reactor planner cooling problem

I have a reactor design which I think is fine and should not overheat, but the reactor planner says will only run for a short time.

http://www.talonfiremage.pwp.blueyonder.co.uk/v3/reactorplan…9dntr9ubfetqu4q

Am I misunderstanding something or is the reactor planner wrong?

Thanks.

Ah, I get it. I assumed all the vents would draw heat equally, but they don't. The upper left vents draw their max which overheats them, and the lower right vents are underutilized.

Thanks.