- the most common trouble call by service and repair HVAC companies
- about thermostat wire with a resource for more information
- the best advice for changing your thermostat
- the most common mistakes made by DIY thermostat changers
- the importance of identifying the type of HVAC system you have so you will select the proper thermostat.
- advisory and disclaimer for installing a new thermostat
- list of tools commonly needed to install a new thermostat
- step by step instructions on changing your thermostat along with hints and tips for making the job easier
- lots of extra links to other articles to make sure you completely understand everything you need to know. (and more about thermostats, thermostat wire, and installing a new thermostat)
- Resource to color codes to make it easier for installation.
- some thermostat wire diagrams at the end of the article
- You can save energy for your heating and cooling system by installing an energy saving 7 day programmable thermostat or a setback thermostat.
- lots of comments from our readers
- thermostat wiring instructions are good for most low voltage room thermostats including a smart thermostat.
How to Wire a Thermostat
Your thermostat or programmable thermostat is an integral part of your comfort system. These thermostats, whichever type of thermostat or thermostats you have, require little maintenance. Often, I find that the only thing people know about their AC and Heating systems is how to turn the thermostat on and off. And change the temperature setting.
The purpose of this site is to help people go beyond the thermostat. To learn more about their system as a whole. And not just a dial on the wall that adjusts the temperature when they are not comfortable. However, we’ll delve into the world of thermostats on this page and hopefully, this will help many people.
We have many other related articles. Use the search feature to the right to help you find other related articles.
HVAC Service Calls and Thermostats
A very common occurrence with service calls is the complaint that the thermostat isn’t working properly. Sometimes this is true but often it is something entirely different than the thermostat. Because some people believe the only problem is with the thermostat. They go down to the local hardware store and purchase a brand new thermostat.
They get home and dust off the toolbox. Never reading any thermostat installation directions, they proceed to change the thermostat. Some are successful at changing the thermostat and some are not. Those that are most likely read some thermostat installation directions or had someone read the thermostats installation directions for them. The ones that are not successful end up calling a professional in to install a new thermostat.
Successful Thermostat Installation Advice Article (opens in a new window)
Thermostat Installation Advice
The bottom line advice to most people is to call an HVAC professional if something is wrong. Whether it is with your thermostat or HVAC system. The problem may not be with the thermostat. And you may exasperate the problem which will cost more in the long run. Additionally, If you have a multi-zone system, a high-efficiency heat pump or even just a heat pump, a regular split-system AC and a boiler for heat (and you have one thermostat), or an Apollo-based system (hot water heated in a water heater) with a split AC system……. call a professional.
These systems can be very complex and may require special sub-bases so the thermostat control circuit will work properly. Additionally, for heat pumps, there are different controls, and wires for these controls run into the thermostat, they are multi-colored thermostat wires. These controls can be very complex. Each wire must go to the correct terminal on the thermostat or the unit will not run correctly.
Variations with Thermostat Installations
There are so many different variations of this High Performance HVAC will not attempt to describe them here. We will describe the common type (at least for this region (mid-Atlantic) and if you see that you have the system described you can proceed cautiously at your own risk. Here is a list of possible thermostat wiring combinations beginning with the most common first:
5-Wire Thermostat Wiring
This is the most common and covers many central air conditioners with an air handler or gas furnace. The 5 wires likely cover Red for 24-volt hot, white for heat, yellow for cooling, green for the fan, and blue for common (common could be another color).
4-Wire Thermostat Wiring
This is typically for a thermostat that is battery powered or a heat only digital thermostat. It covers the same control or color features as the 5-Wire thermostat wiring above except for heat only. With heat only, you likely will have a 24-volt hot and common (red and blue), a white wire for heat and a green wire for a fan.
3-Wire Thermostat Wiring
This is common for a digital thermostat that controls a boiler. It has 24-volt hot and 24-volt common along with the wire for the heat which is likely white.
2-Wire Thermostat Wiring
This likely a heat only thermostat that is either digital or non-digital. If it is digital it has battery power to power the thermostat. The two wires likely are red for 24-volt hot and white for the heat.
If you have more than five wires then you have more control points or you have a heat pump. Refer to our pages about heat pump wiring on how to wire a heat pump thermostat or our thermostat wiring color page to see the additional control points for your thermostat.
Thermostat Wiring Color Code Table (opens in a new window)
Additionally, be aware that thermostats are equipped with heating and cooling anticipators. Cooling anticipators are not adjustable where heat anticipators are adjustable in mechanical thermostats. Setting the heat anticipator is important for your heating system. And for the thermostat to function properly (only if you have a mechanical non-digital thermostat).
It is set according to the amp draw on the control heating circuit. Make sure the heat anticipator is set properly. It is set in the thermostat or thermostats. Proper settings ensure you will get the best out of your heating system and your thermostats.
How to Wire a Thermostat - Selecting the Proper Tstat
AC split or package system with gas, oil, or electric heat
The first thing you should do before changing the thermostat is to select the proper thermostat for your system. Selecting the proper thermostat.
