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Community and Q&A

Minimum Hot Water Temp and Mini-Split Cycling

pNJRiLZxNH | Posted in Energy Efficiency and Durability on

Hello, I recently completed a home energy audit and a few questions arose during the process…
– I was advised to raise the water temp in my standard 50gl electric water heater. My water source is a single user well that gets softened then carbon filtered…what’s the Min temp I can set?
– Is it best to turn the mini’s off during the day and let the house temp go down OR simply leave the mini’s on at a lower temp and adjust for comfort when occupied?

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  1. wjrobinson | | #1

    140 degrees for water heater safety which then should run through a mixing valve to lower to a safer temp at faucets.

    Heat pumps to me should be left at desired temperature. They are more efficient the higher the outside temperature.

    I don't understand why your home audit professional didn't give you this information.

  2. pNJRiLZxNH | | #2

    Thanks for your reply. I was advised no lower than 120 (and then by another person NLT 110). Is the concern micro-organisms in the tank?
    Mini's are just starting to become more common...I've had a few varying responses on the best way to apply them.
    The audit I received was a free community sponsored energy audit. The person conducting the walk-around was concerned, but not a certified auditor ...we generated a few questions in our discussion. thanks again.

  3. davidmeiland | | #3

    Water heater temperature is an interesting question. The medically correct answer for those concerned about legionella appears to be that you want the water in the bottom of your tank not lower than 132 F. Seems like that's going to be hard to accomplish on a continuous basis since cold water is deposited at the bottom, cooling the already-heated water in the tank.

    If you're out of the house, turn your heat down or off. Assuming that recovery time is acceptable and that resistance heat isn't kicked on while trying to get a large temperature rise, that's the way to save energy.

  4. wjrobinson | | #4

    There are two seemingly conflicting safety issues around water heater temperature—the risk of scalding from excessively hot water greater than 55 °C (131 °F), and the risk of incubating bacteria colonies, particularly Legionella, in water that is not hot enough to kill them. Both risks are potentially life threatening and are balanced by setting the water heater's thermostat to at least 54.4 °C (130 °F). The European Guidelines for Control and Prevention of Travel Associated Legionnaires’ Disease recommend that hot water should be stored at 60 °C (140 °F) and distributed such that a temperature of at least 50 °C and preferably 55 °C is

    achieved within one minute at outlets. [15] If there is a dishwasher without a booster heater, it may require a water temperature within a range of 57 °C (134.6 °F) to 60 °C

    (140 °F) for optimum cleaning, [16] in which case tempering valves set to no more than 55 °C can be applied to faucets to avoid scalding. (Note: Tank temperatures above 60 °C may produce limescale deposits, which could later harbor bacteria, in the water tank. Temperatures above 60 °C may also cause gradual erosion of glassware in a dishwasher.)

    In the renewable energy industry (solar and heat pumps, in particular) the conflict between daily thermal Legionella control and high temperatures, which may drop system performance, is subject to heated debate. In a paper seeking a green exemption from normal Legionellosis safety standards, Europe's top CEN solar thermal technical committee TC 312 asserts that a 50% fall in performance would occur if solar water heating systems were heated to the base daily. However some solar simulator analysis work using Polysun 5 suggests that an 11% energy penalty is a more likely figure. Whatever the context, bothenergy efficiency and scalding safety requirements push in the direction of considerably lower water temperatures than the legionella pasteurisation temperature of around 60° C.

    However, legionella can be safely and easily controlled with good design and engineering protocols. For instance raising the temperature of water heaters once a day or even once every few days to 55 °C (131 °F) at the coldest part of the water heater for 30 minutes will effective control legionella. In all cases and in particular energy efficient applications, Legionnaires' disease is more often than not the result of engineering design issues that do not take into consideration the impact of stratification or low flow.

  5. user-965582 | | #5

    Here are some temperature data published at the Tree Hugger website (
    * 70 to 80 °C (158 to 176 °F): Disinfection range
    * At 66 °C (151 °F): Legionellae die within 2 minutes
    * At 60 °C (140 °F): Legionellae die within 32 minutes
    * At 55 °C (131 °F): Legionellae die within 5 to 6 hours
    * Above 50 °C (122 °F): They can survive but do not multiply
    * 35 to 46 °C (95 to 115 °F): Ideal growth range

    I cannot speak for the veracity of these numbers, nor do I know whether the addition of chlorine to municipal water supplies has any effect on legionellae; but it is obvious that the widespread advice to save energy by setting hot water temps to 120°F at the most remote faucet is much too low.

    Is 125°F safe (on the assumption that there is a temperature drop between remote faucets andthe hot water tank, of would it be better to aim for a 130°F reading at a remote fuacet?

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