What size AC a 2,000 sq ft Mesa home needs

The rule of thumb

This Old House gives 20 BTUs per square foot of living space as a common starting point. For 2,000 sq ft that is 40,000 BTUs. One ton of cooling is 12,000 BTUs per hour, so that is about 3.3 tons, which in practice means a 3 or 3.5 ton system. This Old House also recommends a load calculation.

Why a load calculation decides it

The Department of Energy says sizing should start from the home's actual heating and cooling loads, calculated with ACCA Manual J, and warns that rules of thumb lead to oversizing, which wastes energy and makes a system cycle on and off too often. A DOE contractor guide says contractors sometimes oversize because they use rules of thumb instead of load calculations.

In Mesa the things that move the number are:

• West and south glass. A wall of afternoon sun adds a lot of load.
• Attic insulation and roof color.
• Ducts in the attic, and how much they leak.
• Ceiling height and how open the floor plan is.

Two 2,000 sq ft houses on the same street in Dobson Ranch can need different systems, depending on which way they face.

Bigger is not better

An oversized system cools the air fast and shuts off before it has run long enough to work efficiently, then starts again. That wears parts and costs more to buy. If a house can't keep up on a 112 degree afternoon, the cause is often ducts or insulation, not the size of the unit. Short cycling.

What it costs

Fixr's installed prices: 3 ton $5,000 to $9,150, 3.5 ton $5,450 to $9,700, 4 ton $6,950 to $11,600. Fixr's $8,750 national average assumes a 3 ton, 16 SEER split system with new ductwork in a 2,000 sq ft home. Replacement costs.