Hot Water Temperature Control: Thermostatic Mixing Valve

The Importance of Thermostatic Mixing Valves to Protect Against Scalding: Part One
Anyone that has been scalded knows how painful it can be. When water is hot, it doesn’t take long for flesh to burn, and children’s skin burns more quickly. Also, at higher risk are older people who react more slowly, and are more susceptible to burns as a result. As inspectors, our home inspector training teaches us about important components like this to provide advice that will protect our clients’ well-being. Let’s take a look at how quickly a serious burn can occur:

As a result of the speed at which people can get burned, legislation was introduced to require thermostatic mixing valves on any new or replacement water heater installation. The requirement in Ontario (Ontario Building Code 7.6.5) went into effect in the fall of 2004.
Why Now?
Every year there are hundreds of people burned by domestic hot water.
A hot water tap delivering 140°F water can cause a burn quickly. Single lever faucets mix hot and cold water, but can still deliver straight hot water.
There’s an additional problem with single-lever shower valves. If a person sets the mix of hot and cold water to produce a comfortable temperature, they may get a blast of pure hot water if the cold water pressure drops suddenly. This often happens when someone flushes a toilet or turns on another tap, for example.
Before thermostatic mixing valves, protection at tubs and showers was provided via pressure balanced or thermostatic control valves.
Pressure balanced valves are the more common of the two and are usually a single lever in a tub or shower. As the handle is rotated, the ratio of hot to cold water changes until the desired temperature is reached. When that ratio is impacted (e.g. by a toilet being flushed), the valve adjusts (restricts the hot water coming out of the faucet) to maintain the initial temperature.

The problem with these valves is that they don’t sense the temperature; only the ratio of hot to cold. Therefore, if you turn the valve all the way to hot, water comes out at the same temperature as a water heater that was set too high.
Thermostatic valves, on the other hand, have two controls: one for the volume of water and one for temperature. These are safer because the manufacturer or installer can set the limit within the valve itself to prevent an unsafe setting. They are also more convenient because the temperature can be left in your favorite position and all you have to do is turn the water on the next time you jump in the shower.
The downside to these units is they are more expensive and can only serve/protect one fixture (tub, shower, sink).

It is important to understand the benefits of using a thermostatic mixing valve, and why turning the water temperature down is not the solution.

Thermostatic Mixing Valves
How They Work:
Safe storage and distribution of domestic hot water is imperative in any application. We provide a complete line of thermostatic mixing valves to ensure safe, tempered water is delivered from source to use.
The Benefits
• Assures constant outlet water temperature even with variations in hot or cold water supply temperature.**
• Allows the water heater to be operated at higher temperatures, extending the effective system flow rate and preventing the growth of Legionella.
• Can reduce the size/BTU requirements needed to provide capacity for peak system demands.
• Assures constant safe hot water temperature at the point of delivery to the fixture.
Design Considerations
Heat Traps
• All mixing valves used at the source of supply should be trapped. Trapping the valve helps reduce mineral deposits and premature component wear.
• Heat traps help reduce convection loss also known as thermosiphoning, increasing system efficiency.
Stop Valves
• All mixing valves should be installed with a means to isolate the valve for service and repair.
Check Valves
• Check valves assure flow of water in one direction. They prevent thermal siphoning of hot water to the cold water supply.
• The addition of check valves should be considered whenever the potential exists for extreme differential pressures between the hot and cold water supply.
• Check valves are a requirement for compliance with ASSE Standard 1017 applications.
Strainers
• Water supply systems should be thoroughly flushed prior to placing mixing valves in service.
• Installation of additional strainers in the supply piping is recommended in systems where water can contain particles that could impede the proper operation of the mixing valve.
What You Need to Know

At the Fixture
Delivery of water to fixtures intended for use in bathing or washing should always be controlled by valves listed to ASSE Standard 1016 Type T or ASSE Standard 1070. ASSE 1016 listed valves provide the user with both scald protection and thermal shock protection. ASSE 1070 listed valves provide the user with scald protection only. These valves should never be set to exceed a maximum temperature of 49C (120F). (Watts recommends the maximum temperature of 43C (110F) for shower and bathing fixtures.)

Supply Systems
The control of water temperature from the source of supply is optimally achieved by ASSE 1017 listed mixing valves. These valves can also be used for re-circulation systems to maintain domestic hot water supply temperatures at levels (temperatures above 55C (131F) that prevent the growth of harmful bacteria such as Legionella.

Radiant Heat Systems
It is important that a boiler's high temperature limits do not exceed the maximum temperature rating of the system components. This will protect against component temperature failure should dirt, sediment, or other mechanical failure cause a mixing valve to become inoperable.

Periodic Inspection
Regular inspection of mixing valves is recommended to assure maximum valve life and a properly functioning system. Corrosive water conditions, unauthorized adjustments or repairs can be detected during inspection and service of the valve. The frequency of cleaning depends upon local water conditions.


** Please refer to specification for the selected model to determine the allowable inlet temperature range to maintain a desired outlet temperature.