Quantifying Methane Emissions from Natural Gas Water Heaters

Abstract

Methane emissions from natural gas appliances remain the least characterized portion of the fossil-fuel supply chain. Here we examine water heaters from 64 northern California homes to (1) quantify methane emissions from natural gas leaks and incomplete combustion while off, turning on or off, and in steady-state operation from 35 homes; and (2) characterize daily usage patterns over ∼1-2 months per water heater to estimate activity factors from 46 homes. Individual tankless water heaters emitted 2390 [95% CI: 2250, 2540] g CH₄ yr^-1 on average, 0.93% [0.87%, 0.99%] of their natural gas consumed, primarily from on/off pulses. Storage water heaters emitted 1400 [1240, 1560] g CH₄ yr^-1 on average, 0.39% [0.34%, 0.43%] of their natural gas consumption. Despite higher methane emissions, tankless water heaters generate 29% less CO₂e₂₀ than storage water heaters because they use less energy to heat a unit of water. Scaling our measured emissions by the number of storage and tankless water heaters in the United States (56.8 and 1.2 million, respectively), water heaters overall emitted an estimated 82.3 [73.2, 91.5] Gg CH₄ yr^-1, 0.40% [0.35%, 0.44%] of all natural gas consumed by these appliances, comparable in percentage to the EPA's estimate of methane emissions from upstream natural gas production.