Updating Thermostat Wiring

When I started researching replacement thermostats that will interface with my home automation system, I noticed that most of these thermostats require 24 volts to operate in lieu of the battery operated model that is currently installed. The 24 volts is normally provided by the 120VAC to 24VAC furnace transformer.

The reason that this was a problem for me is that I only had 4 wires coming to my thermostat; a 5th wire is needed in order to provide 24VAC to the thermostat. There are workarounds to this problem, such as this "add-a-wire" adapter. However, my house is a ranch with a full basement, and I was confident that I could easily run new wiring.

While I could have purchased 18/5 (a cable with 5 #18 AWG wires), it made more sense to run additional wires for use in the future, since many modern high efficiency HVAC systems are two-stage. I ended up purchasing 18/8 wire, which is 64 cents/foot at Lowes as of January 2014. I only needed 45 feet, so this worked out to cost less than purchasing a a 4 to 5 wire adapter, and I don't have another electrical component in the system that could fail in the future.

18/8 thermostat wire

If you are running into this same situation, and have a basic understanding of your furnace and AC control wiring, I would recommend putting in 18/8 in order to future-proof your system. That being said, if you have no way to fish new wires to your thermostat locations, the add-a-wire adapter is probably your best bet.