Never, ever run DI water through your coils. On fleets, NASCAR fleets and the RV's we detail (although this only accounts for less than 5% of our business), we do our final rinsing with water run through our DI tanks. My DI tanks cost me $80.00 to recharge, and I average maybe 400 gallons, per tank. No way could I make the profits I insist on doing dealerships. Doing RV's and the fleets we service, we meet or exceed our base man hour rate each and every time.
The tap water here in Phoenix is as hard as it gets, and will literally etch glass withing 48 hours. I bought another new truck a couple years ago, brought it home. First time I washed he I noticed the hard water spots all over the exterior glass. Took it back to the dealership, they tried everything to restore the glass, ended up replacing every single piece of glass. Now if that cost was a charge back to the car lot fleet washers, I assume that would wipe out a day or two's profit.
My wife worked for Chapman Auto group for a dozen years, a good friend of mine still owns several dealerships here and in Chicago. In Phoenix the average $0.45 to $0.55 per unit, show-up with about 8 illegals with chamois's.
True Bryan, rainwater is pure, until it comes in contact with that cloud of pollution that hovers over each metropolitan area. Then many Al Gore types refer to it as "toxic rain".