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


NOx emissions standards

Australia is one of the few countries that does have emission standards for NO2 or NOx from domestic appliances. Where the literature search was successful in locating other standards, comparisons with the Australian standards were often difficult because of assumptions that have to be made in converting between units. Sometimes the applicability of the standards to particular appliances was not clear as a result of translation problems (e.g., "air heaters" may mean central heaters, not space heaters, in Australian usage). In many cases full details of the units used were not known (e.g., the temperature to which gas volumes are standardised, which can cause differences of 10% or more in unit conversions).

In spite of these difficulties, standards found for natural gas fired domestic appliances are summarised in Table 21 and Table 22. The assumptions used in converting between different units are also detailed below. The third columns of the tables contain converted standards in units of ng(NOx)/J or ng(NO2)/J consistent with the specification of the standard. It is not possible to convert from a NOx basis to an NO2 basis without knowing the applicable NO2/NOx ratio. It is important to note that entries in column 3 expressed on a NOx basis cannot be compared with other entries on an NO2 basis as the conversion may vary by a factor of 10 or more, depending on the NO2/NOx ratio.

Table 21: Summary of NOx emissions standards for natural gas fired room heaters
Flued heaters   ng/ J(heat input)
Austria 60 ng(NOx)/J 60
Europe – automatic forced draft burners 170 mg(NOx)/kWh 47
– fan assisted burner
200 mg(NOx)/kWh
150 mg(NOx)/kWh
Poland 60 g(NOx)/GJ 60
Unflued Heaters   ng/ J(heat input)
Australia 5 ng(NO2)/J 5
Austria 30 ng(NOx)/J 30
Germany – radiant heater 60 mg(NOx)/kWh 17
Japan 10 ppm(NO2) 5.7
Table 22: Summary of NOx emissions standards for natural gas fired water heaters
Flued heaters   ng/J(heat input)
Austria 60 ng(NOx)/J 60
Belgium – central heating boilers 100 mg(NOx)/m³ (0% O2) 28
Czech Republic
– atmospheric
– fan assisted

200 mg(NOx)/m³ (3% O2)
150 mg(NOx)/m³ (3% O2)

Europe – automatic forced draft burners 170 mg(NOx)/kWh 47
– central heating boiler
– wall mounted boiler

200 mg(NOx)/kWh
200 mg(NOx)/kWh

Japan 60 ppm(NOx) @ 0% O2 34
Poland 35 g(NOx)/GJ 35
USA (California AQMDs)
– residential
– mobile home

40 ng(NOx)/J(output)
50 ng(NOx)/J(output)



This study contributes to Environment Australia's program to improve ambient air quality. It will be important in the formulation of policy and regulations to integrate ambient air considerations with those of indoor air quality, greenhouse effect, energy efficiency and building insulation, noise and pollutants other than NOx.

NOx control technologies and applications

The three basic principles used for primary control are:

These may involve either or both of the following:

Many primary control technologies have been applied commercially to water heaters and air heaters, but few to cooking appliances. Current regulations have NOx emissions limits set close to the minimum achieved by some technologies. If the NOx emissions limits are going to be set lower in the future, some technologies may require further development or may even be unacceptable anymore. Other technologies that have already achieved lower NOx levels could still be used readily. The status of these primary NOx control technologies are summarised in Table 23.

Table 23: Comparison of primary NOx control strategies for residential gas appliances*
Primary NOx control technology Likely lowest NOx (ppm, O2-free) * Likely change in CO emissions * Likely change in thermal efficiency * Technology status for domestic application *
Premixed, high excess air ~ 20 Decrease Decrease Current
Flue-gas recirculation ~ 25 Increase Decrease Not commercialised
Staged combustion ~ 25 Increase Decrease Current
Delayed combustion ~ 25 Increase Decrease Not commercialised
Humidified combustion ~ 25 Increase Decrease Not commercialised
Flame inserts ~ 40 Increase Decrease Current
Thermally active burner ~ 65 Decrease Increase Current
Port-loading reduction ~ 50 Increase Increase Current
Port redesign ~ 45 Decrease Increase Current
Radiant combustion ~ 4 ** Decrease Increase Current
Catalytic combustion ~ 5 Decrease Decrease Not commercialised
Pulse combustion ~ 20 Increase Increase Current

* Some information could be superseded in 1999.
** This low level is achieved by the Bowin technology which is put into this category by the author. Otherwise, the "Likely Lowest NOx" would be ~ 10 ppm claimed by Acotech.

