Smart thermostats require a C-wire that 40% of US homes built before 2000 do not have, and no one discovers this until mid-installation

Smart thermostats like Nest, Ecobee, and Honeywell Home require a C-wire (common wire) to provide continuous 24V power for their Wi-Fi radios, displays, and processors. Approximately 40% of homes in the United States built before 2000 only have 2-wire or 4-wire thermostat cables that lack a dedicated C-wire. Homeowners discover this problem only after purchasing the thermostat, removing their old thermostat from the wall, and finding the wrong number of wires behind it. This matters because at that point the homeowner is stuck. The old thermostat is already off the wall. The new thermostat will not work without modification. The options are all bad: run a new thermostat cable through the wall (requires cutting drywall and potentially an electrician at $150-$300), use an 'add-a-wire' adapter kit that repurposes an existing wire (confusing for non-electricians and can cause HVAC damage if done wrong), or use a power-stealing method where the thermostat trickles power through the heating wire (causes the furnace to short-cycle, clicking on and off, which damages the equipment over time). Heat pump systems are even worse — they require an O/B reversing valve wire in addition to the C-wire, and connecting wires incorrectly can cause the system to heat when it should cool, running up energy bills for weeks before the homeowner notices. This problem persists because thermostat manufacturers list 'C-wire required' in small print on page 12 of the installation guide, but the product marketing and packaging show a smiling person holding a thermostat with the tagline 'installs in 30 minutes.' There is a fundamental information asymmetry: the manufacturer knows exactly which wiring configurations their product supports, but they do not surface this information at the point of purchase. The online compatibility checkers ask questions that most homeowners cannot answer without first pulling their thermostat off the wall. Ecobee ships a 'Power Extender Kit' in the box — which is an admission that the problem is widespread — but installing it requires wiring work at the furnace, which most people are not comfortable doing.