What is the difference between a Category 3 vertical exhaust and the Category 1?

Hello, thank you for reaching out. A Category 1 vertical vent uses natural draft and is designed for negative-pressure venting. Which allows the use of the approved Type B vent pipe when the installation meets Reznor's venting requirements. A Category 3 vertical vent is required for positive-pressure venting & uses sealed, UL 1738-listed stainless steel vent pipe to contain the flue gases and resist corrosion from condensate.If you have additional questions, please feel free to reach out over the phone to our Customer Service line at 1-888-901-5460. Have a great rest of your day.