Earlier today myself and another inspector were at an inspection and saw that the exhaust flue above a water heater did not have an air gap or a draft hood for the house air to be drawn into the flue with the exhaust. We knew that this was an incorrect way to set up the exhaust, but we couldn’t figure out WHY it’s wrong. Our assumption was that without that air gap, the combustion air rising through the flue would create too high negative pressure in the combustion chamber. This would lead to combustion air being sucked into the combustion chamber much faster than the burners could handle, leading to inefficient combustion (I.e. more energy lost than necessary. think of a candle flickering in the wind) and higher levels of carbon monoxide (because the flames aren’t getting all the oxygen they need). Is this correct?
Your help is greatly appreciated!