On May 3, I wrote a column about the dangers of potentially deadly legionella bacteria. Legionella bacteria can thrive in hot-water systems...

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On May 3, I wrote a column about the dangers of potentially deadly legionella bacteria. Legionella bacteria can thrive in hot-water systems. Hotter water kills the bacteria, and the hotter the better for keeping it out. As with most things, there are two sides to an issue and some actions may have unintended consequences.

I had a conversation recently with a reader from Lake Forest Park, Michal Ann McAllister, about another side of this issue:

McAllister told me about how her daughter’s scalding accident 25 years ago helped to pass legislation requiring all new water heaters to be initially set at 120 degrees Fahrenheit. She told me how it happened.

“It was between Christmas and New Year’s 1981, and she was 17 months old,” she said. “We were at her grandparent’s where, unbeknownst to me, the water temperature was set to 150 Fahrenheit, about the temperature of hot coffee. The tank was located just behind the wall of the bathroom. I had quickly rinsed her in the sink, and for just a few seconds I went into the next room to throw away a diaper. She pulled on the hot-water handle, I heard her scream, and ran in almost instantly. The scalding water had blasted the skin right off her butt. It was literally hanging off.”

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I can see how a terrible tragedy like that could occur. A third-degree burn occurs with 10 minutes’ exposure to 120 F water. Only slightly warmer at 130 F, it takes 30 seconds, and at 150 F, there’s not much time to get out of the way because third-degree burns occur within five seconds!

McAllister, who said she keeps up with the issue even today, said since the legislation passed, tap-water scalds have decreased in our state dramatically.

Of course, the other side of it is that the potential for legionella-bacteria infections increases as the temperature in these same tanks decreases.

Much research has been done on this issue and the latest information from many plumbing-industry and public-health groups is that 122 F is the best compromise, providing the best protection against scalding and legionella.

The thing to remember is that that number is the delivered temperature at the tap, not at the tank. There can be from 3 to 5 degrees of drop in the plumbing system, so the tank could be slightly higher than 122.

Anti-scald technology has come a long way of late. There are a plethora of products available with prices from $15 to $1,500.

The best protection from both threats is a tank that keeps water at 140 F, but tempers it down in the lines with a whole-house mixing valve to allow delivery temperature at 120-122 F.

These tempering valves, or thermostatic mixing valves, are commonly installed on the open hydronic heating systems I mentioned May 3. One brand commonly seen is Aqua-Mix, and others are available. Whole-house valves can be retrofit on other buildings when supplied with a cold feed line.

Mixer valves in showers now are “balanced” to eliminate the infamous flush-burn. When cold-water flow is reduced, hot is reduced a commensurate amount, preventing the burn. These valves are generally either on or off, which can be a drawback if you want a lower flow of water or want to turn it off to soap up and have it come back on at the same temperature.

Several manufacturers make homeowner-installed devices called temperature-activated flow restrictors that screw onto just about any fixture and will literally shut off the flow of water when it exceeds a given temperature. They are generally very affordable. Hot Stop is one brand commonly seen in hardware stores.

Thermostatic valves are the most expensive genre, but give the most flexibility, allowing water flow to be regulated and preset to a given temperature, sometimes even digitally. The highest-end fixtures have this kind of scald-prevention valve.

Most anti-scald devices reduce the available flow and pressure somewhat, and can be affected by grit, rust, Teflon and other debris in older plumbing systems.

Darrell Hay is a local home inspector and manages several rental properties. Send e-mail to dhay@seattletimes.com. Sorry, no personal replies.