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%.