Thermostatic Mixing Valve Basics

What is a thermostatic mixing valve?

A thermostatic mixing valve is a unit that mixes hot and cold water. Using a thermostat, the mixing valve adjusts the hot and cold water supplies accordingly to maintain a constant outlet water temperature.

Is tepid water required with emergency equipment? If yes, why?

Yes. Tepid water, defined as water between 60ºF and 100ºF, is required for 15 minutes at 20 GPM (30 PSI) in drench showers, at 0.4 GPM (30 PSI) in eyewash stations, and at 3.0 GPM (30 PSI) for eye/face wash stations. Using water outside these temperatures poses potential health hazards to emergency equipment users. For example, using colder water than 60ºF can cause shock or hypothermia. Additionally, it discourages using the emergency equipment for the full 15 minutes required. On the other hand, using water warmer than 100ºF can actually increase chemical irritation, causing further damage to the eyes or skin. Thus, thermostatic mixing valves are highly recommended to guarantee a stable supply of tepid water.

Is there a difference between emergency thermostatic mixing valves and standard thermostatic mixing valves?

Yes. Emergency thermostatic mixing valves include an integral cold water bypass. Consequently, if the hot water supply fails, cold water will still be supplied. Emergency thermostatic mixing valves also include a positive means of hot water shut-off in the case of cold water supply failure.

How do I properly size thermostatic mixing valves? (NOTE: This is extremely important for facilities with multiple emergency equipment units.)

Determine the manufacturer’s minimum incoming flow rate required to maintain the appropriate temperature for each fixture. Then, determine the incoming pressure in your building. Line pressure can affect the size of the valve required. It is recommended to contact the valve manufacturer for specific pressure drop information. Finally, calculate the product of each fixture multiplied by its actual flow rate. Total each product to determine the minimum total output for a thermostatic mixing valve.

Example:

• 3 eyewash stations * 3 GPM = 9 GPM
• 4 eye/face wash stations * 3.5 GPM = 14 GPM
• 9 GPM + 14 GPM = 23 GPM minimum total output required to supply tepid water to all fixtures if used simultaneously

What incoming water supply pipe size should I use?

The minimum flow rate requirements are: 1/2” incoming supply pipe for eye/face wash stations, 1” incoming supply pipe for drench showers, and 1-1/4” incoming supply pipe for combination drench shower & eyewash stations.

Where should emergency thermostatic mixing valves be installed?

Emergency thermostatic mixing valves should be installed as close to the emergency equipment as possible. It is recommended to avoid long piping runs when time is critical.