Aprilaire #4851 Blower Activation Relay. Allows digital control (#58) to turn on system fan when there is a call for humidification (but not heat).
For Aprilaire series 500, 600, and 700 humidifiers that use the Automatic Digital Humidifier Controls (# 56 and # 58). This device will initiate HVAC fan operation when there is a call for humidity, even when there is no heat call. Use this relay in applications where humidifier operation is required beyond a call for heat, e.g. when installed on a heat pump or during times of the year where there is a need for humidity but not a need for heat.
Note: Aprilaire recommends that humidifer is plumbed to hot water when using this relay.