Wire letter guide
L, S and T Thermostat Terminals: Can You Skip Them?
L (or L/A) lights a "check heat pump" indicator on some older thermostats. S/S1/S2 connect an outdoor or remote sensor. T shows up on some older thermostats for an outdoor sensor or anticipator. None of these normally turn equipment on, and Google lists S and L as unnecessary for Nest.
Cap each unused wire individually (electrical tape or a small wire nut) so bare ends can’t touch. The Honeywell T9 can use L/A if you want the service indicator.
Before you cap these wires
- Make sure they really are L, S or T and not a mislabeled control wire: note where each one lands at the furnace or air handler.
- If an outdoor sensor is on S, check whether your new thermostat can use its own weather data instead. Most smart thermostats can.
- Cap each wire separately and fold them back into the wall so bare copper can’t touch the other terminals.
What you lose by capping them
- L: the “check heat pump” or service light. Smart thermostats show their own alerts in the app, but they won’t see the outdoor unit’s fault signal. The Honeywell T9 can use it on its L/A terminal.
- S, S1, S2: a wired outdoor or remote sensor. Smart thermostats normally use online weather for outdoor temperature, and wireless room sensors for other rooms, so this is rarely missed.
- T: on some older thermostats, a connection for an outdoor sensor or a heat anticipator. Smart thermostats don’t need either.
Leave the other end of these wires connected at the equipment as it was. Capping them at the wall is enough, and it keeps the option to reconnect them later.