[CaberTosser]

You're talking about the screen on the inlet to an instantaneous water heater I take it? Those are there to prevent the small-diameter heat exchanger passages from plugging off. Its possible to put in a cartridge filter upstream of that which will not only filter much finer particulate, but have way more capacity to filter before plugging up.

Oops, I think I misinterpreted that when you mentioned screen, you're talking combustion air side and literal freezing rather than figurative plug 'freezing' on the water inlet filter of an instantaneous tank, you might have a power direct vent heater or instantaneous. That depends on the chimney set-up, I try to provide as much physical separation between the intake and exhaust so that re-ingestion of exhaust gases is minimized. I'll usually take into account prevailing winds as well. With the concentric vents this is difficult but when a unit has separate pipes they can easily be run separated by a few feet. For multiple appliances I will bank all of the intakes together upwind of the exhausts.