The size of an AC unit you need for an 1800 square foot house depends on what zone you live in. However, a 3 ton unit will work for that size house in any of the zones. For Zone 1 and 2 a three ton unit will work for a house with 1501-1800 square feet. For Zone 3, the limits are 1601-1900 square feet. For Zone 4, this unit would work for a house with 1601-2000 and for Zone 4 it works for 1651-2100 square feet.