Why some fixtures must never be directly piped to the drain system — and why an ice machine's drain line is deliberately left dangling in open air.
Almost every fixture in a building — sinks, toilets, showers, tubs — is piped straight into the sanitary drainage (DWV) system. That's a direct waste connection, and it's the standard, default approach for ordinary plumbing. But a specific category of equipment — commercial ice machines, food-prep sinks, dishwashers, certain medical and laboratory equipment — is required by code to connect a completely different way: through open air, into a separate receptor. Understanding why comes down to a single question: what happens to that connection if the drainage system downstream ever backs up?