Maryland climate zones and HVAC design temperatures
Maryland spans 2 IECC climate zones. 98% of people live in zone 4A (mixed, humid). In Baltimore, heating and cooling equipment is sized for 92°F in summer and 19°F in winter.
Design temperatures for major cities
| City | Zone | Summer 1% | Winter 99% | Water in |
|---|---|---|---|---|
| Baltimore | 4A | 92°F | 19°F | 60°F |
| Frederick | 4A | 90°F | 16°F | 58°F |
| Rockville | 4A | 90°F | 16°F | 58°F |
| Annapolis | 4A | 92°F | 19°F | 60°F |
| Hagerstown | 4A | 90°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 (Anne Arundel County); coldest winter design: 7°F (Allegany County).
Typical equipment sizes in Baltimore
One-story house with average insulation (R-13 walls, R-30 attic, double-pane windows), at Baltimore'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 tons + 5 kW backup |
| 2,000 sq ft | 2.5 tons | 40,000 BTU | 2.5 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 (22 counties, 98% of people)
| County | Summer 1% | Winter 99% |
|---|---|---|
| Anne Arundel County | 92°F | 19°F |
| Baltimore city | 92°F | 19°F |
| Baltimore County | 92°F | 19°F |
| Calvert County | 91°F | 22°F |
| Caroline County | 90°F | 19°F |
| Carroll County | 90°F | 16°F |
| Cecil County | 90°F | 18°F |
| Charles County | 92°F | 19°F |
| Dorchester County | 91°F | 22°F |
| Frederick County | 90°F | 16°F |
| Harford County | 92°F | 19°F |
| Howard County | 92°F | 19°F |
| Kent County | 90°F | 19°F |
| Montgomery County | 90°F | 16°F |
| Prince George's County | 91°F | 19°F |
| Queen Anne's County | 92°F | 19°F |
| Somerset County | 90°F | 21°F |
| St. Mary's County | 91°F | 22°F |
| Talbot County | 91°F | 22°F |
| Washington County | 90°F | 17°F |
| Wicomico County | 90°F | 21°F |
| Worcester County | 87°F | 21°F |
Zone 5A: cool, humid (2 counties, 2% of people)
| County | Summer 1% | Winter 99% |
|---|---|---|
| Allegany County | 82°F | 7°F |
| Garrett County | 88°F | 13°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.