Characteristics of methane emissions from gas-fired tankless water heaters

Methane is the second most prevalent greenhouse gas on earth. The number of residential natural gas users in China is enormous, and the emissions from tankless water heaters accounts for a certain proportion of methane emissions. This paper examines unburned methane emissions in the flue gas of brand new and in-service gas-fired tankless water heaters. We derived methane emission characteristics for tankless water heaters in the start-up, steady operation, and shutdown phases. Also, we calculated the differences in methane emissions from tankless gas water heaters of different ages. The results showed that gas-fired tankless water heaters of varying service ages had similar emission characteristics for different combustion stages. However, methane emissions from gas-fired tankless water heaters increased with usage time. We calculated that the emission rate of the new gas-fired tankless water heater was 0.18%, the average emission rate of the tankless water heater in use was 0.25%, and for water heaters exceeding eight years, the rate was 2.25%.