The Camco -100 Boiler Coolant available at Amazon is the correct coolant. The product that they have available is a 'concentrate' and should be mixed with 2 parts distilled water (33% Camco -100 coolant concentrate / 66% distilled water). This one quart bottle will make about 3/4 gallon of ready to use coolant.
Turn off your Hydro-Hot and allow it to cool for 1 hour. Remove radiator cap and fill to top of neck. Replace cap, bring unit to operating temperature, then fill overflow bottle to "Hot" level.
You will find manual for your Hydro-Hot on the Aqua-Hot website, here: https://www.aquahotparts.com/eSource/ecom/eSource/default/ArchievedManuals.aspx
If you want shop manual, download the one for the HHE-500-07E.
- Roger Berke -