The professional carpet cleaning industry is invariably struggling with the issue of whether to clean carpets with cold or hot water. 
While there are pluses and minuses supporting both methods, in the long run, it appears that hot water wins out if the goal is faster and more effective least according to the Department of Physics at the University of Illinois Urbana-Champaign campus.
In a question and answer format, volunteers from the Department of Physics answered the following questions:
Question: Aren't cold and hot water actually similar?
Answer: Although cold and warm water are pretty similar, there are some ways that temperature makes a big difference. For instance, many materials can dissolve better in hot water than in cold water. Also, while hot and cold water are made of the same type of molecules - one oxygen and two hydrogen atoms - the difference between them is the speed of the molecules "jiggling around."
Q. Why do materials dissolve better in hot water?
A. The basic reason is that it often takes energy to pull a molecule (such as soil) away from other similar molecules. The hotter things are, the more often you get that [needed] energy. Also, chemical processes almost always happen more quickly at high temperature. Often for a molecule to go from one place to another it has to go through a state of higher energy, like going over a little bump. Again, the hotter things are the more often that happens.
Q. Are cleaning chemicals more effective in hot water?
A. Yes. Chemical processes almost always happen more quickly at high temperature.
"This tells us that there is considerable science behind the benefits of hot water carpet extraction," say Bob Abrams, carpet care product manager for Nilfisk-Advance commercial business, makers of U.S. Products brand professional carpet extractors. "This allows carpet cleaning technicians considering this issue to make fact-based decisions."