There should be a simple:
Question: I am replacing an old mercury thermostat with a honeywell RTH5100B Non Programmable Digital Thermostat.
The old thermostat had two wires 1) white and 1) red. No markings on the plate or terminals.
The Honeywell Thermostatl has instructions but when in doubt ask. I only need to control hot water radiators, no fan, no hrv, no airconditioning, just the room temperature to turn the hot water radiators off or on.

The honeywell options are terminals labeled: Rc, R, Y, not used, O/B W, G.
My ideal answer would be.. put the old white wire on the terminal marked..?. And put the old red wire on the terminal marked..?.

Someone has the simple answer. Thanks

That is actually a thermostat for a heat pump system. It can be used for what you want but you have some stuff on the front that is not needed and could get accidentally switched to where you system would not work in that setting. So if you could I would trade it for a heating only type.

To use the one that you have connect the red wire to the terminal marked R, connect the white wire to the terminal marked W.
There is a scale on the inside of the thermostat called a heat anticipator. It will have readings like .6 or .5 or on up or down the scale, the little slide on the anticipator should be set to match the amp reading printed on the side of the gas valve of the unit that heats your water for the hot water radiators.