Water around furnace: culprit?
This looks like a newer “vent “ &amp; exhaust, I haven’t seen one quite like that but it looks like a coaxial vent &amp; air supply for a closed system. I wonder if it is rain or a plugged condensate drain that is the problem because that small area would require much more rain to provide what I think you reported even at biblical proportions, I don’t think it has rained that much in our life times. So will a cap help , maybe but probably not. A hvac pro can probably find a problem in short order but I don’t think it could be rain.