I have radiant in floor heat, 2 Honeywell dial type thermostats. Heat source is an oil fired hot water heater. Old therostats have 2 wires 1 black and 1 white. On the new multi terminal digitals what does the black connect to and also the white. No blower or A/C. There is a removable loop between the RH & RC terminals.
Please Black goes to? white goes to ?

My guess is black to RH, and white to W. Go to the boiler controls to make sure. You want the wire connected to one side of the transformer connected to RH, and the one that goes to the rest of the controls connected to W. Don't worry about RC unless you add A/C.

By transformer do you mean the burner unit, it's a Riello and is wired into my main breaker panel. There are 2 thermastats one for each level of the house and I have a new digital for each.