Depending on your climate and what you mean by "windows cracked", you would be much better off with a heat recovery ventilation (HRV) system. They have the expl

Depending on your climate and what you mean by "windows cracked", you would be much better off with a heat recovery ventilation (HRV) system. They have the explicit purpose of pulling in outside air while only losing about 20% of the heat difference between indoor and out. With slightly open windows, you're losing a bunch of heat for very little ventilation.