W wire but no furnace or heat pump

[Mbs2]

I have a high velocity ac unit, but installing the new thermostat, I have a w wire. 
so,,,if I have no furnace or heat pump, a) what will happen when the nest calls for heat and b) why is there a w wire? In case I wanted to add a heat pump later?

[Patrick_Caezza]

@Mbs2 

Not all wires get used.

Check the air handler to see if the W wire is connected. 

Just use the wires that are required. For an AC only, it's usually R, C, Y, and G

[Mbs2]

Air handler is hard to get to in the attic. When I run the “heat”, it seems the fan comes on. @Patrick_Caezza

[Patrick_Caezza]

@Mbs2 

Do you have a G wire installed? If yes, it's for the fan, which could be turned on in heat mode.

If you have a cooling-only system, don't install the W wire.