Design of an economically sustainable energy recovery system for domestic instant geyser

In recent years, due to high electricity prices in South Asian countries, natural gas-based tankless water heaters (instant geysers) have been widely installed for sustainable household water heating. The wastewater after household use is drained into the sewerage system, so a significant amount of heat energy is wasted with this water (usually at T_h1 = 30–40 °C). In this work, the energy wasted with this wastewater is recovered to preheat the input water (from water tank at T_c1 = 5–10 °C) by using a heat exchanger between this high-temperature water and input water to the geyser. Due to variable input water temperature (T_in), the output temperature (T_out) can fluctuate; so, to obtain a constant output water temperature, a low-cost customized gas flowmeter was designed using servo motors. The difference in heat consumption rates (savings in the amount of gas consumption) in the geyser with and without using a heat exchanger (Q₂) was compared with the speed of energy recovered in the heat exchanger (Q₁). This research will provide some insights and considerations at the household level for sustainable operations of gas-based instant geysers.