To get the dishes clean? I was thinking about this. If you soak a dish in water long enough, the food washes away easier. That's my take on it.

Dishwashers nowadays have smaller pumps and lower spray volume than the water-guzzlers of the past. Longer cycle times, coupled with enzyme detergents to dissolve food soils, are thusly required to maintain cleaning performance. Smaller pumps and less aggressive spray is also a factor in promoting quiet operation. Even with longer operating times, less energy is used than older units.

Thanks dadoes. That's good to know.

Dishwashers need 5 elements in order to clean
Time
Temperature
Water
Chemical (detergent)
Mechanical action

If you lessen one of the 5 elements you need to increase one or more of the others.
If you lessen Mechanical action you may need to inscrease the time or the temperature or both.
If you use less detergent you may need to increase the other elements

It takes a certain amount of time for a dishwasher to heat up the incoming water, especially for a "sanitizing" wash.

-Roger

I don't use the heated dry on our KA DW - us Left Coast greenies are conditioned that way, LOL - and the cycle on the KUDS30IVSS for a normal wash is 1 hr 15 min, which is only 15 min longer than my old (and terrible) 1989 Kenmore/GE. I also notice that even without heated dry, the new DW uses the residual heat to dry the dishes quite well anyway, which is nice.

my asko take an average of 1hr 27 min to do a full load. dishes come out sparkling clean. Leave it over night and they are pretty dry. i dont waste electricity drying plastics...

mom270, good question! I've wondered the same thing.
dadoes & jakvis, good answers :)
Seems backwards tho. Seems like with newer technology it would take 1/2 as long instead of 2x as long :o
Monica

You have to think of it this way.
Let's say the old D/W motor used 150 watts per hour to run and to make the math easy lets say the old D/W had a 1 hour cycle. Total power used = 150 watts
Now say the new D/W has a 50 watt motor. The 50 watt motor doesn't spray as hard so the manufacturer makes the cycle 2 hours. Total power used = 100 watts This means the old D/W used half again more power than the new D/W.
The old D/W had more mechanical action (harder spray) and the new D/W had less mechanical action (softer spray) so it needed more time to do the same job but the motor was so much more efficient it used less energy.
There's more to it of course with flow through heaters verses ring heaters, less water vs more water, etc. etc. but the above is the simple explanation.

When it comes to math, my brain appreciates "simple" :)

I am curious about the long wash time too. But I must say that any dishwasher I've had in the past didn't really clean the dishes... it just made them hot.

Our newer dishwasher actually cleans dirty dishes. Older dishwashers would only clean dishes that were already cleaned a little bit by hand first.

There is a thread about commercial dishwashers. They only take 2-4 minutes to run. But they don't clean, they only sterilize.

So I guess watching TV or reading for 2 hours is better than hand washing dishes for 20 minutes.

~Jay

One reason is that older dishwashers were designed back when hot water heater tanks were routinely kept at 140°F - pre-1990s dishwashers relied on hot incoming water and rarely heated the wash water themselves (several DWs had "sani rinse" options that heated the final *rinse* only, ostensibly to sanitize the dishes). More recently, recommended hot water tank temperature was dropped to 120°F, primarily to prevent scalding injuries at faucets and showers, but also to save energy. 120°F is hot enough for showers and usually for washing clothes, but insufficient for washing dishes in a dishwasher. So newer machines internally heat the water during the wash cycle, and sometimes during the prewash, and often during the final rinse to facilitate drying - all increasing cycle times.

As with clothes washers, this is another instance where U.S. 120V/15A electric circuits have become problematic - the feeble power supply stifles the heating elements and results in *slow* water heating - taking about 15 minutes to heat the water 20 degrees, as well as longer drying times. European-market dishwashers running on 240V power heat the water so quickly that many of them draw water from the cold water line rather than the hot, and still wash and dry the dishes faster than 120V machines sold in North America.

It takes a certain amount of time for a dishwasher to heat up the incoming water, especially for a "sanitizing" wash.

-Roger

I can't accept any of these responses as they are simply excuses to me. The thing that really disappoints me is that I don't have a choice to run a short cycle. Frankly, I rinse my dishes before they go in the dishwasher so a 10-20-minute cycle is what I want. A five minute wash and a five minute rinse would work best for me. I worked at a hotel year's ago and the Concierge suite had a dishwasher that had a cycle of a minute or two.

I can't accept the responses as well. I have a dishwasher with a quick wash of 28 minutes, and it works just fine. The commercial washers use a much higher water temperature so it is quicker, but there is absolutely no need for 1 or 2 hour wash. Wear out the dishwasher fast? seems like it might be the only reason, no wonder appliances dont last very long any more.

In 2015, people are now complaining about 4 hour cycles!! They must have degraded the motors and the detergents again. We are looking for a new dishwasher since our old one is severely clogged with deposits. We are using the SAME water source we have been using for 35 years. The big name detergent we been using has been stripped down until they it is terrible. Our glasses get coated with white mineral stuff that we have to use vinegar to remove it. The gray plastic racks are now white. This is new in the last few years.
Also, the machine has started leaving food particles on everything. The glasses come out dirtier than they went in. I did a few tests and found out that two of the spray arms weren't spinning at all and the other one was barely moving. I took them off and all of them were loaded with chunks of white deposits that had clogged the spray nozzles. Vinegar barely did a thing. I had to grind them up by putting objects into the holes, but I couldn't get most of them out.
Also, the screen on the bottom of the machine was almost totally clogged with a thin crust of white deposits. I scrubbed it over and over with vinegar and it finally started to break up. Bad move. Those little flakes ended up in the spray nozzles and clogged them worse. I can't get one of the nozzles to spin at all. Web searches have told me I have to disassemble the machine to get to the sump, which is probably also clogged.
The top rack's cheap plastic wheels are so clogged with deposits that they barely turn anymore, causing the flimsy sliders to fall off the non-turning cheap plastic wheels they ride on. I have had to take these apart several times a week lately.
Has anyone else had these experiences?
We are throwing it out once we figure which of the 4-hour cycle, cheaply made $800 machines to buy.

You didn't mention how old your dw is. No, I'm not having any of those issues. My dw is 7 years old (I think). If you're looking for suggestions, I recommend my KitchenAid KUDE60. It's the only appliance I talk to <g> I tell it EVERY day that it's not just my favorite dishwasher (in 35 years of marriage) but my favorite appliance in 35 years of marriage! Three racks. I've removed the silverware basket (opening up a huge space) and use the cutlery rack for my silverware. You know, I've never timed it but sometimes (not always) I swear it does seem to take 4 hours but most of the time it doesn't. I don't know why it's different. I have a one hour wash that, so far, seems to clean as well as the normal cycle but I've read it uses more water and energy...go figure! You can do a search on here for this model and see that nearly everyone is very happy with theirs too.