Is It Safe to Turn Down Your Water Heater Temperature?

A hand adjusting the thermostat on a white water heater

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Almost every checklist of energy-saving tips includes the recommendation that you turn the temperature of your water heater down from 140°F (60°C) to 120°F(49°C). Yet up in Canada if you look for recommendations, they will tell you not to set your heater below 140F, as it can become a sort of petri dish for Legionnaires Disease. When I mentioned this in a comment on an earlier post, commenters went a bit crazy on me, so I thought it would merit a closer look.

The Danger of Legionnaires Disease

Legionnaires Disease, or Legeionellosis, is caused by Legionella pneumophila, an aquatic organism that thrives in natural and artificial environments, such as cooling towers and spa pools. It was identified after 34 veterans died after attending an American Legion Convention in the Bellevue-Stratford Hotel in Philadelphia in 1976. It is temperature sensitive:

  • The disinfection range is 70 to 80 °C (158 to 176 °F)
  • At 66 °C (151 °F) Legionellae die within 2 minutes
  • At 60 °C (140 °F) Legionellae die within 32 minutes
  • At 55 °C (131 °F) Legionellae die within 5 to 6 hours
  • Above 50 °C (122 °F) They can survive but do not multiply
  • The ideal growth range is 35 to 46 °C (95 to 115 °F)
  • At 20 to 50 °C (68 to 122 °F) Legionellae are still in a growth range
  • Below 20 °C (68 °F) Legionellae can survive but are dormant

Canadian Recommendations

Because of this, quite a few Canadian sources make the following recommendations.

Hydro Quebec

"To reduce the risk of burns from hot tap water, the temperature setting on the water heater can be turned down. But if the temperature is set too low, bacteria may begin to grow in the tank. Even at 60 °C – the setting on most electric water heaters – an estimated 25% of all water heaters are contaminated by legionella bacteria.

"Legionella bacteria tend to grow in the lower temperatures at the bottom of water heater; such bacteria can cause a form of pneumonia. The organism is generally transmitted when people inhale contaminated water droplets from whirlpool baths, showers or building air conditioning systems. In Québec, about 100 people a year are hospitalized for pneumonia caused by contaminated residential water heaters."

In light of the statistics, it is not advisable to lower the water heater temperature to, say, 49° C. This would not only reduce the hot water supply by some 20%, but it would also put your household at risk of contracting pneumonia. Keep in mind that Hydro Quebec is an electric utility.

The Canada Safety Council

"In 2000, the Walkerton disaster had sent a wake-up call about the safety of Canada’s drinking water. While standards for domestic hot water must consider scald prevention, they must also address the broad spectrum of public health and safety issues. To minimize bacteria contamination, water must be stored at 60 C or higher.

"For example, temperatures under 50 C may increase the risk of Legionnaires’ disease, a form of pneumonia, due to bacterial growth in the tank. That disease is caused by Legionella bacteria, which live in water. Temperature is a critical factor for Legionella to grow. The risk of colonization in hot water tanks is significant between 40 and 50 C.

"Legionella bacteria most often enter the lungs due to aspiration. (Aspiration means choking such that secretions in the mouth bypass the choking reflexes and enter the lung.) Drinking contaminated water is not a major cause of Legionnaire’s disease.

"The Centers for Disease Control and Prevention estimates 8,000 to 18,000 Americans contract the disease annually. Five to 30 percent of the cases are fatal. While Canada has no national statistics, Hydro-Québec says about 100 people a year are hospitalized in that province for pneumonia caused by contaminated residential water heaters."

Safe Kids Canada

"If you have an electric water heater, do not lower the temperature setting below 60° C. The bacteria that causes Legionnaires disease grows more easily in some electric water tanks because of the way they are designed. You can still lower your water temperature by installing safety valves. Talk to a qualified plumber, the company that made your heater, or the rental company for your water heater."

American Recommendations

When one looks for American sources, there are not very many that mention Legionnaires, and they are a little more qualified in their approach:

Q&A With the U.S. Department of Labor's Occupational Safety and Health Administration (OSHA)

Q. Can Legionnaires disease be prevented?

A. "Yes. Avoiding water conditions that allow the organism to grow to high levels is the best means of prevention. Specific preventive steps include maintaining domestic water heaters at 60°C (140°F). The temperature of the water should be 50°C (122°F) or higher at the faucet."

Q. Do you recommend that I operate my home water heater at 60°C (140°F)?

A. "Probably not if you have small children or infirm elderly persons who could be at serious risk of being scalded by the hot water. However, if you have people living with you who are at high risk of contracting the disease, then operating the water heater at a minimum temperature of 60°C (140°F) is probably a good idea. Consider installing a scald-prevention device."

The Energy Star Program of the EPA

This program makes no mention of Legionnaires disease. In regards to water temperature safety, it only says the following:

Set water temperature only as hot as needed (110-120 degrees) to prevent scalds and save energy (check local codes for specific temperatures).

Understanding the Disagreements Over Water Heater Temperatures

So why is there so much concern north of the border and so little to the south? Perhaps there is more concern about the dangers of scalding than there is about the dangers of Legionnaires disease. In Canada, they recommend mixing valves to eliminate the risk of scalding.

I also think that there is a tendency in the States to assume that "green" means primarily "saves energy," and that the Environmental Protection Agency is far more concerned about reducing energy consumption than it is about the health of its citizens. That is why they promote Energy Star houses that are sealed up tight but do not require ventilation systems and heat recovery ventilators, and perhaps why they ignore the issue of Legionnaires disease in water heaters even though another agency, OSHA, recommends otherwise. Although the EPA is part of the Energy Star program, they certainly don't seem very concerned about the environment inside the houses.

Or maybe Canada is over-cautious.

View Article Sources
  1. Legionella and the Prevention of Legionellosis.” World Health Organization.

  2. Hung, Ling-Ling, et al. (Editors). Field Guide for the Determination of Biological Contaminants in Environmental Samples (2nd Edition). AIHA.

  3. Understanding Potential Water Heater Scald Hazards.” ASSE International.

  4. Legionnaires’ Disease.” Centers for Disease Control and Prevention.

  5. OSHA Technical Manual (OTM).” U.S. Department of Labor Occupational Safety and Health Administration.