The Aprilaire 4851 is the correct replacement for the following humidifiers:
The Aprilaire 4851 Humidifier Blower Activation Relay allows the humidifier to energize the HVAC blower on a call for humidity where the humidifier is no longer dependent on a call for heat. Also known as part number 4851, the Aprilaire Blower Activation Relay will deliver the comfort and protection needed for controlled humidification. It is highly recommended to use this Aprilaire 4851 Relay if your humidifier is installed on a heat pump or during certain times of the year where there is a need for humidity, but not heat.
Aprilaire Blower Activation Relay specifications:
Aprilaire advises that you make sure your humidifier is plumbed to hot water when using this blower activation relay. This replacement is recommended for humidifiers that run in conjunction with the heating system in your home, particularly during the winter months. This Aprilaire 4851 humidifier relay fits all 500, 600 and 700 series humidifiers that use the automatic digital humidifier controls (#56 and #58). Please note that the Aprilaire brand is also known by the following spelling variations: April Aire, April Air and Aprilair.