How can I extend the non-heating leads for floor heating products when they are too short and don't reach the thermostat box?
If the existing cold lead is too short to reach the thermostat, run the existing cold lead to an accessible junction box at the base of the wall, then run appropriately-sized Romex, THHN, or other approved house wiring supplied by the electrician, from the thermostat to the junction box in the wall. Conduit may, or may not, be required depending on the requirements of the local Electrical Code. Make connections between the house wiring and the non-heating leads inside the accessible box using approved connectors.
Verify that the floor sensor probe will reach the heated area from the thermostat box. If it won't, a separate run of low voltage wire supplied by the electrician will need to run to a junction box in the wall near the heated area. Depending on the local Code, this wire may need to run in its own conduit or loosely through the wall. Low voltage sensor wiring can never share the same conduit as the line voltage wire. Connect the sensor wire and the extension wire inside the junction box using approved means.