Unvented Combustion Appliances

Scope Images
Unvented gas fireplaces release combustion byproducts into the living space, they should not be installed.
Unvented gas fireplaces release combustion byproducts into the living space, they should not be installed.
Scope

Do not install unvented combustion appliances within the conditioned space of the home. Unvented combustion appliances include unvented fireplaces, also known as ventless or ductless fireplaces. These gas, propane, or ethanol-burning fireplaces have no vent so they draw combustion air from the room they are in and release toxic combustion byproducts and moisture vapor back into the space in which they are located. Their use is banned in many states and municipalities.

See the Compliance Tab for related codes and standards requirements, and criteria to meet national programs such as DOE’s Zero Energy Ready Home program, ENERGY STAR Certified Homes, and Indoor airPLUS.

Description

For aesthetics, builders today will sometimes install a ventless gas, propane, or ethanol-burning fireplace in the home. These ventless appliances have real flames, providing the ambiance of a traditional fireplace, with convenience and cost savings for the builder because no chimney needs to be installed. Manufacturers report that they burn at nearly 100% efficiency releasing fewer harmful gases into the home than other types of fireplaces. However, because they are ventless, any unburned combustion byproducts are released directly into the living space because there is no chimney to vent them out of the home. Also because no air intake is installed, many manufacturers recommend that homeowners open a window during operation of the fireplace, although there is no way to guarantee homeowners will follow this advice. A ventless fireplace that is burning efficiently will have a primarily blue flame. Defects such as plugged burner ports, a cracked burner, excessive gas input, impurities in the gas, or a gas leak somewhere in the unit can impact performance, reducing the efficiency of the burn and increasing the amount of combustion byproducts released.

In addition to possible combustion byproducts, ventless combustion appliances also release significant amounts of water vapor into the air. These products produce 1 gallon of water vapor for every 100,000 Btus, so a 30,000-Btu appliance would release nearly 1 gallon of water vapor for every 3 hours of operation, adding greatly to indoor humidity levels.

Due to safety, health, and moisture concerns, some building scientists recommend that unvented appliances never be installed in homes (BSC 2007, Bailes 2013).

ENERGY STAR Version 3.0 permits their installation but requires that the HERS rater test the appliance (using a portable CO monitoring device) and verify that the ambient CO level does not exceed 35 parts per million (ppm). The rater should also confirm that the room size provides a minimum volume of combustion air for safe operation based on the size of the appliance installed, as specified by the manufacturer and/or code. The National Fuel Gas Code prohibits the installation of ventless combustion heaters in bathrooms or bedrooms.

Some ventless fireplaces come equipped by the manufacturer with an oxygen-detection sensor (ODS) that will automatically shut down the appliance if oxygen levels in the room become too low. It is recommended that the builder install a CO detector in the room near the ventless fireplace (NACHI 2013).

Because of safety concerns, several states and municipalities have banned the use of ventless combustion appliances. There have been no documented cases of fatalities caused by ODS-equipped ventless fireplaces, according to the U.S. Consumer Product Safety Commission (NACHI 2013). However, there have been consumer complaints about illness and humidity (Bailes 2013).

How to test unvented combustion appliances

  1. Turn on or light the ventless combustion fireplace.
  2. Let the appliance operate for 10 minutes.
  3. Use a portable, hand-held CO monitor held in the air within a few feet of the fireplace or other appliance to test the ambient air near the appliance. If the CO level is above 35 ppm, the appliance fails the test and must be serviced and retested, or replaced.
A ventless combustion fireplace has no chimney; it draws combustion air from and releases combustion byproducts to the room in which it is located
Figure 1 - A ventless combustion fireplace has no chimney; it draws combustion air from and releases combustion byproducts to the room in which it is located. 

 

A HERS rater uses a portable CO monitor is used to test for ambient CO near a ventless combustion fireplace
Figure 2 - A HERS rater uses a portable CO monitorto test for ambient CO near a ventless combustion fireplace. 

 

Ensuring Success

In homes with an unvented combustion appliance such as a ventless gas or propane fireplace, the HERS rater should test the appliance by operating the appliance for at least 10 minutes and verifying that the ambient CO level does not exceed 35 ppm.

