Current Sensing Relay, 24V

Aprilaire 50 Current Sensing Relay – 24 VAC for Humidifier Control

The Aprilaire Model 50 Current Sensing Relay is a low-voltage control device designed to activate humidifiers in response to current draw from a blower motor or similar load. Compatible with select Aprilaire models, it enables control of the humidifier without a dedicated HVAC control board output, offering an effective solution for retrofit and standard installations.

Product Application

This relay senses current flow to the HVAC system blower and provides a 24 VAC signal to operate compatible Aprilaire humidifiers. It is typically used in installations where no HUM terminal or dedicated low-voltage output is available on the equipment control board. The relay closes its internal switch when the current exceeds the activation threshold, allowing the humidifier to energize during blower operation.

Compatible Models

• Aprilaire 550
• Aprilaire 550A
• Aprilaire 558
• Aprilaire 560
• Aprilaire 560A
• Aprilaire 568

Key Features

• Current-Sensing Activation: Detects blower motor current draw and enables humidifier operation only during active airflow
• Compact Form Factor: Measures approximately 2-3/4" wide x 1-7/16" high x 1" deep, suitable for limited-space installations
• Simple Integration: Connects inline with the blower power wire and provides an isolated 24 VAC output to the humidifier
• Low Voltage Design: Operates on standard 24 VAC, compatible with most residential control systems
• Solid-State Construction: No mechanical components; improves reliability and reduces failure risk from wear

Technical Specifications

• Control Voltage: 24 VAC
• Control Current Draw: 0.4 A
• Operating Frequency: 50/60 Hz
• Housing Dimensions: 1-7/16" H x 2-3/4" W x 1" D

Installation Considerations

This component should be installed in series with the HVAC blower motor power wire, typically using wire nuts or terminal blocks as applicable. Ensure that the relay's current threshold is compatible with the blower amperage. Mounting is typically done via screw fasteners or adhesive backing (not included). Electrical installation should comply with local code and be performed by a qualified technician.

Contractor Notes

• Used to enable humidifier function where no HUM terminal or relay output is available
• Provides control isolation between equipment power and humidifier transformer
• Ideal for retrofitting older systems or interfacing with non-communicating HVAC units

Summary

The Aprilaire 50 Current Sensing Relay enables reliable control of humidifiers in systems lacking dedicated control outputs. Its compact design, low-voltage operation, and compatibility with multiple legacy Aprilaire models make it a practical and efficient solution for field installations and service work.