HVAC design temperatures by ZIP code, from the climate zone.

How the ZIP lookup works.

The page runs the same lookup the BuildSolver assistant uses when you give it a ZIP. Three steps set the temperatures, so every ZIP in the same climate zone gets the same numbers. A fourth step adds the ZIP's own elevation where the data can pin it down.

1. ZIP to county

ZIP → county FIPSUS Census 2020 ZCTA-to-county relationship file. Only ZIPs with a Census ZCTA are covered, about 33,600. PO Box, unique-use and overseas military ZIPs are not.

2. County to zone

county → IECC 2021 climate zonePNNL county map of IECC 2021 zones. If a ZIP spans more than one zone, one zone is applied and the result lists the others in a warning.

3. Zone to temperatures

zone → worst-case design valuesThe highest cooling and lowest heating design values used for that zone, derived from NREL TMY3 weather data.

4. ZIP to elevation

ZIP → population-weighted USGS ground elevationUSGS 3D Elevation Program ground elevation at the center of population of each 2020 census tract that overlaps the ZIP. Tracts larger than about 10 sq mi are split into block groups. Each point is weighted by its population in the ZIP, so the value sits close to where the houses are. A ZIP with no residents uses its Census center point. The load calculators use this value for the air-density correction. Some ZIPs get no reliable elevation, because the populated areas span a wide elevation range, a town in the ZIP reads far off the average, or too few people live there to check. For those the page says so and the calculators apply no altitude correction.

Cooling 1% and 0.4% dry-bulb

exceeded about 88 h/yr (1%) · 35 h/yr (0.4%)Manual J uses the 1% value for the cooling load. The 0.4% value is shown for reference.

Mean coincident wet-bulb

average wet-bulb at the 1% dry-bulbPair it with the 1% dry-bulb for the latent load.

Heating 99% and 99.6% dry-bulb

colder only about 88 h/yr (99%) · 35 h/yr (99.6%)Manual J uses the 99% value for the heating load. The 99.6% value is colder and more conservative.

Example ZIPs in five climate zones.

Computed by the lookup above when this page was built. Each row is the worst case of the ZIP's climate zone, so any other ZIP in the same zone shows the same numbers.

For estimation only · IECC climate zone worst case · not weather-station values

Standards and data sources.

• IECC 2021 climate zones (PNNL county map) - assigns each county to a climate zone. The lookup uses PNNL's county-level data files.
• US Census 2020 ZCTA-to-county relationship file - links a ZIP code tabulation area to its county.
• NREL TMY3 weather data - the hourly typical-year data the zone design values are derived from.
• USGS 3D Elevation Program (3DEP) and Census 2020 centers of population (tracts and block groups) - the population-weighted ground elevation shown for the ZIP, and the check for whether it is reliable. Both are public-domain federal data.
• ACCA Manual J 8th Edition - the residential load procedure these temperatures feed.
• ASHRAE Handbook - Fundamentals (2021), Ch. 14 - the official station-by-station design conditions. They are not reproduced on this page. Use them, or the values your AHJ accepts, for permit work.

Limitations of the ZIP lookup.

• Values are the worst case of the ZIP's climate zone, not the nearest weather station. The result carries BuildSolver's estimate that actual ZIP-level values may differ by 2-8°F.
• Both numbers lean toward a bigger load: the hottest cooling value and the coldest heating value in the zone. A site in a milder part of the zone gets a larger load than its own station values would give.
• Elevation is the population-weighted average for the ZIP. In a hilly or mountain ZIP a site can sit hundreds of feet higher or lower. Where the data cannot pin a ZIP's elevation down, the page says the elevation was not determined reliably and the calculators apply no altitude correction.
• PO Box, unique-use and overseas military ZIPs, and ZIPs in Puerto Rico, Guam, the US Virgin Islands, the Northern Mariana Islands and American Samoa are not covered.
• ZIPs outside the contiguous US get a warning that the zone mapping is preliminary and local code amendments may apply.
• Preliminary sales-phase data. Not accepted for permit submission.

Common design temperature mistakes.

1. Using record highs and lows. Manual J sizes to design conditions, not the hottest or coldest day on record. A record low as the heating design temperature oversizes the equipment before any other input.
2. Using 0.4% and 99.6% for a residential load. Manual J uses the 1% cooling and 99% heating values. Swapping in the more extreme percentiles adds conservatism the procedure did not ask for.
3. Putting zone values in a permit package. A zone worst case is not a station value. Where a permit needs design conditions, use the ASHRAE or ACCA values the AHJ accepts for that location.
4. Pairing the dry-bulb with a guessed humidity. Latent load depends on the outdoor moisture at design. Use the mean coincident wet-bulb that comes with the 1% dry-bulb.
5. Adding a safety factor on top. Zone worst-case temperatures already push the load up. Rounding the load up and then adding a safety factor stacks margins, oversizes the equipment and invites short-cycling.
6. Ignoring the boundary ZIP warning. When a ZIP spans two zones, the result names both. Check which side the building is on before you rely on the numbers.

ZIP lookup vs ASHRAE station table

What you get here, and what the Handbook gives you.

• This pagereturns one set of values per IECC climate zone, picked as the zone's worst case. It needs only a ZIP and errs toward a larger load.
• ASHRAE Handbook - Fundamentals, Ch. 14 lists values for individual weather stations. You pick the station that best represents the site.
• Switch to station values for permit submissions, heat pumps sized close to the load at the heating design temperature, and loads near the line between two equipment sizes.

Common questions about design temperatures.

Related calculators.

• Manual J calculator Whole-house cooling and heating load from a ZIP and a short home description.
• Furnace size calculator Heating load at the 99% design temperature, then a furnace input size.
• Replacement AC sizing calculator Compare the existing tonnage against the calculated load.
• Psychrometric calculator Humidity ratio, enthalpy and dew point from a dry-bulb and wet-bulb.
• Dew point calculator Check where ducts and grilles will sweat.
• Manual J in Texas Texas metros, climate zones and licensing notes.
• Methodology Standards and data sources behind every BuildSolver tool.
• All tools Every public BuildSolver calculator in one list.