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

Existing NOx emission standards

4.1 General

This study has arisen from consideration of ambient air quality, where the principal concern is the generation of photochemical smog from NOx and hydrocarbons. In contrast, many studies of emissions from gas appliances are concerned with the resulting indoor NO2 concentrations. The difference in emphasis is reflected in the form of the standards listed in this Section – generally speaking, those in terms of NOx are to regulate the ambient environment and apply to flued appliances; those in terms of NO2 are for the indoor environment and apply to unflued appliances. There are exceptions to this, as where standards for unflued appliances are in terms of NOx. Standards are often quoted without specifying whether they are for flued or unflued appliances – in such cases the more common flued alternative should be assumed unless the standard is in terms of NO2.

This report is concerned with emissions from domestic gas appliances to the ambient environment. There are no standards for ambient emissions of NOx in Australia. However, there are Australian industry standards governing emissions to indoors. These are described in the Appendix.

There are no international standards that are universally applicable in all countries (OGSV, 1998). A number of national standards have been found in the course of this project. However, the different bases of standards from different countries makes intercomparisons difficult.

A comparison of standards for room heaters and water heaters is attempted in the Conclusion. When comparing standards, it should be remembered that excessive precision is meaningless if comparable accuracy in NOx measurements cannot be achieved.

Following intercomparison tests showing reproducibility of only about 40%, efforts supported by the European Union and the International Gas Union have been identifying sources of error and working towards improved test methods (Yamada and Desprets, 1997; Schweitzer, et al, 1998).

Only standards applicable to domestic appliances are presented in this report. Where applicability is determined by heat rate limits, standards applying only to air heaters > 150 kW or water heaters > 500 MJ/h (139 kW) have been excluded.

Henceforth in this report where 'NOx' is used in units it is short for 'NOx as NO2', meaning that NO is converted to the equivalent mass of NO2.

4.2 GRI report

In the course of the literature searches conducted for this project, a highly relevant report by Benedek and Goodman (1994) was discovered and a copy imported.

The report surveys emission regulations, consumer-oriented eco-standards and voluntary industry standards affecting emissions from gas-fired residential appliances in North America, Europe, Australia and Japan in 1993.

The work was commissioned by the US Gas Research Institute and the authors have evidently had access to greater time and resources to assemble their information than has been available in the present project. Even so, it is apparent that most of their information has been gathered from personal communications or sources that would not be regarded as authoritative. Most of the references cited are newspaper articles, however an extensive list of contacts is given.

This is consistent with the finding of the present authors that it is difficult to find authoritative references to mandatory or voluntary emission standards. As in most places where standards are quoted, there is often a lack of technical detail, as for example, not stating if a standard is for NOx or NO2, or based on heat input or output.

The work by Benedek and Goodman relates to 1993 and may be out of date. The section on Australia illustrates the shortcomings of the report. A voluntary gas industry standard of 5 ng(NO2)/J for all flueless appliances is mentioned. New South Wales is the only state mentioned. The framework for National Environment Protection Measures was not in place. Nevertheless, the report mentions many standards and implementation practices for which we have no better references.

In the absence of better information, it has been assumed that Benedek and Goodman's information is still current. Even if standards have been superseded by subsequent developments, this reference provides a useful picture of the way things were.

Benedek and Goodman identify three regulatory mechanisms that limit emissions from residential appliances:

The first of these is treated in Section 4.0 of this report. The remaining two fall within Section 6.0. Incentive programs sometimes involve emission limits as qualifying standards.

4.3 IGU Report

Another very useful paper is the report by Yamada and Desprets (1997) of a survey carried out in 1995 to survey methods of NOx measurement and standards in member countries of the International Gas Union (IGU). The report is comprehensive and contributes importantly to the material on NOx emission standards presented in this report. However, it is poorly presented and sometimes judgements have had to be made about likely meanings. This is partly a reflection of the language difficulties found in a number of the source references, where translations have had to be made from the original technical and regulatory documents.

In the case of Yamada and Desprets, the paper produced is in English, which is evidently not the native language of either author. Although the survey was conducted within Committee E of the IGU, which deals with domestic gas utilisation, some of the standards quoted are for larger installations.

