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Thread: hot water temp loss when I pull up the shower diverter

  1. #1
    DIY Junior Member forbins's Avatar
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    Default hot water temp loss when I pull up the shower diverter

    hey all, new the board, please help me if you can

    i'm a jack of all trades master of none kinda guy, i've installed a Bosch Aquastar 125 in my house. We have a small house, only one bathroom. The unit seems to be working great. The kitchen sink, bathroom sink, washing machine and bathtub all get really hot. The problem is when I pull up the show diverter on the bathtub spicket. As soon as I pull it up I can visibly see the flame go down on my tankless unit and the end result is a luke warm shower. I can get it flame up and produce hot water again if I turn on the bathroom sink while the shower is running. Also don't if it's related, but if i have the hot water on in the bathroom sink on and my wife turns on the hot water in the kitchen sink there is significant pressure loss.

    i've been reading about thermostatic shower valves vs pressure balance valves.


    please help, haven't had a decent shower in a while!!!

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    All tankless use a system to detect flow, and only turn on when that flow exceeds a certain value (differs by brand and model). You may have a super saver showerhead, or you like it cool, because it sounds like you aren't getting the flow needed to keep the thing turned on. That's why, when you also turn on a sink, the increased flow triggers it to turn on. You might try a different showerhead.

    A thermostatically controlled shower valve is nice, but it can't magically generate hot water...if it isn't coming in, it can't adjust. But, if the temp varies while the shower is going on, it will try.

    Also note that many tankless systems have a flow restrictor in them, and this can mess up both pressure balanced and thermostatically controlled valves. Read the info carefully, and check with the manufacturer before investing in making a change.
    Last edited by jadnashua; 02-10-2011 at 01:05 PM.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3
    DIY Junior Member Henryj's Avatar
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    Default Water flow

    Quote Originally Posted by forbins View Post
    hey all, new the board, please help me if you can

    i'm a jack of all trades master of none kinda guy, i've installed a Bosch Aquastar 125 in my house. We have a small house, only one bathroom. The unit seems to be working great. The kitchen sink, bathroom sink, washing machine and bathtub all get really hot. The problem is when I pull up the show diverter on the bathtub spicket. As soon as I pull it up I can visibly see the flame go down on my tankless unit and the end result is a luke warm shower. I can get it flame up and produce hot water again if I turn on the bathroom sink while the shower is running. Also don't if it's related, but if i have the hot water on in the bathroom sink on and my wife turns on the hot water in the kitchen sink there is significant pressure loss.

    i've been reading about thermostatic shower valves vs pressure balance valves.


    please help, haven't had a decent shower in a while!!!
    Just change the shower head for one with higher flow, water flow is not sufficient to activate flow switch in water heater.

  4. #4
    DIYer, not in the trades LLigetfa's Avatar
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    Quote Originally Posted by Henryj View Post
    Just change the shower head for one with higher flow, water flow is not sufficient to activate flow switch in water heater.
    Before you run out and buy another shower head, take off the existing head and test it with just the stub pipe to see how much flow there is. Some of the newer valve assemblies put a restriction on the shower line to ensure enough back pressure for the anti-scalding pressure balancing to work well.

    Inspect the inside of the shower head thread fitting. Some place a restrictor there that may be removed or drilled larger.

  5. #5
    Journeyman/Inspector Inspektor Ludwig's Avatar
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    I'll bet that tank is undersized.

  6. #6
    DIYer, not in the trades LLigetfa's Avatar
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    Quote Originally Posted by Inspektor Ludwig View Post
    I'll bet that tank is undersized.
    I know, it's a tankless job. :P

  7. #7
    Plumber Winslow's Avatar
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    Had that problem often with the old Price Pfister 2 handle valves. Remove the restrictor on the shower head. Or buy a new shower head and remove the restrictor before installing it.

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