How to add a C-wire to Weil McLain Boiler

According to the wiring diagram, the transformer should be installed between the T1 and T2 terminals. Though you have not shown the wiring in any of the photos, so it's difficult to give a better description than that.

However, since the transformer is only rated at 40 VA (volt-ampere), the maximum current on the secondary side of the transformer would only be 1.66666667 amperes.

40 VA / 24V = 1.66666666666667 amperes

In most cases the only things on the secondary side of the transformer are switches and relays, so this usually is sufficient. When you go installing thermostats that draw more current than that, you start running into problems.

If you want to install a thermostat that requires constant power, you may have to upgrade the transformer and any fuses that protect it.

According to the control wiring supplement for your boiler, there should be a C terminal on the transformer, adjacent to the R terminal. Look for the terminal on the transformer with the green wires going to it.