During an air conditioning system installation, one of the vital steps our HVAC professionals take is to size the new AC. This doesn’t have anything to do with the physical space the unit takes up. It’s about the cooling capacity of the new system. It’s important for an air conditioner to have the right cooling capacity (i.e. “size”) to match the cooling requirements of the house. It must be neither undersized or oversized.
It’s simple to understand why an undersized air conditioner is bad news. An AC that doesn’t have enough cooling capacity to keep up with the comfort needs of a house will run constantly and still not be able to provide sufficient comfort.
It’s tougher to understand how an air conditioner can be too powerful. After all, why not err on the side of too much cooling? The house may need the extra power in an emergency, and at other times the AC can be turned down.
The truth is that an air conditioner that’s too powerful causes as many problems as one that’s underpowered.
The Dilemma of the Oversized AC
Air conditioning systems run on cooling cycles. When the thermostat signals to the air conditioner that the home requires cooling, the fans and the compressor activate to start the cycle. The cycle will last for fifteen minutes or more, depending on the heat in the house and the thermostat setting. When the thermostat signals the set temperature has been reached, the compressor cycles down.
When an air conditioner is oversized, it throws off the cycling. With the AC sending out a larger capacity of cooling, the thermostat will register that the house has cooled down even though the air conditioner hasn’t been running long enough to distribute the cooled air around the house. The cooling cycle will be cut off before it can finish, usually after ten minutes or less. It will only take a few minutes for the temperature to rise again and the thermostat to restart the air conditioner—and this process will repeat over and over. It’s known as short-cycling, and it’s bad business for the air conditioner.
Short-cycling puts enormous amounts of stress on an AC’s components, particularly the compressor, causing the cooling system to wear down rapidly while costing much more to run. It also affects comfort, because the cooling will be uneven: the center of the house will be too cold while other rooms will be too hot. The air conditioning system won’t stay on long enough to spread cool air to all the rooms.