution to me, but I will be interested to hear if the professionals know if there is a problem doing that.

    What style of washing machine are you using? I ask because the front loaders pull in pulses and I could imagine them dropping the line pressure dramatically (though I haven't put a guage on to check.) At any rate if there is substantially more flow resistance in the cold flow path without the water heater than with it, then I would expect backflow from the water heater in a parallel path.

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    It sounds like your heaters are piped in parallel but they are not sitting side by side. They are installed accross the house from each other and you have a cross connection. Thats a mess.

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    Sure a check valve would stop the crossover. Install one on each heater in the cold line. You will need to be sure that you have thermal expansion control between the water heaters and the check valves. In theory the same thing could happen on the opposite side of the house so thats why I suggest you install check valves at each heater.

    You can do this and prevent the crossover but it will not correct the root of the problem and thats having parallel heaters that are not piped correct. You are going to have temp variations due to pressure fluctuations and the hot water being drawn from different tanks and that means it could be two different temps.

    So the real problem will remain. They are not piped right. To really correct the problem you need to find the pipe or pipes that link to two heaters together and separate the two hot water systems.

    Thats whats up.

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    DIY Senior Member Runs with bison's Avatar
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    Good points. I was thinking that 2 checks (one for each) would be necessary as well, since other problems could turn up in new places with just 1.

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    Moderator & Master Plumber hj's Avatar
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    I cannot be sure that you have done a proper diagnosis, and would have to do my own to make a recommendation. However, it would be better to determine the REAL CAUSE of the problem, since it should NOT happen, than to just prevent the symptoms, and possibly cause them to move somewhere else.
    Licensed residential and commercial plumber

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