42 a slotted cover 43 closes the upper end of the tube 42 the slotted cover 43 provides fluid communication between the tube 42 and a duct portion 45 defined by the housing 32 established by the tube 42 the slotted cover 43 and the duct portion 45 is a vapor passage with an inlet communicating via the opening 41 with the restricted chamber 40 and the cavity 22 and an outlet communicating with the surrounding environment via a discharge opening 46 in a top wall 47 of the housing 32 retained by the housing 32 adjacent to the vapor passage duct portion 45 is electrical control circuitry 51 shown in fig5 also retained by the housing 32 is an air blower 52 with an outlet disposed to discharge air into the duct portion 45 through an air passage exit opening 53 therein air is supplied to the blower 52 through air passage entrance openings 54 in a front wall portion 55 of the housing 32 a control knob 57 for actuating the electrical control circuit 51 is mounted on the front wall portion 55 of the housing 32 a liquid level sensor 61 is supported by the bottom wall 31 of the evaporation unit 12 included in the sensor 61 is an annular float 73 and a magnetic switch 74 surrounded thereby as described hereinafter the switch 74 has active closed and inactive open conditions that respectively energize and deenergize the heating coil 36 inwardly directed collar 75 at the upper end of the float 73.