Direct Vent Equipment

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ENERGY STAR Version 3, (Rev. 07)
HVAC System Quality Checklist, Combustion Appliances. Furnaces, boilers, and water heaters located within the home’s pressure boundary are mechanically drafted or direct-vented. As an exception, naturally drafted equipment is allowed in Climate Zones 1-3.

IECC climate zone map

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Description

When installing combustion furnaces, boilers, and water heaters inside the home’s pressure boundary (i.e., within the conditioned space of the home), the safest (and usually most energy efficient) type of equipment to install is a direct-vent, sealed-combustion appliance.

A direct-vent sealed-combustion furnace, boiler, or water heater brings combustion air directly from outdoors to the sealed combustion chamber through one sealed pipe and exhausts the byproducts of combustion (CO2 and N) to the outdoors through a second sealed pipe (Figure 1). Because of this sealed combustion, the risk of backdrafting is eliminated. Combustion air (oxygen) enters the furnace through the sealed pipe and goes directly from the outdoors to the burner assembly. There it mixes with the fuel and is electronically ignited in the sealed combustion chamber. The heated air passes through a primary heat exchanger, which extracts most of the heat, and then through a secondary heat exchanger, where more heat is extracted, cooling the exhaust gases to the point that they condense. The condensed water and carbon dioxide exit through a condensate drain, while the remaining flue gases exit to the outside through the sealed exhaust pipe, which can be made of PVC because of the low temperature of the gases (Figure 2). Condensing furnaces have annual fuel utilization efficiencies (AFUE) of 90% to 98%.

The National Fuel Gas Code identifies four categories for combustion furnaces and water heaters (see Figure 3). The categories are based on combustion type (sealed or unsealed) and vent pipe temperature. Direct-vent sealed-combustion furnaces are Category 4 appliances. The flue and vent pipe are under a positive pressure, flue gases are under 140°F, and water vapor (a byproduct of combustion) is condensed into water liquid and drained to the outside through a condensate drain. Combustion air is piped directly to the burner, which is sealed off from the CAZ (sealed combustion) and the byproducts of combustion (carbon dioxide and nitrogen) are power exhausted to outdoors.

A Category 2 appliance is a natural aspirating furnace with a vent temperature below 140°F that does some condensing of the flue gases and has negative pressure in the vent; category 2 appliances are huge commercial furnaces. A Category 3 appliance is a furnace with a vent temperature above 140°F (high enough to avoid condensation in the vent) and the vent has a positive static pressure.

A direct-vent sealed-combustion furnace has dedicated pipes for combustion air and exhaust

Figure 1 - A direct-vent sealed-combustion furnace has dedicated pipes for combustion air and exhaust  Reference

Because the flue temperatures are cool, intake and exhaust ducts on a Category 4 direct-vent sealed-combustion condensing furnace can be made of PVC

Figure 2 - Because the flue temperatures are cool, intake and exhaust ducts on a Category 4 direct-vent sealed-combustion condensing furnace can be made of PVC  Reference

The National Fuel Gas Code (NFPA 2012) identifies four categories for combustion furnaces and water heaters based on combustion type (sealed or unsealed) and vent pipe temperature

Figure 3 - The National Fuel Gas Code (NFPA 2012) identifies four categories for combustion furnaces and water heaters based on combustion type (sealed or unsealed) and vent pipe temperature  Reference

Atmospheric (or naturally) drafted furnaces and water heaters and mechanically drafted (or draft-induced) furnaces and water heaters are both considered Category 1 appliances (Figure 4). A Category 1 combustion appliance has a flue gas temperature over 140°F. The flue and vent pipe pressure will be negative with respect to the space in which the appliance is located (referred to as the combustion appliance zone or CAZ). With a Category 1 furnace, there is no barrier between the burner and the CAZ. A Category 1 furnace or water heater draws its combustion and dilution air from the CAZ and the byproducts of combustion (carbon dioxide, nitrogen, and water vapor) are transported outside through the flue. Because both types of appliances draw their combustion air from the CAZ, both are susceptible to backdrafting. 

Naturally drafted (also called atmospheric draft or draft hood) appliances use the stack effect to draw combustion gases up the chimney. Natural draft furnaces are low-efficiency (60%-78%) older model appliances that are no longer sold in the United States but are still found operating in existing homes. Mechanically drafted (also known as induced draft) combustion appliances use a fan to pull air through the combustion chamber and to push exhaust gases out of the flue. Mechanically drafted (also called induced draft) furnaces have AFUEs of 78% to 83%. Natural draft water heaters are still sold, although safer and more efficient water heaters are also available, such as power-vented models, which use a fan to push combustion gases out the flue, and direct vent models, which use a sealed intake for combustion air and a sealed flue for exhaust gases.

Naturally drafted appliances are not recommended inside the home, especially in air-tight, high-efficiency homes, where exhaust fans or other combustion appliances such as fireplaces could depressurize the home and overcome the natural draft of an atmospherically vented appliance, thus backdrafting the appliance and pulling combustion gases into the living space.

