Definition
Heating degree days (HDD) measure how much heating a location needs: each day contributes the number of degrees its average outdoor temperature falls below a 65°F base, and the daily values are summed over a month or year. Cooling degree days count the opposite. Higher totals mean longer, colder heating seasons and more heating demand.
Also called: HDD
Why it matters to you
SW Washington and the Portland metro are heating-dominant: the region's degree-day totals are weighted heavily toward heating, from many cool, damp days rather than a few extreme ones. That long, mild heating season is why heating efficiency (HSPF2 for heat pumps, AFUE for furnaces) usually carries more weight here than cooling ratings.
Practical guidance
Use degree days to compare climates or this winter against an average one, not to predict your own bill or size equipment. Sizing comes from a contractor's Manual J load calculation using local design conditions.
About this definition
Written by the Home Energy Group team — a family-owned HVAC company celebrating 25 years of comfort & care across SW Washington and the Portland metro, licensed in Washington (WA LIC# HOMEEGI003J7) and Oregon (OR CCB# 184255). Definitions inform; an on-site diagnosis decides.
