The Answer Guide

How Hot is Just Right?

What water temperature reduces scalding risks?
What water temperature prevents Legionella?
What water temperature is best for my family?

Each year thousands of children and adults receive scalding injuries from household tap water and bath water. Yet, lowering home water temperatures to prevent scalding accidents increases the risk of Legionella, the bacteria responsible for Legionnaire’s Disease. In this Answer Guide, we offer a brief explanation of risks of water temperatures that are too low, too high, and how plumbing technology can protect your family.

A Few Facts

• 90% of scalding incidents occur in the home.
• At 106°F, water can cause pain. It causes third degree burns at

Source: Honeywell

• The typical skin thickness for an adult is 2.5 mm. The skin thickness for a child under age 5 is 0.56 mm.
• Each year, between 10,000 and 20,000 people in North America contract Legionnaire’s Disease.
• Legionella, the bacteria that causes Legionnaire’s Disease cannot survive water temperatures of 131°F or higher.

What Temperature Will Prevent Scalding?

Pain occurs at a water temperature of 106°F. Frankly, pain is one of the best ways to prevent scalding. It hurts. You move. You get away from the source of the pain. Yet, babies, infants, and many elderly may not be able to easily get away from hot water. In addition, their skin tends to be thinner than the average adult, so it takes less exposure to higher temperature water to cause burns. Children can develop a third degree burn in one third of the time required for adults. Because of the risk of scalding many municipal codes now require plumbers to set water heater temperatures at 120°F. Unfortunately, this increases the risk of Legionella.

What Temperature Prevents Legionella?

Legionnaire’s Disease got its name when 34 people died after contracting the pneumonia like lung disease at a 1976 American Legion convention in Philadelphia. Legionnaire’s Disease and Pontiac Fever, a milder form of the disease, are caused by Legionella, a naturally occurring bacteria often found in lakes, rivers, and streams. When Legionella finds its way into a home’s water system, it thrives. The bacteria performs especially well in warm water (i.e., 105°F to 115°F). Legionella can be easily prevented. Simply maintain a storage water temperature of 140°F or higher. Unfortunately, this increases the risk of scalding.

How Can I Protect My Family From Scalding And Legionella?

Lower water temperatures to reduce the risk of scalding and you increase the risk of Legionella. Fortunately, there’s a solution to this paradox. A thermostatic mixing valve mixes cold water with your home’s hot water supply to deliver your desired water temperature, regardless of the temperature in the storage tank. Thus, the temperature of the storage water heater can be set high enough to kill Legionella and before the water is delivered to your faucets and taps, the valve mixes cold water to lower the temperature reducing the risk of scalding. Mixing valves use a thermostat in the mixing chamber to sense outlet temperature and adjust the hot and cold water accordingly. Modestly priced, thermostatic mixing valves give you the best of both worlds, reducing the risk of scalding while allowing storage water temperatures to be set high enough to prevent the formation of Legionella.

© 2006 Service Roundtable