Thermostat types, wiring, and smart controls field guide

What a thermostat does and the matching problem

A thermostat senses the temperature in a space and switches the HVAC equipment on and off to hold the setpoint you choose. That is the whole job. It is a temperature switch with a brain, sitting between you and the furnace, air conditioner, or heat pump, opening and closing low-voltage circuits that tell the equipment to run heat, run cool, or move air.

The catch is that the thermostat has to match the equipment it controls, and that match is where most of the trouble starts. The equipment has a number of heating and cooling stages, it is either a conventional setup or a heat pump, and it runs on either low-voltage 24 V control or line-voltage power. A thermostat built for one of those does not work right on another. Put a conventional stat on a heat pump and the reversing valve never gets a signal, so the system heats when it should cool. Put a single-stage stat on two-stage equipment and the second stage never fires.

On the job, picking the wrong thermostat for the equipment is the common error, more common than any wiring fault. The thermostat is the cheap part of the system, so people grab whatever is on the shelf, wire it to whatever colors are in the wall, and find out later the equipment is not doing what the stat is asking. For what the equipment on the other end of those wires actually is, see the air handling unit guide, and for how one system gets split across several thermostats, see the HVAC zoning guide.

What are the types of thermostats?

Thermostats sort into four types, and they stack from simplest to smartest: mechanical, programmable, smart, and communicating. Each step up adds control and adds something that can be configured wrong, so more capable is not automatically the right pick for a given job.

A mechanical or manual thermostat is the old bimetal or mercury-bulb stat. It has no schedule and no memory. You set a dial, and a coiled bimetal strip bends with temperature to make or break the contacts, or in the older ones a mercury bulb tilts and bridges them. It is simple, it has no batteries to die, and it holds one temperature until somebody moves the dial. The mercury versions are old enough that you handle the mercury as hazardous waste when you pull one. A programmable thermostat is digital and runs a schedule: 7-day lets every day differ, 5-1-1 gives weekdays one program and Saturday and Sunday their own, 5-2 groups the weekend together. The point of it is setback, dropping the temperature while you sleep or are gone and recovering it before you are back.

A smart or connected thermostat is the modern stat. It has Wi-Fi, an app, and usually some learning, so it watches how you adjust it and how fast the house responds and builds its own schedule. It does remote control from a phone, geofencing that backs the temperature off when your phone leaves home, energy reports, and in many cases enrollment in a utility demand-response program. A communicating thermostat is a different animal. It is the proprietary stat that talks to matched equipment of the same brand over a data bus, trading not just on-off calls but capacity commands, sensor data, and fault codes. That last type works only as a matched set, which gets its own section below.

Low-voltage 24 V vs line-voltage thermostats

Thermostats live in two electrical worlds, and you do not mix them. Low-voltage thermostats run on 24 V control. Line-voltage thermostats switch 120 V or 240 V directly. Wire one type to the other system and you either destroy the thermostat or create a shock and fire hazard, There is also a third, smaller world worth naming: millivolt systems, where a thermopile in a standing pilot generates its own roughly 750 mV and needs a true millivolt thermostat, common on older wall and floor furnaces and gas fireplaces. A power-stealing or line-voltage stat on millivolt gear will not work and can damage the gas valve, so the first question on any thermostat job is which of these the equipment uses.

Low-voltage 24 V is the standard for central systems: gas and oil furnaces, central air conditioning, air handlers, boilers, and heat pumps. A small transformer in the equipment steps the line voltage down to 24 V, and the thermostat just switches that low-voltage control signal over thin wires, the kind that look like phone wire. The thermostat is not carrying the power that runs the blower or the compressor. It sends a signal to a relay or a control board that does the heavy switching. Almost everything in this guide, the terminal letters, the C-wire, the heat pump and staging logic, lives in the low-voltage side.

Line-voltage thermostats are a different job. They control electric resistance heat directly: baseboard heaters, fan-forced wall heaters, electric convectors. There is no transformer and no 24 V signal. The thermostat is a heavy switch carrying the full 120 V or 240 V heater current through thick conductors, single-pole with two wires or double-pole with four. A double-pole line-voltage stat can fully break power to the heater; a single-pole cannot, so it has no true off. Line-voltage stats also react less precisely, often needing a few degrees of swing before they switch, so they hold temperature looser than a low-voltage stat. The blunt rule: a 24 V central-system stat does not go on baseboard heat, and a line-voltage baseboard stat does not go on a furnace.

What do the thermostat terminal letters mean?

