Input
Floor area and ceiling height
Volume counts. A vaulted living room holds more air than the square footage on the listing suggests.
Manual J
Manual J load calculations in Tucson
A Manual J says how big your next air conditioner should be, worked out from your house. I do the calculation and write it up, and the report is yours to show every contractor who bids the job.
A Manual J in plain words
Room by room, how much heat your house takes on in the hottest hour and loses in the coldest.
Manual J is the method ACCA publishes for sizing a system to a house. It adds up the heat coming in through the walls, roof, glass and gaps on a design day, and the answer comes out in BTU per hour, then in tons. That number is the size the house needs. These are the inputs:
Input
Orientation to the sun
A wall of west windows takes the full afternoon sun in July. The same glass facing north barely registers.
Input
Window area, glass type and shading
Single pane or low-e, an overhang or none, a mesquite in front of it or bare gravel.
Input
Insulation in the walls and the attic
What is actually there, written on the report so you can check it.
Input
Duct location
In the attic or inside the cooled part of the house. Attic ducts add a penalty to the load, covered in finding 2.3.
Input
Air leakage
Outside air getting in around doors, windows, can lights and the attic hatch.
Input
People in the house
Each person adds heat, and the method counts them.
Input
Tucson design conditions
Two online calculators disagree about Tucson's summer design temperature, one saying 104 F and the other 108 F. The report uses the published ACCA and ASHRAE figures and names them.
Two companion documents follow from it. Manual S picks equipment that meets the load, and Manual D sizes the ducts to carry the air.
The rule of thumb oversizes a Tucson house
Oversized
400 to 500 square feet per ton
Most quotes size a system this way. The rule ignores which way the house faces, how much glass it has and how well the attic is insulated, and in Tucson it tends to land on a bigger unit than the house needs.
Oversized
ENERGY STAR allows 15 percent
A cooling system can be at most 15 percent bigger than the calculated load. A rule-of-thumb size often lands well past that.
Oversized
15 to 25 percent for attic ducts
Arizona attics reach 140 to 155 F in summer, and ducts running through that heat add 15 to 25 percent to the load. The calculation only counts it if someone climbs up and looks where the ducts go.
An oversized unit cools the room to the thermostat setting and shuts off before it has run long enough to pull water out of the air. From June 15 to September 30, monsoon season, that leaves a house cool and clammy. Every extra start also wears the contactor and the capacitor, the parts behind 11 and 50 of the 100 Tucson repair calls in Rite Way's June 2026 audit.
Brenergy Solutions' 2026 pricing guide warns that a "load calculation" quoted under $75 to $100 is usually rule-of-thumb sizing with a name on it.
Three numbers behind the oversizing
- Rule of thumb
- 400 to 500 sq ft per ton
- ENERGY STAR limit
- At most 15 percent over the load
- Attic ducts
- 15 to 25 percent added to the load
- Attic in July
- 140 to 155 F
The Pima County permit
Pima County's permit application for an air conditioning replacement asks for the Manual J, Manual S and Manual D calculations, attached as a PDF. If the quote you're holding never mentions a Manual J, ask the contractor how the permit will be filed.
Tucson Electric Power's 2026 quality-install rebate form has a box for "Manual J load or equivalent completed", so a replacement that qualifies for a rebate needs the calculation as well.
The written report
A PDF you keep, listing:
- Every input above, with the value used
- The cooling and heating load in BTU per hour
- The load in tons
- The recommended equipment size
- Where the ducts run, and the penalty applied for it
Hand it to any contractor bidding the job and compare their equipment against the same number.
Price
Manual J load calculation
$150 to $300
A flat price, agreed before the visit.
Before you sign a replacement quote
A full system replacement in Tucson usually runs $9,000 to $16,500. The Arizona Attorney General's advice on AC work is to get two or three bids on a large job and to confirm the unit size and efficiency across them. A Manual J gives every bid the same number to answer to, so you compare sizes before you compare brands.
If a home sale inspection finds a system that is oversized or short cycling, ask for the calculation before anyone quotes a new unit.
Common questions
What is a Manual J load calculation?
It is the ACCA method for working out how much heat your house gains in the hottest hour and loses in the coldest, room by room. The answer, in BTU per hour and then tons, is the size of system the house needs.
Does Pima County require a Manual J for an AC replacement permit?
The county's permit application for an air conditioning replacement asks for the Manual J, Manual S and Manual D calculations as a PDF.
Why does an oversized AC cost more in Tucson?
It runs in short bursts. Every start wears the contactor and the capacitor, the bursts are too short to dry the air during monsoon, and the extra starts show up on the electric bill.
How much does a Manual J cost?
$150 to $300, as a flat price agreed before the visit.
Can I give the report to another contractor?
It is yours to hand to anyone, and that is the reason to have it: every bid can be held against the same load.
Call me, or send the details
Call 520-820-5422, or send your name, number, address and what the system is doing, and I'll come back to you with a time.