What is a combustion air vent, and why should I care?

If your house has a big, insulated tube that drops from the ceiling to close to the floor (or bends up at the floor), you have a combustion air vent.

The name kind of says what it is. Pretty short blog today. More? Okay.

The appliances in your house that burn fossil fuels like fuel oil, natural gas and propane burn those fuels using combustion. To do that job, they need oxygen., which comes from our air. We call that combustion air. The vent is an insulated tube that runs from the outside of the building into the furnace room, and the open end is near the floor. Sometimes, it stops a few inches from the floor, but sometimes the installer has bent into a “J”,to let cold air warm up a bit before it hits the floor. If you want the air to get a better chance to warm up, you can put the end into a big bucket, like a wastepaper basket. Don’t use a small bucket; that would block the air flow.

About the only thing you can do wrong is to forget to clean the lint from the screen where it enters the building. Use a gloved hand, a brush or a shop vac. Don’t forget to look for wasps first.

Oh. And check how fine the screen is. The screen is there to keep birds and such from making a nest. A screen that is too fine will quickly fill up with lint and get clogged. The best size for a screen is in the 1/4″ to 1/2″ range.

The combustion vent looks just like another kind of vent, called a “makeup air” vent. That kind of vent is installed when a kitchen exhaust fan is so big that it creates a vacuum in the house. Exhaust hoods designed for restaurants aren’t allowed in home kitchens without a make-up air vent. The requirements for a regular home  is at least 100 cfm intermittent or 25 cfm continuous. Restaurants, on the other hand, often move 400 cfm continuous.

Related to this subject is an item called HRV, which stands for heat recovery ventilation.  Related to THAT subject is something called ERV (energy recovery ventilation), but we see a lot fewer of those in the colder states. You can learn more than you want to at Minnesota’s Sustainable something something by clicking :HRV stuff.

There is a pretty good blog about it, written by a very wise man with a wonderful sense of humor. Just click:

HRV blog

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About the author

Joe has many years of experience as a Home Inspector. Joe is a proud member of ASHI, MAHI, WAHI & SAAR. He follows the Best Practices as described by the American Association of Home Inspectors and the Wisconsin Association of Home Inspectors. He is licensed in Wisconsin and is Radon Certified. Joe also complies with ASHI's Code of Ethics.

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