With "water-to-water" systems the water is run through a tube that is surrounded by a tube with flowing, heated coolant from your vehicle's coolant system. Since the temperature of the coolant (water) in the outer tube can be regulated by the vehicle's thermostat, there is no need for a way to divert the coolant away from the outer heating tube.

Both systems do a fine job of heating the water. But, since it is a simple fact that water heats (anything) faster and more consistently than air, the system to use depends on the type of cleaning you are doing. If you are cleaning residential homes, moving from room to room, moving furniture and cleaning detailed areas at a time, then an "air-to-water" system will deliver enough heat to do the job. On the other hand, if you are cleaning large commercial areas or apartments with a lot of non-stop wand work, then the "water-to-water" system will deliver more consistent heat.
https://www.borchpack.com/html/products/animalfeedpackagingmachine/
"Propane Heat" Is overall the hottest and more easily controlled heat available for truck mount systems. The only negative in comparison to heat exchange is that you have to purchase propane gas to run the heating system. In most propane systems the propane gas is stored in a "belly tank" outside the vehicle. A hose is run from the tank to feed a hot water heater inside the van. Pressurized water from the system pump flows through the hot water heater and is delivered to the wand for cleaning. The temperature of the water is thermostatically controlled by easily adjusting the temperature control knob on the heater thermostat. Other than having to purchase propane gas, there are several positives to be aware of with a propane heated system. Besides the thermostatic control of the water temperature, propane systems are extremely versatile and simple to operate and maintain, and with built-in emergency gas shutoff and venting, can be operated with a high degree of dependability and safety.
https://www.borchpack.com/html/products/foodpackagingmachine/