Thermostat Wiring Help

  The following article assumes the most common colors are used to wire your particular thermostat. Some HVAC installers do not comply with conventional installation practices and do not use the commonly used colors for the terminal designations described in this article. Before you begin removing the old thermostat get a pencil and paper and write down the colors that match the terminal designations on the old thermostat. Also, read Wiringhelp.com's disclaimer. Whenever there is any doubt call a professional to do the job. Calling a professional may save you money by ensuring the new thermostat is installed properly without any incidences or major disruptions in heating or cooling because of improper wiring. 

Thermostat Installation Help Step by Step

1. Ensure you have purchase the correct thermostat for the system. If you have a heat pump you will need a heat pump thermostat. If you have straight air conditioning with a gas furnace you'll need to ensure you purchase the appropriate thermostat for that comfort system. Please see Heating and Cooling Thermostat Types and Thermostat Installation 101 articles before proceeding. These articles will help you choose the correct thermostat for the HVAC system you currently have installed in your home or business.
2. Gather the appropriate tools for the job. You will need: A volt meter, pencil, paper, flat head and Phillips screw driver, level, and wire stripers.
3. Turn the power off to the entire HVAC system. Depending on the type of system you have will depend on whether you turn off one switch or breaker or two switches or breakers. An air conditioning and heating package unit will have one switch or breaker that will kill power to the unit but a split system will have two switches or breakers to kill power to the HVAC system. Another possible combination of switches to turn power off is a split air conditioning system with a boiler where the heating and cooling systems share the same thermostat.  In this case there may be three switches and/or breakers to kill power to the entire system. This type of system will likely have two separate transformers. It is important to kill the power to the heating and cooling systems because you could blow the transformer by grounding out the wires in the thermostat. This usually results in a call to a professional HVAC technician, usually at odd hours, to replace the transformer and get the system running again. This is a common occurrence among do-it-yourself people. Once the transformer is blown the system will not function. Ensure that the power is off by testing the circuit with a volt-meter at the thermostat terminals. The meter should read 0 volts at all thermostat terminals especially the "R" terminal(s).                                                                              
   
4. Get a pencil and paper and write down the wire colors attached to the old thermostat terminals. Also make a note of how many wires there are attached to the terminals on the old thermostat. Make a drawing if necessary just make sure you understand the terminals and what wire colors go to them before proceeding to the next step. If you are replacing a mercury type thermostat with a newer mercury type thermostat make note of the heating anticipator setting. This will look like a little dial with a pointer pointing to some numbers. This is very important. You will not find an anticipator setting on a digital thermostat. The anticipators are solid state and self setting.  
5. Open the box with the new thermostat and find the instructions. Read the instructions fully until you understand them. Pay particular attention to wire color and terminal designations. Look at the old thermostat and match up the colors and terminals. Make sure they match and you can wire the new thermostat just like the old thermostat. If you don't understand how you will wire the new thermostat call an HVAC professional.
6. Remove the old thermostat if all is well and you understand how the new thermostat will be wired. Ensure that the thermostat wires, after they are removed from the old thermostat, do not fall into the hole in the wall behind the thermostat. Wrap the thermostat wires around a pencil to prevent them from falling into the hole in the wall. It is a possibility that there are unused wires for the thermostat. It is simply a case of the installer using a higher number of conductors than necessary for the job. Do not cut these wires off. Simply wrap them around the main wire in a spiral. Keeping these unused thermostat wires for future use could save you money by allowing the HVAC technician to use them if one of the existing in use thermostat wires fails.

7. Sub-base mounted and Wired

   Mount the new thermostat sub-base to the wall. Use the existing holes if they match and have good anchors. Make new holes and use new anchors if necessary. Use a level to make sure the new thermostat is level. This is especially important for mercury type thermostats. If a mercury type thermostat is not level it will not accurately maintain temperatures in the house or business. Ensure that the sub-base to the new thermostat is secure and level.
8. Make sure all the thermostat wires ends are stripped of the insulation and have good integrity. Older thermostat wire gets brittle and breaks easily. Make sure there are no brittle ends. Cut the old ends off the thermostat wire if necessary and strip new ends in the thermostat wire.
9. Insert the wires at the proper terminals and ensure they are tight by giving them a little tug. After double checking all the wiring for tight connections stuff the excess wire back in the hole behind the sub-base.
10. Mount the new thermostat on the sub-base as per the manufacturer's instructions and restore power to the system. Test the new system out but do not short cycle the equipment. After turning the system off allow at least three minutes before turning it back on especially the air conditioning system. Many newer digital thermostats will have a built in delay timer to prevent short cycling of the air conditioning and heating equipment. Remember that especially when you turn the thermostat on and you don't hear or see anything come on. Allow time for the thermostat delay timer to turn the system on.

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