Hi,
If the collector is vertical and mounted on a wall, it won't overheat.
The sun is so high in the summer that the radiation on a vertical collector is by a factor of about 2.5 compared to mid winter.
This is the vertical collector I use:
http://www.builditsolar.com/Projects/SpaceHeating/solar_barn_project.htmmounted on a south facing wall.
It never goes above 185F, which is fine.
I actually included provisions for venting the collector in the summer (holes in the top and bottom sills that could be uncovered for the summer to let a little air flow through the collector), but I've never used them.
If you do put the collector under an eave, be careful to not overdo it. The eave may provide more shading than you want in the spring and fall. There are a couple good tools here:
http://www.builditsolar.com/References/SunChartRS.htm#Overhangs that allow you to see what a given size overhang will do. If you heat only in mid winter, then you can have a pretty good eave without hurting heat production, but if you have a LONGGGG heating season like we do, you have to be careful about not overdoing the eave.
Gary