Yamada and Desprets (1997) concluded that in the three years since a previous IGU survey, standard development progressed in many countries. In Europe, a uniform NOx measuring method for type-testing of gas appliances was developed for use by 18 European Committee for Standardisation (CEN) member countries. In Croatia, Czech Republic and Poland national standards on emission levels had been developed during this period or were being prepared. In some countries like Germany, Japan or the Netherlands standards linked with a quality mark scheme were in use, although the scheme in Japan was applicable only in a certain region. In the USA, national standards for unflued room heaters had been developed and were 'under preparation for enforcement'.

More details are given below under each country's heading. Emission levels associated with environmental quality certification schemes (eco-labels) are listed in Section 6.0 where the low NOx technology adoption strategies are discussed.

4.4 Austria

The Austrian standards and unit conversions in Table 2 are as given by Yamada and Desprets (1997).

Table 2: NOx emission standards in Austria
Flued Appliances ng(NOx)/J (ppm @ 0% O2)
Natural gas water heater < 350 kW input 60 (122)
Natural gas flued room heater < 350 kW input 60 (122)
All other natural gas < 350 kW input 30 (61)
LPG water heater < 350 kW input 80 (161)
LPG room heater < 350 kW input 80 (161)
All other LPG < 350 kW input 40 (80)
Unflued Appliances ng(NOx)/J (ppm @ 0% O2)
Natural gas room heaters and cookers 30 (61)
LPG room heaters and cookers 40 (80)

Source: Yamada and Desprets, 1997

4.5 Belgium

The Belgium standard and unit conversion are presented in Table 3 as given by Yamada and Desprets (1997).

Table 3: NOx emission standard in Belgium
Flued Appliances mg(NOx)/m³(0% O2) (ppm @ 0% O2)
Natural gas central heating boiler >100 kW input 100 (49)

Source: Yamada and Desprets, 1997

4.6 Canada

Each province in Canada is responsible for developing and implementing NOx regulations. Benedek and Goodman (1994) indicated that there were no regulatory requirements in Canada for NOx emissions from residential appliances but requirements for gas-fired fireplaces were under discussion. Unvented gas space heaters were not allowed, except in British Colombia, where they were subject to controls, but not NOx emission limits.

Canada has pursued a policy of harmonising its industry standards with US standards and they are now authorised jointly by the American National Standards Institute and the Canadian Standards Association and distributed by International Approval Services. As no federal US standards are known, it is assumed there are still no Canadian standards for domestic NOx emissions. E Smith (personal communication) confirmed that there is no government mandate to reduce NOx emissions from residential equipment. There are, however, federal guidelines for residential indoor NO2 in Canada (see Appendix).

4.7 Croatia

The Croatian standard proposed for small natural gas furnaces and its unit conversion as given by Yamada and Desprets (1997) is tabulated in Table 4.

Table 4: Proposed NOx emission standards in Croatia
Flued Appliances mg(NOx)/m³(3% O2) (ppm @ 0% O2)
Very small: < 50 kW input 150 (85)
Small and middle: 150 kW–5 MW
Whirl combustion
250 (142)
500 (284)
300 (170)

Source: Yamada and Desprets, 1997

4.8 Czech Republic

The Czechoslovakian standards and unit conversions are summarised in Table 5 as given by Yamada and Desprets (1997):

Table 5: NOx emission standards in Czechoslovakia
Flued Appliances mg(NOx)/m³(3% O2) (ppm @ 0% O2)
Very small LPG furnace 200 (114)
Central heating natural gas boiler (fan assisted) < 0.2 MW output 260 (148)
Natural gas boiler (atmospheric) < 0.2 MW output 200 (114)
Natural gas boiler (fan assisted) < 0.2 MW output 150 (85)
All natural gas boilers > 0.2 MW output 200 (114)
LPG boiler (atmospheric) < 0.2 MW output 315 (179)
LPG boiler (fan assisted) < 0.2 MW output 262 (149)
All LPG boilers > 0.2 MW output 200 (114)

Source: Yamada and Desprets, 1997

4.9 Denmark

The Danish standard and unit conversion is presented in Table 6 as given by Yamada and Desprets (1997):

Table 6: NOx emission standard in Denmark
Flued Appliances mg(NOx)/m³(5% O2) (ppm @ 0% O2)
Natural gas engine and turbine > 120 kW input 650 (415)

Source: Yamada and Desprets, 1997

4.10 Europe

The Gas Appliances Directive (EC, 1990) places requirements on manufacturers relating to the design and performance of gas appliances. The EN series of European standards produced by the European Committee for Standardisation (CEN) are binding on Austria, Belgium, Denmark, Finland, France, Germany, Greece, Iceland, Italy, Luxembourg, the Netherlands, Norway, Portugal, Spain, Sweden, Switzerland and the United Kingdom. Standards harmonised under the Gas Appliances Directive are listed on the website of Directorate General III ( ).

