What size AC do you need?
You need the size a load calculation says your house needs, not the size a chart gives for your square footage. Two houses with the same floor area can need different equipment because of windows, walls, roof, ducts and shade. A licensed pro measures those, then picks equipment to match. The City of Tucson requires load and sizing calculations for new and replacement HVAC equipment. Climate Pros Tucson matches you with licensed pros who run that calculation before quoting.
Why isn’t square footage enough?
A chart that says “so many square feet per ton” ignores what actually drives the load. It treats a shaded stucco house and a home with big west glass as the same. That is how oversized and undersized systems get sold.
What does a load calculation measure?
The standard method is called Manual J. It adds up heat gain and loss from:
- Glass: how much, which direction it faces, and whether it has shade or film.
- Walls: block, frame or adobe, and their insulation.
- The roof: flat built-up roofs and attic insulation behave differently.
- Duct location: ducts in a hot attic or on a roof lose cooling on the way.
- Air leakage and the number of people in the house.
- Local design conditions.
For the City of Tucson, the code lists a summer design dry bulb of 105F and a design wet bulb of 66F. Above 4,000 feet in Pima County, the design is different: zone 5B with a summer dry bulb of 90F, and a winter dry bulb of 4F. Your pro uses the set that fits your address.
Why does Tucson heat make sizing tricky?
Tucson has about 68 days a year at or above 100F (1991-2020 normals at Tucson International Airport). A system sized only for the average day struggles on the worst ones. A system sized for the extreme day runs oversized most of the year. The calculation balances both, and a variable-speed or two-stage unit can soften the tradeoff. See two-stage vs variable-speed AC.
What goes wrong if the unit is too big?
An oversized AC cools the air fast and shuts off before it removes much moisture or evens out the temperature. It then turns on and off in short bursts, which wears parts and leaves hot and cold spots. Read about AC short cycling.
What goes wrong if the unit is too small?
An undersized AC runs almost nonstop on hot afternoons and can’t hold the thermostat setting. If your current unit can’t keep up, size may be one cause among several. See AC can’t keep up with the Tucson heat before you assume a bigger unit is the answer.
What should you ask a pro?
- Will you run a full load calculation, not a chart?
- What did you assume for my windows, roof and ducts?
- Will the existing ducts handle the new size?
- Why did you pick this capacity, and what does it do on the hottest days?
- Who pulls the permit? The contractor pulls it.
Efficiency ratings matter too. Read what SEER2 means. More on the service itself: AC installation and the Tucson HVAC hub.
Before anyone quotes equipment
If you’re replacing a unit or adding a room, a licensed pro should run the calculation before anyone quotes equipment. Call (520) 353-2520 and we’ll match you with a licensed local pro.