an evaporator including a housing defining an air inlet , an air outlet , and a reservoir for a liquid supply ; a blower for producing air flow in a path between the inlet and the outlet ; a vessel retained by the housing above the reservoir and adapted to retain a liquid volume ; a wick means retained by the vessel and comprising a source portion submerged in the liquid volume and an evaporative portion disposed in the air flow path , the wick means adapted to provide by capillary action liquid flow from the source portion to the evaporative portion ; and a pump means for pumping liquid from the reservoir to the vessel so as to maintain the liquid volume therein . the output capacity of the evaporator is increased by pumping liquid from a reservoir upwardly to a supply vessel .