Installation · 7 min read · July 28, 2026

Bigger is not better: sizing a system properly

The most consequential decision in a replacement is made before any equipment is chosen, and it is the one most often skipped.

Ask what size system a house needs and the shortcut answer is square footage divided by a rule-of-thumb figure — commonly somewhere between 400 and 600 square feet per ton in a climate like ours. That gets you a ballpark. It does not get you a specification.

A load calculation, usually referred to as Manual J, accounts for what the shortcut ignores: insulation levels, window area and orientation, ceiling height, air leakage, duct location and condition, shading, and how many people actually live there. Two houses of identical footprint can land a full ton apart.

Oversizing is the more common error and the more damaging one. An oversized system short-cycles — it reaches the set temperature quickly, shuts down, and restarts soon after. Short cycles mean poor dehumidification, uneven temperatures between rooms, more wear on the compressor from repeated starts, and a house that manages to be both cold and clammy.

Undersizing has the more obvious failure mode: the system runs continuously through the worst of summer and still cannot hold the set point. It is less common because installers instinctively round up, which is precisely how oversizing becomes the default.

Ductwork deserves its own attention. A correctly sized system connected to undersized returns or leaky supply runs will not deliver its rated capacity. If a replacement quote does not mention the ducts at all, that is worth asking about.

What a good process looks like: someone measures the house, asks about comfort problems room by room, checks the duct system, and comes back with a recommendation that explains why that capacity and that equipment. If a quote arrives with a tonnage and no reasoning, the reasoning was probably 'same as the old one'.

Placeholder editorial written for this demonstration build.