Maintenance procedure · Written to ASD-STE100 Issue 9 · Formatted to ANSI Z535

Replace a Low-Voltage Room Thermostat

24 V control wiring only — photograph the terminals, move one wire at a time, and stop at the four branches this page will not guess
ApplicabilityLow-voltage room thermostats with lettered screw or push-in terminals, on residential central heating and cooling equipment supplied by a 24 V control transformer
Task time1 hr 29 min to 2 hr 31 min editorial estimate, not a measurement
PersonnelOne
Revisionv1 · 2026-07-29
BEFORE YOU BEGIN

Before you begin. This procedure is general reference information, not professional advice. Equipment varies by manufacturer, model, and year. Always follow the documentation supplied with your equipment — where it differs from this page, follow the manufacturer. Verify that you have the tools, the working conditions, and the ability to do this task safely. If anything is unclear, if the equipment is damaged, or if the task exceeds your experience, stop and use a qualified professional. You are responsible for your own safety and for the outcome. This site accepts no liability for any loss, damage, or injury arising from the use of this information.

Specifications marked unverified could not be traced to a manufacturer source. Do not rely on them — obtain the value from your equipment documentation.

Summary

Swapping a room thermostat is an afternoon of careful labelling wrapped around about ten minutes of actual work. The wires behind the old unit carry a 24 volt control signal rather than the supply that runs the furnace or the air conditioner, and the entire job is deciding which of those wires belongs on which lettered terminal of the new unit. That decision is the page. Everything else is packaging.

Here is the part most pages skip: nobody can tell you which wire goes where, and this one will not pretend otherwise. Resideo says outright that there is no standard for which colour wire controls which function, and that every wire must be identified by the terminal it came off and never by its colour. So the method is to photograph what you have before you disturb it, label each wire with the letter it was attached to, and match letters to letters on the new unit using that unit's own guide. There are also four things you may find behind the old thermostat that change the answer: three of them end the job and send you to a professional, and the fourth — heat pump against conventional — is the one that damages a control board when somebody guesses. All four are checked before a single wire comes off.

Safety

WARNING

REMOVE THE POWER FROM THE HEATING AND COOLING EQUIPMENT BEFORE YOU TOUCH A WIRE.
The wall cavity behind a thermostat can also contain other conductors. Contact with those conductors can kill you.

Necessary equipment

ItemNote
Camera or phoneThe photograph of the terminals is your record of the old wires.
Small flat-blade and cross-head screwdriversUse a screwdriver that fills the head of the terminal screw.
Pen and paperWrite the terminal letter of each wire on paper.
Flashlight optionalThe terminal letters are small.
Installation guide for the new thermostatThis guide is the only correct source of the terminal function of the new thermostat.
Documentation for the heating and cooling equipmentThis documentation gives the system type and the setting for the reversing valve.
Wire labels materialAdhesive tape and a pen are also satisfactory.
Pencil or clothes peg materialAttach this item to the wires to hold them at the face of the wall.

Remove the power from the equipment

Estimated 23 min to 43 min

NOTECopeland and Google give the same instruction. Remove the power before you touch a thermostat wire.
Wait before you confirm that the equipment is off
Manufacturer or standard verified
5 minutes

Source: How to turn the power off to your system · tier 1 · accessed 2026-07-29

  1. Read the installation guide for the new thermostat from the first page to the last page.
    • The terminal function is different for each thermostat.
    • This page cannot replace that guide.
    10m
  2. Find the switch that supplies the heating and cooling equipment.
    • Google says that the switch is frequently on the equipment or near it.
    • This switch looks like a light switch.
    • This switch controls no light.
    • A circuit breaker with the label HVAC or FURNACE can also supply the equipment.
    5m
  3. Set the switch to OFF.
    • Google says that a system with two transformers can have two switches.
    • Set both switches to OFF if the system has two switches.
    <1m
  4. Change the temperature setting on the old thermostat by 5 degrees.
    • In cold weather, increase the setting to start the heat mode.
    • In warm weather, decrease the setting to start the cool mode.
    <1m
  5. Wait 5 minutes.
    • Many systems have a delay before they start.
    • Google and Resideo give the same 5 minute interval.
    5m
  6. Make sure that the equipment does not start.
    • Listen at the equipment.
    • Hold your hand near a supply vent to feel the air.
    • Do not continue if the equipment starts.
    • Find the correct switch first.
    2m

