There are a variety of reasons why downstreaming is the way to go. The first has already been commented on and that is no damage to equipment. Running chemical thru your pump, coil, and hose drastically shortens the lifespan of each. Also, when chemical is run thru the coil it reacts when heated, either by sticking to the inside of the coil, and/or by attacking the coil material and eating it from the inside out. The same is true of your pressure line running out to your wash gun.
Second, it prevents you from getting a clean rinse. Chemical residue sticks to the inside of the pump, coil, and hose and can take considerable time to flush out. This residue is left on the surface of the vehicle and has the potential to damage the finish which, if its owner were to find out that you caused it, you would be liable for. Trucks and trailers aren't cheap to repaint. At best, it lowers your quality which is not acceptable to me.
Some confusion lies in the "you can run hot soap" comment. As Dan said, you run your clean rinse water thru your water heater and then inject your chemical downstream and WALA! you get hot soap. Larry said you can run your soap hotter by upstreaming compared to downstreaming. That amount would be marginal, usually a couple degrees which you would never notice the difference anyway. And how hot do you need the water anyway? The hotter the water, the quicker it evaporates and dries on the vehicle, the greater the chance of causing damage to the paint. We've found that ultra hot water doesn't gain you a whole lot unless you're degreasing, which you would only do with special circumstances involved. Ultra hot water brings me to the most important point of all, which I'm surprised that it hasn't even been mentioned, and that is....
SAFETY!!!!! Upstreaming is very DANGEROUS! You are putting your ultra hot chemical under pressure. What happens when the hose that is now worn out blows? Boom! Chemical right in your eyes, or worse yet, depending on the pressure that you run, directly into your bloodstream. NOT GOOD! Some would say "Well that's the choice I made so I can wash better." Each person has the right to make choices in their life, so people make bad ones like smoking, overeating, overdrinking, drug use, etc...When these suffer the consequences of those we say "It was going to catch up to them sooner or later." But if a person hires an employee and expects them to use equipment with known dangers, the employer bears responsibility. That's the reason the Federal Government created OSHA to protect workers of potential dangers. All I can say is if a person is doing this and their employee is not wearing a full facemask and headgear, along with a full chemical resistance bodysuit, the business owner better have great liability insurance! Then try telling them to wear it in the summertime!