The terminal letters are a code for what each wire does, and while the conventions are widely shared, they are conventions, not law. The manufacturer's wiring diagram is the authority for the equipment in front of you, because brands deviate and old installs were not always wired to the standard. Read the equipment's diagram before you trust the colors in the wall.

The power terminals come first. R is the 24 V hot from the transformer. Some stats split it into RH for the heat side and RC for the cool side, which matters only when the system has two transformers; on a single transformer you tie them with a jumper. C is the common, the 24 V return that completes a constant circuit. Then the calls: W drives heat, Y drives cooling or the compressor, and G runs the indoor fan. On staged equipment, W2 is second-stage heat and Y2 is second-stage cooling. On a heat pump, O or B drives the reversing valve, and E and a separate AUX or W2 handle emergency and auxiliary backup heat.

The two that bite people are C and O/B. The common wire gets its own section because it is the number-one smart-stat problem. The reversing-valve terminal gets covered under heat pumps because getting O and B backward is the classic heat-pump miswire. Label every wire to its terminal before you pull the old stat off the wall, photograph it, and do not assume wire color means what the chart says, because somebody before you may have used whatever was on the truck.

What is a C-wire and do I need one?

A C-wire, or common wire, is the conductor that gives a thermostat constant 24 V power by completing a circuit back to the transformer. The R wire brings power in; the C wire is the return path that lets current flow continuously even when the system is not calling for heat or cool. A simple battery or mechanical stat does not need it. A smart or Wi-Fi stat almost always does, because keeping a screen, a radio, and a processor alive all day takes steady power, not the trickle a call provides.

The no-C-wire problem is the single most common smart-thermostat install issue, and it is why a stat that powered up fine on the bench keeps dropping Wi-Fi or rebooting once it is on the wall. Older homes were wired with four conductors, R, W, Y, G, and no fifth wire for common, because the mercury stat never needed one. So the new smart stat has nowhere to land its C terminal and has to find power some other way.

There are four fixes, in roughly the order a pro prefers them. Run a new wire from the equipment to the thermostat if the cavity allows it; that is the clean answer and gives full, stable power. If you cannot pull new wire, use an add-a-wire adapter, the Venstar-style kit and its equivalents, that splits an existing conductor into two signals at the equipment control board to create a common. Or use a plug-in 24 V adapter that powers the C terminal from a nearby outlet, which is the least tidy but works. Power stealing, where the stat draws a trickle through the heat or cool circuit in standby, is the last resort: it can chatter the equipment, drop the radio, or false-call on sensitive boards, so reach for a real common first. Some manufacturers ship a power-extender module that lands at the board and does the same job as an added common.

Heat pump thermostats and the O/B reversing valve

A heat pump needs a thermostat built for a heat pump, and a conventional stat will not do, because a heat pump heats and cools with the same refrigerant circuit run in opposite directions. The part that flips the direction is the reversing valve, and the thermostat controls it through the O or B terminal. A conventional stat has no O/B output, so the valve never gets told which way to send the refrigerant, and the system heats when you call for cool or cools when you call for heat.

O and B are the same wire to the same valve, configured to energize in opposite modes, and the brand decides which. Most equipment, Carrier, Trane, Lennox, and Goodman among them, uses O, which energizes the reversing valve in cooling. Rheem, Ruud, and some others use B, energizing it in heating. Setting the stat to the wrong one is the classic heat-pump miswire: the unit runs exactly backward, blowing cold on a heat call. When a homeowner says their new thermostat heats in summer, check the O/B setting first.

A heat pump loses capacity as it gets colder outside, so it carries backup heat for the cold end: electric strips, or a gas furnace in a dual-fuel system. The stat handles backup through the W2 or AUX terminal for staged automatic backup and the E terminal for emergency heat, which forces backup and locks out the compressor. Two settings keep the backup honest. The aux lockout holds backup off until the outdoor temperature drops below a threshold, commonly set around 35°F so the heat pump carries most of the season; the compressor lockout, set much colder, shuts the compressor off where it can no longer help. Backup strip heat costs roughly two to three times what the heat pump costs to make the same heat, so a stat that runs strips too eagerly is a money leak even when nothing is broken. The exact lockout numbers come from the equipment data and the local climate, not a rule of thumb.

Single-stage vs multi-stage thermostats

Staging is how many levels of output the equipment has, and the thermostat has to be built for that count or the extra stages go to waste. Single-stage equipment is full-on or full-off and needs only W and Y. Two-stage equipment runs a low stage for mild loads and a high stage when the low stage cannot keep up, which needs W2 for second-stage heat and Y2 for second-stage cooling. Modulating equipment varies output continuously and usually wants a communicating stat to command capacity directly.

