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1 Introduction

The Warm Homes project was set up by the Ministry for the Environment to look at ways to encourage New Zealand households to move to cleaner heating sources and increase household energy efficiency, and overall to encourage warmer and healthier homes.

The aim of this study was to gain a better understanding of emission factors1 for wood burners that are compliant with the National Environmental Standard for Air Quality (NES). The NES requires that all wood burners installed on a property under 2 hectares meet an efficiency of at least 65% and an emissions rate lower than 1.5 g/kg when tested in accordance with the method in AS/NZS 4012:1999 and 4013:1999.

NES-compliant wood burners were chosen because there is very little data on real-life emissions from this type of wood burner. In future, most wood burners will be NES compliant, so new emission factors will need to be established in order to carry out emissions inventories.

This study considerably furthers our knowledge about emissions from NES-compliant wood burners, and in particular enables a comparison to be made between the emission factors from old non-compliant wood burners and the NES-compliant burners.

In 2005 Environment Waikato studied the real-life emissions from pre-1994 wood burners in Tokoroa (Environment Waikato, 2006). The average emissions factor from the 12 pre-1994 wood burners was found to be 14.0 g/kg. Using the same in situ sampling method, the average emissions from the nine NES-compliant burners in this study was found to be 4.6 g/kg, a significant difference. These results support Ministry policy and show that the NES design standard is an effective tool for reducing emissions

The Ministry recommends that the emission factors calculated in this report, for NES compliant wood burners, be applied in air shed modelling. These results represent the best available information on real-life emissions in New Zealand. However, as with all emission factors based on limited data, we advise caution. It is important to take into account the small sample size, test methodology and narrow range of burner designs covered.

1.1 The NES and the Ministry’s performance review

It is not appropriate to compare the real-life results with the NES limit of 1.5 g/kg because the NES is based on laboratory test AS/NZS 4013, and real-life emissions are typically higher than laboratory results due to variability in the wood-burner operation, installation, fuel type, moisture and quality. The sensitivity of certain wood burners to these variables is highlighted under real-life conditions, whereas under laboratory conditions these variables are tightly controlled.

It is important to note that whilst AS/NZS 4013 does imitate real-life conditions, it is limited by the need to control many variables to ensure repeatability across different models. As a consequence, the results in this study should not be interpreted as an assessment of compliance with the NES standard.

After testing the burners in Tokoroa, the Ministry carried out a performance review on a random sample of NES compliant wood burners (Ministry for the Environment, 2007). Of the 35 burners included in the review, 57% failed. The performance review included models that had already been installed in Tokoroa as part of this study. Of the burners included in both the performance review and emissions testing only one was found to have failed. This failure was of a minor nature and would not have impaired the performance of the burner.

1 'Emission factor' refers to a unit of particulate matter (PM10) discharged for every unit of fuel consumed. The emission factors in this report are expressed in grams of particulate per kilogram of fuel burnt where the weight of fuel is expressed on a dry weight basis.