How Dishwashers Work


Dishwashers have become a staple in just about every kitchen.
Dishwashers have become a staple in just about every kitchen.

­Basically, a dishwasher is a robot that cleans and rinses dirty dishes. Humans have to load the dishes, add detergent, set the proper washing cycles and turn it on, but the dishwasher accomplishes a whole series of functions by itself. A dishwasher:

  • Adds water
  • Heats the water to the appropriate temperature
  • Automatically opens the detergent dispenser at the right time
  • Shoots the water through jets to get the dishes clean
  • Drains the dirty water
  • Sprays more water on the dishes to rinse them
  • Drains itself again
  • Heats the air to dry the dishes off, if the user has selected that setting

­In addition, dishwashers monitor themselves to make sure everything is running properly. A timer (or a small computer) regulates the length of each cycle. A sensor detects the water and air ­temperature to prevent the dishwasher from overheating or damaging your dishes. Another sensor can tell if the water level gets too high and activates the draining function to keep the dishwasher from overflowing. Some dishwashers even have sensors that can detect the dirtiness of the water coming off the dishes. When the water is clear enough, the dishwasher knows the dishes are clean.

In this article, we'll discuss exactly how a dishwasher gets the job done, how to use one properly and what features to look for when buying a dishwasher.

Inside a Dishwasher

­Although dishwashers are watertight, they don't actually fill with water. Just a small basin at the bottom fills up. There, heating elements heat the water to 130 to 140 degrees Fahrenheit. Then a pump propels the water up to the water jets, where it is forced out and sprayed against the dirty dishes. Think about a garden hose with no nozzle - if you put your thumb over the end of the hose, decreasing the space for the water to come out, it sprays out more forcefully. The dishwasher's jets work on the same principle. The force of the water also makes the arms that hold the spray jets rotate, just like a lawn sprinkler.

When the washing and rinsing is finished, the water drains down to the basin again, where the pump propels the water out of the dishwashe­r. Depending on the type of dishwasher, the drain water might go right into the pipes under your sink, or travel up a hose into your sink itself.

The final step in a wash cycle is optional - the dry cycle. The heating element at the bottom of the dishwasher heats the air inside to help the dishes dry. Some people just let them dry without heat to save energy.

Dishwashers are not very mechanically complex. In the next section, we'll take a look at the main parts of a basic dishwasher.

Dishwasher Basics

A portable dishwasher connected to a faucet and ready to run
A portable dishwasher connected to a faucet and ready to run

The main parts of a dishwasher are:

Control mechanism

The control mechanism is located inside the door behind the control panel. Many units use a simple electro-mechanical system: a timer determines how long each part of the cycle lasts and activates the proper function at the proper time (such as the detergent dispenser, wash spray and draining functions). Units that are more expensive might have a computerized control system. Modern units also have a door latch that must be closed for the unit to run. Some also have child safety locks.

Intake valve

This is where water from the home's water supply enters the dishwasher. The unit's pump doesn't pump the water into the basin – when the intake valve opens, water pressure drives the water into the unit.

Pump

An electric motor powers the pump. During the pump cycle, the pump forces water up into the spray arms. During the drain cycle, the pump directs the water into the drain hose. The motor-pump assembly is mounted beneath the basin, in the center of the dishwasher. There are two main types of pump:

  • Reversible These pumps switch between pumping water to the spray arms and pumping water to the drain by reversing the direction of the motor. Reversible pumps are usually vertically mounted. Reversible pump
  • Direct-drive The motor runs in one direction, so the direction of flow is switched from spray arms to drain by a solenoid that opens and closes the appropriate valves or switches one hose connection to another. Non-reversible pumps are usually horizontally mounted. Direct-drive pump

Dishwashers can be installed in either a portable or a permanent configuration. Portable units have finished sides and top that can be used as a countertop. When not in use, the machine sits in place next to the wall. When it's time to run a cycle, the unit can be rolled on casters over to the sink, where it connects to the faucet and plugs into a nearby outlet. In a permanent installation, the dishwasher goes underneath the existing countertop and bolts into place. Hoses underneath the kitchen sink connect directly to the hot water line and the drain line, and the unit usually plugs in under the sink as well. Both types of installation require a 120-volt grounded line.