Most of these technologies were developed in Japan, USA and Europe which use a natural gas supply pressure ≥ 2 kPa gauge for domestic use. In Australia, many parts of the country receive at a much lower pressure, e.g., ~ 1.1 kPa gauge in most regions in Victoria. Hence some of the overseas low NOx technologies may require fan assistance for applications under Australian conditions. A possible approach to improve this situation is to raise the gas supply pressure to similar levels used overseas if no change of hardware is required. This could cost millions of dollars, and will require the involvement and cooperation of Australian gas distribution companies.

Secondary control technologies (to remove NOx) are expensive and could create environmental concern. Until now, secondary control is applied mostly to power generation and industrial combustion processes but not domestic gas appliances.

Low NOx technology adoption strategies

In the Australian context, limiting NOx emissions from domestic appliances and encouraging the introduction of low NOx technology will be done most easily by continued use of AGA standards. However, if these standards are to be used to address ambient air quality concerns, full cooperation of the government environmental regulators and the gas industry will be required in the development of the standards, which until now have been based on indoor air quality.

In setting NOx emission standards, a clear distinction can be drawn between criteria based on indoor air quality and criteria based on ambient air quality. Australian AGA codes have been based on the former. The intention has been to limit NO2 concentrations indoors, with little concern for NOx emitted through flues to the outside atmosphere. On the other hand, NOx emission regulations for domestic appliances in the USA and at least some parts of Europe have been driven by ambient air quality concerns. Only "non-attainment areas" for ozone are regulated in the USA, with the objective of reducing photochemical smog. Ozone problems probably do not occur to the same extent in Australia; however a judgement on this is outside the scope of the present study (an overview of Australian ozone levels is given in NEPC (1998b)). It should also be remembered that NOx from gas appliances constitute less than 0.5% of total NOx emissions in Australian capital cities, except for Melbourne, where the figure is 1.75% (Todd et al., 1997).

Various strategies are available for governments to encourage the introduction of low NOx technology in domestic gas appliances. These include "command and control" regulation, voluntary regulation, subsidies, tax concessions, assistance with promotion, public education and tradable emission permits. The Australian practice of calling up voluntary industry codes in legislation has worked well in the past and the gas industry appears to be moving towards introducing low NOx technology in flued appliances (Saxby, 1998). However, it may be necessary to demonstrate a clearer link between domestic emissions and indoor or ambient criteria before further steps can be agreed.

Australia does not have an eco-labelling scheme. Present appliance labels indicate energy efficiency levels. Some overseas eco-labels include pollutant emission levels in their criteria and a scheme including NOx levels from domestic gas appliances could be introduced to assist consumers in their choice of products.

Cost to business

The cost to business is a sensitive subject. The indicative costs and lead time for modifying an existing overseas low NO model, or developing a brand new low NOx model, vary much among different types of appliances and different manufacturers. The information that has been collected is summarised as follows:

There have been a number of concerns and comments raised by the industry:

Price differentials

Based upon the indicated manufacturing cost of low NOx burners provided by two burner manufacturers, the cost of a low NOx burner to end-users is expected to be $A 60–80 approximately. It can be down to $A 40 or less for high volume production.

Two Australian water heater manufacturers expected the cost of a low NOx water heater and the installation cost would be higher than that of a non-low NOx model by 5–15% if a natural draft system can be used. One manufacturer expected that it would be at least 25% higher if a fan-assisted combustion system is required. In an exceptional case, Tokyo Gas in Japan has maintained the retailing price of low NOx water heaters the same as the previous conventional water heaters to encourage end-users using the low NOx water heaters despite the increase in manufacturing cost.

In Victoria, the normal retailing price of an ultra low NOx flued space heater was approximately $250, or 17%, higher than that of a conventional flued space heater from another supplier. Features and performance were not compared but they are expected to contribute to part of the price differential.

In a report in 1997, the retailing prices of low NOx grills under development in Japan and low NOx hotplates under development in Germany were expected to be higher than those of the conventional ones by less than 20%. In the same report, it was indicated that the end-user price of low NOx hotplates under development in Austria was expected to be twice as much as that of the conventional models.

There has been a local concern about little perception of willingness to pay for low NOx technology by consumers. In order to persuade Australian consumers to purchase low emissions appliances, perhaps a possible approach is:

  1. Educational promotion of community awareness of NOx emissions and their effects on environment.
  2. Survey on consumers' awareness of NOx emissions and willingness to pay for low NOx appliances.
  3. Development of NOx emissions standards if the consumers and industry are ready to accept it.

The educational promotion may proceed together with other emissions issues such as greenhouse gases.

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