Guides

Wiring guides

Everything to read, in one place, plus quick answers in the wiring FAQ. Want an answer for your own wires instead? Use the thermostat wiring tool or the home wiring tools.

How-to guides

GuideThermostat Wire Colors Chart (And Why You Shouldn’t Trust It)Typical thermostat wire colors for R, W, Y, G, C and O/B, plus why installers go by terminal letters instead of colors.
GuideNo C Wire? Your Options for Every Smart Thermostat BrandHow to install a Nest, ecobee, Honeywell or Amazon smart thermostat without a C wire: the right adapter for each brand, and when you need new wire.
GuideHeat Pump Thermostat Wiring: O/B, AUX and E Made SimpleA plain-English guide to heat pump thermostat wiring: reversing valve O vs B, auxiliary heat, emergency heat, and dual fuel, with brand-specific notes.
GuideLine Voltage vs Low Voltage Thermostats: Check Before You BuyHow to tell a 120/240 V line-voltage thermostat (baseboard heaters) from a 24 V low-voltage one, and which smart thermostats work with each.
GuideHow to Replace a Thermostat Safely (Step by Step)The safe order of operations for swapping an old thermostat for a smart one: power off, photos, labels, wiring, and testing heat, cool and fan.
GuideSmart Thermostat Not Working After Install? TroubleshootingBlank screen, no heat, AC won’t start, fan won’t stop, short cycling: the usual causes after installing a smart thermostat and how to check each one.
GuideThermostat Clicking but AC Not Turning On: Causes and ChecksThe thermostat clicks but the AC or furnace doesn’t start? Follow the call from the wall to the equipment and find where it stops, one check at a time.
GuideHow to Test Thermostat Wires With a Multimeter (24V)Step-by-step multimeter checks for thermostat wiring: 24 V between R and C, testing heat, cooling and fan signals, and finding a broken or shorted wire.
GuideSmart Thermostat Settings to Check After InstallingThe setup settings that matter after wiring a smart thermostat: equipment type, O/B, stages, aux heat lockout, compressor protection, fan and schedules.
GuideHeat Pump Using Aux Heat Too Much? Causes and FixesWhen aux heat is normal, the thermostat settings and wiring mistakes that turn it on too often, and the heat pump problems that need a technician.
GuideThermostat Wire Gauge, Conductors and Running New CableWhat 18/5 and 18/8 mean, how many conductors your system needs, the right cable rating, and how to replace or add thermostat wire through a wall.
GuideThermostat Wire Not Detected? Fixing Y, G and Rh Wire ErrorsWhy a smart thermostat says a Y, G or R wire isn’t detected or has no power, and how to fix it: seating the wire, setup answers, and testing the wire.
GuideThermostat Not Working After a Power Outage or Battery ChangeThermostat dead or the AC and heat won’t start after a power outage or new batteries? The usual causes, from tripped breakers and fuses to reset settings.
GuideAux Heat vs Emergency Heat: What’s the Difference?Aux heat and emergency heat often use the same backup heater, but one is automatic and one turns the heat pump off. When each is normal, costs, and wiring.
Guide6 Wire Thermostat Wiring: What Each Wire DoesSix thermostat wires usually mean a heat pump with a C wire, a two-stage system or two transformers. Identify yours by the letters and find the right wiring.
Guideecobee PEK Wiring: Installing the Power Extender KitHow the ecobee Power Extender Kit (PEK) adds power without a C wire: which systems it suits, where each wire goes at the furnace and thermostat, and fixes.
GuideUK Smart Thermostat Wiring: Combi Boilers, Hive, Nest and tadoHow UK heating controls are wired: 230 V switched live vs OpenTherm, combi vs system boilers, S-plan and Y-plan, and where Hive, Nest and tado receivers connect.
GuideBaseboard Heater Thermostat Wiring: Single-Pole and Double-PoleHow to wire a 120 V or 240 V baseboard heater thermostat: line and load wires, single-pole vs double-pole, sizing to your heaters’ wattage, and smart options.
GuideThermostat Settings by US Climate Zone: Setbacks and LockoutsStarting points for winter and summer setpoints, setbacks, heat pump aux heat lockout and humidity control in each US climate zone, with cities and states for each zone.
GuideSmart Radiator Valves (TRVs): Fitting, Adapters and BoilersHow smart radiator thermostats fit: M30 x 1.5 and Danfoss adapters, removing the old head, why they still need a boiler control, bypass valves and fixes.
GuideUnderfloor Heating Thermostat Wiring: Electric and Water SystemsHow underfloor heating thermostats are wired: electric mats with floor probes, water systems with manifold actuators and wiring centres, load limits and settings.
GuideSmart Thermostat on a Boiler: TT, Zone Valves and C WireWiring a smart thermostat to a hot water or steam boiler in North America: TT terminals, zone valves and end switches, zone controllers, C wire options and settings.

Wiring guides

How-to guides

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Thermostat settings by state

Thermostat wiring by setup

Smart thermostat guides

Thermostat wire letters

Home wiring diagrams