In November 2008, friends arrived at a mountain vacation home in Colorado and found an entire family dead. They died Thanksgiving night after an allegedly improperly vented boiler spewed toxic carbon monoxide into their bedrooms.

Survivors filed litigation alleging negligence in the installation of the boiler and its accompanying ventilation system. The suit was later settled.

While tragic on its face, the incident and outcome offers a grim reminder that improper ventilation systems can have disastrous consequences.

Proper installation of ventilation systems is complicated, but failure to do so properly can be a costly — and perhaps fatal — mistake. Here’s a primer to ensure contractors know the basics of venting and the consequences of not doing so properly.

Proper venting is a simple yet crucial proposition

At its core, venting is a simple proposition: divert potentially harmful gases — such as carbon monoxide and nitrogen oxide — and particulates away from a room or structure into the external environment. But many contractors commonly have questions on venting types and categories — all of which are stipulated under National Fire Protection Association Code 54 — and keys to properly venting gas-fired products such as water heaters, furnaces and other appliances.

Most contractors know to follow NFPA’s National Fuel Gas Code, which mandates standards such as venting distance from windows or other structural openings. But there are so many options for venting, especially in retrofitted buildings, it can confuse even the most experienced installer.

One of the first things you want to determine before installing a ventilation system is what NFPA category the appliance falls under. The categories guide, for instance, whether vent systems must be horizontal  or vertical; the type of materials used; whether the system must be sealed; and what kind of terminals are necessary.

Know your NFPA ventilation categories

Here’s a brief rundown of the ventilation categories:

• Category I: This includes vertical negative-pressure ventilation systems that channel gases more than 275 degrees. This category covers appliances with an efficiency level of less than 84 percent that produce no condensate. Air-tight and corrosion-resistant vent materials are not required for this category. Double wall vent pipe is recommended, although single wall can be used if the requirements of the National Fuel Gas Codes are followed. Manufacturer-certified vent caps are to be used in this category to permit the escape of gases but prevent downdrafts.

• Category II: This category covers negative-pressure vents of gases less than 275 degrees with an annual efficiency rating of more than 84 percent. Vent systems must be corrosion-resistant and water-tight, and cannot be combined with other ventilation. A vent cap is also used.

• Category III: Category III venting is pressure-positive with less than 84 percent efficiency. This horizontal vent system must be air-tight and must be exclusive to a single unit, no other unit is allowed to be vented into it. The vent gas in this category is above 275 degrees. Single-wall galvanized pipe, double-wall or single-wall stainless steel pipe may be used. For residential vent systems, interlocking stainless-steel material must be used. Manufacturer-certified vent caps are to be used in this category to permit the escape of gases but prevents downdrafts.

• Category IV: This classification is for positive-pressure vents with an efficiency rating above 84 percent that expel gases less than 275 degrees. Systems cannot be combined, materials must be air- and water-tight, and a condensate drain is required to collect condensate from a heat exchanger. This vent category calls for the use of schedule-40 PVC piping. As a caution, the uses of PVC piping on Categories I & III are not permitted.

Knowing the proper ventilation category for your gas-fired appliance can help you ensure it and its ventilation system is properly installed.

Ventilation design goals

Goals of good ventilation design include keeping the vent systems as straight as possible, meeting the venting criteria laid out in the installation manual, and properly following the guidelines laid out in NFPA54.

But the biggest concern with venting products properly is the safety of the people in the building or space. Any gas-fired product produces potentially harmful gases.

It’s important to get the job done right the first time. There have been instances in which installation had to be redone to ensure ventilation systems were up to code, resulting in unnecessary time and expense.