Why Would Your Central Air Unit Blow Hot Air?

What’s in the Box

• The box in the backyard is part of a mechanical system. It contains a compressor and evaporator coils that concentrate heat and begin the evaporation of hydroflourocarbon (HFC) gas. The pressurized liquid begins its cooling as it travels through a set of tubes called evaporation coils. A large fan, often the only part visible, pulls air through vents on the sides or bottom of the box, passes it over the coils and then pushes it out the top of the unit, aiding the liquid’s expansion.

States of Matter

• A central air-conditioning system repeatedly compresses and expands HFC, which has a very low boiling point. Compressor units create heat by pressurization, much as a car piston creates heat in the fuel that runs a car. As the liquid travels through pipes called coils, it evaporates, creating a cool gas inside the coils. At this point, a second fan in an air handler unit inside the house blows over the cold coils and forces cool air through the duct work in the house. As the HFC warms, it condenses slightly and is drawn back outside toward the condensing unit.

Problems

• Hot air that comes out the top of the outdoor central air unit means that the compressor is doing its job. Hot air emanating from the air handler through the ducts, however, means the system has failed somewhere. As with a refrigerator, a worn or faulty compressor means expensive repair. It cannot produce heat in the outdoor unit; the system simply recirculates warm air already present in the system. Modern air-conditioning systems are closed, so HFC cannot be accessed as with the old systems that used chloroflourocarbons (CFCs). A loss of HFC from a closed system signals damage to coils or mechanics. The outdoor unit would expel air of the same temperature as the surrounding air.

Keep the Heat Rising

• Annual checkups keep systems producing cool air for the house and pushing hot air out of the outdoor unit of a central air system. In addition to cleaning air intake louvers and checking for obstructions in the drainage tube — which should be directed away from the house, not toward it — technicians check coils, tubing and fans as well as the motors that operate them and the circuits and controls surrounding the compressor. They may check pressurization and refrigerant level if these are accessible. One of the general tests they run is to crank up the unit, set the thermostat as low as possible and check how long it takes for the blast of hot air to begin issuing from the outdoor box.