Just wanted to pass along my recent Aqua Hot fail and fix in case you had a similar issue. The symptom was that the burner fired, unit came up to temp and delivered heat. The heat continued to diminish out of the radiators but the burner did not attempt to come back on or fire up. The red light that would have indicated that the burner had attempted to start but failed remained lit.
On my unit there are four button type thermostats behind a cover plate above the burner. They control the cut in temperature and the safety off temperature for the burner (2 on the left) and the same for the electric element (2 on the right). I surmised that one of the two thermostats was not opening/closing in the proper temperature range and replaced them both being 10 years old. My unit is the Aqua Hot AHE100-02s model without the external monitor panel. This was an easy fix for about $60 in parts. Wish all Aqua Hot repairs were this easy.... Hope this helps.
Later Ed