Climate

No climate specific information applies.

CAD

Compliance

The Compliance tab contains both program and code information. Code language is excerpted and summarized below. For exact code language, refer to the applicable code, which may require purchase from the publisher. While we continually update our database, links may have changed since posting. Please contact our webmaster if you find broken links.

ENERGY STAR Certified Homes, Version 3/3.1 (Rev. 09)

National Rater Field Checklist

HVAC System.
10. Combustion Appliances.
10.3 If unvented combustion appliances other than cooking ranges or ovens are located inside the home’s pressure boundary, the Rater has followed Section 802 of RESNET’s Standards, encompassing ANSI/ACCA 12 QH-2014, Appendix A, Section A3 (Carbon Monoxide Test), and verified the equipment meets the limits defined within.55, 59

Footnote 55) 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.

Footnote 59) The minimum volume of combustion air required for safe operation by the manufacturer and / or code shall be met or exceeded. Also, in accordance with the National Fuel Gas Code, ANSI Z223.l / NFPA54, unvented room heaters shall not be installed in bathrooms or bedrooms.

Please see the ENERGY STAR Certified Homes Implementation Timeline for the program version and revision currently applicable in in your state.

This Retrofit tab provides information that helps installers apply this “new home” guide to improvement projects for existing homes. This tab is organized with headings that mirror the new home tabs, such as “Scope,” “Description,” “Success,” etc. If there is no retrofit-specific information for a section, that heading is not included.

Existing Homes

Unvented combustion appliances, such as unvented gas fireplaces, should not be installed in a home. If an unvented combustion appliances is found to be installed in an existing home, a HERS rater should test the appliance (using a portable CO monitoring device as described in the Description tab) to verify that the ambient CO level does not exceed 35 parts per million (ppm). The rater should also confirm that the room size provides a minimum volume of combustion air for safe operation based on the size of the appliance installed, as specified by the manufacturer and/or code. If the home is not equipped with a carbon monoxide (CO) monitor, one should be installed in the room near the unvented fireplace.

For more information on combustion appliances, please see the assessment guide, Pre-Retrofit Assessment of Combustion Appliances.

The U.S. Department of Energy’s Standard Work Specifications has additional information on combustion appliances.

More Info.

Access to some references may require purchase from the publisher. While we continually update our database, links may have changed since posting. Please contact our webmaster if you find broken links.

Case Studies
References and Resources*
Author(s)
Lstiburek,
Brennan
Organization(s)
Building Science Corporation
Publication Date
Description
Document with important building science considerations, designed for members of the residential construction and remodeling industries, as well as owners and managers who work in affordable housing.
Author(s)
Gromicko
Organization(s)
International Association of Certified Home Inspectors
Publication Date
Description
Document outlining important safety considerations of ventless fireplaces.
Author(s)
National Fire Protection Association
Organization(s)
National Fire Protection Association
Publication Date
Description
Code outlining minimum safety requirements for the design and installation of fuel gas piping systems in homes and other buildings.
*For non-dated media, such as websites, the date listed is the date accessed.
Contributors to this Guide

The following authors and organizations contributed to the content in this Guide.

Building Science Measures
Building Science-to-Sales Translator

Proper Ventilated Fireplaces = Ventilated Fireplace

Image(s)
Technical Description

Ventilated fireplaces typically burn natural gas or propane and use a vent to bring outside combustion air directly into the firebox. These fireplaces have tight-fitting glass doors across their face to prevent conditioned room air from being used as combustion air and then expelled through the flue. Consuming room air for combustion may deplete oxygen levels in the home and pull conditioned air out of the home. Exhaust gases are vented up a chimney or out a side wall. These closed-combustion systems are much less likely to experience spillage of combustion gases into the home. This improves both health and comfort while eliminating odors.

Ventilated Fireplace
Sales Message

Ventilated fireplaces help ensure dangerous by-products from burning fossil fuel cannot back-draft into homes. What this means to you is your family can breathe better every day knowing your home is equipped to help manage a critical respiratory contaminant. Wouldn’t you agree protecting health is too important to ignore in new homes?

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