Ask five neighbors in Tucson what size air conditioner a house needs and you will get a rule of thumb based on square footage. The rules do not agree with each other, and they do not agree with the houses. A midtown ranch, a two-story plan in Marana with a hot upstairs, and a Catalina Foothills house with long attic ducts can share a listing square footage and want different equipment. Sabino Creek Heating & Cooling sizes replacements from the house and the ducts. The number painted on the old condenser is a clue to what someone installed. It is not a measurement of the glass, the insulation, or the addition you built later.
The method that is actually a standard
ACCA Manual J, in the ANSI-recognized 8th edition, is the national standard for producing residential heating and cooling loads. It applies to single-family houses, townhouses, condos, duplexes, and manufactured homes — the building types we actually walk into around Tucson, including manufactured homes in Green Valley that cannot take a “just go bigger” answer because the ducts will not carry it. A Manual J calculation is not a phone estimate and it is not an app that asks only for ZIP code and square feet. The procedure accounts for the pieces that drive the load: design conditions, windows, opaque walls and roofs, infiltration, internal gains from people and appliances, ventilation, and the ducts. Those are sections of the standard because each one moves the result.
We are not going to pretend a blog post is a room-by-room calculation. What we can do here is show you which inputs change a Tucson house, so the conversation on the estimate is about your glass and your attic instead of a tonnage you heard at a hardware store.
Why Tucson breaks the square-footage habit
The climate station’s 1991–2020 normals put the average June high at 101.2°F and count 68 days a year at or above 100°F. Cooling is not a six-week season. A house that coasts in April, when the normal high is 82.9°F, is under a different load in June, and a 110°F afternoon sits above that normal high. Orientation decides who feels it. West glass in the afternoon is a heater. North glass is not the same heater. A calculation that never asks which way the house faces is not a calculation. Shading matters too: a porch, a neighbor’s wall, or a tree is a real reduction, and a tree that came out last winter is a real increase. Tell us if the shade changed.
Roofs and attics matter as much as floor area. Ducts in a vented attic sit in air hotter than the outdoor air, because the roof is in the sun. Leakage and conduction there are a load on the equipment, which is why Manual J has a duct-load procedure and why “the condenser is the system” is the wrong picture. ENERGY STAR’s description of typical duct leakage — on the order of 20 to 30 percent of the air in a typical house — is the national context. Your attic gets its own test. If the ducts are bad, a larger condenser is the expensive way to cool the attic. That problem has its own guide: ducts and attic heat.
Infiltration is the leaky-house part. Older windows, a fireplace damper that does not close, a canopy range hood, an attic hatch with no weatherstrip. Each one is outdoor air the system has to condition. Insulation that has been disturbed by other trades, or that was never much, shows up the same way. We would rather know the attic insulation looks thin than assume a year from a listing.
Internal gains are boring and they count. More people home all day, which is common in Green Valley and in age-restricted parts of Oro Valley, is a different afternoon load from a house that is empty until six. A kitchen on the west wall with the oven on at four is not the same as a kitchen on the east wall used at breakfast. You do not need to log this for a month. You need to mention it if it is how you live.
Too small and too big feel different
Too small is the system on AC can’t keep up. Supply air is cool. Runtime is long. The indoor temperature still climbs when the outdoor temperature is at its worst. The upstairs or the west rooms go first. People respond by dropping the setpoint, which makes the equipment run even longer and does not create capacity.
Too big is shorter cycles. The thermostat is satisfied and the system stops before it has run long enough to pull moisture out of the air. In dry June you may only notice the starts and stops. When monsoon moisture arrives — July’s normal rainfall at the Tucson station is 2.21 inches, against 0.23 inches in June — the house feels clammy at a temperature that looks fine on the thermostat. Extra starts are also how contactors and compressors wear. Oversizing is not a favor and it is not “room to grow.” If someone offers the next ton up “just in case,” ask what in the load changed to justify it.
Equipment selection has to respect the ducts and the blower, not only the calculated load. A coil and condenser that want more airflow than the ducts can move will be noisy, will leak more at weak connections, and will not deliver the capacity printed on the brochure. Manufactured homes and older package-unit houses are where this mistake is most expensive. We match the equipment to the air the building can actually move.
