Kitchen & Cooking

What actually happens inside your dishwasher?

How heat, chemicals, and blasts of water combine to get out tough food stains

An image of a dishwasher Credit: Getty Images / erserg

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Ever wonder what goes on in a dishwasher? No? Well, until I started writing about home appliances, I didn't either. When I was a kid, I always assumed a dishwasher filled to the top with water, like a swimming pool for dirty dishes. According to the internet, this is a fairly common misconception!

After entering the strange and wonderful world of product testing, I realized that a dishwasher is more like a car wash than a swimming pool—and that understanding how dishwashers work is the key to getting clean dishes.

So whether you’re trying to troubleshoot a problem or just curious, let's take a peek at what happens behind that closed dishwasher door.

Inside the shiny box

Credit: Getty Images / deymos

Photo taken with a dish-eye lens.

One of the key parts of clean, sanitized dishes is hot water. Water temperatures in a dishwasher get as high as 130°F-140°F. The heating doesn’t happen instantaneously, however. The dishwasher pulls in water from your water supply via its external hookup. When a cycle starts, water is pumped into a pool at the bottom of the dishwasher. The heating element below that pool turns on, heating the water.

However, at the same time, the water in that pool is mixed with the detergent and sent into the spray arms found throughout the dishwasher, typically at the bottom and top of the dishwasher and occasionally beneath the top rack. The water surges through the spray arms, and hits the dirty dishes, hopefully taking some nasty food stains with it.

The water eventually drips back into the pool below, where it is filtered, reheated, and sent back out into the spray arm again. The same water is being constantly used and reused, heated and reheated, sprayed and collected. See? It's much more like a carwash than a swimming pool (at least my swimming pool).

Credit: Getty Images / Tinieder

This is what your dirty dishes see.

Once the water hits the desired temperature, the heater turns off, but the pumps continue pushing water through the spray arms. At the end of that portion of the wash cycle, all of the water is drained (that’s the gurgling sound you sometimes hear your dishwasher make), new, unused water takes its place, and the cycle begins anew.

So now that you know how a dishwasher works, we can dig into what actually happens during a dishwasher cycle.

The three stages of cleaning

There are three main stages to a dishwasher cleaning cycle:

1. Pre-wash/rinse: The first initial burst of warm water through the spray arm that gets all of the dishes wet, but isn’t really aiming to do much cleaning.

Credit: / Julia MacDougall

Detergent can be added to the pre-wash part of a dishwasher cycle.

Some dishwashers have a pre-wash detergent compartment to add some additional cleaning power to this cycle, for when your dishes are really, really dirty. The pre-wash is typically only a few minutes long.

2. Main wash: Just what it sounds like: the main part of the wash cycle. The water is heated, sprayed, collected, filtered, heated, sprayed, etc. until the heating unit is turned off, while the spraying continues. At the end of the main wash, all of the water is drained. Depending on the cycle, the main wash can take anywhere from 20-60 minutes, and may repeat multiple times throughout the cycle duration.

3. Final wash and rinse: This part pulls in new, clean water and begins the heating/spraying/filtering/heating cycle anew. Just like the main wash, the heater is eventually shut off while the spraying continues. The final wash/rinse may or may not use detergent. Like the main wash, the final wash/rinse can take 20-60 minutes and may repeat multiple times throughout the cycle duration.

A graph of the temperature inside the dishwasher over time clearly illustrates the three main parts of a cycle:


There's one caveat worth mentioning here. Some dishwashers do not have built-in water heaters—they pull water directly from the source, so whatever temperature water happens to be circulating in the kitchen is the water that gets used to clean the dishes. Washing with cold water may be the culprit behind unclean dishes or an undissolved detergent pods. If this is the case, try running hot water at the kitchen sink (to purge the cold water from the system), then turn off the faucet once the dishwasher cycle starts. If the hot water at the kitchen sink stays on, it will pull hot water away from the dishwasher when it needs it most.

Depending on the manufacturer, additional rinses, heated drying, or other features may also happen, but pre-wash/rinse, main wash, and final wash/rinse are the core components of your dishwasher's cleaning cycle.

At cycle's end

Instead of quick dip in the dirty dish pool, it turns out your lasagna tray was repeatedly attacked by water-gunning spray arms loaded with hot detergent water. Significantly less relaxing, maybe, but much more effective at getting your dishes sparkling clean. Huzzah!

(If your dishes did not come out sparkling clean, we have some suggestions on how to deal with that. One other thing to keep in mind: dish placement is very important, since large dishes loaded in the wrong place could block a spray jet's access to other dishes.)

To wrap up, dishwashers are technological wonders that are extremely convenient and time saving. With judicious usage of spray arms and heated water, they can clean your dishes much more efficiently than you can, without any mess or effort on your part.

So the next time you take your car into a car wash, realize that your dirty dishes are getting a similar treatment in the dishwasher— but minus the impact of the drying flappy curtain.