This is a well known Onan problem and you will find many discussions in these forums
and many solutions offered. I have also had two failures at about 150 hours each and spent a lot of time contorted while accessing the heat exchanger
to pick the impeller blades out.
I seriously suggest the easiest and - so far - successful solution is the run dry impeller:
I would not waste my time with major replumbing, although Capt Bills check valve sound good as it is after the strainer, and non return valves as the impeller will probably still start dry on the L450.
I had the factory NRV, it filled with fine grass weed jammed and water ran out. I replaced with another model, same thing happened. I removed it. The electric pump
pushed water up to the strainer ok before the generator starts. However the air lock will likely mean that the impeller will start dry.
ps don't forget to replace the anode. I figure every 1 year to 18 months max.
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