The house had a two-zone heat system, with two non-programmable thermostats (Mercury based ones). I'm trying to replace them with Honeywell Programmable thermostat Model# RTH221B. However when I opened the old Mercury based thermostat(Robershaw) there are only 2 wires a White and a Black wire. The new Honeywell Thermostat has 5 wire ports, Y, G on left side and Rh, Rc & W on right side. Honeywell instalation mannel says nothing about a black wire so unsure where to connect the Black wire on the new Honeywell thermostat.
I tried to seek advice in the store, but no luck.
Grateful for any guidance.

Try RH/black and W/white on the new stat. Make sure the batterys are good. Hope it works.

Thank You hvac1000... its worked

Glad you got it working.