I am confident that my COOLANT consumption is normal but I would like to reduce it. It seems to me that coolant overflow systems are less than ideal. The air space above the coolant in the bottle must contain some components of evaporated coolant. That air is constantly pumping in and out. Engine heat must be increasing that evaporation. In addition, especially whenever the engine is running, air is blowing over the air-bleed hole and whisking coolant vapor away. I wonder too about the size of the bleed hole. My father's old 1978 Mazda 626 had a pinhole whereas my latest two cars are about 25 sq.mm. ... why so large? I also imagine coolants contain antioxidant to reduce rusting and oxygen from the air would consume some of it.
Ideally I would think a sealed system like a bladder that could expand and contract would be the ultimate solution but apparently not? Is there something against heat shielding the bottle? Is the gradual loss just water, meaning keeping the bottle topped up with distilled water rather than coolant is ok ? Keeping it topped up to the max line would keep the air space/vapor loss to a minimum too? Coolant changes are a pain what with flushing, expelling air pockets, disposal of the old, mess etc. and perhaps the intervals could be extended with system design improvements?