In economizer systems, energy savings depend on which relationship between indoor and outdoor conditions?

• A. Energy savings depend on the temperature and humidity relationship between indoor and outdoor air.
• B. Energy savings depend solely on outdoor temperature.
• C. Energy savings are independent of indoor conditions.
• D. Energy savings depend on air velocity in the duct.

Answer: (A)

Economizer savings come from using outdoor air to satisfy part of the cooling load only when the outdoor air is thermodynamically favorable compared with the indoor air. The key factor is the relationship between outdoor and indoor air states, which means both temperature and humidity matter. If outside air is cooler and drier than the indoor air, its lower enthalpy reduces the energy required to cool and dehumidify the space, yielding savings. If outside air is cooler but humid, the latent load from the humidity can erode or negate those savings. If outside air is warmer or more humid than indoor air, the economizer damper won’t help and may increase energy use. So, energy savings depend on how outdoor air’s temperature and humidity compare to indoor conditions, not on outdoor temperature alone, not on indoor conditions being irrelevant, and not on duct air velocity.