Old Honeywell Thermostat Wiring Diagram

If you own an old Honeywell thermostat and need to replace it or repair it, you might need a wiring diagram to help you with the installation process. With the right wiring diagram, you can easily connect the wires to the new thermostat and ensure it works properly. In this article, we’ll explore the different types of old Honeywell thermostats and provide you with a wiring diagram and some FAQs to help you out.

Types of Old Honeywell Thermostats

There are several types of old Honeywell thermostats, each with a unique wiring configuration. Here are some examples:

Type Description
T87 This is a round thermostat with a mercury switch. It was introduced in the 1950s and is still in use today.
CT87 This is an updated version of the T87 that uses electronic components instead of a mercury switch. It was introduced in the 1990s.
T822 This is a rectangular thermostat that was introduced in the 1980s. It has a simple wiring configuration.
T8400C This is a programmable thermostat that was introduced in the 1990s. It has a more complex wiring configuration.

Wiring Diagram for T87 and CT87 Thermostats

If you have a T87 or CT87 thermostat, the wiring diagram is relatively simple. You only need to connect four wires: the R wire, the W wire, the Y wire, and the G wire.

The R wire is the power wire, which is usually connected to the red or brown wire. The W wire is the heat wire, which is usually connected to the white or orange wire. The Y wire is the cooling wire, which is usually connected to the yellow wire. The G wire is the fan wire, which is usually connected to the green wire.

Here’s a diagram to help you understand the wiring configuration:

R wire Red or brown wire
W wire White or orange wire
Y wire Yellow wire
G wire Green wire

Wiring Diagram for T822 and T8400C Thermostats

If you have a T822 or T8400C thermostat, the wiring diagram is more complex than the T87 and CT87. These thermostats have more wires and require more attention to detail when wiring.

Here are the different wires and their functions:

Wire Function
R wire Power wire
W wire Heat wire
Y wire Cooling wire
G wire Fan wire
O wire Reversing valve wire (for heat pumps)
B wire Reversing valve wire (for heat pumps)
C wire Common wire (optional)

Here’s a diagram to help you understand the wiring configuration:

R wire Red or brown wire
W wire White or orange wire
Y wire Yellow wire
G wire Green wire
O wire Orange wire
B wire Blue wire
C wire Black or white wire (optional)

Frequently Asked Questions

1. What’s the difference between a T87 and CT87 thermostat?

The T87 has a mercury switch, while the CT87 has electronic components. They both have the same wiring configuration, but the CT87 is more accurate and reliable.

2. How do I know which wires to connect to the new thermostat?

Refer to the wiring diagram for your specific thermostat model. If you’re unsure, contact a professional HVAC technician for assistance.

3. Can I install a new thermostat without a C wire?

It depends on the model of your new thermostat. Some thermostats require a C wire for power, while others have a battery backup. Refer to the manufacturer’s instructions for more information.

4. Can I install a programmable thermostat with my old wiring?

Yes, but it might be more complicated than a simple replacement. You might need to rewire some of the connections or modify the wiring to accommodate the new thermostat’s features.

5. Do I need to turn off the power before installing a new thermostat?

Yes, it’s important to turn off the power to the furnace or air conditioner before installing a new thermostat. This will prevent any electrical shocks or damage to the equipment.

Conclusion

Replacing or repairing an old Honeywell thermostat might seem daunting, but with the right wiring diagram and a bit of knowledge, you can do it yourself. Whether you have a T87, CT87, T822, or T8400C thermostat, understanding the wiring configuration is crucial for a successful installation. Follow the instructions carefully, and if you’re unsure, contact a professional HVAC technician to assist you.