Aqua-Hot 450 hydronic heater fault codes
Aqua-Hot heats a boiler tank of propylene glycol antifreeze/water with a 12V diesel burner (primary) and a 120V electric element (secondary, needs shore or generator). That hot fluid heats the zones through fan heat exchangers and makes continuous domestic hot water on demand (no stored hot water). The 450-DE can also preheat the engine coolant.
Aqua-Hot: Low tank-level cutoff (red)
Boiler fluid low. Burner/element shut down. Auto resets when the level is corrected.
Aqua-Hot: Low battery voltage (red)
12V too low. Burner shut down. MUST be manually reset after voltage is restored: burner switch off ~30 seconds then on, or press the Low Voltage Reset button through the faceplate hole with a non-metal pick.
Aqua-Hot: Overload (red)
12V short or controller overheated. Short in the main 12V supply, or high 12V load plus a hot controller. Auto restarts when corrected.
Aqua-Hot: Zone light red
Short in that zone. Zone thermostat or heat exchanger (fan) circuit shorted. Green = zone calling and fans powered.
Aqua-Hot: Pump #1/#2/#3 red
Short in that pump circuit. #1/#2 are zone pumps (run on a zone call). #3 is the boiler stir pump (runs during continuous hot water use or when not up to temp).
Aqua-Hot: Engine preheat red
Short in the preheat pump circuit. Light is only active with preheat switch ON plus burner and/or element ON.
Aqua-Hot: Electric element red
Short in the element's 12V control circuit. Green = element heating.
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Practice it on a service call
Guided diagnosis
Full field guide: Aqua-Hot 450 hydronic heater
Codes come from the maker manuals and public service sheets. Your model's own chart always wins, and LP and 120 volt work belongs to a qualified technician.