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Running New Thermostat Wire – It is possible that you may have to run a new thermostat wire from the air handler or furnace to the location where the existing thermostat is located. Running new thermostat wire can challenging even for professionals. If you must then following these instructions and helpful hints will hopefully help you in this endeavor. Pulling new wire should only be done as a last resort. The purpose of running new thermostat wire is necessary for certain reasons such as:

Running New Thermostat Wire

Running New Thermostat Wire

  • The most obvious reason is that the new thermostat requires an additional wire and there are no other wires available in the wire bundle to use for the new thermostat. This is common so do not fret.
  • The existing wire is very old and brittle which you do not trust.
  • The existing wire was damaged.
  • The existing wire is not long enough for the new terminations. Here is a tip that may solve the problem.
    • As my late Uncle used to tell me, “Here’s an old Indian trick we can use that may solve our problem.” The keyword there is “may” solve our problem. If the wire is not long enough for a proper termination at the new thermostat then take a small piece of thermostat wire about 6 inches long and use a small wire appropriate for #18 solid wire and wire nut the short piece to the 6-inch wire. The six-inch wire should be long enough to make the proper termination and when finished you can push the wire nut back into the wall.
    • Be careful when you take the tstat off the wall especially with the wires to make sure the existing thermostat wire does not fall into the void of the wall. If that happens then you will be pulling wire whether you need to pull the new wire or not as you need the tstat wire but it fell into the void of the wall. Just remember to be careful when you take the old tstat loose from the wall.
      • Another Indian trick for you. Before you take the old thermostat from the wall have a pencil or pen nearby. As soon as you get the wires loose and the old sub-base of the old thermostat out of the way take the pencil or pen and wrap one of the wires around it. This will prevent the tstat wire bundle from falling back into the void of the wall and will save you lots of time and headaches of pulling a new wire or trying to get the wire that fell back out of the void.

By now you should have figured out if you have the proper amount of wires or if any of the above conditions will necessitate you pulling thermostat conductors. It is still not too late to call a professional and let them take care of the headache.

If you are still here and reading this then you are a true DIYer and you are determined to complete this installation by yourself. Now it is time to get things right 100% because you are going to have to make a trip to the local hardware or big box store for some materials. Here is a list of some basics you will possibly need to finish the job.

Running New Thermostat Wire - Tools for the Job

  • Read below for the correct type and number of conductors you will need.
  • A long auger bit.
  • A stud finder
  • A fish tape - preferably the flexible nylon type
  • A flash light
  • Safety glasses
  • A helper
  • & lots of patience.

Running New Thermostat Wire - Important Tips

Running New Thermostat Wire – Important Tip


There may be someone out there thinking they will move the thermostat to another location and HighPerformanceHVAC.com wants to discourage you from doing that as your thermostat location was most likely selected by an engineer, architect, or an HVAC Master designer and the current location of the existing thermostat is the best place in the house. There is no reason to second guess the experts. Some people may think that, “because I need to pull a new wire I may as well go ahead and move the thermostat to a better location”? This is not a good idea as the thermostat needs to be located in an area close to the return (or where you may change your filter), on an inner wall versus a wall that experiences a heat load (outer wall), and out of direct sunlight. Another place where thermostats should not be installed are near sources of heat or sources of something that cools or blows cool air. All these things will cause problems and you will find yourself either too hot or too cold because the thermostat will never work properly if it is not installed in the proper location inside the house.


The tstat senses temperature and the temperature of the air near the tstat should be the temperature of the air going into the return or close to the temperature of the air going into the return. Avoid moving the tstat as it can and often times will cause serious problems and prevent you from being comfortable. It can also increase your utility bills rather than decrease them.


Running New Thermostat Wire - Important Tips

First, you need to read the manual and determine the type of HVAC system you have and the thermostat you are installing to determine the number if wires you will need for the bundle.
load (outer wall), and out of direct sunlight. Another place where thermostats should not be installed near sources of heat or sources of something that cools or blows cool air. All these things will cause problems and you will find yourself either too hot or too cold because the thermostat will never work properly if it is not installed in the proper location inside the house.

After you get the wire and all the tools listed above then you need to fish the wire through the wall and to the either the crawl space, basement, and in some cases the attic. The key is that you must get the new wire from where the tstat is located to where the gas furnace or air handler is located. If you are lucky then you can use the old thermostat wire and attach it to the conductor bundles securely and use the old thermostat wire bundles to pull the new thermostat wire however experience tells me that in most situations this will not be possible because usually when someone was installing the original tstat they put a couple of staples into it to hold in place most likely before the drywall or plaster was put on the walls and the wall was simply studs.

Running New Thermostat Wire - New Installation - Need to Know

Remember it is key that you have the number of conductors needed before you pull the new conductor bundle in the wall. This is extremely important that you get it right because if you are just one wire short this will not work. For example, if you have a gas furnace with two stages of heat and an air conditioner with two stages of cooling and you are installing a digital thermostat you need to make sure of two things before pulling the conductor bundle:

  1. That you have the correct tstat. The tstat needed in this case, will require two stages of heating and two stages of cooling. That is the tstat will have a W-1  and a W-2 terminals plus a Y1 and a Y2 terminals on the tstat to properly control the equipment. The two W terminals will control the two stages of heating and the Y terminals will control the two stages of cooling.
  2. You have the number of conductors in the thermostat wire bundle to properly control the equipment and feed the thermostat with power. All thermostats whether powered or not have the R wire or a red wire (if your thermostat wiring follows the proper protocol) and this red wire is the 24 volts of power needed to control the air conditioner, air handler, or furnace plus it can feed the tstat power if the 24 volt circuit is completed at the tstat. In this case, in addition to the 24 volt red wire, and 24 volt common wire from the transformer or terminal strip board in the air handler or furnace also needs to be ran to the tstat simply to power the digital thermostat. That termination should be done at the C terminal in the tstat and originate at the C terminal on the control circuit board in the furnace or air handler.

Running New Thermostat Wire - Exception for RC and RH Terminals

The only exception to this is if you have split transformers for the heating and cooling systems. For example, a boiler can have its very own transformer while the air conditioning system will its very own thermostat located inside the air handler of the air conditioner. In this case, then the thermostat should only be fed by one power source. A choice needs to be made to use either the air conditioner or the boiler transformer and the wires identified and separated. The thermostat also needs to have two R terminals - RC and RH - where the boiler red wire is terminated at RH and the air conditioner red wire is terminated at RC. This also means if you are going to pull new thermostat wire you are going to have to pull separate wire for the boiler and a separate wire for the air conditioner. The boiler is likely a two wire bundle with a red and white wire inside the bundle with the red wire terminated at the RH terminal and the white wire terminate at the W terminal.

Running New Thermostat Wire - New Installation - Additional Resources

For more information on this subject see our article “How Can I Tell 1 or 2 Transformers for Heating Cooling System”

You can purchase new thermostat wire here

High Performance HVAC

 Running New Thermostat Wire

To learn more about heating systems and HVAC use this resource.

If you are purchasing new HVAC equipment don’t forget to check out our HVAC Consumers Buyers Guide Category for lots of help.

See the About the Author page for information about the author of this article.

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