A water heater including a water container, a combustion chamber adjacent the water container, a burner associated with the combustion chamber, a blower located to receive combustion products generated by the burner, an intake air conduit connected to the combustion chamber and having a combustion air intake opening, a dilution air conduit connected to the blower and having a dilution air intake opening; and a flammable vapor sensor having at least an operative portion positioned in the blower or the dilution air conduit.