Match the stat to the stages or you lose what you paid for. Put a single-stage thermostat on a two-stage furnace and it can only ever call first stage, so on the coldest morning the second stage you bought never fires and the house runs cold. Put a two-stage stat on single-stage equipment and the second-stage terminal does nothing, which is harmless but pointless. The stat also has to be told the stage count in its setup. The terminals alone do not configure it.

There is a comfort reason staging matters beyond capacity. Low-stage running is longer and gentler, which holds temperature steadier and, in cooling, pulls more humidity because the coil stays cold longer. A stat that stages well lets the equipment run low most of the time and step up only when the load demands it. For how staging plays out across a zoned system, where the equipment has to serve several thermostats at once, see the HVAC zoning guide.

Communicating and proprietary systems

A communicating thermostat is a proprietary control that talks to matched equipment of the same brand over a low-voltage data bus, usually four conductors, instead of switching individual call wires. Rather than closing a W contact to ask for heat, it exchanges digital messages: how much capacity to make, what the blower speed should be, what the coil and discharge temperatures are, and what fault the equipment is throwing. This is how a modulating, variable-speed system gets controlled with the precision it was built for.

The hard rule with communicating gear is that it is a matched set. The thermostat, the indoor unit, and the outdoor unit have to be the same communicating family from the same manufacturer, because the data protocol is proprietary and brands do not talk to each other. You cannot drop a Wi-Fi stat from one company onto another company's communicating system and expect the modulation to work, and you cannot mix communicating components across brands. When a communicating stat fails, the replacement has to match the equipment, which is why the model and firmware belong in the job record.

The payoff for the lock-in is control that on-off switching cannot give: smooth modulation down to a fraction of capacity, blower speeds matched to the call, and built-in diagnostics that report a failing component by code instead of by symptom. The cost is the lock-in itself. You are tied to one brand's ecosystem for replacements and repairs, and the parts cost more. Many communicating systems can run in a conventional 24 V mode with an ordinary stat as a fallback, but doing that gives up the modulation, so you are running expensive equipment as if it were basic. Decide with eyes open. The matched system is the best comfort and efficiency on offer, at the price of being married to the brand.

Smart thermostat features and what they are worth

Smart thermostats earn their price through features a programmable stat cannot do, and the value is real but uneven. Scheduling and setback are the base, the same as a programmable. On top of that, learning algorithms watch your adjustments and the house's response and build a schedule without you programming one, which mostly helps people who never bothered to program the old stat.

Remote control through an app lets you change the temperature from anywhere, which matters for a vacation house, a rental, or the night you forgot to set back before a trip. Geofencing uses your phone's location to back the temperature off when you leave and recover it before you return, so the savings happen without you remembering. Energy reports show runtime and usage so you can see what the equipment actually does, not what you assume. Remote sensors in other rooms let the stat average several spaces or follow the room you are in, which fixes the single-location blind spot every wall thermostat has.

The feature that pays cash is utility integration. Many smart stats enroll in utility demand-response programs, letting the utility nudge your setpoint during peak load in exchange for a credit, and many utilities and ENERGY STAR rebate the purchase of a certified smart thermostat outright. ENERGY STAR certifies smart thermostats on field-measured savings, not lab claims, so the certified list is a reasonable filter. The savings depend hard on the building, the equipment, and whether the setbacks actually get used, so treat the manufacturer's savings figure as a documented range, not a guarantee. A smart stat on a house nobody sets back saves about what a dial saves.

Installing a thermostat

Installing a thermostat is short work if you respect two things: kill the power first, and label every wire before you disconnect it. Turn off the system at the breaker or the equipment switch, because even 24 V control can spark across the board, and a shorted R-to-C blows the equipment's low-voltage fuse and leaves you chasing a dead transformer you caused.

Pull the old stat off its base and, before touching a wire, photograph the wiring and write down which conductor lands on which terminal, by letter, not by color. Color is a hint, not a fact, because the last installer may have used a green wire on Y or a spare on C. Note whether RH and RC are jumpered. Then release the conductors, feed them through the new base, and mount the base level on the wall. Level is cosmetic on a digital stat, but it tells you the work was careful. Land each conductor on the matching terminal per your notes, set the C-wire if you have one, and fix the no-common problem now if you do not, rather than discovering it when the stat reboots.

