Hi there,
I'm switching my thermostat for a newer digital one, though when I took out the old thermostat, there were only 2 black wires with white stripes on them.
The new thermostat calls for an RH wire and a W wire, but I have no way of distinguishing the two apart.
How can I tell which wire is which?

Thanks for the help.

*EDIT*
Would it be wise to get myself a Volt Ohm Meter and check to see which wire is sending 24 volts?

It does not mater which wire you put on each terminal. Actually the wire you are working with is actually only one continuous wire that you could say has been cut at the thermostat, so if you touched the two ends together the furnace will come on, it is the same when you connect them to the thermostat... it just acts as a switch.

All the meter would show is 24 volts between them. It is a floating voltage, not like house wiring where the neutral is grounded.

Ah OK, so I guess that wasn't the issue with my thermostat then.
I've just noticed now that I have to turn my thermostat on to like 35 degrees Celsius for it to make the furnace go on at all.
My house is sitting at 9 to 12 degrees Celsius and it won't go any higher. Should I just call the repairman or is there something else I could check out?

Go to the furnace and short the terminals together for the 2 wires there. The furnace should come on and stay on until you disconnect them or the house gets so hot the high limit kicks the furnace off. If it does, the problem is the thermostat. It is possible the thermostat isn't getting the power it needs from the 2 wires. In may need batteries or a third wire to the com terminal.