Which category of venting involves negative pressure more than 140 degrees above the dew point?

The correct category of venting that involves negative pressure and operates at temperatures more than 140 degrees above the dew point is Category Four.

Category Four venting systems are specifically designed for high-efficiency condensing appliances that typically operate at negative pressure and utilize special venting materials due to the presence of flue gas condensate. This temperature and pressure characteristic allows for the safe venting of gases that are cooler than the flue gases produced by traditional combustion processes, minimizing the risks of acidic condensate damaging the venting material.

Other categories of venting do not have the same characteristics. Category One typically refers to non-condensing appliances that operate positively, while Category Two refers to appliances that also operate with non-condensing venting, but without the efficiency of condensing systems. Category Three involves positive pressure venting for specific types of appliances but generally does not exceed the dew point in a manner conducive to the characteristics of Category Four venting.