The purpose of the Gas Appliances Directive is to ensure that gas burning appliances and their associated fittings are safe. The Directive does not include limits on NOx emissions. However, NOx emission limits may be included in some of the individual EN appliance standards (C Brooks, personal communication). Most of the relevant EN standards were scanned for NOx limits without success.

One standard, EN676 (1997), contains NOx limits for a range of gases burnt in automatic forced draft burners. This standard is not associated with any particular appliance and appears to be directed mainly at burners incorporated in industrial or commercial appliances. The limit applicable to Australian natural gas is 170 mg/kWh (47 ng/J) of heat input and for LPG the limit is 230 mg/kWh (64 ng/J). Other standards that were found not to contain emission limits were: EN26 (1995), EN89 (1997), EN297 (1997), EN298 (1994), EN483 (1997), EN613 (1997), EN 1266 (1993), EN1596 (1994), and EN13203 (1998). Some of these contain limits for leakage of combustion products into a room.

Benedek and Goodman (1994) note that EN297, then in draft form, specified NOx emission limits for four classes of appliances with increasing energy efficiency. The proposed limits were 260, 200, 150 and 100 mg/kWh (input). At that time the draft limits were to apply to NOx emissions from boilers, cookers and space heaters.

Yamada and Desprets (1997) confirm that there is no European standard for measurement of NOx emissions but a proposal has been drafted.

4.11 Germany

German air quality is governed by a federal law. State governments are responsible for implementing the federal legislation and drafting clean air plans, similar to the arrangement in the USA.

There are no emission limits on residential appliances in the federal law; emission limits are specified in the standards of the Deutsches Institut fur Normung eV (DIN). Manufactured gas is distributed in West Berlin only and is to be discontinued. German standards and unit conversions quoted by Yamada and Desprets (1997) are shown in Table 7.

Table 7: NOx emission standards in Germany
Flued Appliances mg(NOx)/kWh (ppm @ 0% O2 dry)
Natural gas central heating boiler < 2 MW input 200 (113)
Natural gas room heater < 13.5 kW input 200 (113)
Fan assisted natural gas burner <1.2 MW input 150 (85)
Wall mounted natural gas boiler < 35 kW input 200 (113)
LPG central heating boiler < 2 MW input 300 (169)
LPG room heater < 13.5 kW input 240 (135)
Fan assisted LPG burner <1.2 MW input 300 (169)
Wall mounted LPG boiler < 35 kW input 240 (135)
Unflued Appliances mg(NOx)/kWh (ppm @ 0% O2)
Natural gas central furnace without heat exchanger 1.8 @ high excess air
Natural gas radiant heater < 120 kW input 60 (34)
LPG central furnace without heat exchanger 3 @ high excess air
LPG radiant heater < 120 kW input 90 (51)

Source: Yamada and Desprets, 1997

Blue Angel certification limits quoted in Section 6.7 are more stringent than the DIN standard limits (RAL, 1998).

Benedeck and Goodman (1994) quote a standard of 80 mg(NOx)/kWh input for high-efficiency boilers in Hamburg. This is a criterion level for the Hamburg incentive scheme and the DVGW quality mark described below in Section 6.7.

The contents of the six-volume Clean Air Handbook of VDI (1999) were searched without finding emission standards applicable to domestic appliances.

4.12 Japan

Japan's appliance emission targets are set by industry committees dominated by the gas utilities.

The authority of the targets derives from the cultural preference for voluntary industry regulation and from the large buying power of the utilities, which put their brands on appliances produced by manufacturers and then retail them as complying products.

