This might be better , blue is from boiler, red is vent, green is feed from tank, purple is bypass and orange is return to boilerView attachment 292854
I would reposition the cold feed to between vent and pump. Personally I'm not keen on the vent going straight up from a tee like that, as it's more likely to cause flow out of the vent (the opposite problem to what you have now). I prefer the vent to come off the top of a horizontal run. Might be a bit tight, but hopefully it will go in. The cold feed should go up and under into the pipe, to reduce convection warming the header tank, and encouraging algae growth.
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