Combustion Venting Categories – Short #189

In this short podcast episode, Bryan talks about the four different combustion venting categories for gas appliances as set by ASHRAE and where you'll see them. He also shares some notes about pressurization.

These categories deal with the pressurization and temperature ranges of the vents. Category 1 venting is used for old-school open-combustion gas furnaces; they have high flue gas temperatures and are considered low or mid-efficiency furnaces. This venting category is not positively pressurized, and it has a single-wall flue and operates more like chimneys, as the appliance is usually under negative pressure; a draft is created and draws the flue gas out. It's non-condensing, negative-pressure venting.

Category 2 venting is not common anymore; they operate with negative pressure in the vent, and condensation is still likely.

Category 3 venting is non-condensing positive-pressure venting. These are more common in older through-the-wall appliances.

Category 4 venting is condensing, positive-pressure venting for high-efficiency or condensing gas appliances with lower-temperature flues and sealed combustion. PVC is the most common venting material for these furnaces. We can recover some energy from the condensation process.


Learn more about the 5th Annual HVACR Training Symposium at

If you have an iPhone, subscribe to the podcast HERE, and if you have an Android phone, subscribe HERE.”

Subscribe to our YouTube channel at

Check out our handy calculators HERE or on the HVAC School Mobile App (Google Play Store or App Store).




To continue you need to agree to our terms.

The HVAC School site, podcast and daily tech tips
Made possible by Generous support from