On a recent call to replace an aging gas boiler, the owners reported a persistent gas smell. The gas company had previously inspected the building and couldn’t find a leak. Yet the smell continued. The point is, if you smell gas, however subtle the smell, there is a gas leak. Somewhere. The challenge can indeed be where to find it.

Water is a byproduct of combustion. If the proper venting of flue gases is neglected — as is so often the case — water in the form of condensation triggers serious safety and comfort issues for our customers. As professionals, we must pay more attention to flue gas temperatures and the conditions of the flue during our service calls.

Gas appliances are divided into four venting categories based on vent operating pressure and whether they are condensing or non-condensing. Category I is negative pressure, non-condensing. Category II is negative pressure, condensing. Category III is positive pressure, non-condensing. Category IV is positive pressure, condensing.

This multiple-part article series will discuss the basics, codes and standards, and the science behind chimneys. It will show how back drafting affects chimney performance, while explaining the principles of positive pressure. It will explain how to create and use positive pressure so chimneys will draft for safety and performance.