An air conditioner is not just “BTU’s IN A BOX” sitting out in the back yard. An air conditioner is part of a SYSTEM, and that system consists of the outdoor unit, the line set, the indoor coil, the duct system, AND the indoor blower. All of these need to be properly sized and operating for the air conditioning system to function properly.
Probably, one of the most overlooked operating components of an air conditioner is indoor air flow, yet this is critical to the proper operation of any air conditioner.
Ideally, air conditioners need 400 CFM (+/- 50 CFM) per ton of cooling to provide proper heat exchange and efficiencies of the system. Not enough air flow OR too much air flow can cause problems with the system.
When there is not enough air flow through the evaporator or a restriction in the air flow, the Suction pressure is below normal because the refrigerant flowing through the evaporator picks up less heat than normal resulting in lower pressures. Some of the most common air flow restrictions are dirty filters, dirty blower wheels, dirty coils, under-sized duct work, obstructed grills and registers, and duct leaks. If this is not corrected, liquid could be returning to the compressor which will eventually cause compressor failure.
When there is too much air flow through the evaporator coil, the system will experience low sub-cooling, high discharge pressures/temperatures, and high saturated suction temperatures. The increased load on the coil transfers too much heat to the refrigerant. More refrigerant is vaporized, elevating the temperature and pressure. The hotter refrigerant entering the condenser requires more of the condenser’s surface to reject the heat. More condenser is needed giving less room for sub-cooling and this equates to a lower sub-cooled refrigerant.
When sub-cooling is too low, flash gas can occur in the liquid line which and can cause noise in the system, improper metering at the TXV or orifice, fluctuations in the system capacity, performance issues, even higher discharge pressures causing even lower sub-cooling. It becomes a “vicious circle”.
To properly check air flow, a magnahelic or manometer is needed to measure the E.S.P. (external static pressure) of the system. Two reading are necessary – one after the filter in the return and one before the coil in the supply plenum. Once this value is known, the use of the blower performance charts in the installation instructions can help determine the amount of air flow through the system. Most manufacturers size the CFM capacity of the blower based on one half (0.5 IWC) inches of water column.
As you can see, maintaining proper air flow in an air conditioning system is very important to the proper operation of that system. Don’t overlook this when diagnosing problems. Air flow is critical to the proper operation of an air conditioning system.