Certain wastes like those from food-handling, dishwashing (commercial), and sterile- materials machines should be discharged through an indirect waste pipe. This pipe is not directly connected with the building drainage pipes. Instead, it discharges waste liquids into a plumbing fixture or receptacle from where they flow directly to the building sanitary drainage system. Indirect-waste piping is generally required for the discharge from rinse sinks and such appliances as laundry washers, steam tables, refrigerators, egg boilers, iceboxes, coffee urns, dishwashers, stills, and sterilizers.
It is also required for units that must be fitted with drip or drainage connections but are not ordinarily regarded as plumbing fixtures.
An air gap is generally required between the indirect-waste piping and the regular drainage system. The gap should be at least twice the effective diameter of the drain it services, but not less than 1 in. A common way of providing the required air gap is to lead the indirect-waste line to a floor drain, slop sink, or similar fixture that is open to the air and is vented or trapped in accordance with the governing code. To provide the necessary air gap, the indirect-waste pipe is terminated above the flood level of the fixture.
For a device that discharges only clear water, such as water from engine-cooling jackets, air-handling-unit coil condensate, sprinkler systems, or overflows, an indirect- waste system must be used. Clear water wastes from roof-mounted airconditioning equipment can usually be discharged to roof drains or rainwater gutters.
Although some jurisdictions require clear water wastes to be discharged to sanitary sewers, others allow or require clear water wastes to be discharged to the storm sewer system or dry wells.
Hot water above 140F and steam pipes usually must be arranged for indirect connection into the building drainage system or into an approved interceptor.
To prevent corrosion of plumbing piping and fittings, any chemicals, acids, or corrosive liquids are generally required to be automatically diluted or neutralized before being discharged into the plumbing piping. Sufficient fresh water for satisfactory dilution, or a neutralizing agent, should be available at all times. A similar requirement is contained in most codes for liquids that might form or give off toxic or noxious fumes.