How the HVAC size calculator works
A heating or cooling load is the heat a house loses or gains per hour on a design day. The calculator adds the main paths, each as U × area × temperature difference, where U = 1 ÷ R-value:
- Walls: net wall area (perimeter × height × stories, minus windows) ÷ wall R-value
- Windows: window area × window U-factor (single pane 1.04, double clear 0.49, double low-E 0.3)
- Ceiling: footprint ÷ attic R-value; floor: footprint ÷ floor R-value at half the temperature difference, a stand-in for a basement or crawlspace that stays warmer than outside
- Air leakage: 0.018 Btu per cubic foot per °F (the heat capacity of air) × air changes per hour (1 leaky, 0.5 average, 0.25 tight) × house volume
- Cooling adds sunlight through windows (window area × SHGC × 75 Btu/h per sq ft, an all-orientation average), 230 Btu/h per person and 30% for humidity removal (latent load)
The insulation presets are R-7/11, R-13/30 and R-21/49 for walls and attic. These simplifications, a box-shaped house with a square footprint and fixed solar and occupant gains, are what separate this from a real Manual J, which ACCA publishes as the national ANSI-recognized standard for residential load calculations (8th Edition).[acca]
What size HVAC do I need? It depends on the house
The same 2,000 sq ft house needs very different equipment depending on its shell and climate. Each row changes one input from the example in the lede:
| Change | Heat BTU/h | Cool BTU/h | AC tons |
|---|---|---|---|
| As described (average) | 32,656 | 31,769 | 3.0 |
| Good insulation (R-21 / R-49) | 27,288 | 30,367 | 3.0 |
| Poor insulation (R-7 / R-11) | 47,331 | 35,554 | 3.0 |
| Low-E windows | 28,951 | 21,735 | 2.0 |
| Tight, air-sealed | 27,976 | 30,178 | 3.0 |
| Leaky, drafty | 42,016 | 34,952 | 3.0 |
| Milder winter (25°F) | 22,608 | 31,769 | 3.0 |
| Hotter summer (105°F) | 32,656 | 39,492 | 3.5 |
Insulation and air sealing move the heating load more than anything else; sun through windows dominates the cooling load. Fixing the shell first can drop the system size by a half ton or more, which lowers both the purchase price and the running cost on the heat pump cost page.
Square feet per ton: why the rule of thumb fails
A common shortcut sizes air conditioning at a fixed number of square feet per ton. The table shows how far square feet per ton moves with house size alone, even with identical construction:
| House | Heat BTU/h | Cool BTU/h | AC tons | Sq ft per ton |
|---|---|---|---|---|
| 1,200 sq ft (1 story) | 22,137 | 19,802 | 2.0 | 727 |
| 1,600 sq ft (1 story) | 28,526 | 25,668 | 2.5 | 748 |
| 2,000 sq ft (2 story) | 32,656 | 31,769 | 3.0 | 755 |
| 2,500 sq ft (2 story) | 39,484 | 38,959 | 3.5 | 770 |
| 3,000 sq ft (2 story) | 46,198 | 46,109 | 4.0 | 781 |
Oversized systems short-cycle, remove less humidity and cost more; undersized systems cannot hold temperature on design days. The Department of Energy tells homeowners to insist that the contractor use ACCA Manual J, notes that per-square-foot rules ignore climate and how well a house is sealed and insulated, and lists oversizing as a cause of efficiency losses and discomfort.[doe_guide][doe_hvac][femp] For a single room, the ENERGY STAR chart on the BTU calculator is the right tool.
Design temperatures
Design temperatures are not the coldest or hottest day on record; they are the temperatures a location stays above (winter) or below (summer) for all but a small share of hours a year. Your local building department, or a Manual J report, lists them. Using a record low oversizes the furnace; the furnace size calculator turns the heating load into a furnace input rating. Then compare running cost by fuel on heat pump vs furnace cost.
Frequently asked questions
What size HVAC do I need for a 2,000 sq ft house?
About 3.0 tons of cooling and 32,656 BTU/h of heat for the average-insulation example at 5°F/92°F; a tight new house may need a ton less, a leaky old one more.
How many square feet does a ton of AC cool?
There is no fixed number. In the table above it ranges widely with house size; insulation, windows and climate move it further.
What is a Manual J calculation?
ACCA Manual J (8th Edition) is the ANSI-recognized standard for calculating residential heating and cooling loads, room by room.
Does this work as a heat pump size calculator?
Yes for the load: a heat pump is sized to the cooling load in tons, then checked against the heating load at your winter design temperature, where its output drops. Compare the heating load here with the capacity at that temperature on the manufacturer's data sheet.
How many BTU is a 3-ton AC?
36,000 BTU per hour (12,000 BTU per ton).
Sources
- Manual J Residential Load Calculation, 8th Edition (ANSI/ACCA 2 Manual J - 2016)
- Energy Efficient Heating & Cooling Systems guide ("Insist that your contractor use ACCA Manual J"; rules of thumb ignore climate and insulation)
- Residential HVAC guide: Manual J estimates heat loss and heat gain; contractors sometimes oversize using rules of thumb
- Purchasing Energy-Efficient Residential Air-Source Heat Pumps (cold-climate models; oversizing causes efficiency losses and discomfort)