Furnace Combustion Air Vent: What's With that Big Hole In My Wall?

In my professional experience, I've seen dozens, if not hundreds, of homes with combustion air vents that have been purposely blocked or plugged by well-meaning homeowners.

In too many of those cases, the blocked combustion air vents have resulted in potentially deadly carbon monoxide leaking into the living space.

Those holes in your wall are there for a very important reason!

Heat and Cool

You may have one in your utility room, garage, crawl space, or furnace room. Right there, in your carefully sealed and insulated house is a great big vent hole leading to the great outdoors and spilling all kinds of cold air into your home.

It's completely counter-intuitive. Many of us have spent so much time and money upgrading insulation, sealing openings, investing in good quality windows and doors, installing efficient appliances--then along comes a heating contractor who cuts a great big hole in the wall. It just seems wrong.

Too many people feel all that cold air spilling from a combustion air vent and stuff insulation in them. I've personally pulled insulation from multitudes of combustion air vents, along with towels, t-shirts, high expansion spray foam, pie tins, and in one odd case, a whole dog bed!

Still more homes across the nation were built without a combustion air vent and they desperately need them in order to keep the occupants of the home safe and healthy.

Here Is the Story on the Combustion Air Vent and Why You Need One (or more)

Any combustion appliance open to the indoor air system (think furnace, unit heaters, wood stove, water heater, or gas-powered dryer) requires a supply of air to support the flame within the burner and a create draft up the chimney or out the vent. That air must come from somewhere.

Building codes in many areas of the country used to assume that adequate combustion air would leak through holes and cracks in the building shell. In the case of older construction or homes in warmer climates, this incidental leakage makeup air may be sufficient to support most appliances.

All current nationally-recognized model building codes now require some form of combustion appliance venting. Many local building codes have added additional requirements on top of the national building standards, requiring even more combustion air in a home with natural draft combustion appliances.

The thinking for many years was that the air leakage in a home was adequate for combustion venting. This assumption was probably more appropriate back in the day when energy was cheap and global climate change was unknown to the general population. In the case of newer, well-sealed homes and more efficient homes, fuel burning appliances may become starved for combustion air causing them to malfunction.

Worse, the simultaneous operation of multiple venting appliances may create a negative pressure within the living space of a home that can overcome the stack draft pressures in a natural draft appliance. This dangerous situation can cause exhaust gas spillage and backdrafting of potentially deadly carbon monoxide into the living space.

Extreme pressure differentials caused by winds or severe cold weather can make the effect much worse.

Local Codes Are Catching Up

In many cold regions, building codes have required the installation of combustion air vents whenever combustion appliances use living space air for venting.

Some really restrictive local codes may require that vents be hooked to powered interlock dampers that close when the appliances are not on, reducing the likelihood that homeowners will block the vents. Other codes require multiple vents, reducing the possibility of blockage or pressurization due to wind conditions.

Sealed Combustion Appliances Are Safer

Fuel burning appliances that utilize a sealed combustion air system typically use a power blower to draw their combustion air directly from the outside, largely eliminating the need for a passive air vent. These units operate with a positive stack pressure with reference to the building air and allow no exchange of air between the building air and combustion system.

Sealed combustion appliances such as the gas-fired condensing furnace are far higher efficiency units, making them even more desirable.

Important Combustion Air Vent Points To Remember

  • Always check your local building codes regarding venting requirements, vent sizing, positioning of vents, required number of vents, and vent arrangement.
  • If you already have a combustion air vent in your home, do not block it under any conditions!
  • If you are replacing a combustion appliance, strongly consider investing in a sealed combustion unit.
  • Always have a working Carbon Monoxide (CO) detector in your home if you have any combustion appliance present (oil, gas, wood, etc.).
  • Strongly consider having a professional energy audit performed by an auditor certified through a national organization such as the Building Performance Institute or RESNET. This audit will evaluate your appliance venting and indoor air quality issues. A good HVAC technician may be able to help you with this as well (although the professionalism and quality of this advice varies greatly.)

As always, local codes, laws, and statutes must be consulted and followed. Seek the guidance and services of a licensed contractor.

Safety issues such as these are not a place to take shortcuts!

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