Smart Thermostat Wiring for an Old Boiler

jmconboy

Another C-wire posts here, apologies. I’m trying to add a Honeywell T9 thermostat (and remote sensor) to an old Peerless boiler. The current digital thermostat is battery powered and controls the boiler with just the red and white wires. There is an unused blue wire going from the basement to the current Thermostat That I can use as a C-wire. Honeywell says this thermostat will work with just three wires.
I understand I need the C-wire connection to power the new T9 thermostat. There is an external transformer that is wired into the boiler controls. My first attempt to hook up the new thermostat was not completely successful. I connected the c-wire to the hot (?) side of the transformer (the dashed line in the wiring diagram photo) which allowed me to set up the thermostat through the interface. I thought I had it but the boiler wouldn’t turn on. The message on the T9 screen read along the lines ‘Waiting for equipment to respond…’ . I saw in other posts on this forum that similar installations require a RIBU1C relay. Do I need that? Any help would be appreciated.

HVACNUT

Is the wiring diagram still on the inside cover, or did it burn off?
Just one zone?

jmconboy

I haven’t seen anything there that is legible. I’m trying to track down the exact model number and then dig for a schematic on the internet.