Questions about ventillation for our propane heater in our garage?

We bought a 50,000 BTU propane heater for our one car garage. The instructions say we need 2.5 sq. feet of fresh air opening. We need to know if we put a fan in the window blowing out will that be enough to vent the bad fumes out, or do we need a source of fresh air coming in. It seems if we open the window too much or turn the fan around to blow in, we will be defeating the purpose of trying to heat the garage. We put a carbon minoxide detector out there so we will hopefully get some warning if the levels get too high. We are only using the heater when we are in the garage.