3 Central Heating Thermostat Wiring Tips

A thermostat.

Wiring a central heating thermostat is a straightforward task but requires caution, research, and the proper equipment. HVAC wires are generally color coded, but unfortunately not all manufacturers or installers follow the same codes. Connecting wires in the wrong manner can damage your system’s components and easily triple the cost of the repairs you were trying to make. Always follow proper safety procedures when working with electrical wiring. Follow these tips.

1. Work Safely

Air conditioners use 24 volt wire which is relatively safe to work with, although it can be painful if you get shocked. Heating equipment, on the other hand, often uses 110 volt or 220 volt, alternating current wiring. This type of electricity is dangerous and can cause severe burns or death. Even if you don't get hurt, accidentally crossing live wires could damage system components. Spare no precaution in locking out the system before you begin working.

Disconnect your HVAC system from the circuit by flipping the breaker or removing the fuse. Use the emergency shut-off switch on your equipment if one is present. However, do not assume that this is enough to power down the system. Verify that the lines are dead by using a voltmeter, or attempting to run the equipment by turning the thermostat to “on”.

2. Obtain Manufacturer’s Wiring Diagram

The number and colors of wire your system uses depends on the equipment you have installed as well as the whims of the manufacturer and installer. Air conditioners and furnaces will obviously have separate wiring, but if you have a heat pump it will require additional wires as well.

Some wire colors are consistent across most brands and installations. Red usually connects to the hot side of the transformer. Yellow controls compressor activity, switching the air conditioner on or off. White controls heating elements. Green controls the air handler. Orange usually energizes heating equipment, and blue energizes cooling equipment.

Blue and orange might be reversed on Rheem or Ruud units. On the other hand, blue could be the common side of the transformer in York or Trane equipment. If you have Lennox brand equipment, the color coding may be totally inconsistent with this description. Blue, brown, gray, tan, and pink wires are especially tricky.

In no particular order, they could be a heat pump’s optional emergency relay, an outdoor anticipator reset, an indicator service lamp, or second stage heating and cooling controls. Even with the wiring diagram in hand, never assume that the color coding is correct. Avoid overloading the transformer. Identify the 24-volt control signal by using an 1819 test lamp or clamp-on ammeter. You can usually locate the transformer and control wiring at the site of the air handler.

3. Work Carefully

The old thermostat’s terminals are marked with letters (B, Y, W, etc.) to help you identify the corresponding components. Mark the wires with a piece of masking tape and a pen as you remove them from the terminals, especially if the color coding is inconsistent. Wrap the wires around a pen or pencil to prevent them from slipping back into the wall. Wire the new thermostat so that exposed copper touches nothing except the terminals.