"You could wake up dead" is a Yogi Berra-like expression that a teacher of mine uses when he talks about potential Carbon Monoxide issues from defective exhaust installations. Incorrectly installed flues from furnaces, boilers, hot water heaters and gas dryers are some of the most common issues I find on a Home Inspection.
This photo is an example of an improper exhaust vent installation which could lead to excessive carbon monoxide in the home. The issue was probably created when the water heater was replaced with a unit that was taller than the one it replaced.
Exhaust is a warm gas that wants to flow up. It must get out and up before it cools or moisture will condense on the surface leading to corrosion. To insure that, there should be an upward slope and a run to the chimney which is not too long. The rules for exhaust may vary slightly according to the appliance, and some new, high-efficiency appliances have very liberal compliance rules, but for the water heater pictured a 1/4" per foot of rise is required and is clearly not observed. I often see evidence of the improper venting on top of the water heater: corrosion, discoloration. And, at the burner down below there might be additional evidence like backdrafting. It's really easy to see what causes these issues. Sometimes, the height of a replacement water heater is taller than the old unit and this makes it impossible to use the old chimney termination point for the flue. The correct way to address this would be to make a new and higher termination point in that brick chimney. This would add expense to the water heater replacement and that's why it's so often skipped. The result? Insufficient draft and a strong likelihood of Carbon Monoxide in the basement! In most jurisdictions a permit should be pulled to replace a water heater and often homeowners and plumbers don't pull a permit. With no inspection, shortcuts like the one pictured can result in a dangerous situation.
There are crucial things to consider to insure your family is safe when multiple appliances are tied into the chimney. As a certified Home Inspector these are some of the first and most important things I address when evaluating a home.