Record the old wires

Estimated 10 min to 20 min

Thermostat control circuit voltage
Manufacturer or standard verified
24 V

Source: How do I wire my thermostat? · tier 1 · accessed 2026-07-29

Terminal letter for each wire in your system
Unverified — do not rely on this value
READ EACH LETTER FROM YOUR OWN PHOTOGRAPH OF THE OLD TERMINALS

No tier-1 source was found for this value. Take it from the documentation supplied with your equipment.

  1. Pull the body of the old thermostat from its wall plate.
    • Some bodies lift from the bottom edge.
    • Some bodies have a lock tab.
    • Some bodies have a small screw.
    2m
  2. Before you move a wire, photograph the terminals.
    • Make sure that the terminal letters are legible in the photograph.
    • Take more than one photograph.
    • Copeland and Resideo give this instruction.
    2m
  3. Examine the photograph on a large screen.
    • Write each terminal letter on paper.
    • Write the colour of the wire on the same line.
    • Resideo says that the wire colours are not a standard.
    • Identify each wire only by the letter of its terminal.
    5m
CAUTION

DO NOT GUESS THE TYPE OF YOUR SYSTEM.
A heat pump can supply cool air in the heat mode. This occurs when the setting for the reversing valve is not correct. A home with no heat in cold weather can make a person ill.

  1. Write both letters of a terminal that has two letters.
    • An example is a terminal with the letters W and O/B.
    • Resideo says that the O/B terminal is for the reversing valve of a heat pump.
    2m

Examine the wires for four conditions

Estimated 3 min

NOTEThree of these four conditions end the procedure. Read all four before you disconnect a wire.
WARNING

STOP IF THE WIRES AT THE THERMOSTAT ARE THICK.
Thick wires that are black or red show a system that is not a low-voltage system. Contact with these conductors can kill you. A licensed electrician must do all work on that system.

  1. Stop if the wires at the thermostat are thick.
    • Thermostat control wire is thin.
    • There are usually four to six wires.
    • Resideo says that thick wires show a different type of system.
    1m
  2. Stop if the terminals have the letters A, B and C.
    • The numbers 1, 2 and 3 have the same meaning as those letters.
    • Resideo says that these marks show a communicating system.
    • A communicating system needs a thermostat from the equipment manufacturer.
    1m
  3. Stop if the terminals have the marks G1, G2 and G3.
    • Resideo says that these marks show a fan with more than one speed.
    • A terminal with the single letter G is satisfactory.
    1m

Change the thermostat

Estimated 27 min to 49 min

Jumper position on the new thermostat
Unverified — do not rely on this value
CONSULT THE INSTALLATION GUIDE FOR THE NEW THERMOSTAT

No tier-1 source was found for this value. Take it from the documentation supplied with your equipment.

NOTICE

ATTACH A PENCIL TO THE WIRES BEFORE YOU DISCONNECT THEM.
A loose wire can move into the wall cavity. A wire in the wall cavity is very difficult to find again.

  1. Attach a pencil across the wires at the face of the wall.
    • Use adhesive tape or a clothes peg to hold the wires to the pencil.
    2m
  2. Disconnect one wire from the old wall plate.
    • Copeland says to label each wire as you remove it.
    • Do not disconnect a second wire before you label the first wire.
    1m
  3. Put a label with the terminal letter on the wire that you disconnected.
    • Write both letters if the terminal had two letters.
    • Do not put a label on a wire that has no terminal.
    • Do the same for the other wires, one wire at a time.
    4m
  4. Remove the screws of the old wall plate.
    4m
  5. Attach the new wall plate to the wall.
    • Keep the wires through the hole in the new wall plate.
    • Use the anchors that the manufacturer supplies for a plasterboard wall.
    4m
NOTICE

CONNECT EACH WIRE ONLY TO THE TERMINAL WITH THE SAME LETTER.
A wire on the wrong terminal can make a short circuit. A short circuit can damage the control board and the control transformer.

