A carbonated water dispenser comprises a carbonator with a water inlet and a carbonated water outlet. A backflow preventer module is fluidically coupled to the water inlet and comprises a check valve and a thermal mass flow meter. The thermal mass flow meter is configured to communicate a first signal based on a measured amount of heat transferred from a heater to a temperature sensor by a flow of a fluid through the backflow preventer module. A shut-off valve is fluidically coupled between the carbonated water outlet and a nozzle. The shut-off valve is configured to allow or prevent fluid flow from the carbonated water outlet to the nozzle base on a control signal. A controller is configured to detect a backflow condition based on the first signal and generate the control signal to configure the shut-off valve to prevent dispensing carbonated water upon detection of the backflow condition.