What is the minimum allowable temperature for return air mixed with outdoor air before entering the furnace?

The minimum allowable temperature for return air mixed with outdoor air before entering the furnace is indeed 55 degrees Fahrenheit. This temperature is significant because it helps ensure that the air entering the furnace is above the dew point, which minimizes the risk of condensation within the furnace and ductwork.

Condensation can lead to moisture-related issues such as mold growth, corrosion, and decreased efficiency of the heating system. By ensuring that the mixed air temperature is at least 55 degrees F, it provides a safety margin that helps maintain the integrity of the heating system and promotes more efficient operation.

In cold climates, allowing air that is too cold to mix with the return air can lead to inefficiencies and potential damage to the HVAC system. Therefore, maintaining a minimum air temperature is crucial for the longevity and effectiveness of the heating system.