The thermal exhaust portal was a design flaw of the Death Star. It was originally going to be removed from the plans. It eventually ended up being installed, though. The architect overseeing the build told the Wookiee to not install the exhaust portal. The architect was in the process of amending the plans.
Before the Wookiee had a chance to follow through, he became very sick and was taken to a medcenter. His replacement went ahead and built the portal without knowledge of the amendment.