If a system utilizes 30% outdoor air, with an outdoor temperature of 20°F and a return air temperature of 70°F, what is the mixed air temperature?

If a system utilizes 30% outdoor air, with an outdoor temperature of 20°F and a return air temperature of 70°F, what is the mixed air temperature?

• A. 49°F
• B. 55°F
• C. 60°F
• D. 65°F

Answer: (B)

To find the mixed air temperature when a system utilizes both outdoor air and return air, you can use a weighted average formula. The mixed air temperature is determined by the proportion of outdoor air to return air and their respective temperatures. In this scenario, you have 30% outdoor air at a temperature of 20°F, and 70% return air at a temperature of 70°F. The calculation goes as follows: 1. Calculate the contribution of outdoor air to the mixed temperature: 30% of 20°F = 0.3 × 20 = 6°F. 2. Calculate the contribution of return air to the mixed temperature: 70% of 70°F = 0.7 × 70 = 49°F. 3. Add these two contributions together to find the mixed air temperature: 6°F + 49°F = 55°F. Thus, the mixed air temperature calculated is 55°F, confirming that the choice labeled as 'B' is correct. This temperature reflects the blending of the warmer return air with the cooler outdoor air, which is common in HVAC applications to maintain comfort levels within a space.

To find the mixed air temperature when a system utilizes both outdoor air and return air, you can use a weighted average formula. The mixed air temperature is determined by the proportion of outdoor air to return air and their respective temperatures.

In this scenario, you have 30% outdoor air at a temperature of 20°F, and 70% return air at a temperature of 70°F. The calculation goes as follows:

1. Calculate the contribution of outdoor air to the mixed temperature:

30% of 20°F = 0.3 × 20 = 6°F.

2. Calculate the contribution of return air to the mixed temperature:

70% of 70°F = 0.7 × 70 = 49°F.

3. Add these two contributions together to find the mixed air temperature:

6°F + 49°F = 55°F.

Thus, the mixed air temperature calculated is 55°F, confirming that the choice labeled as 'B' is correct. This temperature reflects the blending of the warmer return air with the cooler outdoor air, which is common in HVAC applications to maintain comfort levels within a space.