Formula
Tons are the block load divided by 12,000. The block load is floor area × a planning cooling rate for the zone × (ceiling ÷ 8) × the insulation multiplier. The shopping size rounds up to the next half ton, and only so you can see what that rounding does. It is not a recommendation to buy the larger unit.
BTU/h = area × zone rate × (ceiling ÷ 8) × insulation
tons = BTU/h ÷ 12,000
The zone rates, from 28 BTU/h per square foot in zone 1 down to 15 in zone 8, are planning midpoints for this site. DOE climate zones say where the weather bands fall. They do not publish this table. Sun, occupants, and kitchens are on the room BTU page, because those ENERGY STAR adjustments are for one room, not a whole house. This is not a Manual J.
Worked examples
2,000 ft², zone 4, 8 ft, average insulation. 2,000 × 22 = 44,000 BTU/h = 3.67 tons. The next half ton is 4.0. Buying the 4-ton machine because of that rounding can oversize it.
The same house with good insulation. The 0.85 multiplier brings the load to 37,400 BTU/h, 3.12 tons, and the next half ton is 3.5. The 0.85 is this site’s factor.
1,800 ft² in zone 1 at 8 ft. The planning rate is 28, so the load is 50,400 BTU/h, 4.20 tons.
Reference
Planning whole-house cooling rates at an 8 ft ceiling. Not a DOE load table.
| Zone | BTU/h per ft² |
|---|---|
| Zone 1 | 28 |
| Zone 2 | 26 |
| Zone 3 | 24 |
| Zone 4 | 22 |
| Zone 5 | 20 |
| Zone 6 | 18 |
| Zone 7 | 16 |
| Zone 8 | 15 |
Questions
How many tons is 2,000 square feet in zone 4?
At the planning rate of 22 BTU/h per square foot, 2,000 × 22 = 44,000 BTU/h, which is 3.67 tons. Rounding up to the next half ton is 4.0. Rounding up can oversize the equipment. A Manual J is the number to buy from.
Is a ton 12,000 BTU/h?
Yes. A refrigeration ton is 12,000 BTU/h. This page divides the block-load estimate by that.
Should I buy the rounded size?
Not from this page. The rounded size is a shopping sketch. Oversizing short-cycles and handles humidity badly. Get a Manual J, or a room-by-room load, before you buy.
Sources
- One refrigeration ton = 12,000 BTU/h.
- DOE Building America / IECC climate zones 1–8. Cited for the zone definitions only.
- The BTU per square foot, the ceiling ratio, and the insulation multipliers are planning assumptions for this site. ACCA Manual J is the load calculation to buy from.