What is Degree Days (HDD/CDD)?
By the HVAC Responder Editorial Team
Degree days quantify climate load on buildings. Each day’s average temperature is compared to a 65°F base: a 40°F day contributes 25 heating degree days (HDD); an 85°F day contributes 20 cooling degree days (CDD). Summed across a year, they express how much heating and cooling a location demands — a northern city logs several times the heating degree days of a southern one.
Why it matters to a homeowner
Degree days are why identical houses cost wildly different amounts to condition in different cities, and why equipment choices are regional: condensing furnaces near-automatic in 6,000-HDD markets, high-SEER2 cooling decisive at 3,500 CDD. Utilities and auditors use them to normalize bills — if your usage per degree day is climbing, the house or the equipment is slipping, whatever the weather did.
What this term helps you decide
Normalise your bills against the weather to see whether the house or the equipment is slipping.
The bookkeeping of weather
Each day’s mean temperature is compared to a 65°F base: a 35°F January day deposits 30 heating degree days; a 85°F July day deposits 20 cooling degree days. Summed across seasons, the totals become climate’s invoice — a northern city logs several times the heating degree days of a southern one — and every equipment-sizing, fuel-budget, and efficiency-payback calculation in this glossary quietly runs on them.
Why 65 degrees
The base assumes a house whose internal gains — bodies, appliances, sun — carry it comfortably when outdoor air sits at 65°F, so only degrees beyond that boundary demand equipment. The convention dates to early utility load forecasting and survives because it works; specialized analyses shift the base for tight modern homes, but the 65°F ledger remains the lingua franca of every weather-normalized energy comparison.
The homeowner analytics tool hiding in plain sight
Divide fuel or kWh use by the period’s degree days (utilities and NOAA publish them) and weather cancels out of your bills: usage-per-HDD creeping upward across winters means the house or equipment is slipping, regardless of how cold it "felt." Our high-heating-bill guide builds its audit on this one ratio — the closest thing home energy has to a fitness tracker.
Reading this site with degree-day eyes
Every market page here carries its regional HDD/CDD pair because they are the why behind climate-conditional advice: a condensing furnace repays its premium faster where heating degree days are high, a high-SEER2 system where cooling degree days are, and dual-fuel logic lives in the overlap. The thresholds are your own bills divided by those numbers, not a figure this site publishes.
Related terms, defined in brief
AFUE — AFUE (Annual Fuel Utilization Efficiency) is the percentage of a furnace’s fuel that becomes usable heat for the house over a season. An 80% AFUE furnace sends 20 cents of every fuel dollar up the flue; a 96% condensing furnace loses only 4 cents, recovering extra heat by condensing water vapor out of its own exhaust.
The 80-versus-95+ decision is the central furnace-buying question. Condensing furnaces cost more and need PVC venting and a condensate drain, but in cold climates the fuel savings typically repay the difference well within the unit’s life. In mild-winter markets the payback stretches — run the math on your actual heating bills, not a national average. Check local code: some jurisdictions require condensing efficiency in new installations.
SEER2 — SEER2 (Seasonal Energy Efficiency Ratio 2) is the federal efficiency metric for air conditioners and heat pumps in cooling mode, in force since 2023. It measures seasonal cooling output divided by electricity consumed, tested under higher external static pressure (the M1 test procedure) than the old SEER standard, which is closer to a real duct system — so a SEER2 number is not directly comparable with an older SEER rating.
Mid-efficiency equipment lands at 15–17 SEER2, and premium variable-speed systems reach 20+. The arithmetic: electricity per unit of cooling falls in proportion to the rating, so going from 15 to 16 SEER2 uses about 6% less electricity for the same cooling — an energy ratio, not a bill promise; your cooling hours and rate decide the dollars. Past the top tiers, you are buying comfort features as much as efficiency. Confirm the quoted model’s rating in the AHRI directory.
Manual J (Load Calculation) — Manual J is the ACCA-standardized method for calculating a home’s heating and cooling loads — the BTUs actually needed on design days. It accounts for insulation levels, window area and orientation, air leakage, occupancy, and local design temperatures, producing the number that equipment sizing should follow.
The alternative — square-footage rules and matching the old unit — is how America’s housing stock ended up systematically oversized. Oversizing costs more up front, short-cycles, dehumidifies poorly, and wears equipment early; sizing from a real load calculation frequently specifies smaller, cheaper machines than the outgoing ones. The homeowner move: ask any replacement bidder for the Manual J report. The reaction tells you plenty.
Where this term does real work
No service to sell you here
This term is explanation, not a reason to book anything. If a heating or cooling problem brought you here, the twelve service guides say what each visit involves and what to ask; the glossary index has the rest of the vocabulary.