Although most building scientists would not recommend them, naturally drafted furnaces, boilers, and water heaters are permitted by ENERGY STAR Version 3.0 in Climate Zones 1 through 3. If they are installed, the builder must ensure that two outside fresh air supply ducts are installed in the CAZ: one 12 inches from the floor and one 12 inches from the ceiling or as required by NFPA 54, Sections 5.3.3 and 5.3.4. These ducts provide combustion and dilution air for safe operation in each combustion appliance zone that exists in the home. If atmospheric vented furnaces, boilers, or water heaters are installed, the rater must test the equipment using combustion safety test procedures for depressurization, spillage, draft pressure, and CO concentration in the flue and in the ambient air. Ambient CO concentration should be < 25 ppm (< 8 ppm is preferred). This combustion safety testing should be conducted according to the procedures of the Building Performance Institute (BPI 2012) or the Mortgage Industry National Home Energy Rating Systems Standards, Chapter 8 - 800 RESNET Standard for Performance Testing and Work Scope: Enclosure and Air Distribution Leakage Testing (RESNET 2013).

Although most building scientists would not recommend them, ENERGY STAR Version 3.0 permits the installation of mechanically drafted furnaces, boilers, and water heaters inside conditioned space in all climate zones. Although not required in ENERGY STAR Version 3.0, the rater should test any Category 1 equipment using combustion safety test procedures for worst case depressurization, spillage, draft pressure, and CO concentration in the flue and in the ambient air. Ambient CO concentration should be < 25 ppm (< 8 ppm is preferred). This combustion safety testing should be conducted according to the procedures of the Building Performance Institute (BPI 2012), or the Mortgage Industry National Home Energy Rating Systems Standards, Chapter 8 - 800 RESNET Standard for Performance Testing and Work Scope: Enclosure and Air Distribution Leakage Testing (RESNET 2013).

The method described below is adapted from BPI and the U.S. Department of Energy’s Weatherization Assistance Project requirements.

The natural draft and induced draft furnace are both Category I appliances that receive combustion air from the combustion appliance zone

Figure 4 - The natural draft and induced draft furnace are both Category I appliances that receive combustion air from the combustion appliance zone  Reference

The raised hood at the base of the vent stack on this water heater shows that it is an atmospheric vented gas water heater

Figure 5 - The raised hood at the base of the vent stack on this water heater shows that it is an atmospheric vented gas water heater  Reference

  1. Select high-efficiency, direct vent, sealed combustion furnaces, boilers, and water heaters for installation inside the home. Use certified installers and install the appliances according to the manufacturer’s instructions.
  2. If the builder chooses to install a Category I combustion appliance (i.e., one that is naturally or mechanically drafted) the rater should test the equipment using combustion safety test procedures for depressurization, spillage, draft pressure, and CO concentration in the ambient air and in the flue (<= 25 ppm). (This is an ENERGY STAR requirement if the home is located in IECC Climate Zone 1, 2 or 3, and if the builder chooses to install a naturally drafted furnace, boiler, or water heater within the home’s pressure boundary (an action which is not recommended).

    ENERGY STAR recommends the rater follow BPI test procedures (or RESNET when approved). The following combustion safety testing steps are based on BPI and U.S. DOE Weatherization Program Procedures: 

Combustion Appliance Zone (combustion appliance safety testing)

Appliance Draft and CO Test

Acceptable Draft Readings for Worst Case Draft Test

Ambient CO Readings

Required CAZ Volume

Combustion Dilution Air

Ensuring Success

When furnaces, boilers, and water heaters are installed within the home’s pressure boundary, ideally this equipment would be direct-vent sealed-combustion equipment. ENERGY STAR also permits mechanically drafted equipment in all climate zones. ENERGY STAR permits naturally drafted (atmospheric vented) equipment in Climate Zones 1, 2, and 3. If atmospheric-vented furnaces, boilers, or water heaters are installed in a confined space or a tight building, two fresh air supply ducts or openings must be installed to provide combustion air in the combustion appliance zone (CAZ): one 12 inches from the floor and one 12 inches from the ceiling, or as required by NFPA 54, Sections 5.3.3 and 5.3.4. If the furnace, boiler, or water heater to be installed is atmospheric or mechanically vented and draws its combustion air from the combustion appliance zone, the rater must test the equipment using the Building Performance Institute procedure (BPI 2012) (or RESNET when approved) for depressurization, spillage, draft pressure, and CO concentration in the ambient air and in the flue. Ambient CO concentration should be < 25 ppm (< 8 ppm is preferred).

Scope

Furnaces, boilers, and water heaters located within the home’s pressure boundary are mechanically drafted or direct-vented

Combustion By-Products

Choose the highest efficiency, direct vent combustion equipment possible.

ENERGY STAR Notes: 

ENERGY STAR Ver. 3, Rev. 07, in the HVAC System Quality Installation Rater Checklist requires the following in Section 10. Combustion Appliances.