If you are retired or if someone is at home during the day you likely do not need a programmable thermostat. If this is the case the only benefit you will get from a programmable thermostat or thermostats is at night. Or I prefer to say in the morning because you can set it so the unit comes on just before you wake.
That way the house is at a comfortable temperature when you get out of bed. I will describe the advantages of the programmable thermostat in full detail in a different section below. Once you have made the thermostat selection you can proceed to the next step.
Before You Begin Replacing the Old Thermostat
Helpful Page to help you select the right thermostat (opens in a new window)
Thermostat Wiring and Installation Advisory……….If you decide to install your own thermostat you do so at your own risk. There are many incidences where the homeowner installed their own thermostat and was successful. There are also many incidences where the homeowner was unsuccessful installing a new thermostat. The people that were unsuccessful wasted part of their day, ruined a thermostat or two and caused the malfunction of an integral part of their system.
That is not mentioning the fact that they ended up calling a professional HVAC Technician to fix the problems. Problems caused by improperly installing a thermostat. The unsuccessful end up paying three to four times what they would have paid if a professional was called first. Factor this into your decision and if there is any doubt call a professional. Call a professional to install and wire the new thermostat.
Before you begin to install your new thermostat turn the power off. For both the condensing unit and the air handler/furnace/or boiler.
Thermostat Installation Tools & Steps
These are the four wires that you need to control the heat, cooling and the blower or fan. This is for most air conditioning and heating systems. If the colors of the wires do not match the colors described here make sure you mark the wires with masking tape. Sometimes you will have extra wires that are not connected. This is common. Thermostat wire comes in many different varieties. The contractor who installed the system probably used 5 wire or 8 wire thermostat wire bundle. They used what they needed and simply twisted or cut the other wires off.
- Get the tools together that you will need to do a proper thermostat installation. You will need:
- A small straight-slot (or flathead) screwdriver
- A small Phillips screwdriver
- A pair of needle-nose pliers
- A utility knife or wire strippers (for small wire)
- Plastic wall anchors (sometimes provided with the thermostat) good for drywall
- A drill with a bit to make the holes for the plastic wall anchors
- A small level
- Two pencils or pens
- A small paper bag and some masking tape. Tape the bag below the area where the thermostat. Any trash or dust will fall into the bag and not onto the floor.
- Some touch-up paint
- Clean hands (don’t do a great job changing the thermostat and leave all those prints all over the wall)
- Wire nuts (maybe)
- Plenty of light
The Power = No Electricity
- Turn the power off to the air conditioning and heating unit at the circuit breaker or the emergency cutoff switch. This should kill any power going to the thermostat. After doing that make sure the power is off by turning the thermostat to the on position. Then go to the unit to make sure it is not on. Labels on circuit breakers are not always correct. and emergency switches are not always hooked up properly. Just make double sure that you have killed power to the unit. This is not only for your safety but also to keep from blowing the transformer. I get calls all the time to replace transformers because the homeowner changed the thermostat and didn’t kill the power. They hooked everything up correctly. However, during the process, they touched the wrong wires together and blew the transformer. The transformer powers the HVAC control circuit including the thermostat.
Steps and Procedures for Installing the new Thermostat
Basic Control Circuits for HVAC Article (opens in a new window)
- Pull the cover off the front of the thermostat. If it is a mechanical thermostat there should be a little adjuster tab in the center of it. This is your heat anticipator. It should have numbers ranging from 1.5 to .1. Take note of this setting. Remember to set the new thermostat to this same setting. However, only if you are replacing a mechanical thermostat with another mechanical thermostat. You probably want to do this now before you proceed further. If you are replacing a mechanical thermostat with a digital thermostat, the digital thermostat should set itself automatically. If not read the instructions on the new thermostat for instructions on how to set the anticipator. This is very important. An improperly set anticipator will cause your furnace or heater to run improperly. Thermostats are equipped with cooling anticipators. Cooling anticipators are often on the sub-base and are non-adjustable.
How to Wire a Thermostat - The Colors and Terminals
- Unscrew the thermostat from the sub-base (wall plate or back plate). Take note of each wire. The following list should match the wires and terminals on your thermostat.
- Red to the thermostat RH or thermostat RC terminal with a jumper wire between thermostat RH and thermostat RC. Or Red to the thermostat R terminal which is shared with both the heating and cooling. It has an internal jumper built into the sub-base. The red wire is the source hot wire from the transformer. All other wires, except the common wire, controls a specific relay or contactor. These components will energize the fan, heating, or cooling depending on the selection. The following are the common wiring colors. However, your system may not be common and different colors could have been used.
- Green to the thermostat G terminal. This is the color that controls the fan or the relay that controls the blower.
- Yellow to the thermostat Y terminal. This is for control of the air conditioning.
- White to the thermostat W terminal. This is for control of the heating.
