It seems to be "common knowledge" that to help prevent Legionella, hot water tanks/heaters should be set at 140F. This temperature is enough to cause terrible burns and scalding so devices like thermostatic mixing valves are also commonly used.

However, in Texas where I am from, people set their water heaters to 120F for scalding safety. I've never seen a thermostatic mixing valve installed in a house and haven't known anyone to set a water heater to what I would consider a very dangerous 140F.

I do notice that people that seem to be from colder climates make the 140F recommendation, so does it mainly come from areas with "boiler" systems? Larger hot water tanks that would hold the water for longer (50 gallon is the max common size in TX)? Is this a newer suggestion that I just haven't seen implemented? Is it a commercial thing?

I'm just trying to find out where the discrepancy is between the people that say it's a hard requirement and the other people that would never consider heating water that hot in a residential system.

  • The issues for this are so complex - just one : the "residence time" needs consideration, as if the water is at 120F and the flow through the tank is high which means the water is not in the tank very long the risk of legionella pneumophilia is reduced...
    – Solar Mike
    Mar 27, 2019 at 15:04
  • I would be less concerned with Legionella and more concerned with Naegleria fowleri if you plan on running your water heater at lower temps. Particularly if you do the sinus rinsing thing.
    – Tim Nevins
    Mar 27, 2019 at 17:10
  • Just googled Naegleria Fowleri, echh gross. Wikipedia says it thrives up to 115°F but for the colloquially known "brain-eating amoeba" I wouldn't want to be storing water anywhere near its comfort zone. I'd rather not die to save a couple of hundred dollars.
    – Joe Fala
    Mar 27, 2019 at 17:33
  • @JoeFala It's not that I don't want to or that I don't get the issues with water at 120F, I just wanted to know if some systems or usage patterns (in different climates) exacerbate the problems.
    – JPhi1618
    Mar 27, 2019 at 17:46
  • I've noticed over the years that techs are satisfied just to get things working. The safety issues with the current systems implemented are a real concern that don't get much attention because it involves more then just getting hot water out of the tap. It complicates things. To me it's an opportunity to elevate plumbers, HVAC technicians and electricians to the level of doctors and lawyers. These are the real problems that everyone encounters on a daily basis. I've only needed a lawyer 5 times in my life but behind the scenes I depend on my electricians, plumbers and HVAC techs every day.
    – Joe Fala
    Mar 27, 2019 at 17:59

2 Answers 2


It's a new problem.

First, do anything you want. It's your health and there's no legal mandate, yet.

The legionella issue is actually quite new. The disease itself wasn't even identifed until 1976 at the US Bicentennial in Philadelphia, when a bunch of people at a hotel got sick and died. Only this mass-casualty event made anyone notice and start to look at microscope slides; the disease has no doubt been active for much longer.

It has taken them decades to figure out how it even operates.

Historically, the advice regarding water heaters has been to keep it cool enough that you can't scald people; particularly children or the elderly who may not have the wherewithal to escape/stop too-hot water. (Consider an Alzheimers sufferer who is protesting anyway; you can't tell a bona-fide "too hot" protest from the rest.)

Only quite recently was it understood that it breeds in water heaters, and can enter your system by breathing atomized/evaporated water in a shower. And it took years more for the science to get solidified to where government agencies feel comfortable making it official messaging. So "only in the last 5-6 years".

What also slowed the government proclamations is the scalding factor. Stopping legionella meant a grisly compromise between scalding risk and legionella risk; however this is resolved either a) by tankless heaters, or b) blending/safety valves on all outlets Only recently have those become commonplace (because the government incentivized these, because of this). So the government was very reluctant to start blasting the "140F/60C" message.

Part of the problem, any time you put out PSA messaging like that on a large scale, is you are speaking to a very, very, very stupid populace, and much of that stupidity comes from the "heard it on the grapevine" method by which information is repeated incorrect or incomplete. That makes the government even more reluctant to blast a message.

As a result, your confusion is understandable.

  • You cannot conquer the legionella problem without also thinking about the scald problem, otherwise you're merely "out of the frying pan, into the fire".
  • Legionella grows when water is at a certain midrange of temperature. Keeping water there as short a time as possible is key.
  • Fit blending valves on all outlets and crank it to 140F/60C; done.

  • fit a master blending valve at the water heater and crank it up.

  • Go with tankless heater(s); done. This assures water doesn't dwell anywhere long enough at the right temperatures to breed legionella. Except if you just slap a tankless where the tank was, that can raise some annoyances and doesn't allow the best benefits of tankless to work in your favor. For that, a little more system design is called for.
  • Thanks a lot Harper. Could it be a good compromise to turn the water heater up to 145 and place a mixing valve right on the outlet so that all points of use don't need to be retrofitted?
    – JPhi1618
    Mar 27, 2019 at 19:29
  • @JPhi1618 Yeah, that would work, as long as there's reasonable mobility in the water. Probably get a better quality unit, too. Mar 27, 2019 at 19:33
  • Good answer. But here, (Alberta, Canada) hot water heaters are equipped with a safety release that opens if pressure or temperature gets above certain limits. I have my circulating hot water set at 135, which also heats an indirect hot water heater. My water pressure is typically about 75. Twice, now, in the last 10 years some combination has tripped the safety release, and I ended up with half a water tank of water to clean up. In addition many conventinal hot water heaters have an upper limit well below 140 degrees. Mar 28, 2019 at 1:07
  • @SherwoodBotsford sounds like those heaters were built prior to understanding of the legionella issue. Canadian agencies are some of the loudest voices in the "watch out for legionella" campaign. Mar 28, 2019 at 1:26
  • This is quoted text but I don't know how to do that in the comments "In Ontario legislation was introduced to require thermostatic mixing valves on any new or replacement water heater installation. The requirement in Ontario (OntarioBuilding Code 7.6.5) went into effect in the fall of 2004" I remember when it came into effect and I was a little more foolish and alot more cowboy back then. I was against it. Now I install almost exclusively Navien tankless, several dozen a year. I only install maybe three tanks a year.
    – Joe Fala
    Mar 28, 2019 at 2:01

This is a long and complicated topic. I've noticed since being on this site that codes in the United States are very relaxed compared to the Canadian requirements. That being said, codes are a minimum requirement. Legionnaires affects the vulnerable. Like old people or sick people. Healthy people are not as susceptible to bacterial infections. I've seen your posts and answers. Have a look at Caleffi idronics journal number, well all of them! But in regards to the mixers, issue 21 and 23 I think. I subscribe to the magazine, it's free and if you are interested in mechanical you'll love the journal.

Mixers have been a requirement on storage tanks in my area for over a decade. Not everyone complies because people often want 130°F outlet and many mixer max out at 120. For a few dollars more you can get one with a higher range. It also allows you to store your water at a much higher temperature effectively increasing the capacity greatly.

Caleffi idronics enter image description here

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.