it is a pretty nice bit of kit, though pricey. You could duplicate the function, but they have wrapped it up in a compact package that would take some work (and expense) to duplicate. It lets you select either, both, or none. You can select one and change the other underway. The only time I have done that, it turned out not to be a clogged filter anyway. I mounted it right near the top of the cockpit locker, easy to get to when you need to: