A fireplace needs adequate combustion air to function properly and safely. Combustion air can come from inside the home or from outdoors. Like other combustion appliances, fireplaces come in three types: natural-draft, mechanically drafted, and direct-vent sealed-combustion. The first two take all or some of their combustion air from the indoor space. The third, direct-vent sealed-combustion, takes 100% of its combustion air directly from outdoors.
A direct-vent fireplace has a dedicated combustion air inlet that ducts air from the outside directly to the fireplace box and tight-fitting glass doors across the face of the fireplace opening to prevent room air from being used as combustion air. Direct van fireplaces are typically gas fueled. Because a direct-vent sealed-combustion fireplace draws its combustion air directly from outdoors, it is not affected by house air tightness or exhaust fan size.
Figure 1 - A direct-vent sealed-combustion fireplace takes its combustion air directly from outside through a dedicated air inlet ducted to the firebox and vents combustion products directly outside. Tightly fitting glass doors (not shown) cover the firebox combustion chamber and prevent the fireplace from back-drafting.
The mechanically drafted fireplace uses an exhaust fan attached to the top of the chimney. The exhaust fan pulls combustion gases up and out of the chimney while the fireplace is in use. Some, if not all, of the combustion air typically comes from the conditioned space. Mechanically drafted fireplaces are less affected by building air tightness or indoor exhaust fans but they do have the ability to back-draft other naturally drafted appliances.
Natural-draft fireplaces are the most affected by the combination of building air tightness and mechanical exhaust systems. When a natural-draft fireplace is in full operation or at maximum burn, the natural draft of the fireplace will be hard to overcome; it can move as much as 400 CFM of air up the chimney. Because of the density expansion factor (about 1.6), only about 250 CFM is actually being exhausted out of the building. Nevertheless, it is possible for a natural-draft fireplace, at full burn, to back-draft other natural-draft-vented combustion appliances in the home.
However, when the chimney is cooler, such as when a fire is just getting started in a cold fireplace or when a fire has burned down or is on a low flame, the natural-draft fireplace is more vulnerable to being back-drafted. During these conditions, a small exhaust fan (50 CFM) could back-draft the combustion pollutants from the fireplace into the home.
How to Test for Proper Ventilation of Natural-Draft Fireplaces
If installing a natural-draft fireplace, install a dedicated combustion air duct that brings outside air directly into the firebox. Install a fireplace that has tight-fitting glass doors across the face of the fireplace opening.
The rater will verify that the total net rated exhaust flow of the two largest exhaust fans is ≤ 15 CFM per 100 ft2 of occupiable space.
Calculate the total occupiable space of the house. Per ASHRAE 62.2-2010, occupiable space is any enclosed space inside the pressure boundary and intended for human activities, including, but not limited to, all habitable spaces, toilets, closets, halls, storage and utility areas, and laundry areas.
Calculate the net rated exhaust flow of the two largest exhaust fans. Per ASHRAE 62.2-2010 and published addenda, the term “net-exhaust flow” is defined as flow through an exhaust system minus the compensating outdoor air flow through any supply system that is interlocked to the exhaust system. Add together the rated exhaust flow of the two largest exhaust fans in the home. Subtract the air flow of any dedicated supply ventilation.
Verify the total net rated exhaust flow is less than or equal to 15 CFM per 100 ft2 of occupiable space:
15 CFM per 100 ft2 ≥ ((Largest Fan Rated Flow) CFM + (Second Largest Fan Rated Flow) CFM - (Supply Outdoor Air Intake) CFM ) / (Occupiable Space) ft2
Or, the rater will verify that the pressure differential is ≤ -5 Pa using BPI’s or RESNET’s worst-case depressurization test procedure. Raters shall use either the Building Performance Institute’s (BPI’s) Combustion Safety Test Procedure for Vented Appliances or RESNET’s Interim Guidelines for Combustion Appliance Testing & Writing Work Scope and be BPI-certified or RESNET-certified to follow the protocol. If using RESNET’s worst-case depressurization protocol to evaluate fireplaces, the blower door shall not be set to exhaust 300 CFM to simulate the fireplace in operation, but the remainder of the protocol shall be followed. Note that the fireplace damper should be closed during the test.