Building Parameters

Floor Area (sq ft)

Climate Zone

Zone 4 — Mixed (Chicago, Denver, Philadelphia)

Ceiling Height (ft)

Insulation Quality

Window Area (% of floor)

Fuel Type

AFUE Efficiency

Climate Severity (Heating Degree Days)

Average climate — standard sizing applies

Existing Heating Capacity (BTU/hr, optional)

Sizing Results

Recommended Furnace Output

80,000 BTU/hr

Natural Gas · 80% AFUE

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Calculated Load

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Furnace Output

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Furnace Input

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BTU/sq ft

Available Sizes

ⓘ Size to calculated load ±15%. Oversizing causes short-cycling, reducing efficiency and comfort.

20-Year Fuel Cost by AFUE Efficiency

Gas Price ($/therm)

AFUE	Annual Fuel Cost	5-Year Cost	20-Year Cost	Savings vs 80%

Costs estimated from load-hours and design temperature data. Actual costs vary with fuel prices and usage patterns.

Annual Fuel Cost Estimate

Fuel Prices

Natural Gas ($/therm)

Propane ($/gallon)

Heating Oil ($/gallon)

Electricity ($/kWh)

Annual Cost Results

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Est. HDD

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Est. Run Hours

DOE Minimum AFUE Requirements (2023+)

Region	Furnace Type	Min AFUE	Effective
South & Southwest (Zones 1–3)	Gas Furnace	80%	Always
North (Zones 4–7)	Gas Furnace	90%	May 2023
All Regions	Oil Furnace	83%	Always
All Regions	Electric Heat Pump	8.8 HSPF	Jan 2023
South (Zones 1–3)	Heat Pump (Split)	15 SEER2	Jan 2023
North (Zones 4–7)	Heat Pump (Split)	14.3 SEER2	Jan 2023

AFUE Efficiency Guide

AFUE	Category	Typical Cost	Notes
80%	Standard	$800–$1,500	Single-stage, PSC blower
92–95%	High-Efficiency	$1,200–$2,200	Two-stage, ECM blower
96–98%	Ultra-High	$1,800–$3,500	Modulating, sealed combustion
300%+	Heat Pump (HSPF)	$2,500–$5,000	Efficiency varies by climate

Federal & State Incentives (2024–2025)

Program	Equipment	Amount	Condition
IRA Tax Credit (25C)	Gas Furnace ≥97% AFUE	Up to $600	Primary residence
IRA Tax Credit (25C)	Heat Pump ≥15 SEER2	Up to $2,000	Primary residence
HOMES Rebate	All upgrades	Up to $4,000	Income-qualified
HEEHRA Rebate	Heat Pump HVAC	Up to $8,000	Low/moderate income
Utility Rebates	Varies by utility	$50–$500	Check local utility

Climate Zone Reference

Zone	Description	Example Cities	HDD (approx)
1	Very Hot	Miami, Honolulu	< 500
2	Hot	Houston, Phoenix	500–2,000
3	Warm	Atlanta, Los Angeles	2,000–4,000
4	Mixed	Chicago, Denver, Philadelphia	4,000–6,000
5	Cool	Minneapolis, Boston	6,000–8,000
6	Cold	Burlington VT, Duluth MN	8,000–10,000
7	Very Cold	Fairbanks AK, northern Canada	> 10,000

How to Size a Furnace

Enter your floor area — total conditioned square footage of your home.

Select your climate zone — IECC zones 1 (very hot) through 7 (very cold). Zone 4 covers most of the Midwest and Mid-Atlantic.

Choose insulation quality — pre-1980 homes typically have poor insulation; modern code-built homes are average; energy-efficient new construction is good or excellent.

Select fuel type and AFUE — higher AFUE means less fuel burned per BTU of heat delivered. Northern states now require 90% AFUE minimum for new installations.

Review recommended size — the calculator rounds up to the nearest standard size. Size within ±15% of the calculated load to avoid short-cycling or undersizing.

Glossary

AFUE

Annual Fuel Utilization Efficiency — the percentage of fuel energy converted to useful heat over a heating season. An 80% AFUE furnace wastes 20% up the flue.

BTU/hr

British Thermal Units per hour — the standard measure of heating capacity. One BTU raises one pound of water by 1°F.

HDD

Heating Degree Days — a measure of how cold a climate is. Calculated as the sum of degrees below 65°F for each day of the year.

Short-cycling

When an oversized furnace reaches the setpoint too quickly and shuts off before the blower distributes heat evenly, causing temperature swings and increased wear.

Climate Zone

IECC/DOE energy code zones 1–7 that define building insulation and equipment efficiency requirements. Higher zones = colder climates.

Manual J

The ACCA/ASHRAE standard residential load calculation method. This calculator uses the simplified BTU/sqft method — a full Manual J is recommended for new construction.

Frequently Asked Questions

Is it better to oversize a furnace for extra heating power?

No. Oversizing is the most common furnace mistake. A furnace too large for the load will short-cycle — heating quickly, then shutting off before completing a full run cycle. This reduces efficiency, causes temperature swings, increases mechanical wear, and creates comfort complaints. Size within 15% of the calculated load.

How accurate is the BTU/sqft method vs. Manual J?

The BTU/sqft method is accurate within 10–20% for typical homes. It accounts for climate zone, insulation quality, ceiling height, and window area — the four biggest variables. A full Manual J calculation from an HVAC contractor is recommended for new construction, unusual floor plans, or homes with high ceilings, lots of glass, or unique building envelopes.

What AFUE should I choose for a northern climate?

For zones 4–7 (north of a line through Kansas City, Louisville, and Baltimore), DOE now requires a minimum 90% AFUE for new gas furnaces. A 95–97% AFUE furnace costs $200–$600 more but saves $100–$300/year in fuel costs depending on your climate and gas prices. Payback is typically 2–5 years.

Should I consider a heat pump instead of a gas furnace?

Modern cold-climate heat pumps can operate efficiently down to -13°F and deliver 200–300% efficiency (vs. max 97% for a gas furnace). In zones 4–7 with electricity under $0.15/kWh, a heat pump is often less expensive to operate than gas. Factor in the $2,000 federal tax credit (IRA 25C) and utility rebates when comparing costs.

Why does my existing furnace seem too large?

Before 2000, it was common practice to oversize heating equipment by 40–100% as a "safety margin." Most homes built before 1990 with original HVAC equipment are significantly oversized. When replacing, use a proper load calculation — you may be able to downsize, which reduces equipment cost and improves comfort.

How to Size a Furnace

Key Formulas

Glossary

Frequently Asked Questions

Related Calculators