Heating & Cooling

Does a Furnace Need a Fresh Air Intake?

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With the constant evolution of furnaces and the architecture of homes in general, fresh air intake is a confusing one for many homeowners. What is the actual function of these systems? Are they still necessary today?

Furnaces need a fresh air intake because they are typically designed to push air out of the home while functioning, no matter the type installed. With the fresh air intake installed, you can ensure that the furnace’s combustion will not reduce your home’s air quality.

The rest of this article will take a closer look at what fresh air intake means and how they are essential for maintaining your furnace’s efficiency. You’ll also see how fresh air intakes are important to the indoor air quality in your home.

Fresh Air Intake Explained

As the name implies, a fresh air intake is a channel designed to draw fresh air into your home from outside. Most of the time, the fresh air intake is a duct that runs from a vent outside into the room housing your furnace. The exact locations and the number of fresh air intakes will vary in different homes, especially in regions where building codes dictate how air is released from the home.

Building regulations that control air loss are there to ensure that your property is energy efficient and conserving energy resources, thus ensuring that you contribute your quota in the fight against climate change. They are also designed to make sure that your home is properly ventilated at all times.

These regulations support the installation of fresh air intakes because they play an important role in keeping your home ventilated, especially while a furnace is used.

Why Is a Fresh Air Intake Important for Furnaces?

Furnaces need oxygen to run, so they need as much fresh air as possible. Without fresh air intake, the furnace will use up the air in its immediate environment. To understand this better, you need to know how the various types of furnaces work.

Standard-Efficiency Furnaces

They have also known as conventional or 80% furnaces, as they have a rating of 80 in their annual fuel utilization efficiency (AFUE). The rating means that such furnaces lose 20% of the natural gas energy that can be utilized for heat during combustion as a result of venting.

Standard-efficiency furnaces work by drawing air from their immediate surroundings. Think of this as some sort of equipment in the middle of your room with a fan channeling air into it and a vent connected from the top of the equipment to the outside carrying out the excess air. This means that your room is being stripped of the air in it, thus creating negative pressure.

The design of these furnaces is such that they will draw air away from your home regardless of what you do. Your focus should be on figuring out where the fresh air will come from. This is where the fresh air intake comes in. If you have one, the air pumped out of your home by the furnace will be replaced almost immediately by the fresh air intake, which draws air from the exterior.

If you are using a conventional furnace without fresh air intake, the air in your home will come from crawl spaces, dryer vents, attics, and other such sources as your furnace runs. This will negatively impact the indoor air quality as air from these spaces is often laced with contaminants.

Apart from worsening the air quality in your house, a standard-efficiency furnace running in a space where there’s no fresh air intake won’t function at optimum efficiency. This is because its blower motor will require more electricity to keep up with the demands of powering the process, and it will also generally run slower.

Additionally, the air sucked in from your attic and other similar sources also typically contain a higher level of contaminants with corrosive properties, which can cause the corrosion of parts inside the combustion chamber of your furnace—especially the heat exchangers and the burners. This reduces the lifespan of the furnace.

High-Efficiency Furnaces

High-efficiency furnaces are becoming more popular today. They are sometimes referred to like 90% furnaces to reflect their AFUE rating, which is 90. This means that such furnaces lose 10% of the natural gas energy that can be utilized for heat during combustion as a result of venting.

The confusion about whether or not to use fresh air intakes is often most visible amongst people that have installed this type of furnace. This is because these 90% furnaces feature a special pipeline that runs straight from an exterior vent into the furnace’s combustion chamber, which is sealed.

They also come with a vent outside of the property to dispose of the gases generated while the furnace is working.

The architecture of high-efficiency furnaces means that they have their own fresh air intake. This means that they don’t use the air inside your home; rather, they draw air from outside. However, this doesn’t mean you don’t need an external fresh air intake because you’ve installed a 90% furnace. Indeed, these furnaces don’t cause the poor indoor air quality problem, but they pose a different concern.

Homes that have 90% furnaces are typically vulnerable to excess moisture. This is not a problem in conventional furnaces, as the open flue allows moisture to escape easily.

For information on SEER ratings and high-efficiency heat pumps, see our article Complete Guide to Heat Pump SEER Ratings.

Should You Block Fresh Air Intake During the Winter?

For many homeowners, blocking fresh air intake during the winter sounds reasonable as it is counter-intuitive to allow cold air in while you are trying to heat any space. This is why you will find many people stuffing the fresh air intake with rags, insulation, or any other materials that they feel can prevent the draft from coming down the fresh air intake duct.

Does this describe you? Then you need to rethink your approach, especially if you have a conventional furnace. Fresh air intakes are designed to ensure your furnace is working effectively and make sure your home is properly ventilated. If you block it off with a rag, then make sure your furnace is not running. Otherwise, there is a high chance of toxic fumes, including the deadly carbon monoxide, flowing back down your flue vent.

Your furnace is designed to work in the presence of any draft that might be coming down the fresh air intake and will most certainly neutralize it.

On the other hand, if you own a 90% furnace, you may think it is a good idea to block the fresh air intake since there is little to no risk of carbon monoxide poisoning. Unfortunately, the excess humidity caused by the trapped moisture can cause a range of health concerns, including the following:

  • The growth and spread of bacteria and viruses
  • Higher levels of indoor allergens such as dust mites and fungi
  • Exacerbation of respiratory conditions such as asthma
  • Increase in airborne chemical contaminants such as formaldehyde


Fresh air intakes are necessary for any home that has a furnace. Whether you have a conventional furnace or a high-efficiency option installed, there are downsides to not having a functional, fresh air intake.

These risks may seem more pronounced with standard furnaces, but the health hazards are every inch as dangerous on the newer furnaces. Fresh air intakes are there to keep your home properly ventilated, so don’t block them.

Some heat pumps use a fresh air exchanger. For more information, see our article Do You Need an Air Exchanger with a Heat Pump?


Hubert Miles | Licensed Home Inspector, CMI, CPI

Hubert Miles is a licensed home inspector (RBI# 2556) with more than two decades of experience in inspection and construction. Since 2008, he has been serving South Carolina through his company, Patriot Home Inspections LLC. As a Certified Master Inspector, Hubert is dedicated to providing his expertise in home inspections, repairs, maintenance, and DIY projects.