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Unvented combustion.

Unvented combustion has been used in the United States for decades. For example, cooking appliances using natural gas or propane are very common in homes. Natural gas, propane, and kerosene heaters also have been used in homes for many years. In the case of cooking, it has become more common in recent years to install exhaust ventilation, although recirculating fans are still found in many homes.

In the case of heating, additional ventilation is rare. The primary changes over the last few decades for heating appliances have been the requirement for safety devices such as oxygen depletion sensors (ODS) and standards that regulate the heating capacity and allowable emissions from these devices.

In the last several years, unvented combustion has garnered increasing attention and controversy. Critics argue that any release of combustion products into the occupied space may be harmful. Others argue that these appliances have a strong safety record and that, of the hundreds of deaths due to carbon monoxide poisoning, none have been shown to be the result of ODS-equipped unvented appliances.

In response to this issue, ASHRAE recently published a position document titled "Unvented Combustion Devices and Indoor Air Quality." This document reviews the current state of knowledge regarding unvented combustion devices and provides a number of recommendations. The focus of this position document is on liquid and gas combustion--specifically natural gas, propane, kerosene, and denatured alcohol--and on modern cooking and heating appliances. It does not address solid fuel combustion or less technologically advanced appliances.

Combustion Products

Four primary products of combustion are considered in the document: carbon monoxide, nitrogen dioxide, moisture, and particles. (Moisture is not a contaminant on its own, but excessive levels can promote problematic conditions.) Carbon dioxide is not discussed in the position document because the health concerns are much less clear. Other contaminants, such as polycyclic aromatic hydrocarbons and aldehydes are also not discussed.

For each of the four primary products of combustion, the position document presents the current understanding of health concerns. For carbon monoxide and nitrogen dioxide, current standard and guideline thresholds are presented (Table 1). The U.S. Environmental Protection Agency's National Ambient Air Quality Standards (NAAQS) contains values that are intended for outdoor air, but in the absence of values for indoor air they are often used indoors. In other cases, such as the World Health Organization (WHO) and Health Canada, the values are said to be applicable to indoor air but are guidelines rather than standards. There are no set standards or guidelines for indoor moisture or particle concentrations.

Appliance Categories and Standards

The position document next describes the types of appliances of interest, including the intended use of each category and the requirements of the pertinent appliance standards. These requirements include such topics as maximum heating capacity and maximum emission rates for combustion products. The allowable emission rates for different appliance categories are often very different for the same combustion product.

The appliance standards are ANSI Z21.1 for natural gas and propane cooking appliances, Z21.11.2 for natural gas and propane space heaters, UL 647 for kerosene heaters, and UL 1370 for denatured alcohol appliances. The maximum heating capacity for natural gas and propane heaters is 40,000 Btu/h (10,000 Btu/h [3 kW]) for bedrooms, and 6,000 Btu/h (2 kW) for bathrooms). For kerosene heaters, the maximum capacity is 30,000 Btu/h (9 kW) for fixed-mounted units and 25,000 Btu/h (7 kW) for portable units. Denatured alcohol appliances, which are intended to be decorative rather than for heating, have a maximum input of 0.25 gallons (0.95 L) of alcohol per hour.

Regarding emissions, ANSI Z21.11.2 requires that natural gas and propane heaters have a maximum of 0.02% (200 ppm) carbon monoxide (air-free), and a maximum of 0.002% (20 ppm) nitrogen dioxide (air-free). The nitrogen dioxide limit is based on a recommendation from the Consumer Product Safety Commission (CPSC) that intended to limit room concentrations to under 300 ppb. This limit was incorporated into the standard in 2003 and enforced starting 2005. Note that the term "air-free" means that this is the amount of the contaminant that would exist if it were completely undiluted by air. This allows emissions to be compared across appliances. It does not mean that the indoor concentrations can reach these levels.

For natural gas and propane cooking appliances ANSI Z21.1 has a maximum carbon monoxide concentration of 0.08% (800 ppm) air-free. Per UL 647, kerosene heaters may have a maximum air-free carbon monoxide concentration of 0.04% (400 ppm) at maximum burner setting and 0.08% (800 ppm) at minimum burner setting. The nitrogen dioxide limit is 0.0003 [in.sup.3]/Btu (0.005 [cm.sup.3]/kJ) at all settings. Denatured alcohol appliances have the same limits as natural gas and propane heaters according to UL 1370.

Current State of Knowledge

The position document next discusses research results on emissions and indoor air concentrations of combustion products from propane and natural gas-fired heating and cooking equipment. Research of emissions and concentrations in kerosene and denatured alcohol are not discussed due to a lack of available material.

For heating appliances, the majority of results come from a field study of 30 homes by Francisco, et al. (1) This study found that a number of users operated their appliances continuously for extended periods of time--contrary to the expected and intended use--and that it was primarily under these conditions that carbon monoxide levels rose to above the EPA threshold of 9 ppm as an eight-hour average.

This condition was found in 20% of test homes. The study also found that 80% of homes had one-hour average nitrogen dioxide levels above the new EPA outdoor threshold of 100 ppb, and 40% were above the 300 ppb level recommended by CPSC. However, the study noted that virtually all of the appliances tested were manufactured before the limits on nitrogen dioxide went into force. The study also did not measure particles.

Also cited in the position document was a modeling study by Whitmyre and Pandian, (2) which showed that there was little risk of excessive moisture in homes with unvented space heaters. This was consistent with the results of Francisco, et al., (1) which pointed out that winters in the geographic area of the study were very dry.

