Exhaust-only ventilation is a one-sided system that typically relies on bathroom and kitchen fans to get rid of stale air but does not provide any dedicated source of fresh air. To make up for the lost air, the house draws outside air through leaks in the floors and walls (this is called “makeup air”).
The problem with exhaust-only systems is that they can create a pressure imbalance in the home. When you exhaust air from the building without a dedicated way to replace it, the home must find a way to equalize this pressure on its own. For example, if you have high negative pressure due to running multiple exhaust fans at the same time, there is a risk that combustion appliances could backdraft, or suck dangerous gases inside through smoke stacks.
Airtight homes should have a balanced, continuous ventilation system, like an HRV. For tight homes with exhaust-only systems, specific vents may need to be installed in the walls (such as Fresh 80s).