I've had nothing but problems with Duravent elbows leaking since my Harmon Advance was installed last October. My configuration is a 4" elbow immediately after the stove exhaust, then 21" strait up of 4" pipe.
We went through 4 ittereations of different tees & elbows. The dealer & I are fed up with Duravent, we are swapping it out for Excel, which is on order. We'll report on this forum the results after it's installed.