What is the purpose of a thermostatic mixing valve above a water heater?
Monday, June 25, 2018
We were inspecting a home owned by a doctor today and noticed a thermostatic mixing valve installed between the water heater and outgoing hot water pipe into the home. The valve blends in cold water with the hot water outflow, as necessary, to maintain a constant but reduced hot water temperature serving the plumbing fixtures in a home.
This was the first one we have seen installed in a residence, so a little research was necessary to figure out why the doctor (who wasn’t home at the time of our inspection) needed a mixing valve. As it turns out, it makes sense that a person dealing with infectious disease daily would want one.
Legionaire’s Disease is the reason. Legionella is the bacteria responsible for Legionaire’s Disease and, although it is well-known for the 1976 outbreak in a Philadelphia hotel during an American Legion convention that caused 34 deaths, Breathing in small droplets of water that contain Legionella is how it enters the body. Legionella is a fairly common water bacteria in lakes and streams. It still gets into the water supply occasionally, and can cause an acute infection of the lower respiratory tract, sometimes leading to death.
Although Legionella thrives in warm water temperatures of between 105º and 115º F, it is killed by water at 131º F and above. To avoid the possibility of Legionella infesting your water heater and making everyone in the home sick, you could raise the setting of the thermostat to 140º F. But the higher setting brings with it the new threat of scalding, which can occur in just a few seconds from the extremely hot water.
One solution is a thermostatic mixing valve that enables you to keep the temperature in the water heater at 140º F, safely above the Legionella-killing level, and send lower temperature water to the plumbing fixtures in your home. Here’s what the installation we saw today looks like.
The building code has required single-handle anti-scald water valves at showers for many years now, but a thermostatic valve at the water heater controls the hot water temperature delivered to all fixtures. It’s a whole-house solution that adds about $350 to the materials and labor cost of a water heater installed by a licensed plumber. But some thermostatic mixing valves, specifically ones that meet an older standard (American Society of Sanitary Engineering, ASSE-1017), are only rated for water temperature adjustment at the hot water source, and not acceptable as an anti-scald device at individual fixtures.
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Legionnaires diagram - CDC
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