Next, we'll look at how to use a dishwasher.

Using a Dishwasher

If your dishwasher isn't getting your dishes clean, you may not be loading it properly.
If your dishwasher isn't getting your dishes clean, you may not be loading it properly.
Photo courtesy DHD Multimedia

Even though the dishwasher does most of the work, humans play a part too. Here are some guidelines that can help your dishwasher operate safely, effectively and efficiently.

  • Don't use regular dish soap. The suds will overflow the dishwasher.
  • Don't overload the dishwasher. You need to leave room for the water jets to spray onto the dishes.
  • Face the dirtiest part of the dishes toward the spray jets, which usually come from the center.
  • Don't mix steel and silver items. Putting two different types of metal in contact in a humid environment is a perfect recipe for corrosion.
  • Try to keep bowls, spoons and other dishes with identical shapes separated. Otherwise, they will tend to nest together, and the water cannot reach every part of the dish.
  • Don't put wood, cast iron, fine china, crystal or hand-painted dishes into the dishwasher. Wash these items by hand.
  • Use the dishwasher at a time of day when water pressure is high, such as late at night. The dishwasher will clean better if you're not using a lot of water for something else, like washing clothes.
  • If your home has hard water, use slightly more detergent.
  • Use a rinse aid to avoid spots and help your dishes dry more quickly.
  • Don't put plastics on the bottom rack, especially if you use the hot drying cycle. The heating element could melt some plastics.
  • Don't pre-clean dishes. Dishwashers actually depend on the bits of food that cling to dirty dishes to maintain an appropriate ph level inside the dishwasher. Large pieces should be scraped into the garbage, though.

Detergent

Detergent is an important consideration when running a dishwasher. Detergents counteract mineral deposits, or hardness, in the water. They contain solvents that help dissolve food, have abrasives that scour away stuck-on gunk and help food slide off dishes more easily. You can't use just any detergent in a dishwasher; only detergents specially formulated for dishwashing machines will work. Other detergents could damage dishes or generate so many suds that the dishwasher would overflow. Which detergent to choose – tablet, powder, or gel – is really based on personal preference. One type hasn't been shown to clean better than another type.

The problem most people encounter with dishwashers is a simple inability to get the dishes clean. There might be stuck-on food or residue from the detergent. A water pressure problem may be the culprit. You may need to replace the water intake valve. Another common problem is mineral build-up. If your house has hard water, the mineral build-up can clog the water jets. Clear each jet with a wire or pin, and run an empty load with some vinegar in the detergent dispenser about once a month.

Sometimes, the dishwasher has problems draining properly. There could be a clog in the drain hose, or a problem with the pump. It's also possible that the dishwater is getting too sudsy, and sensors in the washer aren't detecting the soap foam as water. This causes it to shut down the drain cycle too early. Just use less detergent.

Check out the next section for our dishwasher buyer's guide.

Dishwasher Buyer's Guide

An in-sink dishwasher
An in-sink dishwasher
Photo courtesy Consumer Guide Products
A standard 24-inch dishwasher
Photo courtesy of Consumer Guide Products

Modern dishwashers all function in the same basic way. Even cheaper models do a good job of cleaning dishes. That makes buying one a matter of finding features you'll use and avoiding ones you don't need. Durability, size and convenience are the primary factors that set one model apart from another.

Size

Dishwashers come in a wide variety of sizes. The smallest are in-sink dishwashers. These units fit into one-half of a double kitchen sink, use less water, and can wash a complete load in about 20 minutes. When not in use, a cover on the unit lets it serve as a countertop.

