[Trainer]

The red wire coming from the large plug on the control unit to the bottom connection on the middle relay is called the "Diode Wire". This wire is energized any time the burner motor is running. The bottom connection of the middle relay is also the power source for the middle, zone 2 coolant pump. The energized diode wire makes pump two stir the 16 gallon coolant tank providing uniform temps in the coolant tank to provide continous hot water.

The diode in that wire keeps the operation of the middle zone heating loop from back feeding the control unit. Works great.