A three-way mixing valve is used to maintain return temperature by blending which streams?

• A. To prevent return temperatures from becoming too hot
• B. To control gas pressure
• C. To maintain return temperature by blending supply and return streams
• D. To vent exhaust gases

Answer: (C)

The key idea is that a three-way mixing valve controls the temperature of water returning to the boiler by blending hot supply water with cooler water returning from the heating circuit. By adjusting how much hot water is mixed with the returning water, the valve keeps the return temperature within a set range. This protects boiler components, helps stabilize system temperatures, and improves overall efficiency. It’s not about gas pressure or venting gases—the valve’s job is to mix the supply and return streams to achieve the desired return temperature.