External blower activation relays are eliminated! This feature works well with today's more efficient heating systems that have shorter run cycles and supply lower-temperature air.Also, a manual mode switch is onboard the control, which eliminates the need to use the separate manual mode resistor.The new Aprilaire # 60 automatic humidifier control works in conjunction with your furnace to control the operation of the humidifier.The control senses indoor RH and outdoor temperature and automatically adjusts the operation time of the humidifier to provide the optimum humidity level in the home.The digitial readout on the control displays the RH level inside the cold air return.Can also be used in manual mode when it is not possible to run wire to outdoor sensor.MOUNTS ON RETURN DUCTWORK New lighted display includes indoor relative humidity readout, service indicator light, water panel change light, and "on" indicator light.NOTE: control must be continously powered for proper functionality - separate 24 volt transformer may be required.Unit dimensions - 5 15/*40.6cm W X 10.8cm " H Requires round 1.9cm hole for duct mounting.Note - remote sensor can be purchased separately