Furnace and Furnace Closet Dimensions

Average furnace dimensions range from 16”W x 29”D x 34”H for a 40,000 BTU furnace to 25”x28”x40” for the largest furnaces.

The more BTU output, generally, the larger the furnace. You also need to consider whether the furnace will be installed upflow/downflow or for horizontal flow, in which case you’ll need a larger furnace room.

Furnace Dimensions

Below is a chart that shows some average furnace dimensions of units producing 40,000 BTU to 140,000 BTU. These dimensions have been taken from actual models of furnaces made by Trane, Carrier, Goodman, Lennox, Payne and York.

40,000 – 140,000 BTU Furnace Dimensions

In addition to the physical dimensions of the bare furnace itself to physically fit inside whatever space you have allotted for it, there are a few other things to be considered.

1). Clearance: A typical furnace cabinet is insulated, but the outer surfaces still get hot. As a result, each furnace will need to have a minimum amount of clearance from all surrounding surfaces to prevent overheating.

If the furnace room or closet surfaces are combustible, that clearance will be increased, and must be added to the actual furnace dimensions.

Tip on fire-proofing your furnace room or closet: Choose fire-resistant drywall, and use fire-rated caulk to surround any wiring or venting through the closet framing. This is Code in all areas.

2). Added Equipment: There will need to be space for any apparatus connected with the furnace itself. On a gas (natural or propane) forced air furnace, there will be a hot air supply plenum and a return air plenum.

3). Maintenance and Repair: There should also be some extra space for both routine maintenance and possible repairs that may be required in the future.

This will include not only enough space for a maintenance person to reach in and perform tasks, but also to remove and replace a component that may have failed without having to remove the entire furnace from its enclosure.

If your home has a utility or furnace room where other HVAC units such as a water heater are, this will probably not be as much of an issue as if the furnace will be located in a small closet-type room.

The chart below gives the recommended dimensions for a furnace closet which will contain a typical furnace with its associated components. 

Furnace Closet Dimensions

Furnace Dimension FAQ

How much space do you need for a furnace?

You need 2.5 to 3 feet of width, 3.5 feet of depth and 3.5 to 4 feet of height.

In inches, that is 30” to 36” of width, about 42” of depth and 42” to 48” of height.

How far does a furnace need to be from a wall?

Furnace clearance should be at least 6” to 12” on sides, back and top. That’s the simple answer.

But this is a question that can have many answers that are based on a number of factors. First, every manufacturer will have distance requirements for each model furnace it makes. There will probably be different clearances for the front, back and sides of the furnace.

Second, each municipality may have its own code requirements. These special requirements may include distances from any combustible surface, or distance from a bedroom, for example.

The best answer is to follow the manufacturer’s recommendations, and then check with your local building inspector for local requirements.

Can a furnace be in a closed room?

Yes, a furnace may be installed in a closed room if its dimensions allow enough space to install a fresh air vent to the outside of the building.

A gas-fired furnace requires air to burn the gas that supplies the heat. If this air cannot be obtained from outside, it will be pulled from the room. This could deplete the breathable air in your home and result in asphyxiation of the occupants. This is especially true in mobile homes or other homes of small square footage.

In a larger home with the furnace in a designated furnace room, it would normally have been designed to provide enough fresh air for combustion.

How large is a horizontal furnace?

They are the same size as upflow/downflow vertical furnaces.

A horizontal furnace is actually a standard furnace that has been converted to be installed horizontally.

Referring to the first chart above, in this position, the width becomes the depth, the depth becomes the height, and the height becomes the width.

Written by Rene Langer

Rene has worked 10 years in the HVAC field and now is the Senior Comfort Specialist for PICKHVAC. He holds an HVAC associate degree and EPA & R-410A Certifications.