How to Wire a Thermostat - Removing Wires
- Remove the wires from the terminals on the sub-base. Is the power off? Be careful not to let the wires fall back into the wall. Sometimes there is just enough wire to reach the terminals and that’s it. Try pulling the wires a bit to see if there is more wire behind the wall. Most of the time there is some slack and you can pull the wire out more. Unscrew the sub-base from the wall while holding the wires. When you get the sub-base off wrap the wires around the pencil or pen. This will keep the wires from falling back into the wall.
- Get the new sub-base and compare it to the old one. Hold it up to the wall in the position you want it. Is the old paint that was covered by the old sub-base going to be covered by the new sub-base? If any of the old paint is going to show you may want to make some touch-ups now. After finishing with that, put the new sub-base back on the wall in the position you want it. Make sure it is as level as possible. You can use a level to do this. (This is very important, especially for mechanical thermostats. It must be level or the mercury switch will not keep the proper temperature settings in the house. Make sure it is level.) Mark the new holes through the sub-base where the screws will go into the wall to fasten the sub-base.
How to Wire a Thermostat - Anchors
- It is important in this step to have the proper drill bit size. The bit should match the size of wall anchors you have. Some wall anchor kits come with a bit in them. I recommend the wall anchor kits with the bits in them. The bit is the perfect size for the anchors. The bit should be slightly smaller than the anchor. If the bit is bigger the wall anchor will not hold. And the possibility exists that the thermostat will fall off the wall. Drill the mounting holes you made for mounting the sub-base. Insert the wall anchors and push them hard with your thumb. Approximately 1/16th of an inch on the lip of the anchor will remain to stick out of the hole. If it is more than that use the butt-end of the screwdriver. Push it in until just the tip of the anchor remains visible.
- Undo the wires from the pencil or pen and run them through the center of the sub-base. Insert the screws and screw them only snug tight. Get the level and make sure the sub-base is level. When you are sure that it is level, tighten the screws. Be careful not to allow the sub-base to move when you are tightening the screws.
The Colors and Terminals on the New Tstat
- Using the color code of the thermostat wires, attach each wire to their proper terminal. (or if they didn’t match, the color markings you made with masking tape). Some people like to loop the wire around the terminal screws. This is not necessary. What is necessary is that the wires are attached to the terminals and they are tight. Additionally, make sure that none of the bare wire is touching anything except the terminal. Once the wires are attached you are almost finished completing the task of installing the thermostat. The hard part is over!
- Attach the thermostat to the sub-base. Most thermostats have built-in screws while other simply snap in place. Tighten these screws and check to make sure the heat anticipator is set properly. It should have the same setting as the old anticipator setting. Some modern digital thermostats will not have an anticipator setting. If the thermostat instructions say nothing about the anticipator don’t worry about it. Additionally, some thermostats do not have screws holding the thermostat to the sub-base. Instead, the thermostat snaps onto the sub-base.
- Attach the front cover to the thermostat and restore power. Start and check the heating, air conditioning, and with the heating and air conditioning off, the fan only sequence. All systems should be working properly at this time. (Provided you did the task properly). You are the proud owner of a brand new, properly installed thermostat
Additional answers to your questions concerning thermostats can be found here in our thermostat category index. Other questions about a Honeywell thermostat, Honeywell thermostats, programmable Honeywell thermostats can be found at the Honeywell Thermostat website. Additionally, see thermostat wiring color code chart to help with How to Wire a Thermostat. The Nest Learning Thermostat, Ecobee, Sensi, and other WiFi thermostats including Honeywell WiFi tstats allow remote control and monitoring so you can check or change the temperature when you are away. These newer thermostats allow you to have a smart home by offering precise temperature control and saving energy lowering energy use and lowering energy bills.
How to Wire a Thermostat - Thermostat Wiring Diagram
This thermostat wiring diagram is a split system. These split systems include an air conditioner with a gas furnace, oil furnace, or electric furnace. The wires going to the condensing unit are shown in red and white. However, these colors can be different. One wire originates from the thermostat Y terminal and terminates at the condenser. The other wire originates from the common side of the transformer. The transformer is commonly installed in the air handler.
In rare installations, the condensing unit will have the transformer. The manufacturer did not install the transformer in the condenser. The transformer was blown in the past. It was replaced in the condenser rather than the air handler. Additionally, the colors here are typical. However, they can be different depending on who wired the unit and their color coding system. Most HVAC technicians will use this color code. However, there is an occasional oddball who knows better than everyone else. Or perhaps the wiring color combination was not available for the new installation.
Old Thermostat Disposal
Always keep in mind to properly dispose of hazardous materials responsibly. Older mechanical thermostats contain mercury and mercury are extremely hazardous to human health and our environment. Most municipalities have a hazardous waste collection site. They will be happy to accept your old thermostat and dispose of it properly.
Additionally, there are state and federal laws that are explicit about properly disposing of hazardous waste including products containing mercury. Please do not throw the old thermostat in the regular waste bin. Be responsible and properly dispose of the old thermostat. For more information please see HVAC Products Containing Mercury and Proper Disposal. Finally, additional information on products containing mercury can be found on the EPA website.
Thermostat Wiring Diagrams
Your Resource for HVAC Thermostat Wiring Information – How to Wire a Thermostat
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