Fireplaces/Stoves with Proper Ventilation

Scope

Combustion By-Products

For fireplaces that are not mechanically drafted or direct-vented to outdoors, total net rated exhaust flow of the two largest exhaust fans (excluding summer cooling fans) is ≤ 15 CFM per 100 ft² of occupiable space when at full capacity, or the rater has verified that the pressure differential is ≤ -5 Pa using BPI’s or RESNET’s worst-case depressurization test procedure.

1. Calculate the total occupiable space of the house.
2. Calculate the net rated exhaust flow of the two largest exhaust fans.
3. Verify the total net rated exhaust flow is less than or equal to 15 CFM per 100 sq. ft. of occupiable space.

Notes:

Equation for Net Flow per Square Foot 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

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 airflow through any supply system that is interlocked to the exhaust system. “Net supply flow” is intended to represent the inverse. If net exhaust flow exceeds allowable limit, it shall be reduced or compensating outdoor airflow provided.

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.

Per ASHRAE 62.2-2010, an exhaust system is one or more fans that remove air from the building, causing outdoor air to enter by ventilation inlets or normal leakage paths through the building envelope (e.g., bath exhaust fans, range hoods, clothes dryers).

The pressure boundary is the primary enclosure boundary separating indoor and outdoor air. For example, a volume that has more leakage to outside than to conditioned space would be outside the pressure boundary.

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.

Description

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

1. 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.
2. 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.

Ensuring Success

If a fireplace is installed, make sure it has adequate ventilation. If the fireplace is naturally drafted, the rater must verify that the total net rated exhaust flow of the two largest exhaust fans is ≤ 15 CFM per 100 ft² of occupiable space, or the rater must verify that the pressure differential is ≤ -5 Pa using BPI’s or RESNET’s worst-case depressurization test procedure.

Climate

No climate specific information applies.

Right and Wrong Images

Presentations

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Compliance

ENERGY STAR Version 3, (Rev. 6)

HVAC System Quality Checklist, Combustion Appliances. For fireplaces that are not mechanically drafted or direct-vented to outdoors, total net rated exhaust flow of the two largest exhaust fans (excluding summer cooling fans) is ≤ 15 CFM per 100 ft2 of occupiable space when at full capacity or the Rater has verified that the pressure differential is ≤ -5 Pa using BPI’s or RESNET’s worst-case depressurization test procedure. Per ASHRAE 62.2-2010, an exhaust system is one or more fans that remove air from the building, causing outdoor air to enter by ventilation inlets or normal leakage paths through the building envelope (e.g., bath exhaust fans, range hoods, clothes dryers). The pressure boundary is the primary enclosure boundary separating indoor and outdoor air. For example, a volume that has more leakage to outside than to conditioned space would be outside the pressure boundary. 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, per Item 10.2, 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. Per ASHRAE 62.2-2010 and pub. addenda, the term “net-exhaust flow” is defined as flow through an exhaust system minus the compensating outdoor airflow through any supply system that is interlocked to the exhaust system. “Net supply flow” is intended to represent the inverse. If net exhaust flow exceeds allowable limit, it shall be reduced or compensating outdoor airflow provided. 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.

BPI’s Combustion Safety Test Procedure for Vented Appliances is available HERE.

RESNET’s Interim Guidelines for Combustion Appliance Testing and Writing Work Scope are available HERE.

DOE Challenge Home

Exhibit 1: Mandatory Requirements. Certified under ENERGY STAR Qualified Homes Version 3.

ANSI/ASHRAE Standard 62.2.-2010

Ventilation and Acceptable Indoor Air Quality in Low-Rise Residential Buildings.  The standard applies to spaces intended for human occupancy in single-family homes and multifamily homes that are three stories or less, including manufactured and modular homes.  The standard provides minimum acceptable requirements for mechanical and natural ventilation in these spaces. 

2009 IECC

This topic is not specifically addressed in the 2009 IECC.

2009 IRC

This topic is not specifically addressed in the 2009 IRC.

2012 IECC

This topic is not specifically addressed in the 2012 IECC. 

2012 IRC

This topic is not specifically addressed in the 2012 IRC.

Case Studies

References