Is It Bad to Put Furnace In Crawl Space?

Can you install a furnace in the crawl space? It’s a question a lot of people ask. From a simplistic look, you’d argue – why not. In any case, the crawl space has plenty of space doing nothing.

 It’s also reasonably safe and tends to get extremely cold in winter. So, why not have the furnace there to deal with the freezing conditions at the “source?” Unfortunately, it’s not as straightforward as it seems.

 For one, building codes in most locations don’t permit it. Moreover, even where it’s permitted, running your furnace from the crawl space presents many technical challenges for which most homeowners aren’t prepared. This guide looks at what the law says, the pros and cons of installing a furnace in the crawl space, and how to install it in the crawl space (where it’s allowed).

We’ll also explain the best furnace types for crawl space installation.

Can You Install a Furnace in the Crawl Space?

Let’s begin by answering the question on everyone’s mind. Yes, you can install a furnace in the crawl space. Although crawl space furnace installations come with many challenges, including insulation concerns, you can still have the unit installed in the crawl area with great success.

Furnace in Crawl Space Pros and Cons 

Installing a furnace in the crawl space comes with several advantages and several challenges worth keeping in mind.


  • Save space on the yard

Most people install the furnace in the yard next to the main house. Unfortunately, the appliance takes up valuable space in the yard. This is a significant concern in tight spaces where every inch counts.

Moreover, most zoning laws prohibit locating a swimming pool close to the furnace. So, you may not have a swimming pool even if you want and can afford one. You can avoid all these problems by installing the furnace in the crawl space.

  • Aesthetic considerations 

Additionally, installing the furnace in the front yard (the most common location) can negatively impact the aesthetics of your home. That’s because furnaces are huge and not so aesthetically pleasing. They stick out like a sore thumb when you’re decorating, or landscaping, and the loud noise doesn’t help either. Installing the unit in the crawlspace can also go away to solving these problems.

  • Shield it from the weather

 Direct exposure to weather elements can quickly take a toll on your furnace. For one, constant exposure to UV light easily causes fading. Rains and strong winds are even worse as they constantly expose the unit to bombardment by pebbles, stones, and twigs.

Some of these elements can enter the furnace, causing filter blockages and other damages. Again, crawlspace installation shields your furnace from such issues.

  • Reduced mold and mildew

Finally, installing the furnace in the crawlspace may be the best way to deal with mold and mildew issues in your home. Although the two fungi types can originate from anywhere, the crawlspace naturally offers the best conditions for the growth and spread of both. Mold especially thrives in dark, moist conditions.

Installing the furnace in the area once and for all resolves moisture problems in the crawlspace, consequently terminating mold growth.


  • Risk of Radon gas poisoning

Radon is a naturally occurring radioactive gas found in the soil. It forms naturally when uranium, thorium, or radium break down in rocks, soil, and groundwater. As such, the element is common in unencapsulated crawlspaces.

Now, imagine introducing a furnace in the same area! The furnace would pull the radon-filled gas and disperse it throughout your home, exposing your family and guests to many health issues, especially respiratory cancer. NB: Radon is the #2 cause of respiratory cancer, only after smoking.

  • Consequences of poor ventilation

It’s no secret that most crawlspaces are ill-maintained and poorly ventilated. In fact, many homeowners have never gone down there to find out how the crawlspace looks. Installing the furnace in such conditions is asking for trouble, given that most furnaces merely re-cycle the air in the space where the appliance is located.

 You’d be exposed to unclean, unhealthy, and highly unsafe air whenever the furnace is running.

What Kind of Furnace Do I Need for a Crawl Space?

A Horizontal furnace. Also known as horizontal-flow furnaces, the horizontal furnace is placed on its side and draws air from one end while exhausting vent fumes through the opposite side. This makes it ideal for tight/small spaces.

Another identifying feature of the horizontal furnace is the smaller inner pipes. It’s worth noting that horizontal furnaces can be up-flow or down-flow systems. This means you can have either an up-flow horizontal or down-flow horizontal system.

If you’re wondering, up-flow and down-flow characteristics define the direction of cold and warm air through the furnace. Up-flow furnaces take cold air in through the bottom and release warm/hot air at the top of the furnace. Meanwhile, down-flow systems draw cold air from the top and release cold air at the bottom of the unit.

If you still feel a little lost, an up-flow horizontal furnace has the air return air plenum at the bottom of the air intake side, while the supply air plenum is located at the top of the opposite side. So, you’d have a diagonal rather than a horizontal line if you connect the two air plenums.

The other thing we must mention is that horizontal furnaces typically use direct vent configurations. A direct vent system features two venting pipes of different sizes that lead outside the house. The first pipe draws oxygen-rich air from the outdoors into the furnace to facilitate combustion.

Meanwhile, the second vent carries exhaust fumes outside the house.

Best Gas Furnace for Crawl Space 

The following are two of the best gas furnaces for the crawl space. We’ve only picked units from Goodman as it’s one of the best furnace brands. Moreover, reliable horizontal spaces that work effectively in the crawlspace are not easy to come by.

1. Goodman 93% Efficient Downflow/Horizontal 70,000 BTU Furnace


  • High-efficiency condensing furnace (93% AFUE)
  • Dual fuel unit that readily converts to a propane heater
  • Two-stage heating and variable speed blower
  • Simplified installation with convenient left or right connection


  • Expensive at about $2,000
  • Fuel conversion kit sold separately for $99

If you want a small horizontal furnace, the 70,000 BTU model #GCV90704CXA unit from Goodman is one of the best choices. It’s a two-stage variable-speed furnace with a 93% efficiency, which tells you everything you need to know about the furnace. Additionally, Goodman packs all the latest technologies in their modern furnaces.

