Here’s what you need to know to properly install and configure the Nest Learning Thermostat (3rd gen and 4th gen) to control a system with multiple fan wires.
Which PTAC and fan coil systems work with the Nest Learning Thermostat (3rd gen and 4th gen)
The Nest Learning Thermostat (3rd gen and 4th gen) works with many 24V low voltage PTAC and fan coil systems. You can check compatibility here.
However, other types of PTAC and fan coil systems are incompatible or aren’t fully compatible:
- High voltage systems
- Heat pump systems that have an O/B wire and G2/G3 wires
- Systems that require a separate temperature sensor
- Systems that include auto-changeover via an aquastat or a thermal switch
- Systems with motorized dampers
- Systems that use a reheat coil to support dehumidification
- Systems that use modulating valves
- Systems that vary voltage to control fan speed
Know your current thermostat's voltage.
How to set up the Nest Learning Thermostat to work with systems that have multiple fan speeds
Once you’ve verified that the system you’re installing is a 24V system, follow these steps from start to finish to properly install and configure the thermostat to work with a PTAC or fan coil system.
1. Check the thermostat’s serial number.
Note: Follow this step for the Nest Learning Thermostat (3rd gen) only.
To verify that the thermostat has software 5.6 or later, check the serial number before wiring a PTAC or fan coil system to it.
There are a couple of places to find the thermostat’s serial number.
- On the outside of the box.
- On the back of the thermostat’s display.
The thermostat’s serial number normally looks something like this: 09AA01ACXXXXYYYY
2. Look for digits 9 through 12 (XXXX in the example above).
Digits 9 through 12 represent the week and year that the thermostat was produced. If these digits are 4516 (representing week 45 of 2016) or greater, then the thermostat already has the necessary software update. You can safely skip to step 3 and wire the thermostat.
If you don’t have any thermostats with serial numbers that indicate that the thermostat is pre-installed with software 5.6 or later, call our VIP support number, 855-VIP-NEST for help. We’re happy to help you update the thermostat manually.
3. Wire the thermostat:
After the thermostat’s software has been updated to version 5.6 or greater, you can safely wire the thermostat to the HVAC system.
Supported systems with multiple fan wires can have different labeling standards. Typical wire labels are: G, G1, GL, LO, G2, GM, GH, G3, or HI.
The proper wiring for systems with multiple fan speeds depends on how many fan wires the system has and how the wires are labeled.
| How to wire the thermostat | |
| 2 fan wires | 3 fan wires |
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Connect wires with other labels (Y1, W1, etc.) to their appropriate connectors.
4. Configure the fan wire connectors with Pro Setup:
After the wires have been connected to the thermostat’s base, switch on the power to the HVAC system. Follow the normal out-of-box setup process on the thermostat. During the out-of-box setup, use Pro Setup to configure the fan wire connectors.
If you’re not sure how to use Pro Setup to configure the connectors, see the following article:
| How to configure the thermostat in Pro Setup | |
| 2 fan wires | 3 fan wires |
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5. Test the system:
At the end of the out-of-box setup, you can run a system test to make sure that everything is running fine and that all of the fan speeds work properly. If there are any problems, the thermostat can detect them and let you know. For more information, refer to Troubleshooting fan errors during a System Test.