Hot Water Heater Temperature | What’s Right & Why

ByKen J. Last Update
48 Comments
Best hot water heater temperature to avoid Legionnaires.

Temperatures maintained below 140°F (60°C ) encourage growth of  Legionella bacteria and other microorganisms!

There is an apparent misconception that keeping a hot water heater set at 120°F is the right or best temperature.

Wrong. Read on to discover why…

But you might ask, “What about hot water scalding at 140°F ?”

You should be concerned, especially if there are children in the house. In fact some codes won’t allow a plumber to set the hot water temperature higher than 120°F .

Quick answer: Set hot water tank to 140°F (I’ll explain why in a minute). Install a mix valve at the output of the water tank to bring it back down to 120°F . I have a recommendation at the end of this article.

Domestic hot-water systems are frequently linked to Legionnaires’ outbreaks.

Water heaters that are maintained below 60°C (140°F) and contain scale and sediment tend to harbor the bacteria and provide essential nutrients for commensal micro-organisms that foster growth of L. pneumophila.

To minimize the growth of Legionella in the system, domestic hot water should be stored at a minimum of 60°C (140°F)…

~ OSHA

Why Are Hot Water Heaters Set To 120 Degrees F?

You might be one (of many) who have heard that the best hot water temperature setting should be 120°F. So I investigated – why is that?

For one, setting the temperature lower does save some energy dollars. I’ve read on the order of 5 – 10% annual. Depending on your tank and usage, this may range from $25 to $50 savings per year. But is the risk worth it?

Another reason is for scalding safety, particularly if there are young children in the house. I get that. But read on…

The problem is, a water temperature of 120°F does NOT kill the Legionella bacteria.

What is Legionella?

Legionella is a type of bacteria that is found primarily in warm or hot water environments. These bacteria can cause Legionellosis.

Legionellosis comes in two forms, Pontiac fever, the lesser of the forms, and also Legionnaires’ disease which is a more severe multi-system illness with a deadly type of pneumonia.

Thousands of people get Legionnaires’ Disease in the United States each year. Of the approximated 2.4 million cases of pneumonia that that are diagnosed in hospital patients each year in the United States, about 18,000 cases are confirmed as Legionnaires’ Disease and up to 600,000 cases of Legionnaires’ Disease are misdiagnosed as pneumonia because the hospitals do not perform the tests for Legionella.

~ legionellaprevention.org

Prevent Leigonnaires While Minimizing Hot Water Scalding

If you do not have small children in the house who may accidentally scald / burn themselves with 140°F hot water, you might consider setting your hot water heater to 140°F. Simply get used to adjusting hot-cold at the faucet or shower as necessary. But there’s an even better way…

Another way to deal with this is as follows:

Set the temperature of the water reservoir (hot water heater) to 140°F but deliver the water at 120°F by way of a mixing valve. The mixing valve takes the hot water from the newly set heater (140°F) and mixes it with cold water until it can be released from the valve at a safe 120°F temperature.

Best Hot Water Mixing Valve for Hot Water Tank

In my opinion, this particular valve kit from “Cache Acme” (check it here) as pictured below is a easy-to-install solution to this problem. The reviews and recommendations look great.

It automatically adjusts (via internal thermostat) the amount of cold water to output 120°F.

I like (this accessory) which includes a shutoff valve.

If you can’t install a mixing valve kit yourself, no problem. I’m sure that a plumber won’t charge too much to get it done.

Here’s their spec sheet on this valve.

Easy to install hot water heater tank mixing valve for 120 degree temperature.

How To Measure Hot Water Temperature

After having adjusted your hot water heater temperature, wait until it has stabilized. A gas hot water heater heats up pretty quick. Electric may take a bit longer.

At your kitchen sink, turn on the hot water. Let it run long enough for the distribution pipes to achieve equilibrium (depends on distance from faucet to tank). Maybe a minute.

Use a fast acting temperature probe. Place the tip into the hot water stream. Wait until temperature readout settles down. Note the temperature. That’s it.

If you don’t already have a fast acting (instant read) meat thermometer for your grillin, here’s your excuse to get one.

Continue reading: Average Gallons of Water People Consume Each Day

The Berkey Filter is Expensive – Is it Worth the Money?


Ready Made Resources prepping and preparedness supplies
USA Berkey Filters
Fire Steel dot com
EMP Shield
Golden Eagle Coins gold and silver online
Peak Refuel authorized distributor

48 Comments

    1. On my hot water heater tank, on the temperature dial, the center position (which is marked separately with a white color notch as opposed to blue to red gradient), turned out to be exactly 140 degrees. Seems like they ‘knew’ ;)

    2. JMHO, but seems like probabilities are being ignored here. 140°F wastes a LOT of energy and $$, that’s absolute. It’s only a very slim probability you’re going to have Legionella in your water to begin with, and a much higher probability you’re going to scald old/young people if they are in the household. At the very least, call your county health department and ask about Legionella cases in your county before you go adjusting the thermostat on your water heater. My local department said they had never had a case here they they were aware of.

