The thermostat has nothing to do with your fan.
In short: it opens / closes (depending on temp) to let coolant into the engine.
You DO have one installed, it just may not be working properly. IE, if it is stuck open then collant will flow constantly, not allowing the engine to get to proper running temp, thus causig it to run in open loop mode, which in turn consumes a lot of your gasolineeeessss.
:albino: