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Thread: Tankless water too hot for first minute - is this normal?

  1. #1
    DIY Junior Member
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    Jan 2005
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    Default Tankless water too hot for first minute - is this normal?

    I understand why hot water tanks can deliver water that's hotter for the first minute or so (heat rises to top of tank), but I have tankless hot water from an oil burner and for the first minute the water is very hot (160F) and then sometimes drops down to a much cooler temp (as low as 130-ish) but eventually settles down to about 145F and stays there. Is this sort of behavior normal from a tankless? If not what could cause this? We have a Watts Regulator mixing valve at the boiler that is set to the hottest setting--any chance the mix valve is too old and not working right?

    Mike J.
    Boston

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    Sep 2004
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    yes...also, it takes a moment or so for the tempering valve to react. Last, it could have heated up some of the nearby water, so there is not much cold to actually flow to temper it for a moment. Unless you are in a commercial situation, over 130 is risky. My unprofessional opinion.
    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
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    Thanks for the feedback. In considering your idea about the water in the cold pipe being heated, it suddenly occurred to me why the initial water is so hot. Tankless has no thermostat--the water is heated by simply passing through the coil, and when you aren't running the water it sits in the coil getting hotter in hotter as the furnace runs to heat the house. Between that and the two extra things you mentioned it' so obvious I'm embarassed I had to ask

    Quote Originally Posted by jadnashua
    Unless you are in a commercial situation, over 130 is risky.
    I totally agree--I think the mixing valve was turned all the way hot because the shower was too cold, which I fixed by simply adjusting the max temp limiter on the shower valve (previous owner should have read these forums before trying to fix things himself!)

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