What size AC unit do I need for my house?

Why square-footage rules of thumb fail

Two houses with identical floor plans can need very different cooling capacity. A shaded stone twin with thick walls holds heat differently than a vinyl-sided colonial with big west-facing windows. Insulation levels, attic conditions, ductwork location, the number of occupants, and even how the house faces the afternoon sun all shift the load. That is why the industry standard is the ACCA Manual J calculation, which models the actual house instead of applying a tons-per-square-foot shortcut. Capacity is measured in tons, where one ton equals 12,000 BTU per hour of cooling.

What the wrong size costs you in comfort

• ✓Oversized: short run cycles, poor humidity removal, clammy rooms, temperature swings, and extra wear from constant starting
• ✓Oversized: louder operation and uneven cooling between floors
• ✓Undersized: marathon run times on hot days and a house that never reaches the setpoint
• ✓Undersized: higher energy bills from a system that cannot rest
• ✓Either way: shorter equipment life than a correctly sized unit

Getting sized before you buy

A proper installation quote should start with measurements and questions about the home, not just a glance at the old unit's nameplate, because the old system may have been sized wrong from day one or the house may have changed with new windows or insulation. Duct capacity matters too, since even a perfectly sized condenser underperforms on starved ductwork. PJ MAC HVAC performs free installation estimates, sizes equipment to the home, and installs all major brands, including Lennox, Carrier, Trane, and Goodman.

Questions about sizing do not keep business hours, and neither does PJ MAC HVAC; the phone is answered 24/7.