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5.7: Dishwashers

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    A dishwasher typically uses the equivalent of 700–850 kWh of electricity annually, or nearly as much energy as a clothes dryer or freezer. About 80 percent of this energy is used, not to run the machine, but to heat the water for washing the dishes.

    • Older dishwashers use about 8–14 gallons of water for a complete wash cycle.
    • Newer dishwashers, built in the past 10 years, have been using 7–10 gallons per cycle.

    The dishwasher is the only device at home that requires a water heater temperature that is about 140°F. The units built recently have supplemental heaters in the dishwashers to bump up the temperature so that the main water heater temperature can be set at 120°F or less. Remember that each 10°F reduction in water heater temperature lowers the water heater energy cost by 3 percent to 5 percent.

    How a Dishwasher Works

    A dishwasher is essentially an insulated water tight box. The dirty dishes are systematically arranged in the dishwasher. As shown in Figure 5.7.1, hot water is sprayed on to the dishes as jets. Repeated jets of water emanating from a spray arm clean the dishes. Some models have two spray arms: one at the bottom of the dish washer (lower spray arm) and one at the top (upper spray arm). The dirty water passes through a filter and re-circulates until the dishes are finished. Fresh water is then spayed during the rinse cycle to remove the soapy water. Then the dishes are dried with either electric heat or simply with air.

    Figure 5.7.1. The inside of a dishwasher

    The following video illustrates how a dishwasher works.

    Features of a Dishwasher

    Dishwashers can be built-in or portable. Built-ins are mounted under a kitchen countertop usually next to a sink. Portables are on wheels with finished tops and sides. Most models can be converted into under-counter mounting. However, because of the additional connection hardware and finished sides, portables usually cost more than similar built-in models.

    Some of the additional features that are offered are:

    • Interior layout—configuration of sliding racks, baskets, and trays. Does the washing arm reduce the amount of loadable space?
    • Water heating—Most homes have water heaters set to 110 degrees. However, to clean well, a dishwasher should use water at 140 degrees. Many budget units now offer a water heating feature.
    • Number of cycles—light cycles, normal, heavy or pans, and rinse and hold to remove food if dishes will sit in the washer a while before the wash cycle is run.
    • Water-saving cycles—If you live in an area where fresh water is scarce, you’ll want to consider this feature.
    • Sound insulation—The sound level will vary from one model to another. Consider how important a quiet wash cycle is before you purchase.
    • Build in food disposers—will grind up food similar to in-sink units, allowing the user to spend less time cleaning dishes before they go into the dishwasher.
    • Controls—Entry-level machines feature knob and dial controls. On mid- to upper-end models, you will find push-button switches hidden behind smooth, one-piece, plastic console covers. Some of the highest priced dishwashers feature electronic touchpad controls with lighted displays for an uncluttered, high-tech look. And the highest-end European models now integrate controls on the top of the door so they can't be seen when the machine is closed.
      • Countdown timer — lets you know how much time is left in a cycle.
      • Clean light — signals that cycle is complete and dishes are clean.
      • Soil sensors — take the guesswork out of cycle selection. Sensors optically analyze dirtiness of water and adjust water level and wash length accordingly.
      • Delay-start — timer that allows starting dishwasher automatically; lets you take advantage of late-night off-peak power rates or run the dishwasher after everyone has taken a shower.
    • Color and Appearance—Does the dishwasher fit in your kitchen? Do you like its appearance?
    • Delay Start Timer—Allows the user to load the washer and have it start a few hours later.

    Energy Efficiency and Test Criteria

    Energy Factor (EF) is the dishwasher energy performance metric and is calculated using

    \[ EF \, (kWh) = \dfrac{1}{M + W} \]

    Equation 5.7.1 may vary based on dishwasher features such as water-heating boosters or truncated cycles. The greater the EF, the more efficient the dishwasher is.

    The EF is the energy performance metric of both the federal standard and the ENERGY STAR qualified dishwasher program. The federal EnergyGuide label on dishwashers shows the annual energy consumption and cost. These figures use the energy factor, average cycles per year, and the average cost of energy to make the energy and cost estimates. The EF may not appear on the EnergyGuide label.

    Test Criteria for ENERGY STAR Qualified Dishwashers

    Dishwasher manufacturers must self-test their equipment according to the new Department of Energy (DOE) test procedure defined in 10 CFR 430, Subpart B, Appendix C. This DOE test procedure was announced on August 29, 2003, and all models had to be tested using the new procedure by February 25, 2004.

    This test procedure establishes a separate test for soil-sensing machines. Included in the final rule was a decision to add standby energy consumption to the annual energy and cost calculation, but not to the energy factor calculation. Also, the average cycles per year has been lowered from 264 cycles per year to 215 cycles per year. Energy Star dishwashers are at least 25 percent more energy efficient than minimum federal government standards.

    Table 5.7.1 lists the standard and the ENERGY STAR approved dishwasher energy factors.

    Table 5.7.1. Energy standards for dishwashers

    Product Type Federal Standard EF ENERGY STAR EF
    Standard (>8 place settings + 6 serving pieces) >0.46 >0.58
    Compact (<8 place settings + 6 serving pieces) >0.62 N/A

    The current ENERGY STAR criteria for dishwashers became effective January 1, 2001. This criteria of at least 25 percent above the federal standard and applies only to models manufactured after January 1, 2001. The previous ENERGY STAR criterion was 13 percent above the federal standard.

    Dishwashers and Environmental Protection

    Buying the correct size appliance for your needs is critical to saving money, energy, and water. In dishwashers, there are compact and standard-capacity units. Compact models use less energy and water per load, but you may actually consume more energy operating them more frequently. The following tips help you to save even more:

    • Avoid rinsing dishes before you load them in the dishwasher or, if you must rinse, use cold water.
    • Run your dishwasher with a full load. Most of the energy used by a dishwasher goes to heat water. Since you can't decrease the amount of water used per cycle, fill your dishwasher to get the most from the energy used to run it.
    • Avoid using the heat-dry, rinse-hold, and pre-rinse features. Instead use your dishwasher's air-dry option. If your dishwasher does not have an air-dry option, prop the door open after the final rinse to dry the dishes.

    5.7: Dishwashers is shared under a not declared license and was authored, remixed, and/or curated by LibreTexts.

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