The standard size for a dishwasher is 24 inches wide. However, 18-inch units are available, sometimes known as "apartment-sized" dishwashers. Obviously, the wider the dishwasher, the more dishes it can hold. If you have a large family, a 30-inch model might be the right size. Any larger than that, and you're probably looking at a commercial dishwasher.

The Dish Drawer is a small dishwasher the size of a large kitchen drawer. It also uses less water and energy than a full-size dishwasher uses, and is suited for small kitchens that don't have enough room for a full-size unit. They also come in double drawer models that function independently.

Basins and Racks

Lower-end models have plastic basins, while some mid- and all high-priced units have stainless steel basins. In cheaper models, bits of food settle into a filter that must be manually cleaned on a regular basis. Models that are more expensive have self-cleaning filters, and some include small grinders that grind up large chunks so they drain with the dirty water. Dish racks come in many configurations. The more you pay for the dishwasher, the more flexibility and adjustability you'll get, with collapsible racks, folding tines, extra shelves, and removable racks for loading outside the machine. If you have large or oddly shaped dishes that you'll be washing regularly, bring them along to the appliance store to make sure they'll fit the racks.

Extra Features

Controls and cycle types are among the extra features that can drive up the cost of a dishwasher. Cheaper units have mechanical controls, using a dial and a timer to regulate the different cycles. Push-button controls with digital readouts and computerized timers are more expensive, but they don't clean dishes any better. Some dishwashers offer a cornucopia of cycle options, including high pressure super heavy-duty, crystal/china and sanitizing modes. However, three basic cycles will cover most of your dishwashing needs: Light, Normal and Pots & Pans. A Rinse and Hold cycle lets you clear extra food off dishes that will be sitting in the washer for a few days before they are cleaned. Hot Dry cycles dry dishes more quickly, but use extra energy.

This high-end dishwasher has its controls hidden inside the top edge of the door.
Photo courtesy of Consumer Guide Products

Expensive dishwashers can be stylish, with fronts designed to look like kitchen cabinets. Noise suppression might be worth the extra cost if your kitchen is close to your living room. Dishwashers that are more expensive have heavy insulation against noise. Note that dishwashers with grinders for large food chunks are louder than those without them.

Energy Use and Cost

One final consideration is water and energy use. The U.S. government's Energy Star guidelines have a chart that lets you compare annual energy usage of many different dishwasher models. If you're keeping an eye on your utility bills and are concerned about the environment, this chart can help you find the right model. How much can you expect to pay for a modern dishwasher? A reliable, functional model without any extra features costs around $200. The $300 to $700 price range contains models with extra cycles, adjustable racks, larger tubs and better materials. Prices go all the way to about $2,000 for European dishwashers with every possible feature, including built-in water softeners and water cleanliness sensors.

Government regulations have greatly improved the efficiency of modern dishwashers, so unless your dishwasher is a pre-1990 model, it's probably efficient. Using a dishwasher saves energy and water, and it certainly saves time. This depends on individual dish washing habits, as some people take more time and water than others. The Association of Home Appliance Manufacturers estimates that a dishwasher saves about six hours per week, and uses just over eight gallons of water for one load of dishes. It takes roughly 16 gallons to wash them by hand [ref].

For lots more information on dishwashers, check out the links on the next page.

Related HowStuffWorks Articles

More Great Links

Sources

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  • Brown, Scott M. "Appliance Tip of the Day: Dishwasher Drainology." FixItNow.com, February 2003. http://fixitnow.com/2003/02/appliance-tip-of-day-dishwasher.htm
  • "Buying advice: Dishwashers." ConsumerReports.org. http://www.consumerreports.org/cro/appliances/ dishwashers/reports/how-to-choose.htm
  • "Dishwashers." EnergyStar.gov. http://www.energystar.gov/index.cfm?c= dishwash.pr_dishwashers
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