I HAVE 3 wires from the wall: bLK, WHITE AND a bare gnd wire. There is 240v between the blk and white wires. The Thermostat of Fahrenheat has a pair of Red, and blk and a second pair of red and blk with green(Gnd) wires. Can you please decribe how I should connect the wires? Thanks.

One set goes to your heater,,,,
red/black gr
the other set is either your voltage in, or your t-stat will control 2 units

If you haven't fixed it yet, the best way is that the two black wire will break your feed beween the black of the line in and the load and the same for the red.
It should look like this:
line side black to thermostat and out of thermostat on the black to the black of the heater
line side red to thermostat and out of thermostat on the red to the red of the heater.
Each line in are 120 volts so you should get 240volts at the thermostat and when the demand ask for it you should get 240 volts at the heater.

Hi, I believe you are controlling only one Hydronic Baseboard heater, even though you have 240 volts at the wall for a high line voltage t-stat. One wire from the wall wht. or blk., doesn't matter which as they are both the same voltage, 120 ea. and terminate one pair of the black wires to one hot, and one pair of red wire to the other hot. Cap of the othr 4 wires and see if this will control the heater. If nothing works, terminate all 4 black to one hot and the same with red, all to the other hot. One of these combinations will control it. Unless you have a diagram showing other wise on the stat wires, it has to wire this way, one of the 2 options. Best of Luck and safety first.Sincerely, Shastalaker7
PS, you would think on a high votage stat for 240 control, tou would have only 2 hot from the stat and most do. Also, terminate the grounds if you will.