I've been told that the self rentals don't compare to the truck mounted units used by the pros. I also was told that laundry detergent works better (or just as good) than the carpet shampoo they sell with the rental.
You are correct in saying that self rentals don't compare to truck mount units. Just in the same way that car engines are classified according to the number of cylinders and the horsepower they can produce, vaccuum motors are classified according to the number of "stages" they have and the height of a column of water they can "suck up" at sea level.
Your typical Hoover canister vaccuum cleaner will have a single stage vaccuum motor that's lucky to be able to life a column of water 30 inches high.
Your typical wet/dry vaccuum cleaner, car wash vaccuum cleaner and rental carpet shampoo'er will have a two stage vaccuum motor that will draw a column of water about 60 inches high at sea level.
Your typical entry level professional carpet cleaning equipment will have a three stage vaccuum motor that will lift a column of water about 90 inches high at sea level.
My carpet extractor has two 3 stage vaccuum motors piped in parallel and will lift a column of water 113 inches high at sea level (according to it's specs).
Your typical truck mount unit will have 6 two or three stage vaccuum motors piped in parallel and all driven by a gasoline engine, and will suck a golf ball through a garden hose.
The quest for suction comes from the blindingly obvious fact that the more soiled soap solution you can get out of the carpet, the cleaner the carpet will be when it dries.
Both Rug Doctor and Easy Off recommend using 1 to 2 fluid ounces of their carpet soap per gallon of soap solution. That means, 5 to 10 ounces for a machine with a 5 gallon solution tank.
My TMI extractor has a 5 gallon solution tank, but the Chem-Spec Formula 77 that I use calls for only 1 to 2 fluid ounces of soap for every 5 gallons of cleaning solution. That means, that Chem-Spec calls for about 1/5 of the amount of soap that RugDoctor or Easy-Off recommend.
And, because professional extractors will have more powerful vaccuum motors than rental machines, you can bet that the professional equipment will remove MORE of the soiled soap solution from the carpet than a rental machine.
So, if you follow RugDoctor or Easy-Off's advice, you'll spray way more soap on your carpet than a pro would, and you'll suck up much less of that soap solution (cuz a rental machine will only have ONE two stage vaccuum motor whereas the pro's equipment will have several of them).
And, as that carpet dries, the residual soap that remains behind forms a sticky film on the surface of the carpet fibers. Dirt sticks to that soapy film, making normal vaccuum cleaning ineffective at removing dirt and dust from the carpet.
In fact, about the only way of removing the dirt sticking to the soapy film on a carpet is to shampoo the carpet again. And, of course, the homeowner renting the carpet shampoo'er is going to be impressed with how much dirt the carpet shampoo'er has removed from the carpet (judging by the colour of the water in the recovery tank), that he's likely going to start using a rented carpet shampoo'er on a regular basis.
It doesn't cross that homeowner's mind that the real reason that shampoo'ing the carpet removed so much dirt was because the carpet was sticky with residual soap to begin with. If the homeowner had not followed the rental machine's instructions, he would have had far less of a soap film on his carpet to begin with, and normal vaccuuming would have removed most of the dirt that is now in the carpet shampoo'er recovery tank.
So, if you feel compelled to rent a carpet shampoo'er, then:
A.) please don't follow RugDoctor or Easy Off's recommendations to use 1 to 2 gallons of their carpet soap per gallon. Instead, buy your gallon of carpet soap from any place listed under "Janitorial Equipment & Supplies" in your yellow pages phone directory, and follow the dilution instructions on that gallon.
B.) If you do follow RugDoctor or Easy Off's instructions to use 1 to 2 fluid ounces of soap per gallon of cleaning solution, then make a final pass over the carpet with just clean water in the solution tank to remove most of the residual soap from the carpet.
In fact, it'd be best to do that final cleaning with a cup of vinegar in the 5 gallons of solution tank water. The reason why is that carpet cleaning soaps are naturally alkaline in nature, whereas most foods are slightly acidic in nature. The acidic food is attracted to the alkalinity caused by the residual soap in the carpet pile. By putting a cup of vinegar in the final 5 gallons of pure water in the shampoo'er's solution tank, any residual alkaline soap in the carpet will be neutralized, and the residual acidity left behind on the carpet pile will help prevent future food spills from sticking to the carpet pile.
This is one subject I consider myself to be very knowledgeable about, so if you were confused by anything, please post again and outline what you're confused about.
You can do better removing stains from a carpet using a wet/dry vaccuum cleaner and a spray bottle than hiring some professional monkey with a truck mount vaccuum unit in his van.
A "carpet shampoo'er" has some form of mechanical agitation of the carpet pile. A "carpet extractor" has a pump and vaccuum motor just like a shampoo'er, but doesn't have any form of mechanical agitation. Most carpet cleaning contractors use extractors instead of shampoo'ers. That's because you really don't need to vibrate the carpet pile to lift dissolved liquids out of a carpet. With a vaccuum cleaner that vibration is important to allow solid soils to be released from the carpet, but it's not important for removing liquids from the carpet.
