Is storing hot water over 140F a regional requirement?

Question

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.

Answer 1

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.

Answer 2

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