Formula
Cooling looks up the floor area on the ENERGY STAR room-air-conditioner chart. The chart assumes an 8 ft ceiling and starts at 100 ft². Below that, this page uses 5,000 BTU/h and says so. Above 2,500 ft² it scales 34,000 by the area ratio and says so. An area that lands on a boundary stays in the lower band, so 400 ft² is 9,000, not 10,000.
adjusted = chart × (ceiling ÷ 8) × insulation × sun
then + 600 × people past two, + 4,000 if it is a kitchen
shopping size = adjusted, rounded down to 1,000
Sun, the extra people, and the kitchen are ENERGY STAR’s own adjustments. The ceiling ratio and the insulation multiplier (poor 1.20, average 1.00, good 0.85) are this site’s, and average insulation leaves the chart untouched. Heating is area × a planning rate for the zone × the same ceiling and insulation factors. The rates are 25, 30, 35, 40, 45, 50, 55, and 60 BTU/h per square foot for zones 1 through 8. DOE defines those zones. It does not publish this table. This is not a Manual J. Get a load calculation before you buy equipment.
Worked examples
400 ft², 8 ft ceiling, zone 4, average sun, two people, not a kitchen. The chart is 9,000 BTU/h. Nothing else moves it, so the shopping size is 9,000. Heating at the zone 4 rate of 40 is 16,000 BTU/h.
The same room, 10 ft ceiling, very sunny, four people, and a kitchen. 9,000 × 10/8 × 1.10 = 12,375, plus 1,200 for two extra people, plus 4,000 for the kitchen, is 17,575 BTU/h. The window-unit shopping size rounds down to 17,000.
400 ft² with poor insulation. The chart is multiplied by 1.20, so cooling is 10,800 and the shopping size is 10,000. That 1.20 is not on the ENERGY STAR page.
Reference
ENERGY STAR room-air-conditioner chart, 8 ft ceilings. The band includes its upper area.
| Area up to | BTU/h |
|---|---|
| 150 ft² | 5,000 |
| 250 | 6,000 |
| 300 | 7,000 |
| 350 | 8,000 |
| 400 | 9,000 |
| 450 | 10,000 |
| 550 | 12,000 |
| 700 | 14,000 |
| 1,000 | 18,000 |
| 1,200 | 21,000 |
| 1,400 | 23,000 |
| 1,500 | 24,000 |
| 2,000 | 30,000 |
| 2,500 | 34,000 |
Planning heating rates, BTU/h per square foot at 8 ft. Not a DOE publication.
| Zone | Heating |
|---|---|
| Zone 1 | 25 |
| Zone 2 | 30 |
| Zone 3 | 35 |
| Zone 4 | 40 |
| Zone 5 | 45 |
| Zone 6 | 50 |
| Zone 7 | 55 |
| Zone 8 | 60 |
Questions
Is this a Manual J?
No. Cooling starts from the ENERGY STAR room-air-conditioner chart. Heating uses a planning rate by climate zone. Neither is an ACCA Manual J. Get a load calculation before you buy equipment.
Why are there two cooling numbers?
The adjusted figure is the chart after sun, people, a kitchen, ceiling height, and insulation. ENERGY STAR then tells a window-unit shopper to round that total down to the nearest 1,000. A 17,575 BTU/h adjustment becomes a 17,000 BTU/h shopping size.
Where do the heating rates come from?
DOE and IECC define climate zones 1 through 8. They do not publish BTU per square foot. The rates here, 25 through 60 BTU/h per square foot, are planning midpoints for this site. A Manual J replaces them.
Sources
- ENERGY STAR, Room Air Conditioners, the sizing chart and the shaded, sunny, occupant, and kitchen adjustments. The chart assumes 8 ft ceilings. Step 5 rounds the adjusted total down to the nearest 1,000 for a window unit. energystar.gov.
- DOE Building America climate zones 1–8, as mapped for the IECC. The zones are the source. The heating BTU per square foot is not.
- Ceiling height × (height ÷ 8) and the insulation multipliers 1.20 / 1.00 / 0.85 are planning factors chosen for this site.