How could it not take much energy to heat up cold air? That plus blowing air is the entirety of what goes on in a forced air ventilation system. The efficiency

How could it not take much energy to heat up cold air? That plus blowing air is the entirety of what goes on in a forced air ventilation system. The efficiency rating of a gas furnace assumes the incoming air temperature is close to the desired temperature of the house- that's why it is negligible in the artificial efficiency ratings. If the incoming air is below freezing the efficiency must be different. I wish I could find a study that properly quantified this.