Restore power and move to configuration, which is where the install actually succeeds or fails. The wiring can be perfect and the stat still run the equipment wrong if the setup tells it the wrong equipment type. Set the system type, the stage count, and on a heat pump the O/B convention and the backup type before you call it done. Then prove every mode: call heat and confirm heat, call cool and confirm cool, run the fan alone, and on a heat pump confirm the air is hot on a heat call, not cold. The wiring and the config are two separate jobs, and both have to be right.

How do you configure a thermostat for the equipment?

You configure a thermostat by telling it, in its setup menu, exactly what equipment it is controlling, and this step is separate from the wiring. The terminals carry the signals; the configuration decides how the stat uses them. A correctly wired stat with the wrong configuration runs the equipment wrong, and the misconfiguration is harder to find than a loose wire, because everything looks connected.

The settings that matter most: system type (conventional gas, electric, or oil versus heat pump), the number of heating and cooling stages, whether the fan is controlled by the stat or by the equipment (gas furnaces usually run their own blower on heat, so the stat should not energize G on a heat call), and on a heat pump the reversing-valve direction, O or B. Get the system type wrong and a heat pump runs as if it were a furnace, ignoring the compressor for heat and leaning on backup. Get O/B wrong and it heats when you cool.

The common misconfiguration is leaving the stat on its default, which is usually a single-stage conventional system, and walking away. On a heat pump or a two-stage system, that default quietly disables half the equipment. The other frequent miss is the heat-pump fan and aux settings, which decide when backup heat is allowed and whether the stat or the furnace runs the blower. Work the setup menu deliberately, match each item to the equipment nameplate and the wiring diagram, and verify in every mode before you leave the house.

Setback, scheduling, and the heat pump caution

Setback is letting the temperature drift while a space is empty or everyone is asleep, then recovering it before it is needed, and it is where most of a programmable or smart stat's savings come from. Less time spent holding full setpoint is less equipment runtime. On conventional gas or AC equipment, deeper and longer setbacks save more, within the comfort you will tolerate, and the recovery is cheap because the furnace or AC just runs to catch up.

On a heat pump, deep setback can backfire, and this is the caution that gets missed. When the stat recovers from a big overnight setback, it sees a wide gap between the room and the setpoint and stages up to backup heat to close it fast. That backup is the expensive electric strip or furnace you were trying not to run. A deep setback on a heat pump can spend more on recovery strip heat than the setback saved on the compressor, so the night comes out a wash or worse.

The fix is to keep heat-pump setbacks shallow, a few degrees, or to use a stat with adaptive or intelligent recovery that ramps the temperature back gradually on the compressor alone instead of calling backup. Some heat-pump stats let you lock out auxiliary heat during recovery for exactly this reason. The general rule holds: setback saves on conventional equipment, and on a heat pump you set back gently and watch that recovery is not riding the strips. Confirm the recovery behavior against the stat's heat-pump settings, because the defaults are not always tuned for it.

Where should a thermostat be placed?

A thermostat should go on an interior wall, in a room you actually use, roughly 5 ft off the floor, away from anything that lies to it about the room temperature. The stat controls the whole space from the one spot it can feel, so a bad location means it satisfies on a false reading and the real rooms run wrong no matter how good the equipment is.

Keep it off exterior walls, which run hot or cold with the weather and pull the reading away from the room. Keep it out of direct sun, which cooks the sensor and makes the stat think the house is warmer than it is, so cooling overruns. Keep it away from supply registers, where the conditioned air blows right on it and satisfies the stat before the room is done, leaving the rest of the space short. And keep it away from drafts off doors and windows, and away from heat sources, a lamp, a TV, a kitchen, the warm wall behind a refrigerator, all of which throw the reading off.

The classic bad-placement failures are predictable. A stat in a sunny entry cools the whole house to keep that one hot spot comfortable. A stat next to a supply register short-cycles because it keeps getting blasted with conditioned air. A stat over a return or near the kitchen reads warm and overcools. When a system runs fine but the comfort is wrong, look at where the stat is before you blame the equipment. If the only available wall is a bad one, a remote sensor in a better location is the cleaner fix than moving the box.

Remote and room sensors

Remote sensors solve the problem that a wall thermostat can feel only one spot. A sensor is a small temperature probe placed in another room that reports back to the stat, so the system can read where the people are instead of where the box happens to be mounted. Most current smart thermostats support a few of them, and some include one in the box.

