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   Commonly Asked Questions about
       Vent-Free Gas Supplemental
 Heating  Appliances Provided by the
    Vent-Free Gas  Products Alliance






February 1,2000 -Arlington, V A- 1999 sales statistics for vent-free gas products continue to increase and vent-free gas heating products are in more than 8 million U.S. homes. The immense popularity and the acceptance of vent-free products in most states have inspired a strong demand for information about these "smart technology" gas appliances. To help provide the where's, why's and how's of vent-free gas products, here is a list of commonly-asked questions and their answers. This "Q & A" has been prepared by the Vent-Free Gas Products Alliance, a coalition of members of the Vent-Free Gas Products Division of the Gas Appliance Manufacturers Association (GAMA).


1. What exactly are "vent-free" gas products?

            Vent-free gas products are gas appliances that are designed to be installed and operated without the addition of a vent or chimney. They are intended to provide supplemental heat to a home. Because there is no vent, flue, or chimney required, all of the heat generated by the product goes directly into the home. That's why vent-free products are 99 percent energy efficient.

 2. What types of vent-free gas products are available?

 Vent-free gas appliances are available as gas space heaters, gas logs, gas stoves, gas fireplaces, and gas fireplace inserts. They operate on natural or propane gas.

 3. Can I use vent-free logs in any wood-burning fireplace---masonry or prefab?

Vent-free logs are certified for installation in the following enclosures:

o A masonry fireplace with a suitable chimney;
o A listed factory-built fireplace, assembled with a working chimney, in accordance with the terms of the listing, and not including a marking prohibiting use with unvented gas logs;
o A vent-free firebox enclosure, tested and certified to accept any ANSI Z21.11.2 approved vent-free gas logs; or
o A vent-free firebox enclosure specifically tested and approved for use with the vent-free gas logs being installed.
 o Wood-burning stoves are not tested to allow for conversion to gas burning appliances.

Customers need to follow individual manufacturers' instructions on the fireboxes that they purchase.

 4. In an existing wood-burning fireplace with vent-free logs, in what circumstances do I need to use a hood?

A hood is used to allow deeper mantels to be installed at lower heights above a fireplace opening and to reduce wall temperatures above the fireplace opening. A hood should only be used in accordance with the manufacturer's installation instructions and in accordance with the manufacturer's specified clearances from combustible construction. 

5. If I have a wood-burning fireplace and I put vent-free logs in it can I seal off the flue? What is the best way to do it? 

No, we do not recommend any modification to a wood-burning fireplace in which vent-free gas logs are installed, other than closing the flue damper. If the logs give off too much heat and cause discomfort, the damper can be slightly opened. 

6. Most feel that in order for an appliance to be vent-free it must have a blue flame. If that is so, why do so many units have a yellow flame? 

Historically it has been believed that only a hard, blue flame is allowable on gas appliances. However, recent technology has emerged to enable yellow flames that have clean combustion. Several hearth products incorporate this aesthetic feature. 

7. The general rule has been that there is not to be any flame impingement on the logs, yet several manufacturers have impingement on the front log on their sets. How can they do this? 

Recent technological developments have made flame impingement on (contact with) the front logs possible. Vent-free appliances with front log impingement typically utilize both blue flame and yellow flame burner design technologies. The blue flame burners impinge and provide a significant source of radiant heat and a glowing effect or charcoal appearance. The yellow flames in the rear of the appliance do not normally impinge on the logs, but rather burn between the logs and twigs, completing the "wood-burning" aesthetic effect of the appliance. 

8. Some units now incorporate a catalyst. How does a catalyst work in a vent-free appliance? 

The catalyst used on a vent-free appliance is similar in concept to that used on a wood stove, with only slight differences. With a vent-free catalyst, flue gases passing through the combustion module heat up the catalytic material to the minimum conversion of  "light off'* temperature. Once this temperature is reached, the catalytic material combines any CO with O 2 from the catalyst (a noble metal) forming CO2.

 * "Light-off" is the activation temperature of the combustor catalyst.


I. Are vent-free gas products safe to use in the home?

Yes. There is a national safety standard for vent-free gas appliances, ANSI Z21.11.2, which requires the products to satisfy numerous construction and performance requirements. The safety standard is approved by the American National Standard Institute (ANSI) and is developed by an independent Committee comprised of representatives from various interests including state and federal regulatory authorities, utilities, manufacturers, consumers and general interests. Vent-free gas products are certified for compliance with this standard to assure their safety.

 2. Is it safer to use a vented model instead of an unvented one? The fact is, both vented and unvented gas products are safe provided they have been certified to their appropriate product safety standard and they are installed and operated in accordance with manufacturers instructions. In addition to the Oxygen Detection Safety-Pilot (ODS), every vent-free gas appliance is equipped with:

o an automatic shut-off valve, which shuts off the gas flow if the pilot extinguishes or the gas flow to the main burner is interrupted in any way;

o burners that are precision-engineered to produce the cleanest, most complete gas combustion, with no lift-off or flash-back when subjected to low input rates, low gas pressure, or drafts;

 o a non-adjustable pressure regulator that prevents overfiring in case of increased gas pressure;

o an automatic ignition device, usually a Piezo igniter, which eliminates the need for matches in starting the appliance;

o design features that limit cabinet temperatures for wall- or floor-mounted heaters;

o instructions for minimum clearances to combustible surfaces; and

o safety grills for heaters and fire screens for gas logs.

