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All About Combustion Air Ducts

Home Inspector with Structure Tech Home Inspections

Have you ever noticed a big insulated tube dropping down next to the floor near your furnace or boiler in the basement?

Makeup Air Duct

If you trace this duct down, you'll find that it connects to an opening at the exterior of the building.  This is essentially just a hole in the side of the building that brings in fresh outdoor air.  Homeowners, builders, and insulation contractors spend lots of  time trying to seal up every little air leak in to a house, but then the building code requires this big hole that allows cold air to just dump in to the basement.  Silly, right?

I'll try to help make some sense of this.

Houses need air

This opening is a passive intake that provides needed air to the home.  There are several items in a home that remove air - here's a partial list of common items found in Minnesota homes that remove air from the house:

  • Furnaces and boilers that are not direct vent / sealed combustion type
  • Water heaters that are not direct vent / sealed combustion type (at least 99%)
  • Bathroom exhaust fans
  • Kitchen exhaust fans
  • Clothes dryers
  • Wood burning fireplaces

The stack effect in a home, wind, and radon mitigation fans may also remove air.  The most common and obvious problem with too much air being removed from a house is a backdrafting water heater, but there's a lot more to it than just this.

Houses leak

When air is removed from a house, it has to be replaced.  If a house is not built tight, the air will get replaced from every little hole in the envelope in the house; the photos below show a few examples.  These are the things that get corrected to make houses "tighter".   The first photo below shows an outlet box at an exterior wall that hadn't yet been sealed.  Those openings get sealed in new houses today, but this never used to happen.

Leaky Outlet

The photo below shows the furnace vents going through the rim joist.  Daylight is visible around these penetrations, which means air leakage.

Leaks in rim joist

The opening around the faucet is obvious.

Air leak at sillcock

Of course, windows and doors are also a huge source of air leakage.  Daylight showing through is a dead giveaway.

Daylight visible below door

Leaking door with IR overlay

Unsealed openings in the exterior walls equates to uncontrolled air leakage.  Every time the wind blows, air will leak in or out through these openings.  Even without any air moving at the exterior, the stack effect in a home will cause air to leak in through the lower openings in the envelope of a home, and back out through the upper openings, such as attic bypasses.  The image below, used with permission © 2013 E Source, gives a visual example of the stack effect.

Stack Effect

The line of neutral pressure plane will be different in every home.  Some of the factors that affect this are differences in indoor / outdoor temperatures, wind, the height of the home, and how much air is leaking.  For the upper 'positive pressure' leaks, one of the most obvious that can be viewed from inside the house is a loose-fitting attic access panel.

Leak at attic access panel

Other attic air leaks, most of which can only be seen from inside the attic, are also major contributors.  These include leaks around furnace ventselectrical cablesplumbing ventschimneys, etc.

When air is allowed to leak through the house uncontrolled like this, the amount of air leakage and energy loss is typically much more than it needs to be, and it doesn't happen where, when, or how it should.  This can lead to condensation and frost at windows, in the attic, and even inside the walls.

Frost at basement wall

The Combustion Air Duct

To help reduce the effects of uncontrolled air leakage, houses get sealed up as tight as possible and a single hole is created to bring outdoor air in to the basement, usually right next to the furnace.  This is the combustion air duct I showed at the beginning of this post.

When a combustion air duct is properly installed, it will help prevent the house from getting depressurized.  The air is allowed to come in to the house as needed through a large opening, and all of those other holes in the walls can be sealed up.  To see how well this works in a new house, try running all of the exhaust fans for about 5 minutes, then put your hand over the end of the combustion air duct; if it's working properly, you'll feel plenty of air pumping in to the house.  Beautiful.

Problems and Solutions

By far, the most common problem that occurs with combustion air ducts is that they get blocked.  When a combustion air duct is blocked, air needs to 'leak' in to the house through many different undesirable pathways.  I've done a number of home inspections where the windows were completely iced shut throughout the house, and in every case there was a blocked combustion air duct.

Problem: Intentional, ignorant blockage

A combustion air duct brings in fresh outdoor air, which usually means cold outdoor air in Minnesota.  This can create a cold floor where the duct terminates, as well as a cold draft.  I was going to make a nice little drawing of this cold air coming in to the basement around my own combustion air duct, but then I remembered I have an IR camera.  Duh.  Check out the two images below for a nice visual of how the combustion air duct is making my basement floor cold.

Combustion air duct with IR overlay

 To prevent this cold air from dumping in to their home, people sometimes stuff clothes or towels in to the combustion air duct, or the block the intake at the exterior of the home.

Blocked Combustion Air Duct Combustion air covered with duct tape

Solution: Remove any obstructions.  If you want to help cut down on the amount of cold air that just 'dumps' down in to the basement, try creating a trap at the bottom of the combustion air duct.  Make the air have to rise back up again before coming in to the home.  I don't have any hardcore proof that this makes a big difference, but I've convinced myself that it helps, and it's easy enough to do. The two most common ways of creating a trap are to either make a "J" at the bottom of the duct, or to put a bucket  or box underneath the duct.  With either of these methods, the air will need to rise up before coming in to the home.

Combustion air duct with bucket Combustion air duct J

Just make sure that the bucket or box you use isn't so small that it restricts air flow.  I've always just eyeballed this, but if you're super anal, you could make your sixth grade math pay off by measuring the inside diameter of the bucket and the outside diameter of the duct, then calculate the surface areas (Πr²) and make sure the bucket's is at least twice that of the duct's.

Restrictive combustion air duct box Restrictive combustion air bucket

Also, make sure the duct isn't so long that the opening sits flat on the floor, effectively blocking it.

