Honeywell Home RC840/U Electromechanical Relay Without Transformer

The RC840 is a general purpose relay from Honeywell Home.

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 model ships without a transformer. Several relays in the same family carry one built in, and the part numbers look almost alike — so check what is feeding the control side before you order. Here the 24 V power has to come from a transformer already in the system, and that transformer has to have the capacity left to carry this coil on top of what it is already running.

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.

Need the transformer built in? The same relay ships with one: RC840T-120 and RC840T-240.

• Brand: Honeywell Home
• Manufacturer Part #: RC840/U
• Type: General purpose relay
• Internal transformer: No
• UPC: 775264094613
• Category: Relays