Modeling and simulation of tankless gas water heaters to reduce temperature overshoots and undershoots

Abstract

The hot water user’s comfort perception is severely affected by sudden water temperature changes above a certain magnitude. Sudden temperature changes, with overshoots or undershoots, could occur in hot water systems based on domestic tankless gas water heaters (TGWH). These are mainly due to sudden changes on the overall hot water flow rate demand and to the response delays inherent to the heating system, which cannot be anticipated by the equipment controller. This work presents the modeling and simulation of a proposed solution to reduce the temperature undershoots or overshoots to small acceptable values, not perceptible by the users. The proposed solution includes a tank acting as a thermal capacitance, a mixing valve to mix the tank and heat cell water flows and a bypass valve, connected to the output, the latter to eliminate the outlet temperature overshoots. Typical situations originating temperature undershoots and/or overshoots are simulated for validation of the proposed solution, showing that it is able to: a) Substantially reduce the waiting time for hot water in the cold start of the water heater; b) Respond to sudden increases of the hot water flow with only a small acceptable decrease in the outlet water temperature (for hot water flow rates varying from 2 to 14 L/min the temperature undershoots are lower than 0.5 °C); and c) Complete elimination of temperature overshoots for sudden decreases of the hot water flow rate.