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Thermostat wiring
This complete 24-volt thermostat wiring guide shelters all thermostat cabling matters from the simple to the complex. We address them in order from maximum common to minimum common. We do not cover 110/240-volt thermostat wiring and millivolt thermostat installing issues.
Feel free to jump to the section that covers your specific topic:
Is My HVAC Structure a 24-volt Structure?
Achievement starts with knowing what type of thermostat wiring you have or need. Your options contain 24V, 110/240V, and millivolt thermostat. This guide addresses 24-volt systems, the mainstream of HVAC systems.
24V or low-voltage heating and cooling systems contain one or more of these mechanisms:
  • Heating: Gas furnace, moreover natural gas (NG) or propane (LP), oil furnace, heat pump with or without supplementary heat strips
  • Cooling: Air conditioner, PTAC (packaged terminal air conditioner), heat pump
  • Accessories: Humidifier, dehumidifier, ERV/HRV ventilator, air purifier, and other air excellence apparatus
These systems can be outdated split systems, with one constituent outside and others inside, ductless split systems with outside and inside components, and packaged systems with all machinery located in a single large case, usually installed outdoors. They can be single-stage, two-stage, or variable-capacity systems. Dual fuel systems with a heater and heat pump are 24-volt / low-voltage systems.
More than 90% of all HVAC (Heating, Ventilation, and Air Conditioning) systems are low voltage 24V systems. The following systems ARE NOT 24-volt systems:
  • Boilers
  • Electric furnaces
  • Baseboard electric heat
  • Solid fuel stoves (wood, pellets, corn, anthracite, coal, and other biomass materials)
  • Millivolt furnaces that do not necessitate electricity
3 Techniques to Wire a Thermostat
Let’s start with the humblest method. If the old thermostat is still installed, then this will work. If the old thermostat has been detached, then the approach is a bit more multifaceted but can still be a DIY project.
 Substitute the thermostat wire for wire:
If you haven’t reformed your HVAC system machinery but want a new thermostat, take this method.
  • Turn off the electrical circuit to the furnace or air handler.
  • Take a picture of the old wiring associated with the numerous terminals. This will serve as a situation in the event you lose track of the terminal any wire was connected to. You can also use the old-school process of putting tape labels on the thermostat wires and marking them with the letter of the terminal each was involved too.
  • Separate a wire from the old thermostat, and connect it to the terminal with the same letter(s) on the new thermostat. Remove and attach them one at a time, until you’ve altered them all.
  • Note: Your system might not use all the wires in the package. This is common and not a problem. Unused wires are usually warped together and enfolded around the other wires to avoid bare wiring from contacting any bare section of wire being used or a terminal.
  • Turn the circuit back on, and adjust the thermostat to call for heating or cooling.
  • If it works, and it should, you’re done. If it doesn’t, we endorse turning the circuit off again and calling a local HVAC corporation to complete the job.
Discover the wiring connections in the heater or air handler:
If the thermostat has been removed, your job is a bit more complex. One of two approaches might work. Here’s the most convincing approach.
  • Turn off the circuit used by the furnace or air handler.
  • Double-check that the system is off by regulating the thermostat to call for heating or cooling. If the system doesn’t turn on, continue.
  • Take out the cover of the furnace or air handler.
  • Locate where the wire package from the thermostat enters the furnace. It will entail three to eight wires of different colors including red, white, green, yellow, orange, blue, black, etc. The whole bundle might be enfolded with housing.
  • It might be essential to remove a cover over the control board to expose the wiring connections.
  • Once you discover the wiring connection stations on the furnace or air handler, take a picture of them and/or write down what color wire goes to each lettered terminal.
  • Leave the cover(s) off the furnace until you get the thermostat wired.
  • Use your photo and/or the notes you’ve taken to attach the correctly colored wire to the correct terminal on the thermostat.
  • Note: As we said above, your system might not use all the wires in the bundle. This is mutual and not a problem.
  • Turn the circuit back on, and regulate the thermostat to call for heating or cooling.
  • If it works, you’re done. You can put the cover(s) back on. If it doesn’t work, we endorse turning the circuit off again and calling a local HVAC company to wire the thermostat.
Use normal wiring colors to attach the thermostat:
If the thermostat has been detached, so you don’t know which wires were connected to which terminals, and you can’t or don’t feel content getting into the furnace, this method might work. It depends upon the original installer who had used the traditional cabling color code when connecting the thermostat. Note that wiring colors have no characteristic meaning. The wires are all the same: solid copper wire covered with colored PVC insulation. The color code was started to make jobs like you’re undertaking easier. Here’s how to wire a thermostat using the color code and, FYI, each terminal’s purpose.
  • Attach the red wire to the R terminal (Call for heating and/or cooling).
  • Attach the green wire to the G terminal (Fan).
  • Attach the white wire to the W terminal (Heat).
  • Attach the yellow wire to the Y terminal (AC).
  • Attach the blue wire to the C terminal (Common wire – see below for details).
  • Wrap any additional wires around the bundle to avoid them from contacting bare wires or terminals.
Note: If any of the machinery is accomplished of staged heating or cooling, this formation either won’t work or won’t allow for the full presentation of the system. If it doesn’t work or you aren’t sure you’re getting a full recital such as staged heating or cooling, call an HVAC pro. The technician will check the connections in the furnace or air handler and complete the wiring of the thermostat.

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