The 'X' in 3VX450 indicates a notched or cogged construction

preventing catastrophic pressure buildup

or a failing indoor blower motor that is preventing heat dissipation

System Efficiency: Minimizes standby heat loss by closing the damper during the off-cycle

which is standard for most two-story residential systems

Resideo R4222D1013 Relay 318172-701 The 'X' in 3VX450 indicatesThe Resideo R4222D1013 Relay is a heavy duty, field proven switching solution designed for refrigeration, air conditioning, and industrial automation equipment. This 120V DPDT General Purpose Relay features a double pole, double throw contact configuration, allowing for simultaneous control of multiple circuits. Built with a laminated magnet construction for superior operating efficiency and an untaped coil for cooler performance under continuous