Which type of plumbing fixture generally requires an air gap installation?

The type of plumbing fixture that generally requires an air gap installation is the dishwasher. An air gap is a crucial component in preventing the backflow of wastewater into the potable water supply. It creates a physical separation between the water supply and the dishwasher, ensuring that any potential contamination is kept from entering the clean water system. Dishwashers are designed to discharge used water, and without an air gap, there is a risk that the dirty water could siphon back into the drinkable water supply.

In contrast, toilets, sinks, and bathtubs are typically connected without the need for an air gap in their standard configurations, as their drainage and water supply systems are designed differently. Toilets have their own specific requirements for venting and installation, while sinks and bathtubs, generally, have overflow protection features built into their design. Thus, dishwashers stand out as the fixture that specifically mandates an air gap to ensure safety and compliance with plumbing codes.