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Water Heater Optimum Set Temperature

Posted by ronaka (My Page) on
Sat, Feb 2, 13 at 11:51

I recently installed a hot water heater (gas 50 gallon), and did a bit of research to see what the temperature setpoint should be. There have been trends to lower temperatures to save energy, and prevent scalding. Some dishwashers heat the water so they can be less demanding than older models.

I was surprised in what I found. A significant issue is Legionnaires' Disease. Legionella can proliferate in hot water heaters if the temperature is not high enough. See the study at the link below. Short story is that they found contamination in 40% of electric hot water heaters, and none in gas water heaters. So this is a particular concern in electric heaters. They recommend a set point of 60 deg C (140 deg. F), and not the 49 deg C. (120 Deg. F), that is often recommended.

Here is a link that might be useful: Legionella in Hot Water Heaters


Follow-Up Postings:

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RE: Water Heater Optimum Set Temperature

The identified at-risk groups are also at risk to many other things, yet the incidence rate remains pretty low.

The authors concluded that the most significant risk factor came from use of electric heaters. Maybe that can be fixed if the heating elements inside were redesigned? Or, as contributor saltidawg has mentioned his satisfaction with using a heat pump water heater, I wonder if the design of those new products eliminates the suspected problem, a too cold region at the bottom of the tank?

Good information, ronaka. Do note that few people have hot water heaters in their homes, since hot water doesn't need to be heated. Many have cold water heaters (aka water heaters). ;-)


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RE: Water Heater Optimum Set Temperature

I suspect the issue with electric hot water heaters is the on-off deadband is a lot less on an electric. In other words it keeps the water temperature very close to the same temperature through the whole cycle, while the gas overheats, cools down, and then overheats again. Those overheat parts of the cycle kills the bugs.


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RE: Water Heater Optimum Set Temperature

The second to last paragraph says " However, studies in Quebec have shown, even when the thermostat is set at 60 deg C, a high percentage (approximately 40%) of electric water heaters remain contaminated because of the lower temperature, about 30 deg C to 40 deg C at the bottom of the tank."

The cross section diagrams I found from a google search suggest that the lower element in a two element tank is about 25% of the way up from the bottom. There's no way to know if those diagrams are reasonably faithful to scale, but it's easy enough to see why there'd be a cooler water zone at the bottom. And why gas heaters, with the burner at the bottom, wouldn't have such temperature layers.


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RE: Water Heater Optimum Set Temperature

Yes, it would seem best to get an electric heater with the bottom element as low as possible, or just avoid electric models, and get a gas fired one.


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RE: Water Heater Optimum Set Temperature

Some have no choice, many houses are in rural or remote areas where natural gas isn't available. Propane can fill the void but it's nowhere near being a like for like substitution.

A nine year old Canadian medical/public health article wouldn't draw much public attention, either when it came out or now. It's a low profile matter. I'd heard about this but I suspect most haven't. In my area, electric water heaters are very rare because of widespread natural gas availability.


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RE: Water Heater Optimum Set Temperature

I find it interesting that the issue does not get more attention. It is kind of like the flu. Thousands die from it each winter, but unless the WHO decides it is an epidemic variety, and the media fans the flames, nobody really notices.

Legionnaires' made the news in Canada last year (and in fact in Quebec the source of that study) with 13 reported deaths and 181 people affected in an 8 week span. The source was said to be a rooftop cooling A/C cooling tower. An investigation made a number of recommendations but there was no mention of electric water heater contamination issue.

I suspect it comes down to blaming big companies with deep pockets, instead of blaming homeowners for doing it to themselves. It is said mist from these cooling towers can drift up to 4 kilometers, and breathing in the mist is the main way one becomes infected. If that is the case, then I would think taking a hot shower using water from an infected heater would be many more times risky than living down wind from a cooling tower.

In any case I take no chances. I use a gas fired unit and set the temperature at 135 deg F. I've measured it and certainly at times it gets up to 140 F. Probably gets much hotter than that at the bottom of the tank when it is firing. No worries about anything growing in the tank.

Here is a link that might be useful: No Blame Laid in Legionnaires' Deaths


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