For instance, this unit features a reliable hot surface ignition system with a patented adaptive learning algorithm to maximize the igniter life. It also comes in a corrosion-resistant, aluminized, steel tubular heat exchanger with a recuperative coil for maximum efficiency and long life.

The GMV9 is a natural gas heater but readily converts into a propane-burning unit with a field-installed LPM-03 propane conversion kit.

2. Goodman GMEC961004CN 100, 000 BTU Up-flow/Horizontal Gas Furnace 


  • Extremely energy efficient (96% AFUE)
  • Heavy-duty aluminized-steel heat exchanger
  • Multispeed airflow
  • Very quiet


  • Fairly expensive
  • Installation is a nightmare

If you desire a slightly bigger unit, we recommend the 100,000 BTU model GMEC961004CN, also from Goodman. It’s a slightly different furnace type as it’s an up-flow unit, whereas the first one is a downflow model. As stated above, up-flow furnaces have intake vents at the bottom and air outlet vents at the top of the unit.

Another key difference is that it’s a multispeed unit and not necessarily a two-speed system. Multispeed blowers have several set speeds and are closest to variable speed systems. So, you can expect significantly increased convenience.

Above all, the furnace features a heavy-duty aluminized-steel tubular heat exchanger and utilizes a self-diagnostic control board with constant memory fault code history. It’s very quiet too.

How to Install a Furnace in the Crawl Space

Let’s now discuss how to install a furnace in the crawl space. It’s not too different from installing a vertical furnace in the yard. 

Furnace in Crawl Space Codes and Space Requirements 

The first step in installing a horizontal furnace in the crawlspace (or attic) is understanding the requirements and standards plus local codes.

General Requirements

  • Only use the gas specified for the furnace. For instance, if the manufacturer specifies that the unit uses natural gas, don’t connect it to propane or another fuel source.
  • Never install your horizontal gas furnace outdoors or in a mobile home, trailer, or recreational space. Horizontal furnaces are specifically designed for homes built on site.
  • Never install the unit in a corrosive or contaminated environment. To this end, make the crawlspace meets all combustion and ventilation requirements in addition to local codes and ordinances.
  • Ensure ample space for servicing and cleaning. Thus, you need at least one-inch clearance at the top, two inches on the sides, zero inches at the back, and 18 inches at the front. Additionally, you need a one—inch vent clearance for Type-B vents and a six-inch vent clearance for single-wall vents.  

Code Requirements 

Building codes vary from one location to another throughout the country. Make sure you know all applicable codes. Ideally, consult local authorities having jurisdiction (AHJs) for gas piping, electrical wiring, and furnace venting. The installation must conform with ANSI Z223, National Fuel Code, or Canadian Installation Codes.

Crawlspace Gas Furnace Installation Guidelines 

Once you’re familiar with the general requirements and local codes, you can begin the installation process. Begin by choosing a great location. Crawl space furnaces can be hung from floor joists or installed on suitable blocks or a pad.

However, the blocks or pad must be high enough to eliminate the risk of water damage. Also, make sure to observe all the clearance requirements. Next, consider combustion and ventilation air. The total amount of combustion and ventilation air provided within the building must equal the total of all gas appliances in the building.

The best idea is to follow Section 5.3, Air for Combustion and Ventilation, of the National Fuel Code. From there, you can begin the actual installation. First, position the furnace in the selected location and hang or support it as you deem best. Then install the combustion and venting system, followed by the gas piping, and finally the electrical wiring.

Cost of Installing a Furnace in the Crawl Space

The average cost to install a horizontal furnace in the crawlspace ranges from $4,000 to $15,000. However, the actual cost depends on several factors, including the size of the furnace, accessibility of the crawlspace, and the amount of work it takes to complete the process.

Remember that getting the furnace to the crawlspace can also present a headache. You may need to pay extra for the additional work.


Is it okay to install a furnace in the crawl space? Yes. According to the International Gas Fuel Code, you can install a gas or oil-powered furnace in the crawlspace as long as you observe industry regulations and local codes.

Can you put a gas furnace in the crawl space? Yes, you can put a gas furnace in the crawlspace. The International Gas Fuel Code permits it provided follow industry regulations and your local building codes.

Can you put a forced-air furnace in the crawl space? Yes, you can install a forced-air furnace in the crawl space. However, the location of the appliance must not interfere with the circulation of combustion, ventilation, and dilution of air.

What kind of furnace do I need for the crawlspace? The ideal furnace type for crawlspace installation is a horizontal furnace. A horizontal-flow furnace is designed to be positioned on its side. It circulates air at one end and exhausts at the other end.

Can any furnace be installed horizontally? Unfortunately, no. Not all furnaces can be installed horizontally. Only models designed for horizontal installation can be installed horizontally. Installing a vertical furnace horizontally or vice versa can cause a fire.


There you go – everything you need to know about installing a gas furnace in the crawlspace in one place. However, don’t forget that installing your furnace in the crawlspace slightly complicates encapsulation as encapsulation typically leaves an area airtight. So, it’s best to speak with your HVAC technician about it before you begin the installation.