Reuben's Home Inspection Blog


Don't Stuff A Rag In There

Everyone knows that oxygen is required for a fire, right?  So where does the oxygen come from for your gas 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 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 grade, 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 - Email - Minneapolis Home Inspections

Reuben Saltzman, Structure Tech Home Inspections


Comment balloon 3 commentsReuben Saltzman • March 04 2009 06:40AM


Hi Reuben.  Great Information.  I wonder how these combustion ducts show up on the infrared pictures and energy audits.  Air sealing is often a line item with a significant saving amount next to it. 

Posted by Jim Mushinsky (Centsable Inspection) over 11 years ago

Reuben---another case of furnace emphysema:)

Posted by Charles Buell, Seattle Home Inspector (Charles Buell Inspections Inc.) over 11 years ago

I don't think we use intakes here except for the high efficiency power vents. All of our exterior hoods like those in your picture are either laundry exhaust vents or bathroom and kitchen exhaust vents. They all have dampers on them.

Posted by Jim Frimmer, Realtor & CDPE, Mission Valley specialist (HomeSmart Realty West) over 11 years ago