Heating Degree Days: A Simple Guide to Smarter Furnace and Heat Pump Sizing

Heating Degree Days, Explained Like a Pro Talking to a Neighbor

Heating degree days (HDD) are a simple way to estimate how much heating your home will need in a year. We start with a “base” indoor comfort temperature 65°F in the U.S. If a day’s average outdoor temperature is 40°F, that day adds 25 HDD (65–40). Add those daily values across a season and you get your local HDD total. More HDD = a longer, colder heating season and bigger heating bills.
Why it matters: HDD gives you a reality check when choosing between an 80% AFUE single-stage furnace, a high-efficiency modulating model, or a heat pump. It also helps you right-size equipment and set realistic expectations for costs. If you’re new to sizing, our plain-English Sizing Guide and keep it handy while you read.

What Your Region’s HDD Says About Your Heating Needs

HDD varies dramatically across North America. Hawaii sits near zero hardly any heating. South Florida is mild (~600 HDD). Move north and totals rise fast: Texas around 1,500+, Georgia ~2,700, North Carolina ~3,100. The Great Lakes and Northeast run ~5,000+, while the coldest areas Minnesota, North Dakota, Alaska push 7,000–10,000+ HDD.
 What this means for you: As HDD climbs, you lean harder on heating for more months of the year. Equipment that’s “fine” in a mild state can feel underpowered or inefficient up north. If you’re buying for a rental portfolio or multiple properties, note each address will likely sit in a different HDD band, and your equipment choices should reflect that. Not sure where your location lands? Our Design Center can help you translate climate to equipment options.

Turning HDD Into Sizing: BTU Per Square Foot (With a Quick Example)

A common rule of thumb ties climate to BTU per square foot. Typical ranges:

• 0–2,500 HDD (Zones 1–2): ~30–40 BTU/sq ft

• 2,500–5,000 HDD (Zones 3–4): ~40–50 BTU/sq ft

• 5,000–7,500 HDD (Zones 5–6): ~50–60 BTU/sq ft

• 7,500+ HDD (Zones 7–8): ~60–80 BTU/sq ft

Example: A 2,000 sq-ft home in a 5,500 HDD area might use 50–60 BTU/sq ft → 100,000–120,000 BTU as a ballpark input size before efficiency adjustments. If you choose a 95% AFUE furnace, the delivered (output) BTU is ~95% of input.
These are starting points not final answers. Tight homes can downsize; drafty homes may need more. For a stress-free check, see our Sizing Guide.

Matching Equipment to Climate: What Works Best by HDD Range

• 0–1,000 HDD (Hawaii/South FL): Focus on cooling. Heat pumps shine; an 80% AFUE furnace is an occasional backup. See ductless mini-splits for room-by-room control.

• 1,000–2,500 HDD (AZ/TX): Single-stage or two-stage 80% AFUE furnaces work; heat pumps remain efficient and versatile.

• 2,500–4,000 HDD (GA/NC): Two-stage 80–90% AFUE or modulating furnaces balance comfort and costs; heat pumps still strong.

• 4,000–6,000 HDD (Midwest/Mid-Atlantic): Two-stage or modulating 90–95% AFUE furnaces for steady comfort.

• 6,000–8,000 HDD (Northern tier): Modulating 95–98% AFUE with variable-speed blowers. Cold-climate heat pumps can fit with proper design.

• 8,000+ HDD (Alaska/northern Canada): High-efficiency modulating furnaces; dual-fuel (heat pump + gas) is an emerging winner.
  Browse efficient Furnaces to match your band.

Single-Stage vs Two-Stage vs Modulating: Comfort You Can Feel

• Single-stage: Full blast or off. Simple and reliable for mild climates or occasional heating.

• Two-stage: Low stage handles most days; high stage covers real cold snaps. Great for moderate and cold regions where comfort and noise matter.

• Modulating: Fine-tuned gas valve adjusts in tiny increments to hold temperature within ~1–2°F. Ideal in high HDD climates with long runtimes.