Furnaces, boilers, and water heaters located within the home’s pressure boundary are mechanically drafted or direct-vented. As an exception, naturally drafted equipment is allowed in Climate Zones 1-3. For naturally drafted furnaces, boilers, and water heaters, the rater has followed RESNET or BPI combustion safety test procedures and met the selected standard’s limits for depressurization, spillage, draft pressure, and CO concentration in ambient air, as well as a CO concentration in the flue of < 25 ppm.

  1. Install mechanically-drafted or direct-vented appliances.

Naturally drafted appliances are acceptable if ALL of the following are true:

  1. The house is located in Climate Zones 1-3.
  2. The Building Performance Institute (BPI) or RESNET combustion safety test has been performed.
  3. The CO concentration in the flue is less than 25 ppm and the selected standard's limits for depressurization, spillage, draft pressure and CO concentration in ambient air are met.

Per the 2009 International Mechanical Code, a direct-vent appliance is one that is constructed and installed so that all air for combustion is derived from the outdoor atmosphere and all flue gases are discharged to the outside atmosphere; a mechanical draft system is a venting system designed to remove flue or vent gases by mechanical means consisting of an induced draft portion under non-positive static pressure or a forced draft portion under positive static pressure; and a natural draft system is a venting system designed to remove flue or vent gases under non-positive static vent pressure entirely by natural draft.

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.

Naturally Drafted Appliances

All naturally drafted combustion appliances other than fireplaces shall comply with the Building Performance Institute’s (BPI’s) or RESNET's Combustion Safety Test Procedures. In alignment with ASHRAE 62.2-2010, these ENERGY STAR guidelines do not address unvented combustion space heaters. Click here for an Interactive Map.

For additional information and specific duct testing protocols please refer to RESNET Chapter 8 (Standard for Performance Testing and Work Scope: Enclosure and Air Distribution Leakage Testing).

Training

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Presentations

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Compliance

ENERGY STAR Version 3, (Rev. 07)

HVAC System Quality Checklist, Combustion Appliances. Furnaces, boilers, and water heaters located within the home’s pressure boundary are mechanically drafted or direct-vented. As an exception, naturally drafted equipment is allowed in Climate Zones 1-3.  For naturally drafted furnaces, boilers, and water heaters, the Rater has followed RESNET or BPI combustion safety test procedures and met the selected standard’s limits for depressurization, spillage, draft pressure, and CO concentration in ambient air, as well as a CO concentration in the flue of <= 25 ppm. Per the 2009 International Mechanical Code, a direct-vent appliance is one that is constructed and installed so that all air for combustion is derived from the outdoor atmosphere and all flue gases are discharged to the outside atmosphere; a mechanical draft system is a venting system designed to remove flue or vent gases by mechanical means consisting of an induced draft portion under non-positive static pressure or a forced draft portion under positive static pressure; and a natural draft system is a venting system designed to remove flue or vent gases under non-positive static vent pressure entirely by natural draft. 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.

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.

2009 IRC

Section M1801.1 Venting required. Fuel-burning appliances are to be vented to the outdoors per their listing and label and manufacturer’s instructions unless they are appliances listed and labeled for unvented use.

2012 IRC

Section M1801.1 Venting required. Fuel-burning appliances are to be vented to the outdoors per their listing and label and manufacturer’s instructions unless they are appliances listed and labeled for unvented use.

More Info.

Case Studies

  1. Author(s): PNNL
    Organization(s): PNNL
    Publication Date: August 2012

    Case study describing how a properly designed duct system is essential for keeping a home comfortable, managing indoor air quality, and maximizing the performance of heating and cooling equipment.

References and Resources*

  1. Author(s): DOE
    Organization(s): DOE
    Publication Date: January 1995

    Information sheet summarizing inspection and testing of combustion appliances taught during the 2006 Florida whole-house weatherization training as part of the U.S. Department of Energy’s Hot Climate Initiative.

  2. Author(s): DOE
    Organization(s): DOE
    Publication Date: June 2013

    Standard requirements for DOE's Challenge Home national program certification.

  3. Author(s): EPA
    Organization(s): EPA
    Publication Date: June 2013

    Standard document containing the rater checklists and national program requirements for ENERGY STAR Certified Homes, Version 3 (Rev. 7).

  4. Author(s): RESNET
    Organization(s): RESNET
    Publication Date: January 2013

    Standards aimed to ensure that accurate and consistent home energy ratings are performed by accredited home energy rating Providers through their Raters nationwide.

  5. Author(s): National Fire Protection Association
    Organization(s): National Fire Protection Association
    Publication Date: January 2012

    Code outlining minimum safety requirements for the design and installation of fuel gas piping systems in homes and other buildings.

  6. Author(s): Building Performance Institute
    Organization(s): Building Performance Institute
    Publication Date: January 2012

    Standard covering minimum required health and safety diagnostics and specifications for BPI's Building Analyst Professional certification.

Last Updated: 02/28/2014

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