They recommend that it have antifreeze in the water side so it doesn't overheat the coils.
From the manual :
When it’s time to store a motor home for the winter months or when freezing temperatures are expected, it’s crucial to properly winterize the Aqua-Hot to avoid serious damage, requiring a complete system replacement not cov- ered under the Aqua-Hot Limited Warranty Statement. The process of winterization consists of completely draining the domestic water from the system and pumping RV winteriza- tion antifreeze through to flush out the system.
Not winterizing the Aqua-Hot when freezing temperatures are present will result in SERIOUS damage to the Aqua- Hot’s Domestic Water Heating System. Also, be sure to use the FDA approved, “GRAS” rated antifreeze for winteriza- tion.
Please follow these instructions carefully when winterizing the Aqua-Hot’s hot water heating system:
1. Completely drain the fresh water storage tank.
2. Disconnect the domestic water demand pump’s suction
line from the fresh water storage tank.
3. Attach an adequate piece of hose onto the suction side
of the domestic water pump.
4. Place the opposite end of the hose into an adequate
supply of the RV antifreeze, “GRAS” by the FDA and al-
low this to pump through the system.
5. Open and close all interior and exterior water faucets,
one at a time, until ONLY pure RV antifreeze is present.
Perform this procedure for both hot and cold faucets. 6. Remove the hose from the RV antifreeze and reconnect
the domestic water demand pump’s suction line to the fresh water storage tank."
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