Modern furnaces take in outside air for combustion. Using up inside, warm air for combustion is doubly inefficient since you expel exhaust air you already sunk

Modern furnaces take in outside air for combustion. Using up inside, warm air for combustion is doubly inefficient since you expel exhaust air you already sunk energy into and you are sucking cold air that has to be heated from a much lower temperature. Modern code for cold climate houses is to be air sealed. A heat exchanger draws in fresh air, raises the temperature with existing heat and then expels cold, stale air outside. Very efficient. When air sealing was new many buildings developed longevity issues like mold since any moisture issues couldn't dry. All those problems have been worked out and there are homes in extreme cold climates that don't need any energy from the grid they are so efficient.