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 and learn more about their system as a whole, 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.
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 with the system rests with the thermostat, they’ll go down to the local hardware store and purchase a brand new thermostat. They get home and dust off the tool box, never read any thermostat installation directions, and 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.
Thermostat Installation Advice
The bottom line advice to most people is to call an HVAC professional if something is wrong 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 and each wire must go to the correct terminal on the thermostat or the unit will not run correctly. There are so many different variations to 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.
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 thermostat to function properly. It is set according to the amp draw on the control heating circuit. Make sure the heat anticipator is set properly in the thermostat or thermostats so you will get the best out of your heating system and your thermostats.
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.
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 were successful. There are also many incidences were the homeowner was unsuccessful installing a new thermostat. The ones 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 caused by improperly installing a thermostat. The ones that are unsuccessful end up paying three to four times what they would have paid had they called a professional in the first place. Factor this in to your decision and if there is any doubt call a professional to install and wire the new thermostat.
Installation Tools & Steps
These are the four wires that you need to control the heat, cooling and the blower or fan 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. If there are more wires that are not hooked up don’t worry. This is common. Thermostat wire comes in many different varieties and the contractor who installed the system probably used 5 wire or 8 wire thermostat wire. 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) screw driver
- A small Phillips screw driver
- A pair of needle nose pliers
- A utility knife or wire strippers (for small wire)
- Plastic wall anchors (sometimes provided with the thermostat)
- 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 is so that 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)
- Plenty of light
- 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 and going to the unit to make sure it is not on. Not all circuit breakers are labeled correctly and not all emergency switches are hooked up. Just make double sure that you have killed power to the unit 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 but during the process they touched the wrong wires together and blew the transformer which powers the HVAC control circuit including the thermostat.
- 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 and remember to set the new thermostat to this same setting 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. The thermostat is also equipped with a cooling anticipator. Cooling anticipators are often on the sub-base and are non-adjustable.
- Unscrew the thermostat from the sub-base. 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 in to 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 that energizes the fan, heating, or cooling depending on the selection. The following is the common wiring colors but 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 control of 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.
- Remove the wires from the terminals on the sub-base. The power should be off so you shouldn’t have to worry about being shocked. 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.
- It is important in this step to have the proper drill bit size for 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 because it is the perfect size drill bit 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 sticking out of the hole. If it is more than that use the butt-end of the screw driver and push it in until just the lip 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.
- Using the color code of the wires (or if they didn’t match, the color markings you made with masking tape), attach each wire to their proper terminal. 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. The screws for this are built in the thermostat. Tighten these screws and check to make sure the heat anticipator is set to the same setting as the old anticipator setting.
- 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 (if you did the task properly) you are the proud owner of a brand new, properly installed thermostat
Additional answers to your questions concerning thermostats, a Honeywell thermostat, Honeywell thermostats, programmable Honeywell thermostats can be found at the Honeywell Thermostat website.
Thermostat Wiring Diagram
This thermostat wiring diagram is typical for a split system air conditioner with a gas furnace, oil furnace, or electric furnace. The wires going to the condensing unit are shown as red and white however these colors can be different. The important part of these two wires going to the air conditioner condensing unit is that the one wire originates from the thermostat Y terminal and terminates at the condenser and the other wire originates from the common side of the transformer which is most commonly installed in the air handler. On rare occasions the transformer can be found in the condensing unit not because the manufacturer installed the transformer in the condenser but because the transformer was blown and replaced in the condenser instead of the air handler. Additionally, the colors here are typical but can be different depending on who wired the unit and their color coding system. 99% of the HVAC technicians will use this color code but there is an occasional oddball who knows better than everyone else or the wiring color combination was not available for the wire used to wire the equipment.