When I started in the plumbing profession, water heaters were set at 160° F. I can’t remember how many water heaters I installed working for my father’s plumbing and heating company during my youth, but they all were set at 160° when we left the jobsite.

The temperature of 160° was a standard hot water temperature used in the profession. The reason for this temperature was simple. The water heaters could easily heat the water to 160° and there was plenty of hot water available for the building occupants.

The first lowering of the temperature of water heaters occurred after the 1973 oil embargo. The engineering profession was finally faced with being more energy efficient. Engineers used to considered energy cheap and did not really worry about efficiency or energy savings.

The 1975/76 American Society of Plumbing Engineers’ Data Book had hot water temperatures being shown as 140°. However, it wasn’t until about 1978 that the lowering of the hot water temperature to 140° became standardized. The lowering of the water temperature was intended to save energy. All during this time, the majority of the plumbing codes defined hot water as being a temperature at or above 120°.

In the 1981 edition of the Building Officials Code Association’s Basic Plumbing Code, the definition of hot water was changed to water having a temperature greater than or equal to 110°. This lowering of the water temperature was praised by the energy conservation community as a wonderful way to save energy. However, it created an outcry in the plumbing engineering community as being too low a temperature. Concerns were raised regarding the possible growth of microorganisms in hot water.

In the early 1980s, the ASPE Research Foundation conducted research on the temperature of hot water to prevent the growth of Legionnella bacteria. This followed the outbreak of Legionnaires disease in Philadelphia in 1976. The ASPE Research Foundation report identified the temperatures that kill the bacteria causing Legionnaires disease and the time exposure required at that temperature. The recommendation in the report was that water heaters have the water temperature set to between 135° and 140°.

‘Unintended consequences’

By 1984, almost every plumbing code implemented water conservation measures lowering the flow rate through a showerhead to 3 gpm at 80 psi. Previously, showerheads had no flow restriction and typically discharged between 5 and 10 gpm.

One of the buzz terms used in the code arena is “unintended consequences.” That is what occurred with the lowering of the showerhead flow rates. There was a tremendous spike in shower scalding incidents. By the late 1980s, it was reported that there were an average of 88,000 scalding incidents per year that required a visit to the emergency room. Many of the visits to the emergency room involved slips and falls in the shower when someone tried to get out of the way of scalding hot water.

Tom Konen, P.E., conducted research at Stevens Institute of Technology on the impact of shower temperature using a low-flow showerhead. Similar studies were conducted in plumbing labs across the country. What the studies showed is that with a full-flowing showerhead when there is a change in water pressure, the shower water temperature only rises a few degrees — in the 5° to 7° range. However, when a showerhead is restricted to 3 gpm, the spike in temperature is 35° to 45°. Thus, the water is hitting the temperature that causes scalds within a few seconds.

During this same time period, there were a number of lawsuits filed against water heater manufacturers involving scald incidents. Some of the accusations included that the water heater manufacturers failed to warn the building occupants of the hazards of hot water. The result was a change to the labeling on water heaters and the warning being added to the water heater manuals. Water heater labels will either recommend a maximum setting of 125° or 120°. The majority list a warning that setting the temperature about 125° can result in severe scalding and even death.

Under pressure

The scalding incidents also resulted in a change to the plumbing codes. The 1990 BOCA National Plumbing Code was the first model plumbing code to mandate pressure balancing or thermostatic mixing valves (since changed to compensating shower valves) for all showers. The code added a further requirement that the maximum discharge temperature out of the showerhead must be 110°. The upper temperature limit was designed to prevent scalding if someone inadvertently hit the shower valve handle or if children were playing hot and cold with the faucet.

The upper limit of 110° was based on the scald chart where the time needed to be scalded at this temperature was measured in hours – not minutes or seconds. The temperature of 110° did not last long in the Plumbing Code. Another unintended consequence occurred with the upper limit on the shower valve because showers are often installed over bathtubs in a home. If the maximum temperature coming out of the valve is 110°, the bath may not have hot enough water. When a cast-iron bathtub is installed, the cast iron is a heat sink, immediately lowering the temperature of the bath water. Hence, the bath water will not be hot when the maximum temperature of water coming out of the spout is 110°. For any bathtub, if the water cools during the bath, you cannot adequately raise the temperature of the bath water by adding 110° water.

By the next edition of the Plumbing Code, the maximum temperature of hot water out of a shower valve was raised to 120°. Every Plumbing Code adjusted to this upper temperature. The time to scald is still in hours, however the temperature is above the pain threshold temperature. This means there is still the possibility of slips and falls should the water temperature rise to 120°.

Following the scald prevention requirements for shower valves, the plumbing codes started to add requirements for other fixtures to avoid scalding. For the other fixtures, thermostatic mixing valves became required. Upper limits on the temperature of the hot water ranged from 110° to 120°. The only lower temperature was set for emergency fixtures where the temperature ranges from 80° to 100°.

It has been an interesting history in plumbing codes regarding hot water temperature. It is hard to believe 35 years ago there were basically no regulations on the temperature of hot water.

We’ve come a long way in a short period of time.