If the warm air supply temperature exceeds 38°C (100°F) compared to the return air temperature, what could this indicate?

When the warm air supply temperature exceeds 38°C (100°F) compared to the return air temperature, it often indicates that there may be an issue with airflow, particularly relating to the air filters. If the air filters are plugged or dirty, they can restrict airflow through the system. This restriction can cause the furnace to work harder to push air through the ducts, leading to higher temperatures in the supply air as it struggles to maintain adequate airflow and deliver heating effectively.

In a well-maintained heating system, a smaller temperature difference between the supply and return air is typically expected, which signifies efficient heat exchange. When the temperature exceeds the specified threshold, it suggests that the system is not operating optimally, and checking the air filters is a logical first step in troubleshooting.

High supply air temperatures can also lead to discomfort and inefficient energy use, making it essential to ensure that air filters are clean to promote proper airflow and system performance.