What size vent do these use. 4”or 5”

Hello, thank you for reaching out. According to the unit specifications & installation manual, the 125,000 BTU model is equipped with a factory-supplied 5-inch flue collar. A 4-inch vent is not approved for this model. 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.