Why it matters to you
In Houston's heat and humidity, sizing errors show up as short cycling, sticky rooms and uneven temperatures, so the calculation behind the tonnage on your quote matters as much as the brand. Homes that have added insulation, replaced windows or been rebuilt after Harvey flooding may need a different size than the original system.
Practical guidance
Ask each bidder whether they performed a Manual J calculation for your house and request a copy or summary with your quote. ACCA's related Manual S (equipment selection) and Manual D (duct design) build on the Manual J results.
Why does a load calculation matter when replacing an AC?
It sizes the new system to the house as it is today, not to the old unit's label. An oversized system short cycles and leaves Houston homes humid, while an undersized one runs nonstop on the hottest afternoons. A Manual J result gives you a documented basis for the tonnage on a quote.
Is a Manual J calculation required by code?
ACCA says a proper load calculation is required by national building codes and most state and local jurisdictions. Whether an inspector in your city asks to see one varies by jurisdiction. Confirm with your local building department, and ask your contractor how sizing will be documented.
What information goes into a Manual J calculation?
The calculation uses the home's square footage by room, wall and attic insulation, window size, type and direction, air leakage, duct location and condition, and local outdoor design temperatures. Expect a thorough contractor to measure or ask about these rather than estimate from total square feet.
What is the difference between Manual J, Manual S and Manual D?
Manual J calculates the heating and cooling load. Manual S uses that load to select equipment whose real capacity at local conditions fits it. Manual D designs ductwork to deliver the right airflow to each room. All three are ACCA standards and work together.