Thermolic Mini Boiler BT3MBN circulating pump wiring

Wait a minute. You wrote "I have a 20 amp double pole breaker connected to L1 and L2. I have a 15 amp breaker connected to L1 and N"

No. Please no. That wiring is not only not to code, but is horribly unsafe.

If nothing else, opening either breaker alone will not deenergize the wiring in the unit or the wiring from the panel to the unit. This is a fire hazard. It also could very easily kill someone trying to work on the wiring at the panel or at the unit or anywhere in between.

Before you do ANYTHING else, get rid of the 15 amp breaker completely. Disconnect and remove the wire from the 15 amp breaker to the L1 terminal. Check that the neutral wire from the panel has no breakers or switches on it and that it is properly connected to the neutral bus at the panel. Check and make sure the ground is properly connected on the boiler and to the ground bus on the panel.

THats Wrong!

Do not energize those breakers!

You don't need it at all. You DO need the neutral associated with the 20 amp double breaker circuit connected to the N terminal where it says "L1 L2 N" on the terminal block on the far right side of the cabinet, a few inches up from the bottom.

That will provide power to the L and N terminals in the two terminal block near the bottom. Those two terminals are the power output for your circulating pump, and power to that pump is controlled by a relay in the control panel.

Which is, I think, the answer to your question — how to wire the circulating pump to the boiler.

It is, by the way, very likely that the internal 10 amp fuse in the pump power circuit has blown, so you will need to replace it..,

At a first pass, I'm going to go for the cartridge fuse labelled "TR20R" just above the N teminal of the two power output terminals. That's a bit of a guess, however, as the circuit diagram shows the fuse on the L side…