A package unit should be sized by a load calculation (Manual J) for your house, not by square footage or by the label on the old unit. A package unit puts the whole system in one box on the roof or a pad, so three things a split-system buyer can treat separately get decided together: the cooling size, the heating side and the fit on the existing curb. Climate Pros Tucson matches you with licensed pros who run the numbers. For sizing in general, read what size AC you need in Tucson; this page is the package-unit angle.
Can you just match the old unit’s tonnage?
Not safely. The old unit may have been sized by a rule of thumb, or before an addition, new windows or new insulation changed the house. Copying its label repeats whatever mistake was made then. An oversized unit cycles on and off and leaves hot and cold spots (AC short cycling); an undersized one can’t hold the house in July.
What design numbers apply in Tucson?
The City of Tucson’s code lists a summer design dry bulb of 105F, a design wet bulb of 66F and a winter design dry bulb of 35F, in climate zone 2B. Above 4,000 feet in Pima County the numbers differ: zone 5B, 90F summer and 4F winter. The City requires load and sizing calculations for new and replacement HVAC equipment, so a replacement package unit needs one too.
Does the heating side change the size?
In the Tucson basin it rarely drives it. The airport’s 1991-2020 normals show about 2.6 cooling degree days for every heating degree day, so the cooling load usually sets the size. On a package heat pump, that same box then does the heating. On a gas pack, ask the pro to size the furnace section to the heating load rather than taking whatever comes with the cooling size. The trade-offs are in package heat pump vs gas pack.
What pushes a Tucson load up?
- West-facing glass. Afternoon sun comes in hard; shade and film change it.
- Wall type. Block, frame and adobe behave differently, and the calculation should reflect what you have. See cooling a slump block or adobe home.
- Roof and duct exposure. A package unit feeds ducts that often run near a hot roof. ENERGY STAR says a typical forced-air house can lose about 20% to 30% of the air moving through ducts to leaks and poor connections, a national figure.
- Additions the old unit was never sized for.
Will the new size fit the curb?
Ask before you settle on a model. A different size or brand may not match the existing roof curb, and an adapter or a new curb changes the roof work and the quote. A roof unit also needs a crane lift, which belongs in the written quote. Roof package unit replacement walks through that side.
What should you ask?
- Will you run a full load calculation, and can I see it?
- What did you assume for glass, walls and ducts?
- Will the existing ducts carry the new size?
- Does the unit fit my curb, or does it need an adapter?
See AC installation and the Tucson HVAC hub.
Get the calculation first
Ask the pro to show the load calculation before they quote equipment. Call (520) 353-2520 and we’ll match you with a licensed local pro.