Heat Pumps and Electric Heating - replacing mercury 2 wire to digital thermostat

Hi,
i am no expert with household electronics, farthest ive gone is installing a ceiling fan... anyhow, let me get to the point.

i recently purchased a home with has radiant hot water system, with a gas furnace. It has a "white rodgers" mercury thermostat. I believe it is a very low voltage system, because i have two small black wires which attached to the thermostat.
Now i went and purchased a two wire stat, thinking it would work. But the display goes , except no controlling, for example..i cranked the heat.left it for 10 minutes, and nothing happened.

what should i be purchasing ifi want digital controls ?

thank you..

on a 2 wire heat system, connect one wire to R and the other to W. you will HAVE to get a stat that runs off batteries. others need a third wire (common)

is it possible to tap into common somwhere else in the system.
for example , i have two zones, two thermostats... each thermostat goes to what looks like a valve (last component before the hot water travels to the baseboards) which has 4 terminals on the back.
I if there is a common their, can i use it ?

i can get photos also, if more specific info is needed. I just dont like the idea of changing batteries all the time.
typically how long should a good stat last for ??

thank you very much for your reply

mark

Be sure and get the common from the same transformer that is the one for the same zone the tstat is on dont mix the common to the tstat.


ED ;)