3. How does an ODS work?

An Oxygen Detection Safety-Pilot CODS) system reacts to a decrease in a room's oxygen supply in a manner that shuts off the gas supply to the appliance. The gas pilot burns normally when it is exposed to normal air (22- 22.5 percent oxygen). In the rare case when the oxygen level in a room starts to drop to an 18- 18.5 percent level, the pilot flame lifts off of the thermocouple, which causes the gas valve to close. In other words, it shuts off the gas supply and "fails safe." This unit was introduced to the U.S. in 1980 - 1981. All ANSI certified vent-free gas products incorporate an ODS. Based upon data available from manufacturers of the ODS, manufacturers of vent-free gas appliances and data available via legal data bases, there are no documented deaths associated with emissions from vent-free gas products that incorporate an ODS. This data covers a 20 year history during which more than eight million units were sold in the U.S.

 4. How can you ensure the vent-free gas product you purchase operates safely?

You should verify that the product has been certified to ANSI 221.11.2. This can be done by locating the product's certification mark from an accredited safety certification testing agency ( e.g., American Gas Association Blue Star Seal). The installation of vent-free gas products should always be performed by a qualified technician who is able to ensure that all manufacturer instructions have been followed during installation and all local code requirements have been satisfied. Also, the manufacturer is required to provide operating instructions for the consumer to ensure safe operation of the appliance.

 5. Are vent-free gas products safe in terms of indoor air quality?

Yes. This has been verified by research conducted by the American Gas Association Research (AGAR) laboratories. AGAR is the research arm of the International Approval Services, a nationally recognized testing agency, and the most renowned independent research body for gas products. This laboratory has been certifying gas appliances since 1928 to ANSI safety standards. (In 1997, AGAR was purchased by Energy International, Inc. ) The AGAR research shows that when the product is properly sized and installed, vent-free gas heating products meet applicable emissions requirements, even when used over extended time periods, among sensitive populations, and with oversized units. In fact, using oversized appliances actually results in lower average emissions because the unit operates for a shorter period of time. The researchers concluded that under "worst case" conditions, vent-free gas heating products perform well within nationally recognized guidelines for indoor air quality. For instance: When tested for Nitrogen Dioxide (NO2), the vent-free gas products tested by AGAR produced less than one-half of the parts per million (ppm) level of NO2 recommended by the Occupational Safety and Health Administration (OSHA) for a 1 hour period. Also, when tested for carbon monoxide (CO), vent-free gas products emitted less than 20 percent of the current recommended guideline level set by the Consumer Product Safety Commission (CPSC) for an 8 hour period.


1. Where in the home do people install vent-free gas appliances?

Vent-free appliances may be installed in any room in the home. It is important to remember that the appliance should be sized to fit the area in which it is to be installed. However, ANSI Z21.11.2 limits products installed in bathrooms to an input rating no larger than 6,000 Btu/hr and products installed in bedrooms are limited to an input rating no larger than 10,000 Btu/hr.

2. How do I find the correct size of vent-free gas appliance for my home?

  The Vent-Free Gas Products Alliance has published an easy to use set of sizing guidelines that allow the consumer to determine the appropriate size unit that should be installed in their home, depending on the size of the area to be heated, the construction of the home, and the home's geographical location. Because there are broad temperature ranges in all regions of the country, the desired heat output from a vent-free gas appliance will vary dramatically based on the season and usage patterns of the household. All vent-free gas products offer a range of heat settings, whether manually or thermostatically controlled. In terms of indoor air quality, any size of product can be chosen based on personal preference in all applications other than in DOE's region v. The Vent-Free Gas Products Alliance has printed these guidelines, including sizing guidelines for DOE's region V, in a publication entitled Consumer Guide to Vent-Free Gas Products. For a free copy, see the end of this article.

3. What consideration should I give if my home is an extremely tight one?

A home is considered to have tight construction if :

o Walls and ceilings exposed to the outside atmosphere have a continuous water vapor retarder with a rating of 1 perm or less with openings gasketed or sealed; and

 o Weather stripping has been added on operable windows and doors; and

o Caulking or sealant are applied to areas such as joints around window and door frames, between sole plates and floors, between wall and ceiling joints, between wall panels, at penetrations for plumbing, electrical, and gas lines, and at other openings.

In tightly constructed homes, additional provisions for combustion and ventilation air must be provided using one of the methods defined in the National Fuel Gas Code, ANSI Z 223.1/NFPA 54.


1. What causes the odor when the unit is turned on for the first time? What causes odors during operation? N

 Many manufacturers do not require an initial burn-off time, however, some units do emit an odor when operated for the first time due to burn off of excess paint and machine oil. Odors during operation should not be apparent. If there are odors, the cause is best determined by a qualified service person. Homes with many animals, new carpeting, or hobby chemicals sometimes do have odor concerns. The service person can usually identify the problem.

