Code Classroom Continued from page 22
water inlet is located. This is done so the controller can anticipate the need for hot water, sense the incoming cold water and turn the burner on before the heater runs out of hot water. The thermostat turns “on” and “off” the fuel supply to the water heater. There is no way that the control device in the bottom of a heater can sense the temperature leaving the top of the tank. Testing has shown that, when a water heater thermostatic element is set at 120 F, the burner stays on until the water around the thermostat element near the bottom of the heater reaches about 135 F. (See Figures 1 2) The maximum temperature limit test of ANSI
Z21.10.1 gas water heater standard allows the outlet water temperature of the water heater to rise 30 degrees F above the thermostat setting. This provision accounts for the phenomenon known as “stacking” or “layering” of hot water in the top of a water heater. Hot water rises to the top of the water heater, due to recurring short dura- tion heating cycles caused by a frequent number of small quantity hot water uses. Although the above example addresses gas water
heaters, this phenomenon can also occur in other types of storage water heaters. So, at the top of a water heater that is theoretically set for 120 F, the temperatures can easily reach 165 F. The ASSE scald awareness task group is in the process of developing a white paper to put this infor-
mation into laymen’s terms so that the entire plumbing industry and the general public can understand the issue. This scald hazard issue was recently moved up in pri-
ority on the list of topics to be addressed when the State of North Carolina allowed an exception to the code that does not require an ASSE 1016 type pressure balancing or thermostatic type shower valve for showers if the water heater thermostat is set at 120 F. Numerous com- ments from code consultants, industry experts and man- ufacturers were sent to North Carolina during the review period. Ultimately, the committee chose to allow the amendment, which would allow a builder to save a few bucks on every home, in lieu of choosing safety for their constituents. I was told that there was a significant rep- resentation of home builders on the committee that made the decision. n
Ron George is president of Plumb-Tech Design and
Consulting Services LLC. He has served as chairman of the International Residential Plumbing & Mechanical Code Committee. Visit
www.Plumb-TechLLC.com, email
Ron@Plumb-TechLLC.com or phone 734/755-1908.
The views and opinions expressed in this column are those of the author and do not reflect those of Plumbing Engineer nor its publisher, TMB Publishing.
Circle 17 on Reader Reply Form on page 49 Page 24/Plumbing Engineer June 2011
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