An economizer increases the outside air quantity. What is the typical impact on indoor relative humidity?

• A. It increases indoor relative humidity.
• B. It can lead to lower indoor relative humidity.
• C. It has no effect on humidity.
• D. It reduces outdoor humidity.

Answer: (B)

Increasing outside air through an economizer dilutes the indoor air with outdoor air. Relative humidity depends on how much moisture is in the air relative to its capacity at a given temperature. Outdoor air is often drier than indoor air during cooling conditions, so bringing in more of it lowers the mixed air’s moisture content. The cooling/dehumidifying system then reduces humidity as needed, so indoor relative humidity tends to decrease. In typical climates, this means it can lead to lower indoor RH. If outdoor air were very humid, RH could rise instead, but that’s not the common case.