Japanese emission targets for NOx from residential appliances have been set as shown in Table 8 (S Tanaka and K Hase, personal communications). The water heater target does not apply to LPG appliances. Although the target is for application in large cities, most appliances sold in Japan would comply. The target for unflued space heaters was previously 5 ppm NO2 but has been relaxed. Unflued heaters comprise most of the space heater market.

Table 8: Japanese emission targets
Flued Appliances ppm @ 0% O2
Natural gas water heater 60 (NOx)
Unflued Appliances ppm @ 0% O2
Space heaters 10 (NO2)

Source: S Tanaka and K Hase, personal communications

JIS series standards that were scanned and found not to include NOx limits were: JIS S2103 (1991), JIS S2109 (1991) and JIS S2122 (1991).

Yamada and Desprets (1997) quote non-mandatory limits and unit conversions as shown in Table 9. Because of the cultural preference for self-regulation, it is difficult to gauge how enforceable these are.

Table 9: Regional NOx emission standards in Japan
Flued Appliances Issued by ppm(NOx) @ 0% O2
Natural gas water heater (fan assisted and inst.) ≥ 35 kW input Regional Committee on Gas Appliances and Accessory Technical Standards (RCGAATS) 125
Low NOx natural gas water heater (fan assisted and inst.) ≥35 kW input RCGAATS 60

Source: Yamada and Desprets, 1997

Warm water boiler, steam boiler, chiller-heater (natural gas or LPG) = 35 kW output Tokyo Prefecture 60 (Determined from emissions at maximum and minimum load)

4.13 Netherlands

The Netherlands has a high rate of natural gas consumption. NOx emission limits apply for domestic central heating boilers of up to 1,000 kW as described in Table 10 (Benedek and Goodman, 1994; Yamada and Desprets, 1997).

Table 10: NOx emission standards in Netherlands
Central Heating mg(NOx)/m³(3% O2) (ppm @ 0% O2)
Atmospheric burners and air heaters 158 (90)
Fan assisted burners 105 (60)
Fully premixed burners 70 (40)

Source: Benedek and Goodman, 1994; Yamada and Desprets, 1997

A new limit of 40 ppm (70 mg/m³ @ 3% O2) for all burner technologies is under discussion for introduction in 2000. It has not been decided if other appliances will have to meet this limit also. At present there are no limits for other appliances like hot water storage boilers, direct water heaters, stoves and cooktop ranges. (Rob Aptroot, personal communication)

The standard NEN1078 (1987) contains limits for CO2 but not NOx. However, it was updated in 1998.

4.14 Poland

Yamada and Desprets (1997) quote the standards and unit conversions as given in Table 11. The standard for boilers is not mandatory.

Table 11: NOx emission standards in Poland
Natural gas and LPG Appliances g(NOx)/GJ (ppm @ 0% O2)
Burners, heat input between 10 kW and 10 MW 60 (122)
Boilers ≤ 1 MW input 35 (71)

Source: Yamada and Desprets, 1997

4.15 Sweden

Natural gas in Sweden is mainly used in larger scale applications, such as within industry and for combined heat and power production. The town-gas distribution system in the city of Stockholm is still operating and there are about 100,000 households connected to it. Natural gas/air mixtures are used in about 30,000 households in areas which earlier consumed town-gas. The Swedish gas industry is currently performing a study of NOx emissions in kitchens, but there are no results yet (G. Engstrom, personal communication).

There are no specific limits for NOx emissions from boilers. These are set from case to case and common limits for small boilers are 60 ng/J fuel or 100 ng/J for boilers converted to natural gas. (O. Jonsson, personal communication.)

4.16 Switzerland

A Swiss standard called the Zuricher Werte was introduced in 1989 for the Canton of Zurich and integrated into federal law in 1992.

The limits were introduced in response to ambient air quality problems; however, gas and oil use is small in Switzerland due to abundant hydroelectric power.

There are limits for NOx emissions from all stationary combustion equipment with the exception of:

NOx emission limits for gas-fired equipment are shown in Table 12. If requirements are not met, renovation of the system will be forced. However, for smaller systems rated less than 70 kW, if NOx emissions are not within limits, retrofit of the equipment is not required (Benedek and Goodman, 1994; Yamada and Desprets, 1997).

