C-Wire

Equipment and Systems - HVAC Costs pillar

Also known as: common wire, common

Common wire. The 24V power conductor required by most smart thermostats. Older Ontario homes often lack one, requiring an add-a-wire kit or rewire.

The C-wire (common wire) is the 24V power return that smart thermostats need to keep their displays and Wi-Fi running without draining the heating relay battery. Pre-2010 thermostats (mechanical or simple programmable) didn't need a C-wire and most Ontario houses built before that era weren't wired with one. Options when retrofitting a smart thermostat: install a C-wire from the air handler (best, $150-300 from an HVAC tech), use a fused add-a-wire kit (Venstar), pick a smart thermostat that doesn't require C-wire (some Ecobee / Honeywell models).

See the full Ontario HVAC and home energy glossary or browse the HVAC Costs pillar.