Gas Fitter Practice Test

What happens to the system in cooling mode if the prefilter in an electronic air cleaner gets plugged with dirt?

1. The fan motor cycles on overload

2. The cooling coil frosts

3. The furnace shuts off on high limit

4. The air cleaner freezes up

The correct answer is: The cooling coil frosts

When the prefilter in an electronic air cleaner becomes clogged with dirt, it restricts airflow through the system. In cooling mode, the air conditioning unit relies on sufficient airflow across the evaporator (cooling) coil to function properly. If the airflow is diminished due to a plugged prefilter, the cooling coil can become excessively cold. This drop in temperature can lead to moisture in the air freezing on the coil, leading to frost or ice buildup. As the frost accumulates, it further impedes airflow, creating a vicious cycle that exacerbates the problem. Over time, the ice can create significant issues for the air conditioning system, including possible compressor damage. The other options do not accurately describe the consequences of a clogged prefilter in this scenario. For example, while cycling on overload could happen in other contexts, it's not a direct result of a dirty prefilter in cooling mode. High limit shut-off of the furnace pertains to heating mode and would not occur due to issues in the cooling mode. Lastly, “freezing up” in the context of the air cleaner itself doesn’t occur, as that terminology doesn’t apply to the operation described. Thus, the correct understanding focuses specifically on the cooling coil frosting due to restricted airflow from the clogged