You are right Joe,
problems with over heating are usually with upholstery cleaning, not carpet cleaning. The hotter the water for carpets the easier the job, so its better to dump back to the header than the clean tank. Dumping to the waste tank is a rubbish.
But when it comes to cleaning upholstery you may over heat. I have not had this problem yet, but then I have not cleaned upholstery in the summer months as purcahsed machine last November.