The FAMCO IAQ Economizer™ kit is a control system designed to provide cutting-edge economizing, free-cooling, and IAQ functionality to split-system air conditioning systems by using external air to reduce internal temperatures. It allows low temperatures to be realized at times when they can be achieved using less energy. Operator override allows the choice of free cooling beyond automatic operation and allows quick exhaust of household odors. Cooling/fan-off delay (residual cooling) utilizes wasted cooling potential. It works in parallel with existing systems and thermostats. Features include system heating identification, automatic economizing/free-cooling disable-enable function, and self-diagnostics to monitor and report proper operation of the unit.