As high-efficiency gas-fired furnaces and water heaters are retrofitted into existing residential venting systems, the tendency for corrosive flue gas condensate to form increases in vent connectors, common vents, and masonry chimneys. This issue can arise through a range of scenarios, such as the retrofitting of a condensing furnace in a residence with dedicated venting, "orphaning" an atmospheric gas-fired water heater in the vent system from which flue gases reach sub-dew point temperatures from cooler vent walls. The cycling of a retrofitted appliance influences condensation dynamics as well, altering the so-called "wet-time" of the flue interior surface, key to its potential for corrosion. Additionally, the vent itself and its operating conditions are important, including the design, proximity to capacity, and ambient conditions are influential.

Using the combination of computational tools and a full-scale laboratory exterior masonry chimney, the Gas Technology Institute (GTI) has framed this issue facing residential venting systems. Much of the work covered in this paper concerns the use and validation of VENT-II, a residential venting simulation software tool for common vented appliances key to the development of the National Fuel Gas Code venting guidelines. Through targeted use of computational fluid dynamics and full-scale experimental testing, GTI has begun an effort to validate and improve the accuracy and validity of the software, initially focusing on the performance of hot water boilers installed in exterior masonry chimneys. Through this validation, GTI has studied the impact of retrofit scenarios for vent system designs that are on the margins of compliance with the National Fuel Gas Code. As residences approach Zero Energy Designs, the push for higher efficiency appliances will continue to present challenges to safe and efficient venting systems.