One of the methods used in the industry to alleviate the scald concern has been the installation of central thermostatic mixing valves. But these valves may not be providing the level of protection you thought you were getting. I have identified six common problems associated with the installation of thermostatic mixing valves.
The drawing 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.
1. Thermostatic Mixing ValveThe first problem you have often is 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 that statement 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 standard is 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 probably will cost more money.
2. Circulating PumpThere is a well-known scenario in the selection of pumps: The engineer specifies a 1/16-horsepower pump; the plumbing contractor thinks he's an idiot and increases it to a 1/8-horsepower pump; the pump company thinks that plumbing contractors normally undersize and increases the pump order to a 1/4-horsepower pump. The end result: 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.
The energy to flow water in a pipe is pressure. A pump increases the pressure downstream of the pump. When the pump is oversized on a recirculating line, it may prevent the cold water from entering the thermostatic mixing valve.
At valve location No. 5, 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 was sized correctly (very small).
3. Bypass Valve OpenValve 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 No. 2 should be a globe valve since this is the best valve to use for regulating flow. A ball valve or gate valve is terrible 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 ValveThe worst check valve to have missing is check valve 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 pipe size. 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 OffOne 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 ProtectionWhen 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 these valves do not protect against scalding. It is the point-of-use valve that provides this level of protection.
By the way, I am a big fan of installing central thermostatic mixing valves. I just want you to install them correctly so they can properly temper the hot water.