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What is model on your old thermostat?
If your old thermostat is the TCONT402 or similar the BK is a special feature to control the variable speed blower in your air handler.The NEST will not offer this feature.
Although the Nest will work with some variable speed Trane units by depending on dip switches on control board of air handler, it is not compatible with others that depend on the BK terminal at thermostat.
If you choose to try the Nest, your blower will come on if compatible and be controlled by dip switches.If blower does not come on, you will know the Nest is not compatible.