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Reuben Saltzman

Combustion Air Ducts

Everyone knows that oxygen is required for a fire, right?  So where does the oxygen come from for your furnace, gas water heater, fireplace, and other fuel-burning appliances?  Typically, this is supplied through a combustion air duct.  You’ll find combustion air ducts on almost all newer houses, and on many houses that have had new furnaces or water heaters installed.  While it was once thought that these ducts were not as necessary on older, drafty homes, newer research has shown that these leaks are not always reliable, and they are never desirable.

In homes that have bathroom exhaust fans and kitchen exhaust fans, it is especially important to have a combustion air duct installed.  While combustion air ducts are not intended to provide make-up air for exhaust fans, this is really what they end up doing, in addition to replacing the air that gets used by the fuel-burning appliances.  While it’s beyond the scope of this blog to get in to the specifics of how these are all inter-related, there are a couple of simple, but very important things to do as a homeowner.

  • Don’t block the duct opening.  This is the easiest, most obvious thing that you can do (or not do?).  The photo below shows a typical combustion air duct, with the opening un-obstructed.  You’ll feel cold air coming out of this duct on to the floor in the winter, and this is air that needs to come in to the house.  I’ve seen people tape the bottoms of these ducts shut, and I’ve seen rags stuffed in to the duct.  This is very bad idea, as carbon monoxide could be produced from lack of combustion air.  To help keep cold air from dumping in to the house, a loop can be created at the bottom of the duct, or a bucket placed below it, as long as it doesn’t reduce the overall opening.

Combustion Air Duct Combustion Air Duct with a bucket at the bottom

  • Make sure the intake is un-obstructed.  In Minnesota, the intake is required to be located at least 12” above the ground, to help keep it clear from snow, leaves, and other debris.  If the intake is closer than this, consider having it raised.  If there is a damper installed at the opening at the exterior, remove it.  Dampers allow air out, not in! I’ve seen dampers installed on many homes when the vinyl siding installers didn’t know what the opening was for, so they installed a damper, which blocks the combustion air opening.

Combustion air inlet too close to grade

  • Keep the intake clean.  This is something you should check at least once a year.  The intake will be located at the exterior of your home, and looks like the one pictured below.  There should be a 1/4 steel screen installed, which will keep larger pests from entering in to your home.  If this screen is dirty, clean it with a wet/dry vac.

Dirty combustion air intake

When I inspect houses, I check for all these things and I share this information with my clients.

Reuben Saltzman, Structure Tech Home Inspections – EmailMinneapolis Home Inspections

No responses to “Combustion Air Ducts”

  1. Why Is My Water Heater Backdrafting? | Reuben's Home Inspection Blog
    August 31, 2010, 4:04 am

    […] Insufficient combustion air. I start every home inspection the same way – I close up all of the windows and doors, and I turn on all of the ‘motors’ at the house that remove air.  These fans include bathroom exhaust fans, kitchen exhaust fans, and clothes dryers.  Besides giving me the opportunity to make sure the fans are properly working, I’m removing a lot of air from the house at one time, which creates a ‘worst-case’ scenario for the fuel-burning appliances, such as the water heater.  If there isn’t a proper combustion air duct installed, the water heater vent may end up acting as an air intake. This will definitely cause backdrafting. […]

  2. mike
    December 2, 2010, 11:08 pm

    I recent ly installed one of these and have it pouring into a bucket. I find that it always has cold air pouring in, even if there are no appliances drawing air. Any ideas? Is this normal or am I wasting energy?

  3. Reuben Saltzman
    December 3, 2010, 5:22 am

    Mike – yes, that’s normal. The bucket will help to slow down some of the air, but won’t prevent it from coming in entirely.

  4. Jason
    December 20, 2012, 8:42 pm

    Typically, I see interior doors to the furnace/water heater closet that have a register attached to it, but since this is not always the case, I am wondering if this is normally required or if a solid door can be used without the register.

    Any tips would be appreciated.

  5. Reuben Saltzman
    December 21, 2012, 4:49 am

    Hi Jason,

    It’s generally a good idea to have openings in to the room where the combustion equipment is located, but there are a lot of variables involved, such as the age of the house, the size and type of the heating equipment, the size of the room, etc.

  6. Vickie Swenson
    February 20, 2013, 9:03 pm

    This is great infomation. I have a client whose combustion air vent is touching the floor. I’m recommending that it be cut to allow for more air flow, and I see from your post that it should be 12 inches off the ground.

    Last year I saw a vent that was intentionally closed by a renter with fiberglass insualiton. The entire house was condensing so bad that mushrooms were growing in the sliding door in the kitchen. The carpet was wet, the windows were dripping. Two days after the combustion vent was opened, the humidity returned to normal levels and the house dried out.

    I always check the in-take vent for HRVs, but I have never thought to check the vent for this passive combusion air vent. I will from now on!

  7. Vickie Swenson
    February 20, 2013, 9:06 pm

    OK, I re-read the post and realize that the outdoor intake vent needs to be at least 12 inches off the gound. How far from the floor should the bottom of the duct be inside of the house? Thanks!

  8. Reuben Saltzman
    February 21, 2013, 4:40 am

    Ideally, the same thing; about 12″ from the floor.

  9. Renae
    February 26, 2013, 6:26 pm

    I am looking at replacing our furnace, and we currently have one combustion vent. One of the contractors we contacted said that Code required two vents. I have not been able to find this. Is this accurate? I am worried I am being misinformed.

  10. Reuben Saltzman
    February 27, 2013, 4:53 am

    Hi Renae – here in Minnesota, section 304.6.2 of the Minnesota Fuel Gas Code allows one permanent opening.

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