Winchester EP-037 Hamilton Replacement Evaporator Pad for 12HF Humidifier Cartridge design makes the evaporator pad easy to replace Pad is enclosed in plastic frame that ensures water flows through proper path for evaporation As water wicks through the evaporator pad, the coated pad suspends moisture in the air stream for humidity to be added Evaporator pad should be changed once per heating season, or more often in hard water areas to ensure peak performance