Guides / Heating and cooling · Updated
Is my new AC or heat pump the right size?
From ACCA, the International Residential Code as building departments quote it, and ENERGY STAR, checked November 2026.
Short answer
The right size comes from a Manual J load calculation for your house: its insulation, windows, air leaks, orientation and climate. The International Residential Code requires equipment to be sized that way, not by square feet per ton or by matching the old unit. Ask your contractor for the Manual J report.
What the code says
IRC M1401.3, as the Texas Department of Housing and Community Affairs quotes it: "Heating and cooling equipment shall be sized in accordance with ACCA Manual S or other approved sizing methodologies based on building loads calculated in accordance with ACCA Manual J or other approved heating and cooling calculation methodologies." Pima County, Arizona adds that ducts are designed by ACCA Manual D (IRC M1601.1).
Manual J, S and D
- Manual J works out the heating and cooling load, room by room. ACCA calls it "the national ANSI-recognized standard for producing HVAC equipment sizing loads."
- Manual S picks equipment to match that load, with size limits so it doesn't end up oversized.
- Manual D sizes the ducts to deliver the air.
Why bigger isn't better
ENERGY STAR: "a system that's too large will not keep your home comfortable because of frequent 'on/off' cycling." An oversized AC also removes less humidity. Pima County's check is a good yardstick: the chosen unit's sensible cooling capacity, plus half any excess latent capacity, should land within 6,000 Btu/h of the Manual J sensible load.
What to ask the contractor
- Did you do a Manual J for this house, and can I have the report?
- What indoor and outdoor design temperatures did you use?
- Which Manual S selection did the equipment come from?
- Will the existing ducts carry the airflow (Manual D), or do they need work?
Then compare running costs with the SEER2 savings calculator and price it with HVAC replacement cost.