Pennsylvania climate zones and HVAC design temperatures
Pennsylvania spans 2 IECC climate zones. 51% of people live in zone 4A (mixed, humid). In Philadelphia, heating and cooling equipment is sized for 92°F in summer and 18°F in winter.
Design temperatures for major cities
| City | Zone | Summer 1% | Winter 99% | Water in |
|---|---|---|---|---|
| Philadelphia | 4A | 92°F | 18°F | 60°F |
| Pittsburgh | 5A | 86°F | 11°F | 56°F |
| Allentown | 5A | 89°F | 14°F | 56°F |
| Erie | 5A | 85°F | 12°F | 54°F |
| Harrisburg | 4A | 91°F | 17°F | 59°F |
Summer: exceeded in 1% of hours. Winter: colder in only 1% of hours. Water in: estimated groundwater temperature for tankless water heater sizing. Computed for each county from NOAA hourly weather data, 2015-2024; see where our numbers come from. Hottest summer design in the state: 92°F (Philadelphia County); coldest winter design: 4°F (Cameron County).
Typical equipment sizes in Philadelphia
One-story house with average insulation (R-13 walls, R-30 attic, double-pane windows), at Philadelphia's design temperatures. Your house can need more or less; use the calculators for yours.
| House | Central AC | Gas furnace (95% AFUE) | Heat pump |
|---|---|---|---|
| 1,500 sq ft | 2 tons | 40,000 BTU | 2.5 tons + 5 kW backup |
| 2,000 sq ft | 2.5 tons | 40,000 BTU | 3 tons + 5 kW backup |
| 2,500 sq ft | 3.5 tons | 40,000 BTU | 3.5 tons + 5 kW backup |
AC size calculator Furnace size calculator Heat pump size calculator
Climate zone by county
From the 2021 International Energy Conservation Code (IECC), as published by the U.S. Department of Energy. The zone sets the insulation and window requirements in the energy code; the number runs from 1 (hottest) to 8 (coldest), and the letter is A for moist, B for dry and C for marine.
Zone 4A: mixed, humid (14 counties, 51% of people)
| County | Summer 1% | Winter 99% |
|---|---|---|
| Adams County | 89°F | 13°F |
| Berks County | 90°F | 15°F |
| Bucks County | 90°F | 15°F |
| Chester County | 86°F | 16°F |
| Cumberland County | 91°F | 17°F |
| Dauphin County | 91°F | 17°F |
| Delaware County | 91°F | 19°F |
| Franklin County | 90°F | 17°F |
| Lancaster County | 90°F | 15°F |
| Lebanon County | 89°F | 13°F |
| Montgomery County | 89°F | 16°F |
| Perry County | 91°F | 17°F |
| Philadelphia County | 92°F | 18°F |
| York County | 89°F | 13°F |
Zone 5A: cool, humid (53 counties, 49% of people)
| County | Summer 1% | Winter 99% |
|---|---|---|
| Allegheny County | 86°F | 11°F |
| Armstrong County | 86°F | 8°F |
| Beaver County | 87°F | 10°F |
| Bedford County | 86°F | 11°F |
| Blair County | 86°F | 11°F |
| Bradford County | 87°F | 5°F |
| Butler County | 86°F | 8°F |
| Cambria County | 82°F | 7°F |
| Cameron County | 82°F | 4°F |
| Carbon County | 89°F | 14°F |
| Centre County | 86°F | 10°F |
| Clarion County | 83°F | 7°F |
| Clearfield County | 86°F | 8°F |
| Clinton County | 88°F | 12°F |
| Columbia County | 88°F | 12°F |
| Crawford County | 83°F | 7°F |
| Elk County | 84°F | 7°F |
| Erie County | 85°F | 12°F |
| Fayette County | 88°F | 13°F |
| Forest County | 84°F | 7°F |
| Fulton County | 90°F | 17°F |
| Greene County | 88°F | 13°F |
| Huntingdon County | 86°F | 11°F |
| Indiana County | 86°F | 9°F |
| Jefferson County | 84°F | 7°F |
| Juniata County | 88°F | 12°F |
| Lackawanna County | 87°F | 11°F |
| Lawrence County | 86°F | 7°F |
| Lehigh County | 89°F | 14°F |
| Luzerne County | 87°F | 11°F |
| Lycoming County | 88°F | 12°F |
| McKean County | 82°F | 4°F |
| Mercer County | 86°F | 7°F |
| Mifflin County | 86°F | 10°F |
| Monroe County | 82°F | 6°F |
| Montour County | 88°F | 12°F |
| Northampton County | 89°F | 14°F |
| Northumberland County | 88°F | 12°F |
| Pike County | 88°F | 11°F |
| Potter County | 82°F | 5°F |
| Schuylkill County | 90°F | 15°F |
| Snyder County | 88°F | 12°F |
| Somerset County | 82°F | 7°F |
| Sullivan County | 88°F | 12°F |
| Susquehanna County | 82°F | 5°F |
| Tioga County | 87°F | 5°F |
| Union County | 88°F | 12°F |
| Venango County | 83°F | 7°F |
| Warren County | 81°F | 7°F |
| Washington County | 86°F | 10°F |
| Wayne County | 87°F | 11°F |
| Westmoreland County | 86°F | 11°F |
| Wyoming County | 87°F | 11°F |
Climate data as of 2026-10. Design temperatures may differ by a few degrees from the ACCA Manual J tables your contractor uses; winter values run a little warmer because recent winters have been milder.