Your Thermostat Wiring Step by Step Guide

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Last updated: 
September 20, 2017

This guide will walk you through removing your old thermostat and connecting the thermostat wiring to the new one. If you haven’t removed the old one yet, wait!

There’s important information here about what to do with the thermostat wiring when disconnecting it from the old unit. It will help you avoid confusion when wiring the new unit.

thermostat with clock

Why You Might Need to Replace the Thermostat

There are three common reasons to replace a thermostat.

If your HVAC system is having any of these problems, there’s a possibility the thermostat is the cause:

  • Your HVAC system won’t start or it won’t stop: Broken thermostat wiring or terminals prevent the HVAC components from getting the signal to start. A miscalibrated thermostat will misread the temperature and not start the system when it should or start it when it should not.
  • Hot and cold swings: A faulty thermostat can cause heating and cooling cycles to run too long or too short leading to temperature fluctuations.

Secondly, you might have to upgrade the control if you’ve installed two-stage or variable-capacity equipment and your current thermostat wiring doesn’t support it.

Finally, you might want the convenience and potential energy savings of a programmable thermostat or a WiFi thermostat like the Nest, Ecobee or Honeywell Lyric.

Tools & Specs to Keep in Mind

The tool list for replacing a thermostat is short:

  • Narrow flat screwdriver;
  • Phillips screwdriver;
  • Vise-grip pliers;
  • Drill or driver;
  • 3/16” or 1/4" drill bit;
  • 12” to 24” level;
  • Cellphone with camera or masking tape and a marker.

The new control must match your system’s capabilities. If you don’t know these, pull the cover off to check the thermostat wiring on the old unit. Yellow wires are typically for AC. If there is one yellow wire, it’s a single-stage AC; Two yellow wires indicate a two-stage or variable-capacity unit. White wires are for the furnace, and the same principle of one or two wires applies. There might be one or two wires not connected to a terminal. They are for HVAC components or functions your system doesn’t have, such as a heat pump reversing valve typically connected to the orange wire.

When shopping for thermostats, their packaging or the control’s online description should list what type systems each supports. With the preliminaries out of the way, here are step-by-step thermostat wiring instructions. Please also look at the below extra tips extracted from the steps.

Replacing a Thermostat in 14 Steps

  1. Turn off the power;
  2. Remove the thermostat cover;
  3. Open the new thermostat package;
  4. Take a picture and label the wires;
  5. Remove the wiring from the old unit;
  6. Remove the baseplate from the wall;
  7. Attach vise grip pliers to the wires;
  8. Mark the new baseplate holes;
  9. Install the new baseplate;
  10. Attach the wires to the new thermostat;
  11. Install batteries in the thermostat;
  12. Mount the thermostat on the baseplate;
  13. Program the new thermostat;
  14. Turn on the power, and test your HVAC system.

Extra Info and Instructions for the Steps

  • 1: Turn off the circuit or circuits in the electrical box that powers your HVAC system.
  • 2: Most unclip at the top and can be pulled forward and off.
  • 3: You're looking for ready-made labels marked Y, G, W and so forth.
  • 4: Before removing any wires, take a picture showing to what terminal each is attached. Then, use the new labels or marked tape to label each wire with the letter on or next to its terminal connection. These two steps are redundant, but a safeguard against miswiring the new control.
  • 6: Hold the wires with one hand, and pull the baseplate away from the wall with the other. When there’s room, reach behind the baseplate, and grab the wires. Remove the baseplate.
  • 7: This ensures the wires aren’t pulled into the wall
  • 8: Place the new baseplate over the wires and mark the holes.
  • 9. Drill the holes and install wall anchors before securing the baseplate with screws
  • 10. Loosen each terminal screw first. Then, attach the wires to the terminal with the same letter. Double-check yourself using the picture you took of the old thermostat wiring scheme.
  • 11: Most units require one or more batteries for lighting.
  • 13: The thermostat might have a switch indicating the type furnace you have gas/oil is one option, and electric is the other. Position the switch appropriately as well as switches for AC/Heat and Fan On/Auto. Follow the included instructions to program, monitor and use your new control. If there’s a phone app for it, download it, and get familiar with its operation.

Caution Notes

There is an industry standard regarding the color of the wires used for each function, yellow for cooling and white for heat, for example. It is surprising how often the standard isn’t followed. This is why labelling and taking a picture of the thermostat wiring on the old unit are essential. When installing the new unit, connect the wires to the same terminal on the new unit they were connected to on the old thermostat, regardless of color. If you rely on color, and the standard was not followed, your system won’t run. The worst-case scenario is that you might short the control board on the furnace or air handler, and that is a pricey repair.

If your HVAC system doesn’t work properly once the new thermostat is installed, make sure each wire is secure in its terminal. If problems persist, turn off the system and get assistance from a local HVAC professional.

Save Money on Thermostat Wiring

Replacing a thermostat is DIY-friendly. Just go slow, take a picture and label wires. Hiring an HVAC technician for the work will cost $75+, and that’s money that should stay in your pocket.

An easy way to save your family and friends that cash is to share this thermostat wiring guide with them. They’ll appreciate the tips! After you replace your thermostat, feel free to chime in here on how the job went for you. It will encourage others to tackle thermostat wiring rather than spending money on pro installation.

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