• A variable-speed blower often pays you back in quieter operation, fewer temperature swings, and better efficiency. Under ~2,500 HDD, single-stage is usually enough; in between, two-stage is the sweet spot. For small additions or rooms, consider ductless mini-splits to fine-tune comfort without reworking ducts.

AFUE and ENERGY STAR: What Efficiency Rating Makes Sense Where You Live

AFUE tells you how much of your fuel becomes usable heat. 95% AFUE means 95 cents of every dollar warms your home. ENERGY STAR maps this to climate: southern states typically ≥90% AFUE, northern states ≥95% AFUE.
Rule of thumb by HDD:

• Below 3,000 HDD: 80–85% AFUE can be cost-effective.

• 3,000–6,000 HDD: 85–92% AFUE hits the value sweet spot.

• Above 6,000 HDD: 95%+ AFUE usually wins on total cost of ownership.

Remember installation quality, duct losses, and controls can swing real-world results more than a label. Pair high-efficiency furnaces with properly sized air handlers, sealed ducts, and smart thermostats for best outcomes. If you’re pricing options, our HVAC Financing can make high-efficiency upgrades easier to budget.

Heat Pumps in Cool to Cold Climates: What’s Changed

Old wisdom said heat pumps “quit” in the cold. Modern cold-climate heat pumps can deliver meaningful heat down to –20°F when correctly sized. In ≤6,000–7,000 HDD regions, many homes can go heat-pump-only; in colder zones, a dual-fuel setup (electric heat pump + gas furnace backup) blends low operating costs with reliable deep-cold performance.
What to expect: Heat pumps add heat gradually, super comfortable, ultra-efficient, but not “blast heat” like gas. For homes that cool as much as they heat (Mid-Atlantic, many parts of the Midwest), the year-round savings can be compelling. Explore flexible R-32 Heat Pump Systems for additions, and packaged dual-fuel units where space is tight.

House Factors That Change the Math: Insulation, Air Sealing, and Ducts

Two homes on the same street can need very different sizes. Why?

• Insulation & Air Sealing: A tight, well-insulated home can cut heating load 15–25%. Drafty older homes can push required BTU 20–30% higher.

• Windows & Orientation: Big, leaky windows lose heat fast; winter sun can help in south-facing rooms.

• Ducts: Leaky or undersized ducts waste heat and create room-to-room swings.

• Blower Type: Variable-speed blowers improve mixing and comfort, especially in 5,000+ HDD regions with long runtimes.

Before you oversize equipment “just in case,” fix the envelope. You’ll often step down a furnace size, save money upfront, and run quieter. If you’re replacing lines or moving equipment, don’t forget line sets and accessories with small details that protect performance.

Dollars and Sense: When High-Efficiency Pays Back

Fuel use tracks HDD. A 95%+ AFUE furnace in Minnesota (~7,200 HDD) racks up enough runtime to pay back the upgrade faster than the same unit in Texas (~1,500 HDD). In very cold regions, modulating + variable-speed often pencils out when you include comfort and noise.
A simple way to think about it:

1. Estimate your HDD band and square footage.

2. Pick the smallest equipment tier that meets load with margin.

3. Compare annual fuel/electric costs between tiers (80% vs 95% AFUE, or gas vs heat pump).

4. Add maintenance and expected lifespan.

For help comparing lifetime costs, stop by the Help Center.

Planning Ahead: Climate Trends and Flexible Choices

Across much of North America, HDD is slowly trending down while cooling demand rises. If you live in a moderate HDD area, a well-sized heat pump (with or without a gas backup) may future-proof your home, reduce emissions, and simplify maintenance. In extreme-cold regions, a high-efficiency modulating furnace still anchors reliability but pairing it with a cold-climate heat pump (dual-fuel) can reduce shoulder-season gas use. 

If you want a quick, expert sanity check on your plan, our team can review photos and specs via Quote by Photo and suggest right-sized furnaces that match your HDD.