Proper Fireplace Venting: A Complex Issue

By Rodney Twyford, NPI Franchise Owner and Inspector, San Antonio, Texas

Fireplace_shutterstock_172698602One of the biggest controversial issues with home construction has been proper fireplace ventilation. In an effort to prevent indoor air contamination and improve overall efficiencies within modern homes, the home envelopes have become tighter — meaning little to no air leaks between interior and exterior spaces. While the intentions were good, constructing a tight home has caused some other issues, such as poor air change ratios and controlling pressures between interior and exterior spaces. This has resulted in new technologies to provide controlled mechanical ventilation systems.

Since the topic of building ventilation is vast and involves many different systems and components, this article will focus on exterior air supply, specifically for factory-built and masonry-built fireplaces.

This is a subject that is often misunderstood and which has resulted in improper installation of exterior air supply vents. The current International Residential Code (IRC) Section R1006.1 says, “Factory-built or masonry fireplaces … shall be equipped with an exterior air supply to assure proper fuel combustion unless the room is mechanically ventilated and controlled so that the indoor pressure is neutral or positive.”

Most masonry fireplaces are not constructed with an exterior air supply, and factory-built fireplaces are designed to incorporate exterior air supply vents but are often not connected or are installed improperly. During home inspections, we typically find the exterior air vent to be installed in areas that are specifically prohibited by current code — such as on the side of chimney structure or in the attic area above the firebox.

IRC Section R1006.2 states, The exterior air intake shall be capable of supplying all combustion air from the exterior of the dwelling or from spaces within the dwelling ventilated with outside air such as non-mechanically ventilated crawl or attic spaces. The exterior air intake shall not be located within the garage or basement of the dwelling, nor shall the air intake be located at an elevation higher than the firebox.”

So, while the intake vent is allowed to be located in an attic, the IRC prohibits it from being located higher than the firebox.

However, one manufacturer of a factory-built fireplace allows and actually instructs the installer to locate the exterior air supply vent at least 3 feet from the top of the chimney. Knowing that these instructions are contradictory to current IRC provisions, I contacted the manufacturer about this issue and was quickly informed that the local codes governing your municipality shall take precedent over the manufacturer’s installation instructions.

Fireplace3So, why is it even important where the vent is located? Because pressure differentials between interior and exterior spaces can fluctuate depending on installed equipment, weather conditions and where the home is located. This difference in atmospheric pressure could prevent the chimney from drafting or exhausting properly, or it could cause the exterior air supply vent to function as an exhaust vent for which it is not designed for.

Some inappropriate vent locations are in garage and basements, where combustible materials are often stored. Be sure not to locate an exterior air intake in a mechanically ventilated attic or crawl space. Attic and/or crawl space mechanical ventilating systems are primarily used to remove air from those areas by exhausting unwanted air or creating a negative pressure in those areas. If an air intake for a fireplace terminates in a crawl or attic space that has a mechanical ventilation system, then there is potential for the air intake to perform exactly opposite of its designed intent.

Oftentimes, the exterior air vent is properly installed in a non-mechanically ventilated attic as is permitted by code. However, later down the road, an attic fan is installed and thus causes a problem. Also, where combustion air openings are located inside the firebox, the air intake opening on the outside of the dwelling cannot be located higher than the firebox. Such an installation could create a chimney effect, drawing the products of combustion up through the combustion air ducts, which are not generally constructed of materials that can withstand the heat and sparks that could be drawn through them.

So what do you do if your fireplace is located in the center of your home and not on an exterior wall? This is often an issue, and home inspectors will discuss it with the builder or installer. There are actually two ways to address this issue.

First, in the foundation phase, an exterior air supply vent could be installed through the foundation prior to pouring concrete. This would meet the requirements of not being located higher than the firebox.

However, if the foundation has already been poured, then there is a provision in current code at the end of Section R1006.1 that says, “Unless the room is mechanically ventilated and controlled so that the indoor pressure is neutral or positive.” This provision seems to allow elimination of the exterior air vent altogether if the room is “mechanically ventilated and controlled so that the indoor pressure is neutral or positive.” With that said, new air conditioning systems are becoming more sophisticated in their technologies to provide for this controlled mechanical ventilation.

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