BTU SizerClimate Zones and Sizing
Two houses of identical size, one in Houston and one in Minneapolis, need almost the same air conditioner and furnaces that differ by two thirds. That asymmetry is the single most useful thing to understand about sizing.
What a climate zone is
The IECC climate zones run 1 to 8 from the Gulf coast to Alaska, and they exist for building codes: insulation requirements, window specifications and equipment efficiency minimums are all written per zone. Sizing rates follow them because they are already the standard way of saying “how cold does it get here”.
| Zone | Roughly | Example cities |
|---|---|---|
| 1 | Tropical | Miami, Honolulu |
| 2 | Hot–humid / hot–dry | Houston, Phoenix, New Orleans |
| 3 | Warm | Atlanta, Los Angeles, Dallas |
| 4 | Mixed | Baltimore, Seattle, St Louis |
| 5 | Cool | Chicago, Boston, Denver |
| 6 | Cold | Minneapolis, Burlington, Helena |
| 7 | Very cold | Duluth, northern Maine, Fairbanks outskirts |
Why heating doubles and cooling does not
Load is proportional to the temperature difference the equipment works against.
In summer you want about 75 °F inside, and design outdoor temperatures run from roughly 90 °F in mild areas to 105 °F in the desert. That is a difference of 15 to 30 °F — a factor of two at the extremes, and most of the country sits in a narrow band in the middle.
In winter you want about 70 °F inside, and design outdoor temperatures run from 45 °F on the Gulf coast to 0 °F or below in the far north. That is a difference of 25 °F to 70 °F, and the heating rate per square foot follows it: 27 BTU/h per square foot in zone 1, 57 in zone 7.
This is why the central AC tonnage table barely changes across its columns while the furnace table doubles.
Finding your zone without a map
Two shortcuts that work:
- By state. Most states span one or two zones. Florida and Hawaii are 1–2; Texas and the Gulf coast 2–3; the Carolinas, Tennessee and southern California 3; the mid-Atlantic and Pacific Northwest 4; the Midwest, New England and mountain west 5–6; northern Minnesota, northern Maine and most of Alaska 7–8.
- By the coldest week. If a normal winter has nights near 0 °F, you are in zone 6 or 7. If it rarely goes below freezing, you are in 2 or 3. That is the same question the zone is answering.
What the zone does not capture
Elevation, urban heat islands and coastal moderation all move real design temperatures within a zone. A house at 7,000 ft in a zone 5 state is colder than the zone implies; a city centre is warmer than the rural county around it. And the zone says nothing about your building — a well-sealed zone 6 house can have a lower heating load than a leaky zone 4 one, which is exactly what the insulation correction is for.
For anything whole-house, this is a starting point rather than a design. A Manual J calculation uses local design temperatures from weather data rather than a zone band.