Getting Into the Mix
A thermostatic mixing valve tempers hot water down to a lower temperature by:
a. Passing through a chilled water heat exchanger
b. Isentropic expansion
c. Adding cold water
If you guessed “c” you’re correct! It probably wouldn’t surprise you to hear that most plumbing designers would answer this correctly. Why is it then that so much trouble stems from these little devices? Like any other engineered system, the proper design and specification of a mixing valve requires an understanding of how it works. What must follow is a careful analysis of how it will work under varying conditions.
Let’s take the example of a small apartment building with 30 units. Since most model codes require thermostatic mixing valves on hot water systems serving bathtubs, a central mixing valve at the source might be a good choice. Water can be stored at 140 degrees Fahrenheit and blended down to 120 degrees for delivery to the fixtures. Using the lever rule and assuming 60 degree cold water, 25% cold water would be blended with 75% hot water:
(0.25 x 60ºF) + (0.75 x 140ºF) = 120ºF
When everybody is showering, the operation is quite simple, but what about at night when the circulating pump is trying to move water through the valve. Remember, the volume of the system is fixed. If there is no water being drawn out and you must add 1 gallon of cold water for every 3 gallons of hot water that passes through the valve, where does the water go? An improperly detailed hot water return is a common error in the design of hot water systems. Most manufacturer’s suggest a solution similar to figure A.
In this illustration, a large percentage of the returning tempered water would enter the “cold” side of the mixing valve, while a small percentage of the returning tempered water would travel through the water heater and enter the “hot” side of the mixing valve. The ball valve is used to balance this flow. Alternatively, some manufacturers direct the returning tempered water to a return water connection on the thermostatic mixing valve so it is important to follow their installation instructions carefully. Note that only circulated water that passes through the mixing valve would be configured as such. If a separate 140 degree loop was circulated back to the heater, it could join the cold water feed to the heater in the traditional method. It is important not to mix returning hot water of differing temperatures as this creates temperature spikes in the return and may lead to temperature control issues in the supply.
To understand the next issue, one must understand how the valve adjusts to varying inlet temperatures to produce a constant outlet temperature. When hotter water enters the valve, a thermostat within the valve expands, throttling down the hot water inlet while simultaneously opening the cold water inlet. When colder water enters the valve, the thermostat contracts creating the opposite effect.
Another common error in the design of hot water systems is in the sizing of the mixing valve. In most cases, the valve is not the same size as the pipe; it is smaller. Hot water pipes are generously sized to meet the velocity requirements of copper pipe. Unfortunately, an oversized thermostatic mixing valve will not react quick enough to regulate temperatures under low flow conditions. Conversely, an undersized thermostatic mixing valve will create a significant pressure drop in your system. Therefore, the valve must be sized to meet both the highest and lowest anticipated flow rates. The manufacturer’s data will provide the correct solution. Occasionally, two valves are needed as shown in figure B.
In this diagram, a large thermostatic mixing valve is used to handle the high flows while a small thermostatic valve is used to handle the low flows. Note that the high flows must pass through a pressure reducing valve before joining the tempered water. This pressure reducing valve is used to ensure that the low flows pass unrestricted through the small valve until the pressure drop is greater than the pressure drop created by the pressure reducing valve. At that time, water will flow through both valves. The maximum flow through the system is then equal to the sum of the flow through the two valves. Most manufacturers provide a packaged system that includes valve necessary for adjusting and balancing the system.
If a circulated loop is required, the high/low system in Figure B can be substituted for the thermostatic mixing valve in Figure A. At this point, however, the piping may become confusing to the plumbing contractor. To remedy that problem, manufacturers also provide pre-assembled, tested systems with pumps. One such example is the picture to the left.
Thermostatic mixing valves that control the maximum temperature to a fixture or group of fixtures must meet the standards outlined in ASSE 1070. It should be understood that when either the hot or the cold water supply is cut off, the valve defaults to a closed position and the flow of water is stopped. For this reason, a thermostatic mixing valve designed for domestic water use should never be used for emergency eyewash and shower systems. Those fixtures must be specified with a thermostatic mixing valve certified to ASSE 1071 standards. These valves will stop the flow of water when the cold supply is lost, but continue the flow of water when the hot supply is lost.
Most model codes also require thermostatic mixing valves at public lavatories. If these are the only fixtures requiring protection, small point-of-use valves are an easy, inexpensive option. Many of these can handle up to 5 gallons per minute so designers are tempted to use only one to sever a battery of lavatories. Think twice before doing this. A quick check of the volume in the uncirculated fixture supply often reveals unsatisfactory delivery times.
In troubleshooting an installation for a hotel in Orange County, the contractor checked everything mentioned above. Nevertheless, he was unable to get a water temperature out of the valve hotter than 108 degrees Fahrenheit. The drawings and calculations were double checked. Many experts were called in to no avail. Ultimately, the valve manufacturer’s engineer was contacted and he simply pointed out that under the decorative cover, there was a tiny screw to adjust the hot water limit stop; a safety measure designed to adjust the mechanical range of the dial. Oops.
Peter A. Kraut, P.E., is a licensed Mechanical Engineer in 23 states. He founded South Coast Engineering Group, near Los Angeles, California in 2001. He can be reached via email at firstname.lastname@example.org.