Aprilaire 4851 Blower Activation Relay for 500, 600 & 700 Series Humidifiers (Automatic Control) Reviewhttp://tinyurl.com/mpzpa5t Aprilaire 4851 Blower Activation Relay for 500, 600 & 700 Series Humidifiers (Automatic Control) Aprilaire #4851 Blower Activation Relay.For Aprilaire series 500, 600, and 700 humidifiers that use the Automatic Digital Humidifier Controls (# 56 and # 58).Allows digital control (#58) to turn on system fan when there is a call for humidification (but not heat).This device will initiate HVAC fan operation when there is a call for humidity, even when there is no heat call.