Contemporary HVAC controls automatically regulate system output. At minimum, these controls maintain indoor temperature as interior loads and outdoor conditions change. In a small heating-only building, this is done with a thermostat that "compares" indoor temperature with an occupant-selected "setpoint," or desired value. When the indoor temperature drops enough, the thermostat activates the furnace or boiler, and then turns it off when the space reaches the setpoint. The difference between the "start" and setpoint temperatures is the "dead band." If it is set too large, temperatures will vary too much for comfort. If it is too small, the equipment will tend to run in very short cycles, which is very inefficient. Even the simplest systems require judgments to optimize performance.

Conceptually, air conditioning requires a second controller, although usually it is integrated into the same thermostat box. The thermostat controls the A/C compressor as well as the furnace section and air handler (fan). From this point, residential systems also can have controls that become much more complex, even though the external appearance is still "just" a thermostat. In addition to programmable settings (for different days or times of day), some advanced units have separate controls driven by humidity levels.

Controls for smaller commercial unitary equipment are quite similar to residential units, except that the equipment should bring in outdoor air to comply with the ventilation standards of ASHRAE Standard 62.1 (discussed further below) (American Society of Heating, Refrigerating, and Air-conditioning Engineers. 2001. Standard 62-2001-Ventilation for Acceptable Indoor Air Quality (ANSI Approved). Atlanta, Ga.: American Society of Heating, Refrigerating, and Air-conditioning Engineers).

This equipment may also include an economizer that brings in large amounts of outdoor air when it is sufficiently cooler than the indoor air, thus providing cooling at lower cost than operating the refrigeration system. Economizers are crucial, except where the daily and seasonal temperature swings are very small.

Buildings with larger, multi-zone, and multi-unit packaged systems, and those with central systems, are almost always controlled by an energy management system (EMS) with a direct digital controller (DDC). These use zone thermostats, outdoor temperature, and other sensors to determine how to operate all the equipment to provide economy and comfort. Their design, installation, and troubleshooting are generally done by specialist firms. For a large building, it is challenging just to develop a bid specification for an energy management system that can be expected to work correctly! At the level of the building operator, modern systems will typically include features such as:

• Night Setback: This turns off the fans, closes the ventilation dampers, and shuts down the heating and cooling equipment, except as needed to keep the building warm enough or cool enough to return to operating conditions in the morning.

• Seven Day Programming: This keeps track of the day of the week and allows the night setback to continue on Saturdays and Sundays, and offers holiday overrides.

• Optimal Start: When the space switches from unoccupied to occupied, the equipment must go through a warm-up or cool-down period. The optimal start feature recognizes both the outdoor condition and the space temperature and delays the starting of the equipment in the warm-up or cool-down mode as long as possible while still achieving occupied temperature conditions at the desired time.

• Discharge Air Limit: A thermostat can be placed in the unit to limit the temperature of the discharge air during the economizer mode. This is a safety feature that will ensure that the air temperature is not too cold for occupant comfort.