The Aprilaire 4851 is for the 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. For example you would use the Aprilaire 4851 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.