The short answer
Ask for a Manual J load calculation before you buy new equipment. It works out how much heating and cooling your home actually needs from its size, insulation, windows, air leakage, orientation and climate. Equipment chosen this way (Manual S) and ducts designed to match (Manual D) beat copying the size of the old unit or using square feet per ton.
Why sizing matters
An oversized air conditioner cools the air quickly and shuts off before it has removed much humidity – leaving a cold, clammy house, short cycling and extra wear. An oversized furnace cycles more and can be noisier. An undersized system runs constantly and may not keep up on the hottest or coldest days. Heat pumps are especially sensitive, because their capacity changes with outdoor temperature.
Manual J: how much heating and cooling the home needs
Manual J, published by ACCA, calculates heat gain and loss room by room. Inputs include floor area, ceiling heights, insulation levels, window size and type, which way the windows face, air leakage, duct location, number of occupants and your local design temperatures. Software does the arithmetic; the quality depends on the measurements going in.
Manual S and Manual D
Manual S picks equipment whose real capacity at your design conditions matches the load. Manual D sizes the ductwork so each room gets the airflow it needs at reasonable static pressure. A perfect Manual J with undersized ducts still gives a noisy, uneven system.
Why "square feet per ton" fails
Two homes of the same size can need very different equipment: a tight, well-insulated house may need far less capacity than a leaky older one. Rules of thumb also ignore climate and windows. And copying the old unit’s size repeats any mistake made at the last installation.
The ductwork question
Many comfort problems are duct problems: leaks into attics and crawlspaces, undersized returns, or crushed flexible duct. Ask whether the company will measure static pressure, and consider sealing leaky ducts before or during a replacement.
What to ask for
- A written Manual J, or at least the calculated heating and cooling loads.
- Why the proposed size fits that load.
- For heat pumps: heating capacity at your cold design temperature and how backup heat is set up.
- Whether the ducts can carry the required airflow, and what changes are included.
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Questions
What is a Manual J load calculation?
An industry-standard calculation of how much heating and cooling a home needs, based on its construction, windows, air leakage and local climate.
Is a bigger air conditioner better?
No. An oversized AC short-cycles, removes less humidity and wears out sooner. The right size comes from a load calculation.
How many tons of AC do I need?
It depends on your home and climate, not just its square footage. A Manual J load calculation is the reliable way to find out; most homes fall between 1.5 and 5 tons.
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Updated October 2026 by the Heating and Cooling editorial team. General information for homeowners, not professional advice – rules and equipment differ. When gas, carbon monoxide, electricity or anyone’s safety is involved, call 911 or your utility. How we write our guides · HVAC glossary