  1. Connect each labelled wire to the terminal with the same letter.
    • Obey the installation guide for the new thermostat at each step.
    • Do not move a wire to a different terminal later.
    • Make sure that no strand of a wire touches a second terminal.
    8m
  2. Set the jumper of the new thermostat as its installation guide specifies.
    • The jumper connects two power terminals in some systems.
    • Resideo and Copeland do not give the same instruction for their own thermostats.
    • Obey the guide for the thermostat in your hand.
    3m
  3. Attach the body of the new thermostat to its wall plate.
    1m

Start the equipment again

Estimated 26 min to 36 min

Reversing valve setting for a heat pump
Unverified — do not rely on this value
CONSULT THE DOCUMENTATION FOR THE HEATING AND COOLING EQUIPMENT

No tier-1 source was found for this value. Take it from the documentation supplied with your equipment.

System type setting for the new thermostat
Unverified — do not rely on this value
CONSULT THE DOCUMENTATION FOR THE HEATING AND COOLING EQUIPMENT

No tier-1 source was found for this value. Take it from the documentation supplied with your equipment.

  1. Set the switch that supplies the equipment to ON.
    • Set both switches to ON if the system has two switches.
    <1m
  2. Tell the new thermostat the system type from the documentation for the equipment.
    • The documentation for the equipment gives the system type.
    • Do not use the system type of a different home.
    7m
  3. Set the thermostat to the heat mode.
    1m
  4. Increase the temperature setting.
    <1m
  5. Wait 5 minutes.
    5m
  6. Feel the air at a supply vent.
    • The air must be warm in the heat mode.
    1m
  7. Set the thermostat to the cool mode.
    1m
  8. Decrease the temperature setting.
    <1m
  9. Wait 5 minutes.
    5m
  10. Feel the air at a supply vent again.
    • The air must be cool in the cool mode.
    1m
  11. If the air is cool in the heat mode, set the switch to OFF.
    • This result shows that the setting for the reversing valve is not correct.
    • The correct setting is a property of the equipment.
    • Speak to a licensed heating and cooling contractor.
    <1m
  12. Write the terminal list and the date in your records.
    • Keep the photograph of the old terminals with the record.
    • Keep the installation guide with the record.
    3m

Terminology

This page uses one term for each item and holds it throughout, as ASD-STE100 requires. Equipment sellers and other guides use the names below for the same parts.

low-voltage thermostat
Also called: 24 volt thermostat, wall thermostat, room thermostat, furnace thermostat, heating thermostat, AC thermostat, smart thermostat
common wire
Also called: C wire, c-wire, no c wire, common terminal, blue wire
power terminal
Also called: R terminal, Rh, Rc, red wire, R wire, jumper
reversing valve
Also called: O/B, O wire, B wire, changeover valve, heat pump backwards
communicating system
Also called: proprietary thermostat, ABC terminals, 1 2 3 terminals, data bus thermostat

When to use a professional

Stop if the wires at the thermostat are thick. Stop if the terminals have the letters A, B and C. Stop if the terminals have the numbers 1, 2 and 3. Stop if the terminals have the marks G1, G2 and G3. A licensed electrician or a licensed heating and cooling contractor must do the work in each of these conditions.

Stop if the equipment is a heat pump with a furnace for the backup heat. Resideo says to speak to a professional heating and cooling contractor about this type of system. Do not guess the type of your system.

Speak to a licensed heating and cooling contractor if the equipment does not start after this procedure. Speak to the same contractor if the air is cool in the heat mode. Do not open the equipment. Do not disconnect a wire at the equipment. Do not remove a cover from the equipment. This page ends at the thermostat.

Common questions

Which wire goes to which terminal on a thermostat?

There is no general answer, and any page that gives you one is guessing about your house. Resideo says outright that there is no standard for which colour wire controls each function, and that every wire should be identified by the terminal it connects to and never by its colour. That is exactly why the method here is photograph, label, then match letter to letter. If a label has come off and you no longer know where a wire belongs, Resideo's own fallback is to go to the heating and cooling equipment and read the designations printed on its control board. If that is not something you want to do, a heating and cooling contractor will sort it out in one visit.

What is the C wire and why does my new thermostat need one?

