When is it permissible to vent a Category IV furnace into the same chimney or vent as a water heater?

A Category IV furnace is a high-efficiency appliance that typically uses a condensing process to extract as much heat as possible from the combustion gases, which results in the production of a more significant amount of condensate and cooler flue gases. Because of these unique characteristics, proper venting is critical to ensure safety and efficient operation.

Venting a Category IV furnace with a water heater in the same chimney or vent system is generally not permitted due to several safety and operational concerns. The condensation that forms as exhaust gases cool in the venting system can lead to inadequate venting, increased humidity, and potential corrosion in the venting materials. Moreover, the cooler flue gases from a water heater can interfere with the operation of the furnace and lead to issues such as backdrafting, where dangerous gases can enter living spaces instead of being properly vented outside.

The safety codes and standards outlined by organizations like the Technical Standards and Safety Authority (TSSA) are designed to prevent these issues. Proper venting practices are vital for ensuring that both appliances operate safely and effectively, which is why sharing a vent between a Category IV furnace and a water heater is not allowed.

Understanding these crucial safety parameters helps in maintaining compliance with current regulations, ensuring not only the