120v Relay w/ Built In 24V Transformer

The Honeywell Home RC840T-120 combines a 120V relay and a built-in 24V transformer in a single unit, eliminating the need to purchase and wire separate components. This relay delivers immediate activation for improved temperature control and comfort in electric heating systems. Designed for HVAC professionals and contractors, it serves as a direct replacement for any brand-name relay and simplifies installations where line-voltage equipment needs to be controlled by a low-voltage thermostat. The epoxy-soundproofed relay ensures quiet operation, and low current consumption allows multiple relays to be controlled by a single thermostat.

• Built-In 24V Transformer: Integrates the transformer directly into the relay housing, reducing installation time by eliminating separate transformer mounting and wiring.
• Universal Compatibility: Functions as a direct replacement for any brand-name relay, giving you flexibility across different heating systems and reducing inventory needs.
• Immediate Relay Activation: Provides instant response when the thermostat calls for heat, improving temperature control and occupant comfort.
• Epoxy-Soundproofed Relay: Uses epoxy coating to dampen the click sound during switching, ensuring quiet operation in occupied spaces.
• Low Current Consumption: Draws minimal power, allowing a single thermostat to control multiple relays without overloading the circuit.
• Simple Wiring Configuration: Accepts 24V thermostats with 2-wire connections (R, W) or 3-wire connections (R, W, C), accommodating both basic and advanced thermostat models.

The RC840T-120 operates on 120 volts and is compatible with 24V thermostats using either 2-wire (R, W) or 3-wire (R, W, C) connections. It works with standard programmable thermostats, non-programmable thermostats, and line-voltage heating equipment that requires low-voltage control. The relay accepts line-voltage input on the primary side and provides switched 120V output to control heating loads. Installation requires access to 120V power and proper grounding per local electrical codes.

Mount the relay in a standard electrical box or on a suitable surface near the heating equipment. Connect 120V line power to the designated terminals, wire the thermostat leads to the 24V transformer terminals (R, W, and optionally C), and connect the switched output to the heating load. Follow all manufacturer wiring diagrams and local electrical codes. The unit requires no routine maintenance beyond periodic visual inspection for secure connections and physical damage.

• Honeywell Home RC840T-120 relay with integrated 24V transformer
• Installation instructions

The RC840T-120 is backed by Honeywell's standard manufacturer warranty covering defects in materials and workmanship. Refer to the included documentation or contact Honeywell customer support for specific warranty terms and claim procedures.

A: A 120 to 24V transformer is a component that converts standard line voltage (120 volts AC) to low voltage (24 volts AC), which is the power level used by most residential and light commercial thermostats. In the RC840T-120, the transformer is integrated directly into the relay housing, eliminating the need to purchase and wire a separate transformer. This built-in design simplifies installation and reduces the number of components required for the control circuit.

A: The RC840T-120 relay is designed to be controlled by a 24VAC signal from a standard thermostat, not 24VDC. The built-in transformer outputs 24 volts AC, and the relay coil is rated for 24VAC operation. Using a DC signal on an AC-rated relay can result in improper operation, coil damage, or failure to energize. Always match the voltage type (AC or DC) to the relay's specification.