Thermostat Wiring for Dummies – How Anyone Can Do It Heck, now I’m not calling anyone a dummy! Even the most seasoned people who enjoy DIY may come across problems with wiring a thermostat. The main trouble is dealing with those different colored wires, knowing which one does what and if that wasn’t enough, you also have numbers and letters to deal with.The C-wire Issue · Honeywell Wi-Fi Smart · How to Install a Thermostat · Nest air Conditioning Thermostats -- How To Wire A Thermostat W – (White wire) this is the AC thermostat wire from heating unit. The unit could be furnace, oil or gas furnace, boiler or electric furnace. The W wire goes to W terminal on the subbase of air conditioning thermostats. W2 – (Commonly brown wire) This wire is for second stage heating.