The MAIN tank(about 8 gallons) is Propylene Glycol which is Antifreeze. This is what goes thru the Furnace radiators in the RV and what the HOT water coils are immersed in. The Hot Water coils are what you need to be worried about. What you do to winterize is drain the system and then pump RV antifreeze thru the water pump until Pink antifreeze comes out the nearest Hot water spigot, either a faucet or the low Point drain. Then you are done. The Coils are so small Aqua Hot states that blowing them out cannot remove all the moisture in the tubes and they could then freeze. The Coils are about the ONLY non serviceable part of the Aquahot. If they freeze and break you have 2 choices. 1. Buy a new unit(expensive). 2. Send your old unit back for rebuild.(not quite as expensive but close). Doug
PS, you will know when they freeze and burst. The Overflow reservoir will overflow when you have the fresh water system pressurized up.