Nitrogen oxides emissions standards for domestic gas appliances

Background study
Mr Bob Joynt, Environmental Consultant and Mr Stephen Wu, Combustion Engineering Consultant
Environment Australia, February 2000

Appendix: Standards based on indoor air quality


The Australian Gas Association (AGA) 100 series standards relate to domestic gas appliances. Emissions limits are specified for cookers and space heaters in terms of nanograms of nitrogen dioxide per joule of heat input – ng(NO2)/J. The units specifically refer to emissions of NO2, rather than NOx, or NOx calculated as NO2 (for emissions containing 50% NO and NO2 by volume, 1 ng(NOx as NO2) corresponds to 0.5 ng(NO2); for 90% NO and 10% NO2 by volume, 1 ng(NOx as NO2) = 0.1 ng(NO2)). Henceforth in this report where "NOx" is used in units it is short for "NOx as NO2". The heat content is based on the specific energy of the gas burnt (at appliance efficiencies close to 100%, a concentration of 1 ng/J(input) will be close to but larger than 1 ng/J(output)).

AG101 (1998) requires that "the amount of nitrogen dioxide produced by any cooker shall not exceed 15 ng/J when tested at the highest setting". Most gas cookers do not meet the standard of 15 ng/J (Gardiner K., personal communication).

AG102 (1998), for water heaters, contains a limit for carbon monoxide, but not for NOx.

AG103 (1998), for space heaters, specifies an emission limit for NO2 of 5 ng/J. The domestic flueless space heater standard for nitrogen dioxide has been reduced from 15 ng/J in 1987 to 10 ng/J in 1990, to 5 ng/J in 1991, with the standards applying to heaters sold from those dates (Saxby, 1998). The gas industry has adopted an unofficial guideline for indoor air quality of 0.3 ppm, which it expects to be met by indoor appliances emitting 5 ng/J.

The installation code, AG 601 (1998) has no emission limit for NOx but local rules restrict unflued space heaters burning reticulated natural gas in Victoria.

AG501 (1998), for industrial and commercial appliances, requires that "a direct-fired air heater shall not produce … nitrogen dioxide in excess of [0.5 ppm] … with the heater handling the manufacturer's rated air throughput at nominal gas consumption and at the manufacturer's minimum gas consumption."

The standards are based on best practice as determined by committees representative of the natural gas and LPG industries, technical regulators and consumers. AGA and Standards Australia standards are called up in statutory regulations in each jurisdiction, so that detailed requirements vary between the States and Territories. However, approvals are subject to mutual recognition between the States.

In Victoria, new gas regulations have been proposed (OGSV, 1998) and are still under discussion. They proposed regulations to allow flueless gas heaters, as in the rest of Australia. Victoria currently allows flueless LPG heaters but effectively bans reticulated gas heaters without flues. The proposed regulations would allow flueless space heaters meeting NO2 emission standards of 5 ng/J for 12 months after the regulations commence, then 4 ng/J for the next 5 years, and thereafter 2.0 ng/J. The level of 2 ng/J was chosen as equivalent to an indoor concentration of 0.12 ppm (Saxby, 1998).

Various building configurations are being modelled at CSIRO Building, Construction and Engineering Division to establish required heat outputs under different conditions. As a later step it will be possible to relate heat output to NO2 emission, assuming various emission limits, and hence to indoor air quality, by modelling (A Vissenti, personal communication).

Provisions of the proposed Victorian regulations are being opposed by the Department of Human Services and the Environment Protection Authority. Indoor air quality studies indicate that substantially higher levels of NO2 and other pollutants can occur inside domestic buildings than outside and it is the indoor effects that are the cause of the current controversy.

Strategies for the control of indoor air pollution (which have much in common with strategies for the adoption of low NOx technology) were mentioned in Section 6.1. Indoor air quality and its control have been addressed by various authors and summarised by EPAV (1993).


While Canada has no domestic emission standards based on ambient or indoor effects, there are federal guidelines for residential indoor NO2 (quoted in EPAV (1993)):

United Kingdom

Although the UK has no domestic emission standards, Yamada and Desprets (1997) quote a 1990 British Standard for natural gas direct fired unflued air heaters < 2 MW heat input. The limits are 5 ppm (NO) and 1 ppm (NO2) at operating conditions with high excess air.

United States

Benedek and Goodman stated that the Consumer Product Safety Commission was working with the ANSI committee to develop NO2 emission limits for incorporation into the ANSI standard for unvented space heaters (Z21.11.2). The Consumer Product Safety Commission is an independent federal agency that works to reduce unreasonable risks of injuries and deaths associated with 15,000 types of consumer products. It had a concern for indoor air quality to meet a standard of 0.3 ppm (NO2) (1 hour average). Its annual report to Congress for 1994 confirms this activity but there are no other references on its website ( ), suggesting that the revision to the standard was not made. This confirms the result of scanning the 1997 update of ANSI Z21.11.2. Yamada and Desprets (1997) stated that a proposed standard for unflued natural gas or LPG room heaters = 11.723 kW of heat input was being submitted to ANSI for final approval expected in late 1995 or early 1996. The proposed level was 20 ppm (NOx @ 0% O2).

As of January 1999, Montana and Massachusetts prohibited residential use of vent-free gas heating appliances in the states. California was writing regulations and was projected for sales and installations of natural gas vent-free products in the year 2000. Nevada, Arizona, Idaho, Minnesota and Alaska allow vent-free installation in some areas and Wisconsin allows it in homes built before 1990 (US Gas Appliance Manufacturers Association website ). These restrictions appear to be less stringent than in 1993, when Benedek and Goodman (1994) listed eight states prohibiting unvented space heaters, and several other local prohibitions. Most states continue to allow unflued space heaters (J. Joyce, personal communication).

The following six model building codes permit the installation of listed vent-free gas products (Source: US Gas Appliance Manufacturers Association website ):

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