 2. Should I place a limit on the operation of my vent-free gas product to no more than two hours at a stretch, as a rule?

It is important to recognize that vent-free gas appliances are supplemental heating units. They are not to be used as a central heating system but to assist in heating the home. Many units have thermostats and remote controls that will regulate the amount of heat needed based on each individual's comfort levels. A major AGAR research project proved that vent-free products perform well within nationally recognized guidelines for indoor air quality ( see Section B, Safety Issues, Number 5, above). The AGAR research was based on "worst case" operating conditions. Worst case operating conditions consisted of a maximum sized unit (40,000 Btu/hr) emitting the vent-free products' maximum levels of combustion for a minimum of four hours of continuous operation. Even at these extreme operating conditions, the vent-free gas products met all applicable emission requirements, with the exception of a confined space in DOE heating region v. However, this one exception is accounted for in the sizing guidelines, as detailed in the publication Consumer Guide to Vent-Free Gas Products. For a free copy, see the end of this article.

 3. Is it necessary to leave at least one window open in the space where the vent-free gas appliance is operating? 

No. This is an obsolete requirement that no longer applies to products manufactured today which comply with the current safety standard, ANSI 221.11.2. This requirement was based on outdated information that was developed from pre-1980 vent-free units and kerosene heaters. Vent-free gas products now come with an ODS and many other built-in safety features as standard equipment (see Section B, Safety Issues, above). Additionally, in order to satisfy combustion air requirements for vent-free products, ANSI 221.11.2 requires that all vent-free products be installed in accordance with local codes, or in the absence of local codes, the National Fuel Gas Code. These codes specify the appropriate measures to take for ensuring an adequate supply of combustion air for all gas appliances. The ventilation requirements are the same for both vent-free gas products and vented gas products.

 4. How about vent-free appliances in homes at high altitude?

 Vent-free gas appliances installed at high altitudes can operate safely and satisfactorily. Many manufacturers have taken advantage of the International Approval Services and Mountain Fuel Testing Laboratory high altitude testing capabilities to investigate and ensure proper performance of vent-free appliances at high altitudes. The state of Utah decided to allow vent-free equipment throughout the state on January 1, 1999. Because high altitude reduces available oxygen, at extreme elevation, nuisance "fail-safe" shut down may occur. Currently one manufacturer has 123 vent-free gas products working in Vail, CO.

 1 . How much moisture does a vent-free appliance put out?

The amount of moisture is dependent on the gas Btu input. A 100,000 Btu/hr gas input produces about a gallon of water per hour. A 25,000 Btu input heater would produce about a quart of water as vapor per hour. It is important to keep this issue in perspective. When a typical consumer sees condensation on a window, it does not trigger an immediate alarm regarding allergenic organism growth or structural damage. The relative humidity in residential indoor air results from many contributing factors: cooking, showers, dishwashing, humidifiers, etc. The American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE) recommends a maximum relative humidity value of 60 percent for human comfort. The AGAR research project (see Section B, Safety Issues, Number 5, above) demonstrated that, even under "worst case" conditions, the highest relative humidity produced by vent-free gas heating products was only 49 percent, well below the recognized comfort level of 60 percent. In fact, during very cold weather, the air is dry, so many consumers use humidifiers to increase the relative humidity for greater comfort. In this context, vent-free gas heating products serve a useful function by providing some water vapor.

6. What specifically needs to be told to a customer about product usage and care in their home?

As with all gas appliances, the customer should read and follow the instructions and safety warnings provided on all gas appliances and included in the manufacturer's instruction manual for the proper installation, use, and maintenance of the appliance.

7. What maintenance is needed on vent-free appliances?

As with all gas appliances, vent-free appliances should be inspected for proper operation at least annually by a qualified service agency. The burners should be visually inspected and cleaned in accordance with the manufacturer's instructions. The area in which the appliance is installed should always be kept clear and free of combustible materials, gasoline, and other flammable vapors and liquids.


1.  Do the building codes in my area allow for the installation of vent-free gas products?

Most building codes throughout the U.S. now allow for the sale and installation of vent-free gas products. This includes Winston - Salem ,Forsyth County and surrounding counties. In fact, vent-free gas products are allowed in 47 states. However, since this is not the case in some regional or local areas, you should always check with your local code authorities before purchasing and installing a vent-free appliance.

2. Do people need to install a CO alarm when they install a vent-free appliance?

 It is a good idea to install a CO alarm in every home, regardless of an upcoming vent-free appliance installation or whether a home is heated using electricity , oil, or gas. Home-related CO incidents are caused by many sources; some examples are automobiles in attached garages, portable grills or hibachis used indoors, improper use of a gas range for heating, and malfunctioning gas appliances. Whether vented or unvented, following the manufacturers' instructions and having all gas appliances inspected and properly maintained by qualified service personnel will reduce the chance of a CO incident occurring from a malfunctioning gas appliance. CO alarms are recommended by the Consumer Product Safety Commission.