Most building rooftop HVAC units have an economizer. This is a series of dampers, controls and filters designed to bring in outside air, rather than running the compressor. This is free cooling.
When outdoor conditions are right, both temperature and humidity, the economizer opens and draws in fresh, cool air. Typically, if the outside air is 55 degrees or lower the economizer will cool your interior spaces with unconditioned outdoor air first. In Michigan where we’re located, this can be nine months out of the year.
People and activity within buildings generate heat. Even in the dead of winter, spaces where people gather can quickly become uncomfortably warm. We can’t just open a window in most modern buildings. If there is an economizer, you’re getting that benefit while filtering the air. When the space cools off, the dampers are closed. No forgetting to shut the window.
Whenever the economizer is working, your air conditioning compressor is not. This saves on energy bills as well as extending the service life of your mechanical system.
Unfortunately, it is not unusual to find an economizer which has been consciously disabled be a previous technician. Although the controls involved aren’t particularly complicated, ensuring proper operation through testing and adjustment can be time consuming. You’re not going to save any money if it isn’t working.
Since the economizer is a mechanical system, it does require periodic maintenance and occasional repair. This is money well spent as you will spend more on energy than service. Annual inspection and maintenance will ensure the system is functioning properly - bringing in free cooling.