I am a Nest Pro and install Nest thermostats as well as all Nest products. A common wire is really required IMHO and I do NOT recommend nor perform an install without it. I get ~5 call per week from customers with existing installations asking for diagnostic troubleshooting assistance because of failures. The common completes the circuit and provides continuous 24 VAC so the thermostat can retain its battery charge. Call us at Satuit4me.com for all your Smart Home needs.