40,000 BTU Gas Furnaces: Which One to Buy

Need something different?

• 

  Your furnace blows air down (downflow)

  Goodman GD9S960403BN · 97.0% AFUE · Downflow

  For ducts under the floor, a crawl space or a slab. The cheapest condensing downflow furnace at this size.

  $1,831 on Amazon
• 

  Keep your old metal flue pipe

  Goodman GD9S800403AN · 80.0% AFUE · Downflow

  An 80% furnace vents up the metal flue you have. A 90%+ furnace needs new plastic vent pipe and a drain.

  $1,165 on Amazon

Will a 40,000 BTU furnace heat your home?

Replacing a furnace? Read the rating plate inside the door of your old one and match its input BTU. If the old one kept the house warm, the same size will too. For a new home or a big change in insulation, a contractor's load calculation (Manual J) is the real answer. As a rough guide:

The BTU number is the gas it burns. The heat it puts into your home is lower: about 38,800 BTU for the 97.0% pick, and about 32,000 BTU for an 80% furnace.

From Bryant's sizing rule: 30 to 35, 40 to 50, 50 to 60 BTU per square foot for warm, moderate and cold climates. Other sizes: 60,000 BTU.

What it costs

On Amazon a 40,000 BTU furnace costs $1,165 to $2,000 for the furnace alone. A contractor who supplies and fits one typically charges:

Installed prices are U.S. typical for a like-for-like swap, equipment, labor and permit together, from EnergySage, Modernize and other published 2026 data (compiled July 2026). Buying the furnace yourself still leaves the installer's labor, venting, permit and any duct changes to pay.

Before you order one online

1. Book the installer first. Goodman gives no warranty on a furnace bought online unless a licensed dealer installs it to code. Ask the installer whether they will fit a furnace you supply.
2. Match the airflow. Upflow sends air out the top (basements, closets). Downflow sends it out the bottom (ducts under the floor). Horizontal lies on its side (attics, crawl spaces).
3. Match the cabinet width. Goodman furnaces come 14.5, 17.5, 21 and 24.5 in wide. A different width than your old one means sheet-metal work on the ducts.
4. Check your gas. These are natural gas furnaces. On propane, the installer fits the maker's propane (LP) conversion kit.
5. Check the vent. Going from an 80% furnace to a 90%+ one means new plastic vent pipe through the wall or roof and a drain for the water it makes.

Every furnace Amazon sells at 40,000 BTU (5)

40,000 BTU furnaces on Amazon: efficiency, airflow, cabinet width, stages and price

Furnace	AFUE	Airflow	Width	Stages	Price	Buy
Goodman GD9S800403AN	80.0%	Downflow	14 in	Single-stage	$1,165	Buy
Goodman GR9S920403AN	92.1%	Upflow or horizontal	14.5 in	Single-stage	$1,580	Buy
Goodman GR9S960403AN	97.0%	Upflow or horizontal	14.5 in	Single-stage	$1,593.60	Buy
Goodman GD9S960403BN	97.0%	Downflow	17.5 in	Single-stage	$1,831	Buy
Goodman GRVT960403BN	97.5%	Upflow or horizontal	17.5 in	Two-stage	$2,000	Buy

Prices read live on Amazon. Airflow, width and stages are from each Amazon listing; a dash means the listing doesn't say. AFUE is the certified figure.

Questions buyers ask

How many square feet does a 40,000 BTU furnace heat?

About 800 to 1,000 sq ft in a moderate climate, 1,150 to 1,350 in a warm one and 650 to 800 in a cold one, using Bryant's rule of 30 to 60 BTU per square foot. Insulation and windows move it a lot. When you replace a furnace, start from the input BTU on your old one's label.

How much does a 40,000 BTU furnace cost?

On Amazon, $1,165 to $2,000 for the furnace alone (5 listings at our last check). Installed by a contractor, a 96%+ furnace typically runs $7,500 to $12,000, equipment and labor together.

Can I buy a furnace on Amazon and keep the warranty?

Yes, if a licensed dealer installs it. Goodman says it gives no warranty on equipment a homeowner buys online unless the unit is installed by a dealer who follows all federal, state and local codes and licensing rules. Line up the installer before you order, and register the furnace within 60 days of install to get Goodman's 10-year parts warranty.

Can I install a gas furnace myself?

Not safely or legally in most places. The gas line, venting and electrical work need a licensed installer and usually a permit, and Goodman's warranty depends on it. What you can do yourself is buy the furnace, the filter and the thermostat.

80% or 96% AFUE?

At this size the 80% furnace on Amazon burns about $840 of gas a year and the 92.1% one about $730, for a typical winter (40 million Btu of heat) at $1.68 a therm, the U.S. average. Keep 80% only if you want to reuse the metal flue; otherwise buy condensing.

40,000 or a bigger furnace?

Match the heat your home loses, not the biggest number. An oversized furnace turns on and off too often, which wears it out and leaves rooms uneven. If your old 40,000 BTU furnace kept the house warm, buy the same size.