  2. We use well water for everything except cooking. With a new water heater a couple years ago, we began getting a rotten egg smell when using hot water at 120 deg. After some research, I turned it up to 140 deg, and after a few months, the rotten egg smell has disappeared; probably killing off the bacteria that made the smell in the tank.
    Good article.

    1. You can get a test on Amazon (go figure) that will test your well water for 170 different things. It is from Drinking Water Specialists and runs about $170.00.
      For city water users, it costs less, but tests for less. My test on a well that has been dormant for fifteen years came back almost perfect! A slight amount of iron bacteria (which they took care of by sterilizing the well) and a trace of strontium. They also test for heavy metals, and many industrial chemicals. You do one part of the test at home, and send four vials to their lab in New Jersey. The results are e-mailed in about ten-twelve days. Highly recommended!

      1. Yes, I would hope that anyone who has well water, will have their water tested by an accredited source – once in awhile. Water is #1. And is another reason to own a quality drinking water filter, like the Berkey.

        Prior to purchasing my current home, I paid to have the water tested. It’s from a natural spring, pumped up to the house. It didn’t cost near $170 (don’t recall exactly). But maybe it’s ‘inflation’ over the years ;)

        1. Water heaters are set at 120 from the factory.
          As a plumbing contractor, by law, I’m not allowed to turn it up
          The homeowner can do as they wish after I leave.

        2. Randy,
          Wow, that’s interesting – but not surprising – given the litigious nature of our society. A lawsuit from scalding trumps the potential of lawsuit from Legionnaires (because it’s a less likely comparative scenario).

          I wonder if this is a requirement that varies from state to state…

          While doing my research, I believe that Canada now requires tank temperature of 140 (60 Celsius up there) AND a mix valve at the output set to 120 (49 Celsius). This seems like the best way to deal with the situation. I will proactively be replacing my hot water heater fairly soon (surprised it has lasted this long!), and this is how I will do it.

  5. A couple of things, building code and plumbing codes both mandate rhat a water heater my NOT be set above 140.

    Also remember if you have a long “run” from the heater to the sink/shower your losing a LOT of heat and $$$$.

    Next, a “tank” type heater stores that water at 140 for, what 4-8 hours between uses? How bout switching to a “Tankless” water heater? Yes they are pricey but WILL pay for that extra money in 2-4 years.

    AND you will not drain that tank heater and have COLD water when having that romantic/fun shower with the Sweetie.

    BUT absolutely have that sucker set at 140…..

    1. I looked into a tankless heater (about $250) for the bunkhouse. The electrician said it would require a 50A circuit. The current circuit is only 30. He added that for the power company to additional power would cost us thousands.
      I really like the idea of tankless heaters. They do require a lot of power to run.

      1. – extexanwannabe-
        Have you looked into propane or natural gas tankless water heaters? I am quite certain you can find either, you have to look, though.

        – Papa S.

        1. Xtex. I have an ecotemp L5 propane on demand heater and don’t have any complaints. You can buy a combo kit with a water pump included from amazon for ( last time I checked) less than $200 bucks.
          The only thing you need to use this type of set up, is water pressure, propane, and a D cell battery. You can plumb it into your system. Or use it as a portable stand alone heater. Mine came with a handheld shower wand, and quick couplers to connect it to a garden hose.

        2. Thanks, guys. I had not considered LP gas. Will look into it.

      2. Tex,

        Listen to Papa Smurf. I bought a cheapo Marey CNG tankless for my entire house, 8 years ago, for about $250. It’s been among the most reliable appliances I’ve ever owned. One caveat… stay away from single handle faucets. They sometimes make it difficult to get just the right temp.

  6. I am sorry if I am off topic, I really need help. I have a few dozen gallon pickle jars, but I cannot find lids to fit. I have purchased 3 sets online and none fit. Why are my olive jar lids so hard to find?

    1. Km,
      If you post your question over on the open forum (which is for off topic conversation), I’ll bet you’ll get some answers. Thanks.

      Hint: check the Recent Comments page for responses (it’s sometimes easier while used in combination with the Open-Forum article).

  7. Washington State law prohibits landlords from setting hot water tank temp above 120*F. Residents can set it higher.

    This sure make the argument for an on-demand water heater stronger. DFM has propane-fired one at off-grid cabin. Works a treat and never a problem.

