The homeowner had four quotes on her kitchen table. Three of them said the existing 4-ton system was right-sized or undersized for her Queen Creek single-story. One said the system was oversized and the right replacement was a 3-ton. The numbers ranged from $7,800 for a 4-ton replacement to $11,400 for a 5-ton replacement that included new ductwork.
We ran the Manual J. The total cooling load came back at 28,400 BTU. With a 0.9 latent sensible heat ratio and a 1.1 sizing factor for Phoenix design conditions, the right equipment size is a 3-ton. We quoted a 3-ton 16-SEER variable speed system, installed it, and the homeowner has now run it through two Phoenix summers.
The new system has held up to the same heat load the old system struggled with. It uses less electricity than the old system at the same thermostat setting. The homeowner’s summer electric bill is lower than it was with the 4-ton.
What the Other Quotes Were Sizing To
Three of the four quotes were sizing to the existing 4-ton system. “The house was on a 4-ton, so the house needs a 4-ton.” That logic is backward. The right size for the house is the size the house actually needs, not the size the existing equipment happens to be. The existing 4-ton might have been right-sized in 2008 when it was installed, or it might have been oversized by the rule-of-thumb calculation the original installer applied. We did not know without running the calculation.
The fourth quote was sizing to the rule of thumb - about 500 square feet per ton in a hot climate, applied to the 1,950 square foot house, which gives roughly 3.9 tons, rounded up to 4. Three of the four quotes were sizing to existing equipment. One was sizing to a rule of thumb. None of them were sizing to a load calculation.
What the Manual J Actually Said
The inputs that mattered for this house.
Floor area and ceiling height. 1,950 square feet, 9-foot ceilings throughout. Single-story, slab-on-grade, with the attic above and the garage attached on the north side.
Orientation and glass. South-facing great room with a 12-foot by 8-foot sliding glass door and a 6-foot by 4-foot window. East-facing master bedroom with a 6-foot by 4-foot window. West-facing kitchen with a 4-foot by 3-foot window. North-facing rooms have minimal glass.
Construction. Standard 2008 tract construction. R-13 wall insulation, R-30 attic insulation, radiant barrier on the attic decking. Double-pane low-E glass on all windows and the sliding door.
Infiltration. Smoke pencil test at the door and window frames. Conservative infiltration rate based on what we measured.
Internal gains. Standard household - refrigerator, dishwasher, television, lighting, two adults, no cooking appliances adding significant load beyond the kitchen range hood.
Design temperature. Phoenix design cooling temperature at the 1 percent level.
The total sensible cooling load came back at 24,800 BTU. Add the latent load from infiltration and internal gains, factor for design conditions, and the total cooling load was 28,400 BTU. A 3-ton system (36,000 BTU nominal, with a typical sensible capacity around 28,000 to 30,000 BTU at design conditions for a modern system) matches this load.
A 4-ton system (48,000 BTU nominal) would have a sensible capacity around 38,000 to 40,000 BTU at design conditions. That is meaningfully more capacity than the load. The system would short cycle, run inefficiently at part load, and dehumidify less effectively.
What the Homeowner Got From Right-Sizing
Lower equipment cost. A 3-ton system is less expensive than a 4-ton system. The price difference on this install was about $1,200.
Lower operating cost. A 3-ton variable speed system modulating to match the load uses less electricity than a 4-ton system cycling on and off. The homeowner’s summer electric bill dropped by about 18 percent after the install, compared to the previous summer with the 4-ton system running.
Better dehumidification. A right-sized variable speed system runs longer at lower capacity, which extracts more moisture from the air. The homeowner noticed the house felt less clammy at the same thermostat setting.
Quieter operation. A 3-ton variable speed system at part load is quieter than a 4-ton system cycling on and off. The homeowner noticed the difference immediately.
No new ductwork needed. The existing ductwork was sized for the 4-ton system. Right-sizing to a 3-ton meant the existing ducts were now over-sized for the new system, which is a comfortable position to be in. A 4-ton or 5-ton replacement would have required either ductwork modifications or running the system at lower static pressure than rated.
What Right-Sizing Cost Versus the Wrong Sizes
| Quote | Equipment | Equipment cost | Install cost | Total |
|---|---|---|---|---|
| Quote 1 | 4-ton 14-SEER single stage | $3,800 | $4,000 | $7,800 |
| Quote 2 | 4-ton 16-SEER two stage | $4,600 | $4,800 | $9,400 |
| Quote 3 | 5-ton 16-SEER variable speed | $5,800 | $5,600 | $11,400 |
| Quote 4 (our Manual J-based quote) | 3-ton 16-SEER variable speed | $4,400 | $4,000 | $8,400 |
The 3-ton variable speed was not the cheapest quote on equipment. It was the cheapest quote on total installed cost. The 4-ton two stage at $9,400 had a better equipment efficiency rating (16 SEER) than the 3-ton variable speed, but the equipment cost was higher and the operating cost difference was not enough to make up for the lost efficiency from oversizing.
We were not the lowest equipment bid. We were the lowest total bid for the right-sized system. The homeowner signed with us for that reason.
What We Tell Homeowners Considering AC Replacement
Three things to verify before signing an AC quote.
The contractor should have run a Manual J. Not a rule of thumb, not a square-footage calculation, not a “same size as the existing unit.” The Manual J is the standard. If the contractor cannot show you the calculation, ask why.
The quote should specify the equipment size, model, and efficiency rating. A quote that says “3-ton” or “4-ton” without a model number and a SEER rating is not a quote - it is a placeholder. The equipment specification determines the operating cost for the next 15 years.
The quote should include ductwork evaluation. A right-sized system on poorly sealed or poorly designed ducts is leaving efficiency on the table. The contractor should have inspected the ducts, measured the static pressure, and identified any ductwork modifications that the new system requires.
If any of those three are missing from the quote, the install is being undersold. The number on the quote reflects what is being left out.
If you are considering AC replacement and want a quote based on a real Manual J - request a load calculation and quote or read more about what a Manual J load calculation actually covers.
