If you’re thinking about replacing or installing a new thermostat yourself, there is some basic information about thermostat wiring you should be familiar with. But remember, there is no substitute for professional knowledge. Always follow the manufacturer’s installation procedures on any project. The information here is presented only as an overview and should not be relied on for any one specific type or model of thermostat.
Since there are many different brands and models of thermostats on the market, the first step a homeowner needs to do is make sure any replacement thermostat is compatible with your heating and cooling system. Sometimes this is not always easy. Model numbers don’t always tell everything in terms of compatibility. Don’t always rely on the cross-referenced model numbers alone for compatibility.
Most of the time you will be dealing with a conventional forced air heating and cooling system. In an older homes the cooling system may have been added separately but the wiring will be similar to what we describe below.
One of the best ways to determine the compatibly of a replacement thermostat is to first look at the existing installation. Make a note of the number of wires coming in and the terminals in the thermostat they connect to. If you know the number of wires and terminals being used, finding a compatible replacement is much easier.
Let’s consider the diagram above. You can see that there are 4 wires coming into the thermostat. What is important here is the number of wires (4) and the terminals they are connected to. The color of the wires is meaningless. A wire is a wire. While there are some conventions used, my recommendation is to use the wire color only as a guide as to which wire goes to which terminal in your system. Yours may not have the same colors as this so avoid any confusion and focus on the terminals in use.
The wire connected to the RH and RC terminal is for the source coming from the transformer on the heating and cooling system. Sometimes there is just an R terminal instead of the RH and RC as above. If the system shares a transformer for both the heating and cooling you will see a small jumper wire between the two. The jumper wire would be removed if both the heating and cooling systems had their own transformers. This would most like occur if the cooling system was added separately.
The wire to the G terminal is the fan relay which will turn the fan on/off. The wire to the Y terminal is the compressor relay for turning the cooling on/off. The last wire is to the W terminal which is the heating relay to turn the heat on/off.
Since this setup is a very basic configuration you will find a wide selection of replacement thermostats compatible with this setup. There are of course more complex alternatives when you start looking for digital or programmable alternatives. At a minimum these models will require batteries for power unless there is a fifth wire to supply power.
If you have a multi-stage system then the thermostat wiring configuration will be different. In these cases consult a professional or an electrician to be on the safe side.
Ther are many options when replacing your old room thermostat. Determine the features and functions you want, then look for a compatible model for your system. It should be easy to find what you need.