  8. Thanks Ken, but I do have a couple?’s
    1) I have a send home and turn the tank down when not there, which obviously would create an issue. If turned back up, will this kill the bacteria?
    2) how is this bacteria delivered to us? From the air as it comes out of the faucet? Absorbed through our skin? Washing dishes? Hopefully you or someone knows.
    What you are saying makes sense. I’m like the person above with the happy wife – happy husband on this issue, although I will check the temperature.

  9. Having had several patients die from excessively hot water I read the above advice with extreme trepidation. The elderly and and the young would be most at risk.

    It is not uncommon to have apartment supervisors turn up the heat when tenants complain about running out of cold water. It does not re-heat the water faster it just heats it longer.

    When I was stationed at the army hospital in Hawaii many years ago we had an old retired soldier as a patient. He had an incontinent stool and the medic took him into the bathroom to clean him. He got the water running into the tub and went out to get towels. The tub had two set of faucets. One set for the spigot and one set for the shower. They figured out that the patient tried to stand up in the shower and grabbed the hot water faucet for the shower and as he pulled himself up it turned on. The hot water in the hospital was kept at 160 degrees for the dishwasher. He was probably unconscious immediately from the pain. The only places not burned were behind his ears and the crack between his buttocks. We kept him knocked out with morphine dripping into his IV and he dies in less than 24 hours. That was from a scald at160 degrees, I believe the same degree of burn would occur at 140 degrees.

    Peripheral nephropathy is diminished sensation and is very common in diabetics. I had a diabetic spill boiling water on his leg and was not aware that he had been burned until his kid visited him the next day.

    Caveat emptor.

  10. On the tankless water heating systems, something many of you are missing. When you take a bath you turn the water temperature down to your comfort zone each time you shower. Then turn it back up to the setting you use for washing dishes, laundry etc. Ours is set at 125 degrees, an I will turn it up to 140 for the items listed above(general household use). When we had company would leave it at the 125 so that they could mix their own hot water setting for showering.

  11. One more item, be sure you stock pile regular white household vinegar for cleaning your tankless water systems. You use a small sump pump with two hoses that run the pure vinegar through the tank to remove any mold or minerals that have build up over the year. I clean ours every spring/summer depending on the amount of iron or lime in the water system out of the well will depend on how many jugs of vinegar it will require.

    How will you know when it is clean, when the vinegar runs clear through the heating system. In this household I keep 12-16 jugs, or more of vinegar just for the cleansing process. Some years the minerals are higher that infiltrate the system, and when I find time to accomplish this chore each year.

    1. jcb:
      Actually your only 1/10 correct.

      If the water is 1 deg above the incoming water the water can already be considered HOT.

      The concept of heating water rather it be ice or boiling water is all a mater of reference, so heating HOT water or ice water is all in the realm of whomever is making that statement.

      If one is heating ice, would it be called an Ice Heater? You could not call it a water heater because of the thermal state makes it Ice. Same for Steam, if one heats Steam same goes, NOT a water heater. Hence it is for existing Hot Water. If one turns up the stat its already Hot Water, you’re going to make it hotter, hence Hot Water Heater.

      With that is any water in a Water Heater Tank already not hotter than when its coming out of the pipe in the ground? So it’s hotter than the Cold Water, now your adding MORE heat to the hot water……

      Hence, yes it’s a Hot Water Heater.

      PS: you dont need yo use CAPS indicating you’re yelling, we can hear you just fine.

    2. JCB,
      THANKS FOR POINTING THAT OUT! IT’S ONE OF THOSE THINGS THAT SOME PEOPLE CALL A “HOT WATER HEATER” (PERHAPS TECHNICALLY INCORRECT), THOUGH IT ACTUALLY TECHNICALLY IS A WATER HEATER.

      THAT SAID, I DO WONDER AT WHAT TEMPERATURE WATER IS CONSIDERED TO BE “HOT”. ;)

      1. Ken,

        I stand in awe of his (?) brilliance. I bet he (she?) knows that tin foil is actually made from aluminum. Won’t be able to fool him (her?).

        Up next, nit-picking for fun and profit.

        1. WHAT!!! Where can I get true tin foil? Everybody knows that aluminum foil is not effective at keeping the satellites from reading my brain waves!

