Water supply and drain requirements for appliances: pressure, temperature, hoses, air gaps and standpipes

Inlet valves are designed for a pressure window. Manufacturer sheets commonly list 20-120 psi for dishwashers and 20-100 psi for many washers. Below the window, fills are slow and timing codes appear; above it, valves and hoses are stressed.

Plumbing codes cap static house pressure at 80 psi; above that, a pressure-reducing valve is required at the service. A closed system with a PRV also needs thermal-expansion control at the water heater.

Dishwashers want hot water at about 120°F at the unit. DOE also recommends 120°F as the water heater setting for most homes, so a long pipe run that delivers cold water first is a common cause of poor cleaning.

Drains must not siphon or back up. Washer drain hoses go into a standpipe (often 2 in minimum diameter) or laundry tub; the hose end is secured but not sealed into the pipe. Dishwasher drains use a high loop under the counter (accepted by the IRC) or an air gap fitting (required under the UPC and some local codes).

Standpipe height matters both ways: too low and the washer can siphon or overflow; too high and the pump cannot lift the water. The IRC range is measured above the trap weir; manufacturer sheets give their own minimum and maximum heights above the washer base.