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Thread: Tempering Valves for residential water heaters.

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  1. #1
    DIY Member tom12's Avatar
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    Aug 2011
    California & UK

    Default Tempering Valves for residential water heaters.

    Tempering valves(T/V) used with residential water heaters (W/H) - aka ( i believe ) Thermostatic mixing valves.

    1. Their main function appears to be to protect from scalding, ( how often do first line of scald defence, residential w/h thermostats fail without isolating the gas/power?)

    2. And protect against water borne diseases eg. Legionnaires Disease.

    3. Legionnaires apparently thrives in w/h's at 105* to 115*F, and upping the temp to 140*F will kill the disease.

    4. At 140* a t/v is required. Note, the cold blending water has not been "treated" at 140*, more likely, it's been warming in the vicinity of the extremely hot w/h . ie. dubious cold water by-passes the w/h and mixes in at the t/v.

    5. What about mfr's warranties if the w/h is used at 140*F?

    6. What about the extra fuel/power being used to obtain 140*?

    7. There are, of course, other methods of disinfecting the system without using high temps.

    8. At high temp's, increased scale will accumulate along with the usual, introduced silt and sediment. These particles might possibly disable the functioning of the t/v, even with filters and screens. Perhaps the screens will block with the larger particles and the t/v will req. frequent maintenance?

    9. Extra isolators are req'd with a t/v - hence, more sod's law chance of one being left shut and the possible consequences.

    Note: Just a few observations; i'm ignorant and curious, and would welcome criticism or further suggestions. Can anyone cite government stats for Legionnaire infections from residential w/h's?

  2. #2
    In the trades Dana's Avatar
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    Jan 2009


    A thermostatic mixing valve isn't exactly the same thing as a tempering valve, but they're similar in function here.

    Most water-borne pathogens that require 120F+ temps to kill have extremely low rates of reproduction at sub-70F water temps, so the fact that the mixing passes the "un-cooked" water is of little concern if it's from an otherwise clean & tested source. The sweet-spot range for rapid legionella bloom is between 85-115F.

    The extra energy use to maintain water at 140F rather than 120F is only a standby loss issue, but it is indeed an substantial increase in that standby loss. With electric tank heaters that standby is very low compared to the energy for the raw heating of water up to temp, but in atmospheric-drafted gas burners it approaches half the total energy use at 50gallons/day usage (less of the fraction at higher volume use, more at low-volume use.) In the grand scheme of things it's not a very big loss, and insulating the hot water distribution lines as well as any near-tank plumbing (cold, hot, or temperature & pressure valve outflow) with 10' of the tank to R4 (5/8"-3/4" closed cell foam) pipe insulation would more than offset the increased loss of water stored at 140F rather than 120F.

    Scaling is rarely an issue with tempering valves, but I suppose anything is possible hard water areas.

    For most situations setting the storage temp to 120F has very low increased risk if the water source is a treated & monitored municipal supply. In many locations building code requires it to be initially set 130F or 140F when installed by a licensed plumber, but there is no requirement that the homeowner leave it there.

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


    FWIW, I had my tempering valve off when I was checking out a leak...there were some (soft grainy) mineral deposits in it, but not enough to cause any problems. It was off to replace the tank.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  4. #4
    DIY Member tom12's Avatar
    Join Date
    Aug 2011
    California & UK


    Thank you both for your replies. Sorry for the delay, I've been on an enforced absence. I guess this subject is now dead, but i do still wonder.

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