Scalding remains a major issue in the plumbing industry. We have definitely come a long way in preventing scalds in showers, bathtubs, whirlpool tubs, lavatories, sinks, and other fixtures. But we still have work to do and must remain vigilant. There are still too many scald cases that occur on an annual basis. But, plumbing contractors can and must address scalding.
One of the methods used in the industry to alleviate the scald concern has been the installation of central thermostatic mixing valves. These valves may not be providing the level of protection that you thought you were getting. The system must be properly designed and installed in order for central thermostatic mixing valve to provide scald protection. I have identified 6 common problems associated with the installation of thermostatic mixing valves.
The drawing that accompanies this article shows a generic installation design for a thermostatic mixing valve. You will notice that each valve, other than the thermostatic mixing valve, is numbered to help identify where the problem occurs. This system has a hot water recirculation system. The reason for including this in the design is that the recirculation system often is the contributor to a problem. The design that is shown is for a thermostatic mixing valve that does not have a separate port connection for a recirculating hot water line.
1. Thermostatic Mixing Valve: The first problem you have is often the valve itself. Most thermostatic mixing valves are designed to meet the requirements of ASSE 1017. Quite often you will find this designation on the valve or valve label. The label may also state, “This valve is not an anti-scald device.”
Contractors mistakenly believe that the reason the manufacturers place a statement, “Not an anti-scald device,” is because the lawyers told them to put it there for liability protection. Nothing could be further from the truth. The reason it is on the valve is because an ASSE 1017 valve is not an anti-scald device. While it is a thermostatic mixing valve, the valve does not have any failsafe operation that allows it to function as an anti-scald device.
As a result, you can only consider this type of thermostatic mixing valve as a valve for blending water temperatures under ideal conditions. If the right conditions do not exist, the valve will allow the hot water to rise to scalding temperatures.
There is a new standard out for thermostatic mixing valves that can be used for anti-scald purposes. The new valves conform to ASSE 1070. Hence, if you want a central thermostatic mixing valve that has an anti-scald feature, you have to install these newer valves. Yes, they will cost more money, but they will also provide greater safety. In the event that the hot or cold water cannot temper the water, the valve will close off the flow of water. This provides a failsafe feature to prevent scalding. ASSE 1017 does not require valves to have this failsafe feature.
2. Circulating Pump: There is a saying in the selection of pumps--the engineer specifies a 1/16 hp pump, the plumbing contractor thinks he’s an idiot and increases it to a 1/8 hp pump, the pump company thinks that plumbing contractors normally undersize and increases the pump order to specify 1/4 hp pump. The end result is a grossly oversized recirculation pump.
While that is a nice joke, all too often, this really occurs. The vast majority of recirculation pumps are oversized. When they are oversized in an installation of a thermostatic mixing valve, the result can be a significant increase in temperature on the tempered water line.
Keep in mind that a properly sized recirculation pump will not increase the pressure in the piping system downstream of the pump. It is used merely to move the water. The energy to flow water in a pipe is pressure. Of course, all pumps increase the pressure downstream of the pump. The key is to minimize that pressure increase with a recirculation pump. When the pump is oversized on a recirculating line, the increase pressure downstream of the pump may prevent the cold water from entering the thermostatic mixing valve.
At valve location No. 5 on the illustration, if the pressure on the recirculating line is greater than the pressure on the cold-water side, only recirculating water will enter the thermostatic mixing valve. When a fixture on the tempered water line is opened, the cold water may only enter the system through the water heater as hot water. The cold water supply to the thermostatic mixing valve will only be recirculated water. This will cause tempered water downstream of the thermostatic mixing valve to continue to rise in temperature until it is almost the same temperature of hot water.
The only way to correct this problem would be to open more fixtures on the tempered water line, causing a lower pressure to allow the cold water to reach the thermostatic mixing valve. Of course, the problem would never occur if the pump were sized correctly (very small).
3. By-pass Valve Open: Valve No. 2 in the diagram is a bypass valve. The only reason this valve is present is to adjust the thermostatic mixing valve under a no-flow condition. Under a no-flow condition, water is not being used on the tempered water line. However, the circulating pump continues to operate. If hot water is not added to the thermostatic mixing valve, the water temperature will fall.
The bypass valve is supposed to be adjusted during a no-flow condition to maintain the temperature in the tempered water line. I think that valve at location No. 2 should be a globe valve since this is the best valve to use for regulating flow. Ball- or gate valves are terrible valves to use for regulating flow. When nothing is running, you should gradually open valve No. 2 to find the point where the temperature remains constant. I have witnessed many installations where the bypass valve was wide open. When this occurs, the temperature will continue to rise downstream of the thermostatic mixing valve. That is because too much water is being bypassed through the water heaters.
4. Missing Check Valve: The worst check valve to have missing is check valve at location No. 7. This would be the same as leaving bypass valve No. 2 in the wide-open position. Actually, it is worse. The bypass line is normally a smaller-diameter pipe. The cold water supply is a full-size. Thus, under a no-flow condition, an uncontrolled amount of water will flow through the water heater, resulting in a rise in temperature.
If check valve No. 6 is missing, this normally doesn’t create any problems. That is not to say that it should be removed. If the pump is on, check valve No. 6 serves no purpose. It is only when the pump is off that cold water can run back through the recirculating line. This doesn’t result in any scalding, but what happens is that someone turns up the temperature on the thermostatic mixing valve because the water is too cold. Hence, the increased temperature results in a scald.
5. Valve Is Off: One of the dangers with this type of installation is that either valve No. 4 or valve No. 5 may be left in the off position. If you turn either valve off, the system will still operate. Of course, it should be obvious to you that it will not operate correctly. With either valve off, the temperature of the tempered water will rise.
6. Forgot Point-of-Use Protection: When you install a central thermostatic mixing valve, you still need a valve at the point of use to protect against scalding. For example, on a shower or bathtub, you still would have to install either a pressure balancing valve or a thermostatic mixing valve. This goes back to the first problem in that many of these valves (specifically ASSE 1017 valves) do not protect against scalding. It is the point of use valve that provides this level of protection.
Finally, not listed as an item but soon to be a major concern, is conformance to the State of California’s AB 1953. Not all central thermostatic mixing valves meet the no-lead requirements of AB 1953. You need to check the listing to determine compliance. Valve may also be marked with the designation NSF 61, Annex G, or NSF 61-G. Either one of these designations indicates that the valve conforms to AB 1953.
I am a big fan of installing central thermostatic mixing valves. These valves provide an additional level of protection against scalding. I just want you to install them correctly so they can properly temper the hot water.
