Achieving the "perfect" school ventilation system, however, is often easier said than done. Most school systems have only limited resources available for facilities construction or upgrades. Getting the most out of these funds usually requires trade-offs such as not including air conditioning in northern climates, or selecting the same type of equipment used in existing buildings to save on maintenance costs.
For the past 50 years, unit ventilators have balanced these and other considerations. Readily available and relatively simple in both design and operation, unit ventilators have been widely used since the post-World War II school construction boom, and for good reason. They provide a low-cost means of introducing cooler air to classroom spaces without having to install extensive air conditioning equipment. In winter months, unit ventilators compensate for the "cold wall" effect in rooms with large or multiple windows.