"What size is it?" is the first question on almost every replacement call, and the answer is always in tons. Tonnage is the trade's shorthand for cooling capacity. It's useful shorthand, as long as everyone remembers it's a nominal label and not a promise of how much cooling the house will actually get.
Tonnage in plain English
One ton of cooling is 12,000 BTU per hour. NIST lists the ton of refrigeration at exactly 12,000 Btu per hour, or about 3,516.853 watts1. A 2-ton system is nominally 24,000 BTU/h, a 3-ton 36,000 and so on.
It has nothing to do with the equipment's weight. It's a rate of cooling.
Where does the "ton" come from?
Before mechanical air conditioning, buildings were cooled with ice cut from lakes and rivers in winter. ACCA's blog traces the unit to the heat needed to melt a ton of ice2:
- 1
Heat to melt ice
About 143 BTU per pound of ice at 32°F (ACCA's figure).
- 2
Scale to a short ton
2,000 lb × 143 ≈ 286,000 BTU to melt a ton of ice.
- 3
Spread over a day
286,000 ÷ 24 hours ≈ 11,917 BTU/h.
- 4
Round it
The industry settled on 12,000 BTU/h, which is now the defined value.
Tons to BTU, kW and airflow
| Tons | BTU/h | kW (NIST factor) | Airflow at 350 CFM/ton | Airflow at 400 CFM/ton |
|---|---|---|---|---|
| 1.5 | 18,000 | 5.3 | 525 | 600 |
| 2 | 24,000 | 7.0 | 700 | 800 |
| 2.5 | 30,000 | 8.8 | 875 | 1,000 |
| 3 | 36,000 | 10.6 | 1,050 | 1,200 |
| 3.5 | 42,000 | 12.3 | 1,225 | 1,400 |
| 4 | 48,000 | 14.1 | 1,400 | 1,600 |
| 5 | 60,000 | 17.6 | 1,750 | 2,000 |
The airflow columns use the 350 to 400 CFM per ton range that DOE's Building America guide calls typical for design3. The right airflow for a specific system comes from the manufacturer's data. See CFM.
Two size lines matter in the rules. AHRI 210/240, the rating standard DOE's test procedure builds on, covers unitary air conditioners and air-source heat pumps under 65,000 BTU/h4. And DOE's regional efficiency standards for split air conditioners set different SEER2 and EER2 minimums above and below 45,000 BTU/h of certified capacity5. See SEER2.
Nominal vs actual capacity
The tonnage on the box is nominal. Ratings are measured at standard test conditions: 80°F dry bulb and 67°F wet bulb entering the indoor coil, with 95°F air entering the outdoor unit. The ICC presentation on ACCA's design process points out that this rating point is a standard for comparing equipment but inappropriate for sizing6.
At a real house, actual capacity moves with:
- Outdoor temperature: capacity falls as it gets hotter outside.
- Indoor wet bulb: more humidity shifts capacity from sensible cooling toward moisture removal.
- Airflow: low airflow cuts capacity. DOE's guide cites lab testing where 250 CFM per ton cut total capacity to 85% of its value at 400 CFM per ton3.
- Matched indoor coil: the same condenser rates differently with different coils.
That's why ACCA's Manual S has the designer use the manufacturer's performance data at the home's design conditions, and sets upper and lower limits on equipment capacity relative to the load6.
How do you size tonnage correctly?
Formula
Worked example (example numbers)
A Manual J for a house returns a total cooling load of 34,000 BTU/h.
| Step | Result |
|---|---|
| Load ÷ 12,000 | 2.83 tons |
| Nearest nominal sizes | 2.5 or 3 tons |
| Manual S check | Compare each candidate's capacity from the maker's expanded data at the local design conditions against the load and Manual S size limits |
| Outcome (example) | The 3-ton matched system lands inside the limits; the 2.5-ton falls short at design conditions |
Tonnage vs BTU vs SEER2
| What it tells you | Unit | |
|---|---|---|
| Tonnage | How much cooling the system is built to deliver (nominal) | Tons; 1 ton = 12,000 BTU/h |
| BTU/h | The same capacity, or a calculated load, in heat per hour | BTU per hour |
| SEER2 | How efficiently it delivers that cooling over a season | BTU per watt-hour |
A bigger tonnage isn't a better system. Capacity should match the load; efficiency is a separate choice. See BTU and Manual J.
How tonnage shows up in TradesWork
The data-plate scan in the TradesWork technician app reads make, model, serial, size and refrigerant from one photo, and works out the unit's age from the serial for common brands, so the equipment record shows tonnage and age before the replacement conversation starts. Replacement options can then go to the customer as a good-better-best proposal, with rebates and proof photos on each option.
Footnotes
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NIST Special Publication 811, Guide for the Use of the International System of Units, Appendix B.8. https://www.nist.gov/pml/special-publication-811/nist-guide-si-appendix-b-conversion-factors/nist-guide-si-appendix-b8 ↩
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A. Bailes III, "A Little History Behind Tons of Air Conditioner Capacity," ACCA HVAC Blog, June 11, 2015. https://hvac-blog.acca.org/little-history-behind-tons-air-conditioner-capacity/ ↩
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D. Springer and B. Dakin, Measure Guideline: Air Conditioner Diagnostics, Maintenance, and Replacement, U.S. DOE Building America, DOE/GO-102013-3766, March 2013, pp. 8–11. https://www1.eere.energy.gov/buildings/publications/pdfs/building_america/measure_guide_air_cond_diagnostics.pdf ↩ ↩2
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AHRI 210/240 (I-P), Performance Rating of Unitary Air-conditioning and Air-source Heat Pump Equipment, scope. https://www.ahrinet.org/search-standards/ahri-210240-performance-rating-unitary-air-conditioning-air-source-heat-pump-equipment ↩
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10 CFR 430.32(c)(5) and (c)(6), Energy conservation standards for central air conditioners and heat pumps. https://www.ecfr.gov/current/title-10/chapter-II/subchapter-D/part-430/subpart-C/section-430.32 ↩
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J. Sedine, "An Overview on ACCA's Residential HVAC System Design Process," 2018 ICC Annual Conference. https://iccsafe.org/wp-content/uploads/Residential-HVAC-Design-Process.pdf ↩ ↩2
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U.S. Department of Energy, Building Science Education, "HVAC: Proper Sizing of HVAC Systems." https://bsesc.energy.gov/energy-basics/hvac-proper-sizing-hvac-systems ↩
