BAC RV Club Forum
General Boards => Technical Support => Topic started by: Roger Milne on April 29, 2026, 12:48:48 AM
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So the Aqua Hot suddenly quit running on the Diesel Burner side a couple days ago and I found the return hose from the expansion tank to the pressure cap had split and drained the expansion tank. So repaired that and topped up the tank with about 1/2 gal of Propylene Glycol.
All indicators on the control module are showing green, but when I flip the kitchen panel diesel switch to "on", the indicator comes and the burner lights off, but then turns off and goes through its purge / cool down cycle and after that the indicator light in the kitchen panel goes out. The switch is still in the "on" position and the #3 Stir pump still runs as indicated on the control panel, until I switch the off in the kitchen.
I have checked the expansion tank since running it and it seems to have drawn in about 1/4 of a gal based on the fact I filled it to the "hot" line and it now shows half way full between the Cold and Hot marks.
There's no Red indications on any of the system monitor lights, so I don't know if I have air in the system or if something else is going on. I'm currently away from the RV for the next 14 days and my wife can use the electric element for hot water, but I wanted to see if I maybe missed something or what direction of troubleshooting I need to take when I get home.
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Roger,
One of the possibilities could be the "float switch" inside the coolant reservoir. If it has detected low coolant, it will give you the symptoms you are experiencing. Turn off the electric and let the coolant cool off and remove the radiator cap. If the coolant is not right there in the neck, add coolant. Hope this helps, Fred
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Thanks for info. I’ll look into it and see if that helps.
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To Fred's comments:
The float switch in my HydroHot wore/broke a few years ago. It wasn't something I could dig out of the top of the tank, but if you lower the coolant level to below the switch port, you can remove the switch (the float may not be with it if it broke off). Replacement switch/float assemblies, as I recall, are "rare" but available from certain vendors. My replacement was an improved version as I recall, and less likely to "disintegrate" and or the float leak.
The job was messy, but I finagled a drain line from the drain port down and out an existing nearby hole in the bay floor. Sans that it could've been worse. I probably followed instructions proffered on a YouTube video, but recall my configuration was slightly different, requiring some common sense thinking. I know I was concerned about the missing float part sashaying around inside the tank, but ultimately decided, after failed fishing attempts through the top, that it wouldn't interfere with operations. It's imperative that everything be kept clean, like around the top cap/port and the switch port. You don't want dirt or debris getting in the tank. Properly done, you can reuse the fluid you have to drain to get below the switch port.
It was a matter of getting just the right deep socket as I remember (finally found at Autozone) - and I had to modify it to accommodate the new switch's wires, carefully balled up and stuffed inside undamaged while turning the wrench. Then it was just a matter of hooking up a couple of wires.
I think a person can jumper a couple points on the board and bypass the float switch until you have time to do the repair or get it to a service facility. I apologize for my dim and aged memory, but others here may offer further details. And somewhere I think I have pix of my fix that I'll try to add.
Joel
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Hydronics float switch photo and tank drain location photo.