There are two ways a stat uses remote sensors. It can average several rooms so no single space dominates, which evens out a house where the hallway the stat sits in is not representative. Or it can prioritize a room on a schedule, following the bedrooms at night and the living areas by day, so the stat controls to the room that matters at the time. Either mode fixes the single-location blind spot without rewiring or moving the thermostat.

Sensors are the practical answer to a stat stuck on a bad wall, a hot upstairs the downstairs stat never feels, or a primary suite that runs different from the rest of the floor. They are not zoning. They change which temperature the one system controls to; they do not let different areas hold different setpoints at the same time. When the rooms genuinely need independent control, that is zoning, covered in the HVAC zoning guide.

One thermostat per zone

When a building is zoned, each zone gets its own thermostat, and that stat owns the conditioning for its area instead of one stat speaking for the whole house. The thermostats report to a zone control panel rather than straight to the equipment, and the panel opens the dampers for the zones that are calling and runs the equipment to suit. The thermostat type still has to match the equipment and the panel, conventional 24 V or communicating, the same matching rule as any single-stat system.

Two thermostat details change in a zoned system. The stat has to be compatible with the zone panel, which most conventional 24 V stats are, while communicating zone panels demand matched communicating stats. And a remote sensor or a smart stat per zone is where the per-zone setback scheduling lives, which is a real part of the energy savings zoning promises. The damper, panel, static-pressure, and bypass side of zoning is its own subject. See the HVAC zoning guide for how the thermostats fit into the larger system.

Commercial space sensors and BMS controllers

Commercial buildings mostly do not use a standalone thermostat. They use a space temperature sensor wired back to a DDC controller, and the controller, part of the building management system, makes the decisions a thermostat would make on a house. The sensor on the wall is just a probe, maybe with a setpoint adjustment and an override button. The intelligence lives in the controller and the BMS, not on the wall.

The reason is scale and coordination. A commercial system runs many zones, VAV boxes, economizers, and central plant equipment that have to work together, schedule together, and report together, which is more than a wall stat can do. The DDC controller modulates the VAV damper and reheat for its zone, takes a static-pressure reset from the air handler, and rolls up to a head-end the operator watches and trends. A standalone thermostat still shows up in small commercial, a rooftop unit on a strip-mall suite, but once the building has a BMS, the wall device is a sensor, not a thermostat.

Precision spaces push this further. A data center or a lab controls space temperature on tight tolerances with continuous sensing and modulation rather than the on-off cycling a thermostat does, because the load is dense and constant and the band is narrow. The principle is the same as any DDC zone, just held closer, and it leans on the building controls, not a wall stat. For the air handler those controllers drive, see the air handling unit guide.

Why is my thermostat blank or not working?

A blank thermostat is a power problem first. On a battery stat, the batteries are dead; change them. On a hardwired or smart stat, a blank screen means the 24 V is not reaching it, and the usual causes are a blown low-voltage fuse on the equipment control board (often a 3 A or 5 A blade fuse), a tripped float switch on the condensate drain that cuts the control circuit, a missing or broken C-wire, or a loose connection at the stat or the board. Pull the cover and check for 24 V between R and C before you condemn the stat.

The other common failures sort by symptom. A stat that does not call at all may be miswired, misconfigured for the wrong equipment, or set to the wrong mode, heat in summer or cool in winter, the simplest miss there is. A stat that short-cycles, switching the equipment on and off too fast, is usually a placement problem, sun or a register hitting it, a cycle-rate or anticipator set wrong for the equipment, or on an old mechanical stat a heat anticipator out of adjustment. A heat pump running backup heat too much points at a deep-setback recovery calling the strips, an aux lockout set too warm, or the O/B and system-type configuration wrong.

Work it in order: power, then configuration, then placement, then the equipment. Confirm 24 V at the stat. Confirm the stat is set for the right equipment type, stages, and O/B. Confirm it is not sitting in the sun or over a register. Only then chase the equipment itself. Most thermostat callbacks are not a dead thermostat. They are power, a setting, or a location, and the meter and the setup menu find all three faster than swapping the box.

Checking thermostat compatibility before you buy

Check that a thermostat works with the system before you buy it, because the cheap part on the shelf is useless if it cannot control your equipment. The checks are quick and they save a return trip. First, confirm the world: low-voltage 24 V central system, or line-voltage baseboard. A smart stat is a 24 V device and will not run baseboard heat.

