Understanding Honeywell Mercury Thermostats
Honeywell's mercury thermostats operate based on the temperature of the surrounding environment. They typically contain a small glass vial filled with mercury, which moves in response to temperature changes. This movement opens or closes electrical contacts, thereby regulating the heating or cooling systems.
Common Models of Honeywell Mercury Thermostats
There are several models of Honeywell mercury thermostats, including but not limited to:
1. Honeywell RTH9585WF: A programmable thermostat that offers Wi-Fi connectivity.
2. Honeywell T87: A classic round thermostat often found in older homes.
3. Honeywell CT87N: Another popular round model, known for its reliability.
Each model may have different wiring configurations, but the basic principles of operation and wiring are generally similar.
Wiring Basics for Honeywell Mercury Thermostats
Before diving into the wiring diagram, it's essential to understand the basic components involved in the wiring of a Honeywell mercury thermostat. The key terminals you'll encounter are:
- R (Red): Power from the transformer.
- W (White): Heating control.
- Y (Yellow): Cooling control.
- G (Green): Fan control.
- C (Common): Common wire for power (not always present).
Tools and Materials Needed
Before beginning the wiring process, gather the following tools and materials:
- Screwdriver (flathead and Phillips)
- Wire stripper
- Voltage tester
- Electrical tape
- New thermostat (if replacing)
- Wiring diagram (specific to your thermostat model)
Diagram of Honeywell Mercury Thermostat Wiring
While wiring diagrams can vary slightly based on the specific model and age of the thermostat, the general wiring process remains consistent. Below is a simple representation of the wiring connections.
Typical Wiring Diagram
```
+-----------------+
| Thermostat |
| |
| R (Red) |-----------(Power)
| W (White) |-----------(Heating)
| Y (Yellow) |-----------(Cooling)
| G (Green) |-----------(Fan)
| C (Common) |-----------(Common)
+-----------------+
```
Note: The above diagram is a simplified version. Always refer to your specific model’s manual for accurate wiring instructions.
Steps for Wiring Honeywell Mercury Thermostats
1. Turn Off Power: Before beginning any electrical work, turn off the power to your HVAC system at the breaker box to avoid electric shock.
2. Remove the Old Thermostat:
- Carefully detach the thermostat from the wall.
- Take a picture of the wiring connections for reference.
- Note which wires are connected to which terminals.
3. Label Wires: Use masking tape to label each wire according to its terminal (R, W, Y, G, C) for easy reconnection.
4. Disconnect Wires: Disconnect the wires from the old thermostat.
5. Prepare the New Thermostat: If you're replacing the thermostat, remove the new unit from its packaging and attach the mounting plate to the wall.
6. Connect the Wires:
- Connect the R wire to the R terminal.
- Connect the W wire to the W terminal.
- Connect the Y wire to the Y terminal.
- Connect the G wire to the G terminal.
- If applicable, connect the C wire to the C terminal.
7. Secure the Thermostat: After all connections are made, attach the thermostat to the mounting plate securely.
8. Restore Power: Turn the power back on at the breaker.
9. Test the Thermostat: Set the thermostat to a desired temperature and observe if the heating or cooling system activates accordingly.
Common Problems and Troubleshooting
Even with a proper wiring setup, you may encounter issues with your Honeywell mercury thermostat. Here are some common problems and how to troubleshoot them:
Thermostat Not Responding
- Check Power Supply: Ensure that the circuit breaker is on and that there is power to the thermostat.
- Inspect Wiring Connections: Make sure all wires are securely connected to their respective terminals.
Heating or Cooling Not Functioning
- Test the System: Set the thermostat to the appropriate mode (heat or cool) and adjust the temperature setting.
- Inspect Equipment: Check if the furnace or air conditioner is functioning properly. Look for any blown fuses or tripped breakers in the HVAC system.
Erratic Temperature Readings
- Calibrate the Thermostat: Sometimes, the mercury can become stuck. Gently tap the thermostat to see if it frees the mercury.
- Check Location: Ensure that the thermostat is not located near drafts, direct sunlight, or heat sources, which can affect its accuracy.
Safety Considerations
When working with mercury thermostats, safety is paramount due to the toxic nature of mercury. Here are some essential safety tips:
- Avoid Breakage: Handle the thermostat with care to avoid breaking the glass vial containing mercury.
- Disposal: If you need to dispose of an old mercury thermostat, check local regulations for hazardous waste disposal.
- Consider Upgrading: If your thermostat contains mercury, consider upgrading to a mercury-free model for safety and environmental reasons.
Conclusion
Understanding the diagram old Honeywell mercury thermostat wiring is vital for anyone looking to install or replace one of these classic devices. With the right tools, knowledge of the wiring process, and appropriate safety measures, you can successfully wire your Honeywell thermostat and ensure your heating and cooling systems operate efficiently. While these thermostats have served many homes well, it is essential to consider the safety and environmental implications of mercury use and to explore modern alternatives when possible.
Frequently Asked Questions
What are the common wire colors in an old Honeywell mercury thermostat wiring diagram?
Common wire colors include red (R for power), white (W for heat), yellow (Y for cooling), and green (G for fan).
How can I identify the terminals on an old Honeywell mercury thermostat?
The terminals are usually labeled with letters such as R, W, Y, and G. You can refer to the thermostat's manual for specific terminal designations.
Is it safe to work on old Honeywell mercury thermostats without turning off the power?
No, it is essential to turn off the power to the HVAC system before working on any thermostat to avoid electrical shock or damage.
What should I do if the old Honeywell mercury thermostat is not functioning properly?
Check the wiring connections, ensure the mercury bulb is level, and consider replacing the thermostat if it is outdated or malfunctioning.
Can I replace an old Honeywell mercury thermostat with a digital one?
Yes, you can replace it with a digital thermostat, but you may need to rewire or use an adapter depending on the existing wiring configuration.
What tools do I need to rewire an old Honeywell mercury thermostat?
You will need a screwdriver, wire stripper, voltage tester, and possibly a drill if you need to mount a new thermostat.
How do I dispose of an old Honeywell mercury thermostat safely?
Check local regulations for hazardous waste disposal, as mercury is toxic. Many areas have specific drop-off locations for hazardous materials.
