Single Conductor vs. Twin Conductor Identification
A single conductor floor heating system is a loop of one core wire, it starts at the thermostat and ends at the thermostat - this is the original method of manufacturing heating cable. Most single conductor floor heating systems have the same color cold lead wire connected to both load terminals/pigtails of the thermostat. Some early single conductor floor heating systems were produced with a white and a yellow cold lead wire.
A twin conductor floor heating system terminates two core wires at the end, both wires start at the thermostat - this is the current method of manufacturing heating cable. Most twin conductor floor heating systems will have two different colored cold lead wires, and one of these is black. The other wire in a WarmlyYours floor heating system is either yellow (120V system) or red (240V system).  A few early Environ systems had two black cold lead wires.
Note:
- Not all heating cables are wired directly to the thermostat. It may have been necessary during installation to terminate the heating cable in a junction box, and then run a separate (contractor-supplied) wire from the junction box to the thermostat.
- WarmlyYours single conductor cold leads were produced with a 13‘ length at the start of the loop, and a 26‘ length at the end.  The 120V floor heating systems had a yellow and a white label attached to the cold leads, and the 240V floor heating systems had a red and a white label attached.
- If you have a single conductor floor heating system and notice that one cold lead is thicker or feels hotter than the other, please contact technical support at +1 (800) 875-5285