Honeywell Home RC840T-120/U Electromechanical Relay with 24 V to 120 V Transformer

The RC840T-120 is a general purpose relay from Honeywell Home, 24 V.

A relay lets a small control signal switch a much larger load. The thermostat or control board energizes a coil; the coil pulls contacts closed; the contacts carry the current that runs a blower, a circulator or an electric heat element. Nothing on the thermostat side ever sees line voltage.

This relay carries an internal transformer, so it makes its own 24 V control power instead of borrowing it from the system. On a retrofit that is often the deciding feature: adding a relay to a circuit that is already at the limit of its transformer causes exactly the intermittent faults that are hardest to chase.

Match the coil voltage and the contact rating to what you are replacing. A relay with the right coil but an undersized contact will work on the bench and weld shut in service.

The same relay without a transformer is the RC840, for systems that already have 24 V power available.

• Brand: Honeywell Home
• Manufacturer Part #: RC840T-120/U
• Type: General purpose relay
• Voltage: 24 V
• Internal transformer: Yes
• UPC: 775264840005
• Category: Relays