Table 12: NOx emission standards in Switzerland
Appliance mg(NOx)/m³(3% O2) (ppm @ 0% O2)
Atmospheric (natural draft) natural gas burner < 12kW output 120 (68)
Atmospheric (natural draft) natural gas burner 12 kW < heat output < 350 kW 80 (45)
Fan assisted natural gas burner ≤ 350 kW output 80 (45)
Atmospheric (natural draft) LPG burner < 12kW output 120 (68)
Atmospheric (natural draft) LPG burner 12 kW < heat output < 350 kW 90 (51)

Source: Benedek and Goodman, 1994; Yamada and Desprets, 1997

4.17 United Kingdom

The UK has harmonised with the European EN series standards (see above, Section 4.10). At present the only standards that apply are for CO from domestic appliances (L Sloss, personal communication); this confirms the conclusion drawn from scanning the European standards that there are no NOx limits for domestic appliances.

4.18 United States

The Department of Energy is assembling data in support of proposed rulemakings for water heaters and central air conditioners ( ).

Domestic NOx restrictions in the USA are driven by compliance with National Ambient Air Quality Standards, which are implemented by states. Each state is divided into regions that are designated 'attainment' or 'non-attainment' areas. Non-attainment areas for ozone in California, the Northeast, Texas and the Midwest states have considered introducing stricter NOx emission controls, including for domestic appliances.

California leads the way in appliance regulations, under requirements from the US EPA, to review the standards regularly and update them as necessary. No specific plans for appliance regulations are known in any other states, including Michigan, Massachusetts and New Jersey (P George, personal communication).

NOx emissions standards are in force for central heating systems and water heaters in the South Coast Air Quality Management District (AQMD), which covers the Los Angeles area of California. The statutory rules can be downloaded from the website :

Table 13: Rule 1111 for NOx emissions from central heating furnaces in districts of California
Central Heating Furnaces ng(NOx)/J (output)
Central heaters < 175,000 Btu/h input 40
Combination heating and cooling units with a cooling rate < 65,000 Btu/h 40

Most of the domestic heating in California is by central heating systems and there is no emission standard for space heaters in the room (M Haimov, personal communication).

Table 14: Rule 1121 for NOx emissions from natural gas water heaters in districts of California.
Water Heaters < 75,000 Btu/h(input) ng(NOx)/J (output)
Residential 40
Mobile home 50


  • 40 ng/J = 72 ppm air free @ 80% efficiency according to Benedek and Goodman (1994). 40 ng/J = 90 ppm air free according to Yamada and Desprets (1997).
  • 40 ng/J = 0.093 lb/million Btu, which is the reference emission rate for water heaters and central residential furnaces quoted in the Area Source Credits rule (see Section 6.15 below).
  • Emissions are for NOx (NO2 + NO) expressed as NO2.

A proposal to amend this standard is currently being discussed (SCAQMD, 1999). The AQMD is proposing to amend Rule 1121 as follows:

In the San Francisco area (Bay Area Air Quality Management District) the same standards for furnaces (Regulation 9 Rule 4) and water heaters (Regulation 9, Rule 6) apply and can be downloaded from the website . 40 ng(NOx)/J(output) limits for water heaters also apply in the California districts of Sacramento, Ventura, San Joaquin Valley and San Diego (Benedek and Goodman, 1994).

Benedek and Goodman (1994) reported that in 1993 the states of Massachusetts, Illinois, Texas and the Northeast Ozone Transport Region were considering NOx controls similar to those in California, in order to achieve attainment with the ozone standard.

Personal communications during this project continued to suggest that other states may have domestic NOx controls but none were actually known of. On balance of evidence and after limited checks of the home pages of state environmental authorities, it is concluded that there are no domestic NOx standards in states other than California.

Standards of the American National Standards Institute are mostly harmonised with those of the Canadian Standards Association. The following standards were found not to include NOx limits: ANSI Z21.10.1 (1998), ANSI Z21.10.3 (1998), ANSI Z21.11.1 (1993), ANSI Z21.11.2 (1997), ANSI Z21.86 (1998). Each of these contains limits for carbon moNOxide and ANSI Z21.10.3 (1998) contains limits for leaks to a room. Although the study of ANSI standards was not exhaustive, the conclusion that they contain no NOx limits is consistent with Benedek and Goodman (1994).

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