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Thread: Thermostatic valve - Only cold water, valve does not call for hot water

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  1. #1
    DIY Junior Member psperanza's Avatar
    Join Date
    Dec 2012

    Unhappy Thermostatic valve - Only cold water, valve does not call for hot water

    I've searched all over the net, and I cannot seem to find anyone with the exact same problem. I have found similar problems, but this seems to be a little unique. I am not certain if my plumber crossed the lines on the install, but i was wondering if there was any other troubleshooting for this problem. Basically, when the shower is turned on, the pressure is just fine, but it does not call for hot water. I use a tankless water heater, and it does not fire up when the shower is turned on. Only ice cold water will come out, not matter where I set the thermostatic valve. After trying a thousand different things, we realized that if I manually cut off the cold supply to the shower (by closing the shut off valve), and leave only the hot running to it, then I will get scalding hot water only. I cannot seem to get it to mix at all. Is this a symptom of reversing the hot and cold lines running to the valve? Or could it be anything else? Thanks in advance for your help.

    BTW, it is an Aquabrass 1/2" thermostatic valve

  2. #2
    Moderator & Master Plumber hj's Avatar
    Join Date
    Aug 2004
    Cave Creek, Arizona


    It is a problem in the valve itself, but without being there and doing our own troubleshooting, we cannot tell you exactly what the problem is. The "apparent" problem seems to be that even when calling for just hot water, the flow is not adequate to cause the burner to turn on. This COULD be caused by reversed connections. IF this were the case, when the valve is trying to get "hot" water but the water is cold, it shuts down the "cold" side, which would actually be hot and just gets colder. I this is the situation, then it is usually just a case of reversing the control unit.
    Licensed residential and commercial plumber

  3. #3
    Retired Defense Industry Engineer jadnashua's Avatar
    Join Date
    Sep 2004
    New England


    Sometimes the tankless systems have lower pressure/volume through them than the cold, and that pressure/volume difference can disrupt the operation of a themostatic valve. I'd verify it is plumbed properly first, though. Some valves all you need to do is rotate the cartridge 180-degrees as HJ indicated, some would require replumbing or maybe they have a 'reversed' cartridge available.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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