Thermostats vary in the features they offer. There are also differences when it comes to wiring one to control the heating and cooling system in your home. If you check the Honeywell thermostat ct31a1003 wiring diagram, you'll see that it requires only two wires because it's a very basic thermostat designed only to control a heating system. If you have a system that both heats and cools your house, you need a more complex thermostat with more wire terminals, and if your system incorporates a heat pump, you need even more wire terminals.
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You can use a multifunction thermostat to control a simple system, but not the other way around. It helps to have a basic understanding of your home's heating and cooling system as well as how thermostats work before you buy a thermostat to control it. If your HVAC knowledge is limited, you can also simply count the wires connected to your old thermostat. Your new Honeywell thermostat should be able to accommodate the same number of wires.
Meaning of Wiring Terminal Letters
Honeywell thermostats follow the industry standard for terminal identification. Each terminal is marked with a letter that usually corresponds to the color of the wire that should be used to connect it. Here's a list of the basic terminal letters and their functions:
- W (white) controls the auxiliary heating elements.
- Y (yellow) controls the compressor on the cooling system.
- G (green) controls the fan.
- C (blue or black) provides a return path to the transformer. This terminal is needed when the thermostat uses 24V power from the system control panel. This terminal won't be present if the thermostat has batteries or operates mechanically.
- O (orange) controls the reversing valve on a heat pump to switch the pump back and forth from heating to cooling.
- Rh (red) supplies 24VAC power to the thermostat and switches the heating system on and off.
- Rc (red) supplies 24VAC power and switches the cooling system on and off.
Some thermostats may have additional terminals, including W2 for the second-stage heater, Y2 for the second-stage cooler and X2 for the second-stage indicator lights. The wire colors for these extra terminals aren't specified.
Other thermostats have fewer wires. Again referring to the Honeywell thermostat ct31a1003 wiring diagram, you can see it requires only two wires, R and W. It doesn't control cooling and, because it's a mechanical thermostat, it doesn't need power for internal functions, so it has no C wire.
Metal Jumper for Thermostat Terminals
If your HVAC system has both heating and cooling functions, you'll need to connect both the Rc and Rh terminals with a red wire. Because many thermostats come with a pre-installed thermostat jumper wire that connects these terminals, you only need one red wire, which could be the thermostat Rc wire or the Rh wire. If this jumper wire isn't present, you can install one yourself. If you don't, you'll need two red wires so you can connect each terminal separately to the system transformer.
Don't Rely on Color Code
When installing a thermostat, many people — even HVAC pros — don't always pay attention to wire colors, so you can't assume that the colored wires sticking out of your wall perform their standardized functions. The best way to identify the wires is to look on your system control panel. It has the same terminal designations as the thermostat does, and you can simply note the color of the wire attached to each terminal.
If you're replacing an existing thermostat, note the color of the wire attached to each terminal before you disconnect it. Label each wire with painter's tape for easy identification. Connect the wires to your new Honeywell thermostat in the same way.
Chris Deziel is a contractor, builder and general fix-it pro who has been active in the construction trades for 40 years. He has degrees in science and humanities and years of teaching experience. An avid craftsman and musician, Deziel began writing on home improvement topics in 2010. He worked as an expert consultant with eHow Now and Pro Referral -- a Home Depot site. A DIYer by nature, Deziel regularly shares tips and tricks for a better home and garden at Hunker and Family Handyman.