      3. Who is this, certainly not the Ken I know and love, for our Leader in this extravagant circus would not be escalating his calm demeanor and over emphasizing his tone….. 😎

        1. Silly. I have no intention of putting a fire in it, and I like the look of polished wood. Why shouldn’t it be wood? :)

      5. Ken (and anyone one else who gives a rat’s nose what jcb so snobbishly stated),

        Many inventions are designed to heat water, each for a specific, desired goal. A boiler heats water. It’s purpose is to change water to steam, not to produce hot water. Steam can power an engine, dissolve dirt and grime, kill germs, heat a home with radiators, etc. One cannot take a bath in steam, wash their hands in steam without grave bodily injury. For that you need “hot” water. For “hot water” you need a water heater that will not heat water past it’s boiling point, where it becomes steam. Thus it actually is a “hot water heater” as opposed to a “steam producing water heater” or “boiler”. Both heat water, each has their unique end goal. One produces hot water (a relative terminology), the other steam. Each product has it’s own unique use. Both are “water heaters”.

        Some folks get a thrill from pointing out what they see as a mistake on your part, while totally oblivious to their own lack of knowledge or understanding.

        Reminds me of an anecdote from my basic training days at Ft. Polk. The majority of the recruits/draftees in my training company were from the northeast. During some idle time in the barracks, a young guy from Philadelphia engaged me in conversation. He, with a straight face and with all sincerity said, “Tex, youse guys just slay me when youse say y’all.”…………how do you respond to that total lack of awareness? (By the way, he was a good guy). I responded by telling him I was 21 years old and had never heard the term “youse guys”, but I immediately understood what he meant. Communication uses words and terminology. Words and terminology vary from region to region. Later found out, talking to other recruits from Pennsylvania, that “youse guys” and just “youse” was almost exclusively a slang confined to the Philadelphia area.

        1. Dennis:
          Being from Ohio, Cincinnati, I moved to Cough Cough, California, it took me 2 years ti figure out there was no “R” in Wash, always said it as “Warsh”.
          BTW where did that expression “rats nose” come from? Wrong end of the Rat buddy… LOLOL

        3. – I always thought it was called a “rat’s nether hind parts”. As being the last part of the rat to get away in the hole.

          – Papa S.

  13. Golly gee, some topics sure get people riled up. When the SHTF, we will all be glad if we just have a “cold water heater” :) How’s that herd of cats Ken?

      1. NRP
        You are close to the river so water should not be a problem. A kids wading pool with a black plastic liner and the temperature should reach 120 quickly – two buckets should do it figuring the Archimedes Principle and your girth. Or does the river dry up?

        1. hermit us:
          Water wi nada be a problem and the Orange River has bever dried up completely, had a few low-flow years but nada stopped.

  14. Thanks the good info about on-demand water heaters. I will consider using a gas unit if I purchase one.

    As for my use of the English language, dude, I grew up in Kalifornia so it is fun to talk like I am from the San Fernando Valley ( valley-girl Mall talk vernacular.) or like the surfer dude who just got spliffed on some really good weed.

    This coming from an old, bald asian dude really cracks people up here in Oregon. For more info, see video of Frank Zappa and his 80’s hit Valley Girl.

    I can hardly wait to hear what tmgyver has to say about this topic.

  15. For those considering an “On-Demand” unit.
    I have been on a Rinnai for going on 10 years now (currenthome), not a single problem.
    I actually have 2 units, one for domestic water and a commerical unit for the in floor heating system.
    Both propane and very efficient. These cost wholesale at $1400 & $1700 respectively. NOT cheap units but I expect to get 20+ years out of them.
    Last house I had was an old Tank “Hot Water Heater” 50 gallon, that sucker was a energy hog. Put Insulation blankets and all the BS, year before I sold the house I put in a tankless, droped the propane useage by right at 25%.
    AND never ran out of 140deg water.
    PS: If tshtf, make sure ya have a few Black Buckets, a day in the sun and you have a warm bath 😎

    1. – BTW, NRP-
      Being as how you are there in the Four Corners area, you are probably aware that among the detritus and debris at the famous Robbers Roost where Butch & Sundance hid out, they had the remains of a black-painted bucket on the site until at least the mid-sixties when I was there; even the wanted, wanted to be clean.

      – Papa S.

  16. Cali
    Like OMG
    Like you would be an awesome dude to hang with. Like yah.
    Like, the water heater settings and like, all the problems with, like, low
    temp settings, as like Ken pointed out….
    Gag me with a spoon.
    (I can’t believe I use to listen to some of that stuff, like ya know)

  18. Just as an fyi,in NH it is illegal for a licensed plumber to set the temp above 120 by state law. Yes if the right officials decide to you the homeowner can be prosecuted as well as a plumber. A young woman near Berlin NH was charged with child endangerment, when she went to ans phone the child turned the hot water faucet up.

    Sad case but true. Better to install a mixing valve to temper the temp to 120.

    Retired NH plumber,I got the story from the st official who charged her and as I recall he was proud he did!

    Everyone just be careful and check your local code!

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