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Community and Q&A

Water heater temperature

NormanWB | Posted in Energy Efficiency and Durability on

My Rheem heat pump water heater has the ability to change temperature based on a schedule. It was originally set to 120 F, but I changed it so it was 110 F (the lowest setting) except when we normally need hot water.  This has resulted in a 1/3 reduction in the power usage. 

However, it is recommended that water heaters be run at 140 to kill any bacteria, including legionella.

Since mine can be programmed, I would like to find an optimal setting to get the best of both worlds, efficiency and safety.

I am thinking something like, run the water heater at 140 x number of times a day for y number of minutes too kill bacteria, 120 around showers and such, and 110 the rest of the time.

Has there been any research in this area or any guidelines?

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  1. Jon_R | | #1

    I'd get it to 135F or 140F at least some of the time.

  2. jackofalltrades777 | | #2

    I run my Rheem at 135F. You definitely don't want bacteria to grow as your medical expenses $$ will offset any energy savings you get by running it at 110F. Just keep it in HEAT PUMP ONLY mode.

  3. this_page_left_blank | | #3

    I don't know about any research into the specific control pattern you're considering. But 90% of legionella dies within 2 minutes at 140F. It survives, but does not grow, at 120F. So something like 1hr a day at 140F, 120F the rest would seem to be pretty iron clad. 110F is very close to the ideal growth range, so that's more complicated a proposition. How much does it grow, and how many times in a 24 hour period would you want to raise it up to 140F in that time are open questions. Most cases of illness are related to stagnating water, which makes me suspect that bringing up to 140F even once a day would probably be protective, but I have only inference to base that on.

  4. BrianPontolilo | | #4

    Hi Norman.

    You may find this FHB article informative: A Debate Steeped in Hot Water

    1. NormanWB | | #6

      Unfortunately, the article is behind a pay wall. I have let my subscription lapse, but maybe I have the original issue somewhere.

  5. CollieGuy | | #5

    My understanding is that these bacteria colonies can survive at the bottom of the tank even at these higher temperature settings due to thermal stratification and sediment build-up.

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