I doubt "leaving the door open" is enough, unless the house is REALLY well sealed and most are not. IN FACT near the time I got out of HVAC in the mid 90's air-air ventilation heat exchangers were popular. These (in one form) are essentially forced air low pressure systems that bring in outdoor air, exhaust indoor air, for ventilation, while exchanging heat on the way in/ out. Now that intake MUST BE SOMWHERE, hopefully "not near" the garage!!!
ANOTHER ANNOYING STORY from the old days.
In my HVAC days we did lots of upgrades and conversions. Nice older guy, nice little simple house with a basement, old oil furnace, and we are putting in high efficiency nat. gas. I'm down there plumbing up the AC and controls, and here sits an old generator, with a FLEXIBLE EXHAUST PIPE (just like automotive "flex" only smaller). This was not connected, just sitting there. AND I WARNED HIM "there is no way" I'd use that down here in the basement.
About 1-2? years later I was back for routine maintenance of the system. He told me, "I did not listen to what you said, and installed that generator. (Showed me a plugged hole in the basement wall for exhaust) Said he WAS smart enough to get a CO detector and it "went nuts". Said it took him all day with fans running to get the basement cleared out!!!
