Trying to work out same issue on an Instant On Takagi Hot Water Heater using for radiant floor heat.
As soon as gets cold outside it would ice up. I had a fix that worked until it was below zero F, and since my install is high on ground level floor and exhaust exits just under soffits, was to add 2' extension to the exhaust. More condensate was forming inside the pipe and then dripping to ground vs turning side of cabin into a cloud making machine like a dryer vent.
My 'furnace' can handle up to 15' of venting before I need to flip some DIP switches to change and can handle vents longer than my house, so adding this small section was of no concern.
Now that it is -25F out there, and near zero wind, the small amount of plume is now a concern again and I was outside middle of night clearing ice off intake, and again 6 hours later.
I was tempted to add small heater to intake, as I bought one to warm up my ATV, small 15w unit, but think will just extend intake when can make another trip to hardware store.
Add a 90 and run away from exhaust. If I didn't just have the whole house sided and new soffits I would build some sort of diverter between the 2.
I thought going 3" when 2" was only required due to short length would work, 1st heating season new system, guess I was wrong. At least with in floor heating, if it kicks off middle of night I only lose a few degrees of internal temps. They need to design a better intake/exhaust system for use in colder climates. Some low volt DC heater on intake with humidistat and thermostat sensors to trigger the need for heating. Or possibly reversing the intake and exhaust so warm air blows out intake and melts the buildup? Engineers get to work, this should not be upon the homeowner.
And if those ideas makes someone rich here, please come out and upgrade my system, I am too old for late night trips out in -40F windchills.