To read an Aquastat wiring diagram, begin with the external connections, such as the thermostat, which is wired to the internal T terminals. Moving clockwise, note the B1 and B2 lines that run power to an oil burner relay or gas valve. The circulator located next to those lines connects to the C1 and C2 lines. Moving further, the low-limit controllers R, W and B ports wire to the internal L1, 3 and B ports, respectively.
The last portion of the external diagram shows the ground wire connecting to an Earth ground. This is done using the provided grounding screw.
The internal diagram shows the electrical connections starting in the lower right corner. B2 connects with C2, which then connects to L2. L2 runs to L1, off of which is a jumper connected to point 3 and, after a break, runs to terminal C1. The high limit box, which contains the B and R terminals that connect to B1, appears after a break from the L1 line. Inside that outline, the T terminals connect.
The diagram indicates that the power supply should be disconnected during the wiring assembly. Additionally, the jumper should be removed as the low-limit controller is installed.