ASSE 1069 - Automatic Temperature Control Mixing Valves.
The ASSE 1069 Automatic Temperature Control Mixing Valves are designed for gang shower applications such as those in a school, prison or health club with On-Off controls at each shower station. A single thermostatic mixing valve is exposed to a wide range of flow rates. This valve standard was developed to deal with a wide range of flows with tight temperature tolerances at low flows. If only one shower is flowing, the flow rate could be as low as two gallons per minute. In larger installations, it may be necessary to install a hi-low system that incorporates two or more thermostatic mixing valves. A smaller valve is used to compensate and maintain temperature during low flow conditions, and multiple valves work together as the flow rate increases.
ASSE 1070 - Water Temperature Limiting Devices.
In-line devices such as the ASSE 1070 thermostatic mixing valve limit the water temperature of the hot water supply to a shower valve, but they do not provide thermal shock protection, since additional unprotected cold water is mixed in downstream of the device. This device limits the water temperature to fixtures such as sinks, lavatories or bathtubs and reduces the risk of scalding. The device shall be either the final temperature regulation or have water further tempered downstream of the device with the addition of cold water.
Water heater thermostats
Water heater thermostats do not control temperature. I cringe every time I hear a radio talk show host or some one tell people to turn their water heater thermostats down to prevent scalding. Turning the thermostat down will not prevent scalding. I see the same mis-guided advice dispensed in newspaper handyman columns that talk about water heaters or home safety. Water heater thermostats cannot be relied upon to control the hot water temperature to a hot water system. Although water heater manufacturers recommend that installers set thermostats at 120 - 125 F, and although most of them ship the water heaters at lower temperature settings, it is impossible to accurately control the water heater temperature with a water heater thermostat. The plumbing engineering community continues to recommend storing hot water at 140 degrees Fahrenheit and mixing the hot water with an ASSE 1017 thermostatic valve to deliver hot water to the plumbing system at about 120 degrees Fahrenheit. This reduces the threat of Legionella bacteria growth in the tank, and allows a smaller water heater than one with a lower stored temperature.
Hot water systems are designed and water heaters are generally sized to store hot water at 140 F. When the thermostat is set at a lower temperature, the water heater has a reduced capacity to deliver hot water. As a result, when users run out of hot water, and the water heater thermostat is re-adjusted to a temperature above 120 F, this increases the temperature to a scalding temperature and in some cases it could be adjusted even higher than the original setting. The water heater will continue to store and deliver water at scalding hot temperatures. The solution is using a thermostatic mixing valve not tinkering with the water heater burner controls.
Water heater thermostats were never intended to provide precise temperature controls for hot water systems. For example: the thermostat dial calibration test of ANSI Z21.10.1-1998, which is the applicable standard for gas-fired water heaters, allows the temperature to vary 10 degrees above or below the thermostat setting. I have talked to water heater manufacturers that have indicated that the controls can vary as much as 15 to 18 degrees Fahrenheit above or below the set point of he thermostat. The thermostat is inserted into the lower portion of a water heater tank and turns the fuel supply to the heater on and off. Most newer water heater thermostat dials have no way to know what the temperature in the tank is. There are letter designations, numbers, or words like, warm, hot and very hot. There is generally never a fixed temperature indicated on the dial.
Theoretically, if the water heater thermostatic element is set at 120 degrees Fahrenheit, the burner would come on when the temperature at the thermostat reaches about 105 degrees Fahrenheit. The burner stays on until the water around the thermostat element near the bottom of the heater reaches about 135 degrees Fahrenheit (about 30 degrees higher than when the burner came on and 15 degrees above the theoretical set point of the thermostat).
Most people don't realize that the maximum temperature limit test of the ANSI Z21.10.1 Gas Water Heater Standard allows the outlet water temperature of the water heater to rise 30 F above the thermostat setting. This provision in the standard accounts for the phenomenon known as "stacking" or "thermal layering". The hot water is less dense and rises to the top of the hot water tank. The cooler water drops to the bottom of the tank. Stacking or layering occurs when hot water rises to the top of the heater due to recurring short duration heating cycles caused by a frequent number of small quantity hot water uses. This phenomenon can occur in any type of storage water heater and generally is more significant in vertical heaters.
At the top of a water heater that is theoretically set for 120 F to prevent scalding, the temperatures can easily reach 150 to 165 degrees Fahrenheit. These extremely high temperatures will cause third degrees burns and severe scald injuries in an instant upon contact with the skin. This is why I highly recommend installing the proper type of an ASSE 1017 thermostatic mixing valve on the outlet piping of a water heater to limit the hot water distribution temperatures to a maximum safe delivery temperature of 120 F. If high temperature hot water uses are required in a building, I recommend installing an ASSE 1070 thermostatic mixing valve on the local branch piping serving a fixture or group of fixtures. The mixing valve can then reduce the hot water temperature to a safe temperature.
I hope that these hot water system design recommendations will help keep you out of hot water.
By
Plumb-Tech Design & Consulting Services LLC
Plumbing, Mechanical and Fuel Gas Systems Forensic Expert Witness
ABOUT THE AUTHOR: Mr. Ron George - Article First Published on April 16, 2008
Mr. George is President of Ron George Design & Consulting Services. His company specializes in plumbing, piping, fire protection and HVAC system design. He also provides forensic investigations and litigation support for plumbing and mechanical system failures. He is Chairman of the International Residential Plumbing & Mechanical Code Committee and he is a member of the International Association of Plumbing & Mechanical Officials Techical Council which oversees the development of the Uniform Plumbing Code and the Uniform Mechanical Code. He is a member of numerous plumbing product standard committees with ASME, ASTM, ASPE, ASSE and NSF. He also provides plumbing and mechanical code product standard consulting services.
Copyright Plumb-Tech Design & Consulting Services LLC
Disclaimer: While every effort has been made to ensure the accuracy of this publication, it is not intended to provide legal advice as individual situations will differ and should be discussed with an expert and/or lawyer. For specific technical or legal advice on the information provided and related topics, please contact the author.