C stands for common, and that is the name Resideo's terminal list gives it. On an older mechanical or battery thermostat there was often nothing for it to do, so plenty of houses have a C wire pushed back into the wall unconnected, and plenty have none at all. A thermostat with a screen and a radio in it wants continuous power rather than a battery, and the C wire is what supplies that. Check the installation guide for your new unit before you buy it: some models tolerate no C wire, some ship with an adapter, and Resideo sells adapters for the case where the wall genuinely has no spare conductor. When you take your photograph, look for a wire that is in the wall but on no terminal, because that is very often the C wire somebody left behind.

How do I know whether I have a heat pump or a conventional system?

Resideo gives the practical test: a heat pump runs the compressor for heating as well as for cooling, so the outdoor unit runs in winter. The wiring hint is a terminal marked O, B, or O/B, which Resideo identifies as the reversing valve connection on heat pump systems. If you are not certain, the documentation for the equipment settles it, and this page tells you to stop rather than guess. Resideo takes the same line, and tells readers who are unsure to pause and contact a professional heating and cooling contractor.

My new thermostat blows cold air when I set it to heat. What did I do wrong?

On a heat pump this is almost always the reversing valve setting rather than the wiring, and it is a software setting inside the thermostat rather than a wire you moved. The reason this page will not tell you which way to set it is that the correct answer belongs to your outdoor unit and not to your thermostat, and no primary source we could open states a universal rule. Everything a general search turns up on the question is a forum thread, a trade blog or a paid-answer site, and that class of source can tell you a problem is common but can never tell you a value. Read the documentation for your equipment, or ask a heating and cooling contractor, and set it once from an authority instead of by trial.

Do I really have to cut the power for a 24 volt thermostat?

Yes, and both manufacturers say so without hedging. Google's instruction is to always turn the power off before you connect or disconnect any wires, and then to test that it really is off. Copeland's installation guide says the same thing before it says anything else. There are two reasons. A stray wire across two terminals can short the control transformer and take a board with it, and the wall cavity behind a thermostat is not guaranteed to hold only thermostat wire. The check costs five minutes and a control board costs a service call.

Can I use this page for the thermostat on my electric baseboard heater?

No. That kind of thermostat switches the heater supply directly instead of sending a 24 volt signal, and Resideo's tell is the one used on this page: thick black or red wires at the thermostat rather than thin control wire. If that is what you find, close the page and call a licensed electrician. The same applies if you find terminals marked A B C or 1 2 3, which Resideo identifies as a communicating system needing a thermostat from the equipment manufacturer.

Sources

Every specification on this page is traced to one of these sources. Tier 1 is a manufacturer document, a published standard, or a government safety authority. See Standards for the full tiering rule.

  1. Tier 1How do I wire my thermostat?Resideo Technologies, Inc. (Honeywell Home) · accessed 2026-07-29 · re-verification due 2027-01-25
  2. Tier 1How to turn the power off to your systemGoogle LLC (Google Home and Nest Help) · accessed 2026-07-29 · re-verification due 2027-01-25
  3. Tier 1Sensi Smart Thermostat Installation Guide (models 1F87U-42WF, ST55; form R-5020-4)Copeland LP (Sensi; formerly Emerson Electric Co.) · accessed 2026-07-29 · re-verification due 2027-01-25

Revision history

Current version 1 · updated 2026-07-29 · next review due 2027-07-29

  1. v1 · 2026-07-29First draft. Terminal legend, the no-standard-colour rule and the three incompatibility branches from Resideo's own support article; isolation and the five-minute confirmation from Google Nest, corroborated independently by Resideo; photograph-and-label from Copeland's Sensi installation guide. The reversing valve setting, the system type and the jumper position all publish as consult-the-documentation: no primary source states a universal rule and everything else on those questions is tier 3.

Corrections to this page are listed in the public corrections log. Reports that are credible and not yet verified are listed in the public errata register, and an open one renders above the step it concerns.

Comments and corrections

Found an error? A correction with a manufacturer source is applied and logged publicly. You can also write to corrections@fixchecklist.com.

Every step above carries its own report control, which tags the report with that step so a correction reaches the right place. For anything about the procedure as a whole — the order of the sections, a step that is missing, something that did not match your equipment — use this: report something about this whole procedure.

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