Running the two appliances that use the most water at the same time may seem risky but actually may hold some advantages. The water use of the two appliances falls within the capabilities of most water delivery systems, and combining the two chores may improve the efficiency of the hot water system.
An older dishwasher uses up to 16 gallons of water per load with the modern high efficiency dishwasher using about 7.5 gallons. The older washing machine uses about 50 gallons of water while the more modern units use less than 20 gallons. Even running an older dishwasher and washing machine at the same time uses less than 70 gallons of water during a one-hour cycle, which compares to running a sink at 1 1/2 gallons per minute for about 50 minutes and falls within the normal capacity of a home water system.
Hot Water Use
The dishwasher uses all hot water while the washing machine may use a mix of hot and cold water. Home water heaters often maintain up to 50 gallons of hot water in reserve. The heater also begins heating additional water as soon as water is drawn from the tank. Tankless water heaters often have capacities of as high as 4 gallons per minute or 240 gallons per hour.
Water Heater Efficiency
Running the dishwasher and washing machine at the same time may improve the efficiency of the water heater. The logic is that drawing down the heater once rather than twice and running the two appliances at separate times allows the water heater to cycle just once to heat the water in the tank, which may lower the cost of operations for the water heater. This is not an issue with a tankless or on-demand water heater.
Running the dishwasher and the washing machine only with full loads reduces the number of loads the household will wash during any given period. This reduces the utility cost for heating the water and the cost of the water itself.