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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Here’s links to a collection of more blog posts about WATER HEATERS:

Are water heaters required to be raised off the floor? 

Can a Temperature and Pressure Relief (TPR or T&P) valve be mounted to discharge horizontally?

Where are gas water heaters not allowed to be installed?  

Is the minimum size water heater inlet pipe 1/2" or 3/4" according to the building code? 

Can you use a light switch for a water heater disconnect? 

How can I tell if a water heater is HUD-approved for mobile/manufactured homes?

Can you wire a 240-volt water heater with 120 volts? 

Is it alright to have a shut-off valve on both the hot and cold water pipes at a water heater?  

What is the minimum clearance to doors and windows for an outdoor tankless gas water heater? 

What is required clearance for access and working space in front of an electric water heater? 

Why is the water heater older than the house? 

Does a water heater need a shut-off valve? 

Why should a tankless water heater have an isolator/service valve kit installed?

When was a gas water heater first required to be elevated 18 inches above a garage floor?  

Can the temperature and pressure relief (TPR) valve discharge pipe for a mobile/manufactured home water heater terminate under the home?  

Does a tankless gas water heater still work with no electricity during a power outage?  

•  Why do water heaters have a sacrificial anode? 

What is the difference between a manufactured/mobile home water heater and a regular water heater? 

Does a tankless water heater require a pressure relief valve?

When is a water heater drain pan required?

Why is there water in my water heater drain pan?

What does it mean when a water heater Temperature/Pressure Relief (TPR) discharge pipe is "trapped"? 

Can I leave a gas water heater in place when remodeling a garage into a family room or bedroom? 

Where do I find the water heater in a mobile home?

Does a tankless water heater in an attic require a drain pan? 

Does an electric water heater require a disconnect? 

Is a catch pan and drain piping required for a replacement water heater?

What is the difference between a single element and dual element electric water heater? 

What is an FVIR water heater?

What is a heat pump water heater?

What is a dielectric union?

What's that powdery crust on the pipe connections at the water heater? 

What are the most common installation mistakes with water heater replacement?

Why is my water heater making strange (rumbling, gurgling, knocking or banging) noises? 

What can I do to make my water heater last longer?

How can I determine the age of a water heater if the serial number is missing or decoding it is impossible? 

How does a hydronic heating system work?

What is the difference between a regular water heater and a direct vent water heater? 

What is the difference between a regular water heater and a power vent water heater?

What is backdrafting at a gas water heater? 

How do I determine if a water heater is gas or electric?

What does it mean when a gas appliance (water heater, furnace, or range) has been "red tagged"?  

• What's the valve with the flip-up handle on the water heater for?

• Why is an older water heater an insurance problem?

   Visit our WATER HEATERS page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles. 

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