I often get asked, "Does it make any difference if the water supply line is hooked up to cold water line rather than hot?" To understand this fully, first watch this video on how to install a dishwasher.
If you have to pull out the old machine, watch this video on how to remove a dishwasher. 
Are you not sure which water pipe you hooked up the dishwasher to? Or are we just figuring out which water valve we need to connect the dishwasher supply hose to before we start the install. For either one, we need to understand dishwasher soap to grasp the whole picture here.
The new formulations of dishwasher detergent have gotten very good at cleaning dishes, and its now common practice to not rinse the dishes before putting them in the dishwasher. Yes, knock off the large stuff, but just put them in. Rinsing dishes before putting them in the dishwasher is a waste of water and your time.
Make sure the dishes face toward the center of the machine. If you have a large pan that needs cleaning, put it face down so the spray jets can do their best cleaning. Plastic items and glasses should go on the top rack. Consumer Reports has found that the detergent pods are the best detergents right now.
Most dishwashers have a water pre-heat function, sometimes it is a feature you can turn on or off, but leave it turned on. Dishwasher detergent, and your dishwasher, work best with very hot water. And when you first turn on your dishwasher, the water in the hot water supply is probably not very hot. The dishwasher pre-heater brings it up to a higher temperature, allowing the detergent to work at its best. The pre-heater supplements the water coming in from the hot water line, don't think you can hook it up to the cold water line and rely on the pre-heater.
This is why, for dishwashers, its always the hot water supply line.
When doing a dishwasher installation, I have found the existing water supply line hooked up to the wrong pipe. Check under the sink that the dishwasher line is actually hooked up to hot water pipe. If it is not, you will have to switch out the valves under the sink to you can attach the washer line to the hot water pipe. Here's a good book on basic plumbing that I learned a lot from.
DAn
My 10' drain pipe ABS DWV from under the sink to waste pipe is crack, leaked. The only reason I can think of and 100% sure is hot water discharge from dishwasher. I look at ABS DWV pipes that r selling at Homedepot, they are rated at max 140F, that is below average because my hot water tank temp is set at the middle line 12 o'clock position is heated up to 145F. The local plumbing code says hot water tank must set at 165 to 180F. The whole thing just makesd no sense. Anyway, I just lower my hot water tank temp to 135F and will get the pipe replaced. Any comments on this?
Eric
@dan, if you hot water heater is set too low, it can breed bad bacteria, from what I understand. thx!
Scott Sauers
Water temp too low can cause legionaries diesease.
Jim Navotney
The biggest myth on the internet is that you should hook up your dishwasher to the cold line.
Doing this may result in early preheater failure and poorly washed dishes.
Both will result in greater sales of dishwashers so i suspect this myth is driven by the industry.
George S
Practically speaking, it doesn't make that much of a difference, not in my experience. If it makes you feel better, hook it up to the hot water when installing. But don't bother redoing it if you are already hooked to cold water.
My dishwasher at our old house was hooked to COLD water (lived there for 10 years, same dishwasher). My Mom's dishwasher is hooked to cold water. My current house dishwasher is hooked to cold water.
And guess what? The dishes come out clean, and grease free. And the pre-heat hasn't failed on any of them.
Daniel
This is an interesting conclusion for the question of hot or cold water for your Dishwasher however, your assessment leaves me a few concerns.
for starters, at what point do you introduce a distance calculation for the length of pipe from water heater to dishwasher? For example, the modern dishwasher only uses 2-3 gal per cycle. The typical 1 1/2" water pipe holds 1gal per 18' of pipe. If the dishwasher is 50' from the water heater, no hot water will actually reach the machine during filling. As the primed water in line can cool to ambient temperature in as few a 15 minutes, this means you would have to bleed the hot water line before each wash cycle. You could easily end up doubling or tripling your water consumption per load to maintain hot water, to say nothing of the additional time spent babysitting the washer to bleed the line before each fill.
This brings us to our second issue of temperature regulation. Assuming on was willing to go through the additional hassle of priming the hot water multiple times per load, how would you prevent the machine from overheating? Modern dishwashers, and most older models, are designed an internal heating element for the purpose of efficiency. As stated in my last question, relying on your home water heater to heat your dishwasher is wasteful. Heating within the machine itself is simpler and easier. The only problem with this is that it is hard to get an accurate thermostat reading in a semi-aquatic environment. This is why the heating element is most often regulated by timer rather than by temperature. This introduces the potential for overheating the dishwasher which can not only damage you dishes but any plastic/rubber components of the dishwasher itself.
Finally that brings me to the issue of the detergent. In this article, you claim that the higher temperature allows the detergent to work at its best but that is simply not true. Modern dishwasher detergents are designed to work best at a temperature between 120-140°F. If you are filling the machine with hot water, usually around 120°F, The internal temperature can often exceed boiling during the cycle. While those high temperatures are great for disinfecting, they do have the potential to cook the detergent onto the dishes along with any residual food particles. That is the very reason why dishwashers are designed to wash the dishes at a lower temperature, only increasing the heat for the final rince as a sanitization measure.
These are just a few variables for your consideration. In my experience, connecting your dishwasher to the hot tap only leads to additional expenses down the road in the form of higher utilities, reduced lifespan of dishes and more frequent maintenance on the machine itself. Best case scenario, you experience no difference. Everywhere I have lived save 1 address, the dishwasher has been connected to the cold tap and it has made no difference in cleanliness. The 1 time my dishwasher was connected to the hot tap however, my ceramic dishes became too brittle and shattered within 2 years while my plastic dishes needed constant replacement every 3-6 months.
Thank you for your time