Carbon emissions of electric-resistance water heaters, gas-fired boilers, and heat pumps for multi-unit residential buildings under real-world hot-water demands

Abstract

Although water heating accounts for 14% of residential electricity usage, the impacts of different water-heating technologies on greenhouse gas emissions are poorly quantified. This article compares carbon emissions from three water-heating technologies—including electric-resistance water heaters (ERWHs), heat-pump water heaters (HPWHs), and gas-fired boilers—by combining high-frequency measurements with physically based models. Models based on heat transfer principles are forced with measured hot-water demands from multi-unit residential buildings and then paired with spatially explicit marginal emissions data to compute carbon intensities. A technology warming potential analysis is then conducted to assess impacts over a 100-year period accounting for both site and source emissions. The use of real-world data at high temporal resolution reveals variations in emissions that are overlooked by studies that use precomputed factors. While, on average, ERWHs emit 1.9–2.2 times and 3.8–4.1 times more than gas-fired boilers and HPWHs, respectively, relative performance varies by season. Estimates of carbon emissions obtained using high-frequency data are found to be 8.9–9.6% higher than those obtained using average emissions factors, owing to alignment between marginal emissions and water demand peaks. These observations are discussed in the context of recent policy changes affecting water-heating technologies.