Your HVAC
What size AC does a house need?
Learn why square-foot rules are only screening estimates and what a defensible Manual J load calculation and equipment selection should consider.
Why square feet are not enough
Floor area is only one input. Cooling load also changes with:
- local design temperature and humidity;
- roof, wall, floor, and attic insulation;
- air leakage and ventilation;
- window area, orientation, shading, and glass performance;
- ceiling height and room geometry;
- occupants, lighting, and appliances;
- duct location, leakage, insulation, and airflow.
A quick “tons per square foot” rule cannot see those differences. It may be useful for detecting an implausible proposal, but not for buying equipment.
What Manual J and equipment selection do
ACCA Manual J is the accepted residential load-calculation method referenced by ENERGY STAR. ENERGY STAR tells buyers to have a contractor verify size with Manual J rather than guessing (ENERGY STAR). A useful report should show house inputs, design conditions, and room-by-room sensible and latent loads, along with the final tonnage.
Load is not equipment. After calculating it, the contractor selects a specific matched system using manufacturer performance at design conditions. ENERGY STAR points to the connected use of ACCA Manuals J and S and quality-installation standards (ENERGY STAR). Duct capacity and room airflow must also support the selection.
What to check
A larger unit can reach the thermostat setting quickly and shut off before providing the runtime needed for steady moisture removal and mixing. It may also require airflow the existing ducts cannot deliver. ENERGY STAR states that both too-large and too-small equipment can struggle to meet comfort needs (ENERGY STAR).
An undersized system may run continuously and lose temperature at design conditions—but low refrigerant charge, poor airflow, duct leakage, excessive house load, or equipment faults can look similar. Runtime alone does not prove sizing.
Questions to ask with a replacement quote
- May I see the Manual J inputs and room-by-room results?
- What outdoor and indoor design conditions were used?
- Were additions, insulation, window changes, leakage, and ducts inspected?
- Which exact indoor/outdoor equipment combination was selected?
- How does its sensible and latent capacity match the calculated loads?
- Can the existing ducts carry the required airflow to each room?
- What commissioning measurements will verify airflow and charge?
What an online calculator can do
It can organize inputs, explain why load changes, and produce a broad planning range. It cannot inspect the enclosure, test ducts, select a matched system, or replace a professional calculation. Treat any output that asks only for square footage and ZIP code as a screening estimate.
Sources
- ENERGY STAR — Ductless heating and cooling buying guidance, accessed 8/12/2026
- ENERGY STAR — Clean heating and cooling, accessed 8/12/2026
- ENERGY STAR — HVAC quality installation, accessed 8/12/2026