Then count and read the wires. Pull the existing stat and note the terminals in use. Do you have a C-wire? Do you have O or B, which means a heat pump? Do you have W2 or Y2, which means staging? Identify the equipment type from the nameplate, conventional or heat pump, single or multi-stage, modulating or communicating. A communicating system needs its matched brand of stat; a third-party smart stat either will not work or runs it in a dumbed-down conventional mode. Most smart-stat makers publish a compatibility checker that takes your terminal list and tells you yes, no, or yes-with-an-adapter.

The two checks that catch most mismatches: is there a C-wire or a plan to create one, and is the equipment a heat pump or communicating. Miss the first and the smart stat will not stay powered. Miss the second and you bought a stat that cannot control what you own. Five minutes at the wall with the cover off answers both.

What to document

A thermostat install lives or dies on the wiring record, because the wires disappear behind the stat and the next tech inherits whatever you leave. The photo of the old wiring and the new terminal map is the most useful thing in the file, and it is the first thing you will wish you had when a stat dies in three years and the colors do not match any chart.

Record the equipment type and stage count, the heat-pump O or B convention, the wire-to-terminal map with a photo, where the C-wire came from or which adapter created it, the aux and compressor lockout temperatures you set, and the thermostat model and firmware so a communicating replacement can match. If you changed any default in setup, write down what and why. The person reading this later is often you, and the system that was wired by feel and never written down is the one that eats an afternoon to sort out.

Common mistakes

• Putting a conventional thermostat on a heat pump, so the reversing valve never gets a signal and the system runs backward.
• Wiring or configuring O and B backward on a heat pump, so it heats on a cool call and cools on a heat call.
• Installing a smart or Wi-Fi stat with no C-wire and relying on power stealing, so it drops Wi-Fi, reboots, or chatters the equipment.
• Setting the wrong system type or stage count in setup, so half the equipment never fires even though the wiring is right.
• Mixing a low-voltage 24 V stat with line-voltage baseboard heat, or the reverse, which destroys the stat or makes a hazard.
• Mounting the stat in sun, over a supply register, near a heat source, or on an exterior wall, so it controls to a false reading.
• Deep overnight setback on a heat pump, so recovery calls the backup strips and erases the savings.
• Pulling the old stat without labeling and photographing the wires, leaving the colors to lie to the next tech.

Field checklist

Standards and references

The thermostat itself is governed less by a code section than by the equipment it controls, and the manufacturer's installation instructions for that equipment are the controlling document. The wiring diagram on the furnace, air handler, or heat pump defines the terminals, the O or B convention, and the staging, and where the stat's defaults and the equipment differ, the equipment data wins. Wire and configure to the unit's instructions, not to a generic chart.

The control wiring is low-voltage Class 2 under the NEC, NFPA 70, which sets how the 24 V conductors are run and protected; line-voltage baseboard stats fall under the standard branch-circuit rules for the heater they switch. The adopted code edition and local amendments control, so confirm them for the jurisdiction. On the efficiency side, ENERGY STAR certifies smart thermostats on field-measured savings, and many utilities tie rebates and demand-response credits to that certification, so the ENERGY STAR list and the local utility program are the references for the incentive side.

Two more bodies show up at the edges. ACCA Manual Zr covers the residential zoning that puts a stat in every zone, and on commercial work the controls move from a standalone stat to DDC and a building management system, where ASHRAE guidance on ventilation and energy and the controls-integration standards apply. Cite the document that controls the point: the equipment instructions for the wiring and config, the NEC for the Class 2 control wiring, ENERGY STAR and the utility for the incentive, and verify each against the current edition.

Units and terms

A thermostat job borrows vocabulary from controls, refrigeration, and electrical work, and the same idea shows up under more than one name across an equipment manual, a stat menu, and a spec.

Control voltage is nominal 24 V AC for central systems and 120 V or 240 V for line-voltage baseboard stats. Temperature is in Fahrenheit on most North American stats and Celsius on metric ones, and the setpoint is the temperature you ask the stat to hold. The terms below cover the parts that confuse people most.

Setpoint

The temperature the thermostat is told to hold

Setback

Letting the temperature drift while a space is empty, then recovering it before it is needed

Cycle rate

How often the stat lets the equipment switch on per hour, set to the equipment type

Heat anticipator

An adjustment on older stats that shortens the heat cycle to hold temperature steadier

Reversing valve (O/B)

The heat-pump valve that switches between heating and cooling, driven by the O or B terminal

Auxiliary / emergency heat

Backup heat on a heat pump; aux stages in automatically, emergency forces it and locks out the compressor

C-wire (common)

The conductor that gives the thermostat constant 24 V power

Differential / swing

The temperature gap between switching on and off, wider on line-voltage stats

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