Recently moved into a new condo, the unit has hydronics baseboard heaters with a mercury Tstat.

Bought a new programmable Tstat from Honeywell and installed it myself. Thought it would be pretty simple! There were only 2 wires connected to the old mercury Tstat, one to the R terminal and one to Y. I labelled them and reconnected them to the same terminals on the new Tstat.

Programmed new Tstat, even set the mode to ‘radiant heating’. Unfortunate doesn’t matter how I play with the settings, the heat stays on. I eventually discovered that if I set it to COOL mode, and when ‘cool on’ comes on, the heater will turn off.

What am I doing wrong? Anyway I can get it to work correctly in heat mode? Thanks!