My thermostat for this installation also lacked the c wire so I had to choose between the work around provided by Honeywell or bring in a pro to rewire the connection. Opted for the professional installer, but would have saved $400 on the DIY. Know that the workaround means you'll lose manual fan control, but the fan runs fine when the system calls for heat or cool (just can't run only fan or fan in circulate mode). Great product and the fifth gen I've installed in various homes over the past few years.