Problem: Lack of maintenance

The opening at the exterior for the combustion air duct will bring air in to the home, and with that comes dust, dirt, insects, leaves, etc.   I've found that the closer the combustion air duct is located to the ground, the more likely it's going to get blocked with debris.

Dirty combustion air intake

Solution: take a peek underneath your combustion air duct every year to make sure it stays clean.  If you do this during the summer or fall, watch out for wasps.  They love to make nests in this opening.  If the opening is dirty, vacuum it off.  If you have an HRV, check the HRV intake at the same time.

Problem: Small Mesh at the Exterior

The opening at the exterior of the home needs to be covered with a steel mesh having openings not less than 1/4", and not more than 1/2".   When standard window screen is used here, it will get dirty very quickly.  Click on the photo below for a larger view; you'll see the opening is actually covered with a window screen, which should be removed.

Window screen at combustion air intake

Solution: Remove any restrictive mesh or material, and replace it with 1/4" hardware cloth or something similar if it's not already present.

Problem: Unintentional, ignorant blockage

Every so often, vinyl siding installers will forget which opening was meant for the combustion air intake, and they'll install a damper at this opening instead of a screen.  These dampers allow air out, not in.

Damper at combustion air inlet

Solution: Replace the exterior terminal with a type that is designed for a combustion air intake, or remove the damper and cover the opening with 1/4" hardware cloth.

Problem: Inlet installed too close to the ground

The inlet for the combustion air duct needs to be installed at least 12" above grade.  When it's too close to the ground, it can get dirty very quickly, and can get blocked over with snow.

Combustion Air inlet too close to grade Combustion air inlet blocked with snow

Solution: When the combustion air inlet is installed this close to the ground, it's usually done because that's where the rim joist was located, so making a higher hole in the side of the house isn't an option.  The solution is to install what Milind calls a 'snorkel'.  I laughed the first time I heard this, but I like this term.  I think the photo below is pretty self-explanatory.

Combustion air snorkel

That's about all I know.

Jay Markanich
Jay Markanich Real Estate Inspections, LLC - Bristow, VA
Home Inspector - servicing all Northern Virginia

Well, I suggested you before I read the whole post Reubs.  Great discussion.  But houses need to breath!  In the winter it breathes cold air, but in the summer it breathes warm, so it evens out.  Even is good...



Mar 26, 2013 08:46 PM
Reuben Saltzman
Structure Tech Home Inspections - Minneapolis, MN
Delivering the Unbiased Truth.

Thanks Jay!  Gee, I never thought of it that way ;-)

Mar 26, 2013 09:04 PM
Clint Mckie
Desert Sun Home, commercial Inspections - Carlsbad, NM
Desert Sun Home, Comm. Inspection 1-575-706-5586

Hi Reuben,

There are a lot of homes that are messed up when it comes to proper breathing.

I write this stuff up all the time.

Have a great day in Minneapolis.

Best, Clint McKie

Mar 26, 2013 09:54 PM
James Quarello
JRV Home Inspection Services, LLC - Wallingford, CT
Connecticut Home Inspector

Great overview of the importance of combustion / dilution air intake. This is from my experience a not well understood concept. 

Mar 26, 2013 10:18 PM
Michael Setunsky
Woodbridge, VA
Your Commercial Real Estate Link to Northern VA

Reuben, learned a lot today about combustion air ducts.Thanks for the information.

Mar 26, 2013 10:23 PM
Reuben Saltzman
Structure Tech Home Inspections - Minneapolis, MN
Delivering the Unbiased Truth.

Clint - You guys need this stuff down in the South too?  ;-)

James - given the number of ducts stuffed shut, you're definitely right.

Michael - thanks!

Mar 26, 2013 10:50 PM
Scott Seaton Jr. SLS Home Insp
SLS Home Inspections-Kankakee Will Iroquois Grundy Counties - Bourbonnais, IL
The Home Inspector With a Heart!

As always, you are a fountain of useful knowledge. Or should I say a breath of fresh information?

Mar 26, 2013 11:50 PM
Jim Gilbert
Samson Properties - Fairfax, VA
The Gold Homes Team

Most informative and helpful posting.

Mar 27, 2013 12:06 AM
Richie Alan Naggar
people first...then business Ran Right Realty - Riverside, CA
agent & author

This post is on par with a college class or a book on the subject...Well done and shared...A gift of giving here...thank you

Mar 27, 2013 12:24 AM
Robert Butler
Aspect Inspection - Montreal West Island, QC
Montreal Home Inspector | Aspect Inspection
Great article Reuben. This is a good comprehensive discussion of the issues relating to this. It's one of those things that people don't 'see' and don't think about, but nevertheless the less is important. Around here progressive developers and builders are accommodating these requirements by providing HRV systems.
Mar 27, 2013 08:04 AM
Pamela Seley
West Coast Realty Division - Murrieta, CA
Residential Real Estate Agent serving SW RivCo CA

Lots of information here about combustion air ducts. I wouldn't know, but I do know homes need to breathe. Thanks for your post today,

Mar 27, 2013 10:46 AM
Paul S. Henderson, REALTOR®, CRS
Fathom Realty Washington LLC - Tacoma, WA
South Puget Sound Washington Agent/Broker!

Wow that is a lot of information Reuben. I learn so much from each home inspection I attend...

Mar 27, 2013 05:37 PM
Ray Stockwell
ZipperAgent - Boston, MA
Director of Marketing

I agree with paul Henderson ~ every time I read one of your blogs I feel that I learn so much.  Wish I had the time, money and knowledge to make my 1896 house tighter.

Apr 04, 2013 07:14 AM
Earl Bell

What is the code for drawing inlet air from the attic and ducting it to the dryer. In the south we have temps in the 145 degree range up there. Florida .

Sep 16, 2015 06:17 AM