The old nameplate is a clue, not a verdict
Photograph the outdoor data plate and the indoor one. Tonnage, refrigerant, and the model number tell us what is there and whether the indoor and outdoor sections were even designed to work together. They do not tell us that the original installer ran a load calculation. Production homes often got whatever the builder specified for the plan, with ducts that were adequate on paper and marginal once bedroom doors shut and the filter loaded. Builder-grade systems in newer Marana tracts are a frequent version of this. The upstairs thermostat, if there is only one and it is downstairs, never represented the hot floor.
Additions break the nameplate completely. A casita, a closed-in patio, a garage that became a bedroom: if they were tied on with a flex run, the original tonnage is now covering a larger, usually leakier, usually worse-ducted house. Sometimes the right answer is to leave the original system alone and cool the new space separately. That discussion is mini-splits for casitas and additions. Sometimes the right answer is a new central system and new ducts. Square footage of the addition, added to square footage of the house, added to a rule of thumb, is how both of those answers come out wrong.
Window replacements cut load. A west-facing slider that used to be single-pane and is now a decent dual-pane unit is a reason not to copy the old tonnage. So is a shade structure. So is the opposite: a remodel that added glass. Tell us what changed since the condenser was set.
Two systems, one house
Foothills houses often have two systems, or a zone panel nobody has opened in years. Sizing is per system, for the rooms that system actually serves. If one wing is comfortable and the other is not, we need to know there are two outdoor units before we condemn the wrong one. A zone damper stuck closed looks like “the system is too small” from the hot room and looks like short cycling from the thermostat. Do not solve that by adding a ton. Identify which rooms each thermostat controls, and whether you can hear one air handler and not the other.
Heat pumps and straight air conditioners impose the same cooling load. The heating side is a separate question, and Tucson’s mild winters do not automatically mean the heating load is trivial in the foothills or Green Valley. If you are replacing both functions, the comparison is on heat pump versus gas furnace. Do not let a cooling-tonnage conversation quietly pick your heat.
What to gather before you ask for an estimate
You can collect this without tools. Rough square footage and whether the house is one story or two. Which rooms face west. Whether the air handler and ducts are in the attic, a closet, or a package unit outside. How last July behaved: held the setpoint, climbed every afternoon, short-cycled, or only failed upstairs. Photos of both data plates. Age, if you know it. Any addition. The ZIP.
Do not buy equipment from a width-of-the-house glance. Do not accept a size chosen to match a rebate tier if the load does not support it. Tucson Electric Power’s Efficient Home Program lists rebates for high-efficiency equipment and for duct sealing, described on our replacement cost page and on TEP’s own site. A rebate is not a sizing method. Eligibility depends on the equipment, the installation, and a participating contractor, and the dollar amounts change.
Where you stop guessing and we measure
The load calculation, the duct look, and the electrical service are the visit, or the desk work that follows a visit. Homeowners should not be in a summer attic taking measurements at two in the afternoon, and homeowners should not be inside the electrical panel comparing breaker sizes to a condenser they have not bought yet. Send the form with what you gathered. We follow up the same business day. Say if you are planning ahead or if the current system is already failing. Planning is installation. A system that has already quit is repair first, and replacement only if the fault and the rest of the equipment say so — that decision is repair or replace, and it still does not skip the load.
Include the ZIP. Coverage is confirmed from it, not from a neighborhood nickname. The list is on service areas. If the house is in Oro Valley, the elevation and the HOA screen around the condenser are part of the job, not a footnote; see Oro Valley for how those houses differ from midtown. A screen that blocks airflow will make a correctly sized system behave like a small one.
Sources for the figures on this page
- ACCA, ANSI/ACCA 2 Manual J – 2016, residential load calculation: acca.org Manual J.
- National Weather Service Tucson 1991–2020 normals: June high 101.2°F, April high 82.9°F, 68 days at or above 100°F, June rainfall 0.23 inches, July rainfall 2.21 inches. weather.gov/twc/TucsonMonthlyNormalExtremes.
- ENERGY STAR, typical duct air loss of about 20 to 30 percent: energystar.gov duct sealing.
Nothing here is a tonnage for your house. A room-by-room load and a look at the ducts are how a size gets chosen.