Even though a water temperature of 110° F is ‘relatively-safe’, exposure can be painful; the human pain threshold is around 106-108° F.

As the chart reproduced below reveals, the severity of a burn is a function of the temperature of the water and the duration of the exposure and the condition of the skin. Children and older people, who typically have thinner skin, suffer more severe burns in a shorter time and at lower temperatures than adults. A child can suffer a third-degree burn in 124°F water in less than three minutes. Children and adults can be burned this badly in two seconds
or sooner in 149°F water.

It is worth noting that this chart indicates how quickly third-degree burns, the most serious burns, can occur. These burns involve extensive tissue damage and have the potential for serious disfigurement, deformities, loss of function and death. First and second degree burns occur even more quickly, and, when sizable areas of the body are involved, also require immediate and skillful medical treatment.

Estimated Times/Temperatures Causing a
Full Thickness (3°) Burn in Adults/Children

Water
Temperature

Adults
(skin thickness of 2.5 mm)

Children 0-5 Years
(skin thickness of 0.56 mm)

<160°F 1 second
<149°F 2 seconds 0.5 seconds
<140°F 5 seconds 1 second
135°F 10 seconds 4 seconds
133°F 16 seconds
<130°F 35 seconds 10 seconds
127°F 1 minute
125°F 2 minutes
124°F 3 minutes
120°F 10 minutes

Source: National Burn Victim Foundation

Facts and Recommendations

Domestic water scald injuries are the second most common cause of serious burn injuries in all age groups. Each year, about 112,000 people are treated in hospital emergency rooms with scald burns, and about 6 percent of them are hospitalized. About 80 percent of hot tap water burns are among young children, the elderly, and the physically impaired. Many of the scaldings are a result of water heaters being set at temperatures above 120 degrees F.

Unfortunately, scald-type injuries can occur for a variety of reasons. No one solution or set of equipment can safeguard all users. However, the following recommendations can help you reduce the possibility of such injuries in the hot water systems you design:

  • In institutional and commercial facilities, large volumes of hot water at varying temperatures may be required. Water is often heated to high temperatures necessary for sanitizing and other processes. Another reason for elevating water heater temperatures is for an attempt to prevent bacterial growth in the water system. These temperatures are too high for human contact and should be adequately isolated or controlled to prevent being allowed to enter the distribution piping supplying plumbing fixtures.
  • Specify readily accessible and easily readable thermometers at stratigic locations within the system to allow monitoring of temperatures. Minimally, provide thermometers at each heater discharge, master mixing valve outlets and at each circulating pump suction.
  • Consider providing high-temperature alarms on the discharge of water heaters and master thermostatic mixing valves when water is heated above 120 degrees F.
  • Consider providing solenoid valves on the discharge of water heaters and master thermostatic mixing valves that will shut-off hot water supply when set temperatures are exceeded.
  • Specify that all piping supplying master mixing valves are thoroughly flushed prior to being connected to the TMV.
  • Specify that installation, start-up and commissioning of all master mixing valves be certified and approved in writing by a trained manufacturer’s representative.
  • Where a bypass of temperature control devices is provided to facilitate repair work, specify that the valve handle to open the bypass be removed or otherwise secured in the closed position.
  • Consider installing thermostatic control devices at each hot water fixture outlet which will interrupt the flow of hot water when the temperature becomes hot enough to scald. This provides an additional safeguard for high water temperature, even after the originally installed system has been altered.

No system of temperature-regulating equipment, regardless how comprehensive or sophisticated, can function effectively if it is not properly operated or maintained. Educate your clients on the proper use and maintenance of the systems you design. The following are a few suggestions you may consider recommending:

  • Regular inspections of hot water systems should be in place to insure that they are operating correctly and is in good condition. The facility’s operations staff should be trained to understand system design and recognize signs of problems.
  • Train building staff to report any instance in which the water from plumbing fixture outlets is too hot for comfort or safety. Insure that an employee is assigned the duty of periodically checking and recording temperatures in each tub and shower when only the hot water tap is opened. Finding water hotter than 110°F flowing from a tap accessible to users should trigger a check by maintenance workers to adjust or repair the system.
  • Insure that only authorized staff are given access to open bypass valves at temperature control devices.
  • Insure that only authorized staff are allowed to adjust temperature control devices.