You can’t really overemphasize the importance of making sure a boiler is properly vented. If it isn’t, the equipment can become damaged, and the people living in the home can become sick or even die.
The Department of Energy (DOE) has put the squeeze on manufacturers to design boilers with higher and higher efficiencies, which hasn’t necessarily been good from a venting aspect. Hot air rises, and the hotter the air, the better it rises. Many years ago, boilers weren’t very efficient and a lot of the heat went into the chimney, which meant the chimney heated up nicely and proper venting occurred.