A third study cited in the position document on space heaters was by Hedrick and Krug, (3) which evaluated emissions from different types of unvented heaters and showed that different technologies produce greater or lesser levels of carbon monoxide and nitrogen dioxide, with there often being a tradeoff between the two combustion products.

For cooking appliances, the research has not evaluated the relative importance of the combustion process compared to the cooking itself. Cooking has been identified as an important source of pollutants with respiratory impacts. The relative impact of broilers, ovens, and range top burners depends on the contaminant of interest. The position document also makes clear that many of the past studies were done on older units with pilot lights, which are substantially less common today.


The position document next discusses the role of ventilation. Although it does not make quantitative recommendations regarding ventilation, it does reinforce the value of good ventilation when unvented combustion is present.


While the background and review of current knowledge are important for context, the primary product of any position document is the set of positions themselves. Due to the complexity of the issue of unvented combustion, combined with the range of appliances covered by the document, there are several positions. These can be grouped as pertaining to installation/ maintenance; the buildings in which the appliances are located; cooking; gas/propane heaters; kerosene and denatured alcohol appliances; standards; and public awareness. Within product categories, there are positions relating to the appliances and positions regarding needed research.


The position document reiterates industry recommendations regarding installation and maintenance, including that appliances should be installed by a qualified installer; they should be serviced annually; and they should not be the sole source of heating. The document also suggests some changes to current industry guidance. These positions include that sizing guidance for these appliances should be reviewed due to changes in housing stock (primarily that homes are tighter than they used to be) and that code- and standard-making bodies should require installers to be certified.


The two primary positions relating to the building are that there should be a carbon monoxide alarm in every home and that ventilation should be reassessed when air sealing of a building has been done. In the case of the former, this is consistent with a recent addendum incorporated into ASHRAE Standard 62.2, which will be published in the 2013 edition of that standard.


The only position taken regarding the use of cooking appliances is that electronic ignition should be preferred over pilot lights. There are, however, several recommendations regarding research, which could potentially lead to further recommendations. These research recommendations include disaggregating the IAQ impact of the cooking process from the fuel combustion; determining emissions from modern units; and evaluating the difference between the impacts of ovens vs. rangetop burners.

Gas/Propane Heaters

The document has two positions regarding appliance selection. These include that appliances without oxygen depletion sensors (ODS) should not be used and that any appliance that is used should be listed to ANSI Standard Z21.11.2. There are three research needs identified, which could inform future positions. These include evaluating the suitability of the nitrogen dioxide thresholds in the ANSI appliance standard; measuring the nitrogen dioxide levels in homes using units manufactured since 2005 when the nitrogen dioxide emissions requirements went into enforcement; and measuring particle emissions.

Kerosene/Denatured Alcohol Appliances

Due to the lack of research information available, the position document simply states that any kerosene or denatured alcohol appliances should be listed to the relevant standard (UL 647 for kerosene, UL 1370 for denatured alcohol), and that further installations should be avoided until such time as research demonstrates the safety of these appliances.


There are three positions taken with regard to standards. One is that appliance standards should be reviewed in light of recently published standards on nitrogen dioxide, most prominently the new EPA standard for outdoor air. The second is that heating appliance standards should be reviewed regarding carbon monoxide in light of evidence in field research that extended continuous use (one hour or more) is not uncommon. The third is that ventilation standards should include unvented combustion in their scopes and set appropriate requirements. This has led to a change proposal in ASHRAE Standard 62.2 to include unvented combustion in the scope; this proposal is currently working its way through the public review process.

Public Awareness

The document takes two positions with regard to public awareness. One is that a public education program should be developed to reinforce the information regarding health and safety in industry literature. The second is that those who wish to minimize the risk of adverse health effects due to combustion products should not use unvented combustion appliances.


ASHRAE's position document on unvented combustion and IAQ covers a range of issues and appliance types. It is based on those research results that are available and identifies gaps in the knowledge base. It takes meaningful positions based on the current knowledge regarding potential improvements in appliance standards and recommendations as well as ventilation standards. While it is not expected that this position document will settle the debate on these appliances, it is a valuable resource regarding what is known about these appliances and what measures should be taken to minimize the risk from exposure to combustion products. The position document can be downloaded for free on ASHRAE's website at http://tinyurl. com/az9dvk2.


(1.) Francisco, P.W., J.R. Gordon, B.Rose. 2010. "Measured Concentrations of Combustion Gases from the Use of Unvented Gas Fireplaces." Indoor Air 20(5):370-379.

(2.) Whitmyre, G.K., M.D. Pandian. 2004. "Probabilistic assessment of the potential impacts of vent-free gas products on indoor relative humidity." Building and Environment 39:1179-1185.

(3.) Hedrick, R.L., E.K. Krug. 1995. "Conventional Research House Measurements of Emissions from Unvented Gas Space Heaters." Gas

Research Institute Topical Report, GRI-95/0205.

By Paul Francisco, Member ASHRAE

Paul Francisco is a research engineer at the University of Illinois at Urbana-Champaign. Francisco is vice-chair of SSPC 62.2 and is a member of the Environmental Health Committee.
Table 1: Relevant thresholds for carbon monoxide and
nitrogen dioxide.

Contaminant              U.S. EPA     WHO     Health Canada

Carbon Monoxide           35 ppm    25 ppm       25 ppm
  (One Hour Average)
Carbon Monoxide           9 ppm      9 ppm       10 ppm
  (Eight Hour Average)
Nitrogen Dioxide         100 ppb    110 ppb      250 ppb
  (One Hour Average)
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Author:Francisco, Paul
Publication:ASHRAE Journal
Geographic Code:1USA
Date:Apr 1, 2013
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