Arrived on site to return for revenue on a 2015 carrier unit. Installed and high limit unit was operating and heating in fort and second stage. Once flame were created and fluttering. Advised that’s can be a ventilation issues. Once unit was running cycled for a couple minutes to react how it would trip and ram longer but now the combustion vent high limit is tripping and over heating. Inspected venting in attic and on roof to ensure it’s not clogged. Found vent is really pinched and has been since they installed new unit. Vent goes from a 4 inch vent to 2 inch or less as it should be opened up and venting. Ram unit with out vent cap and unit ram longer but advised can not leave cap off unit as it can rain. Advised client now of findings and showed him photos and he is aware of the issues due to bad design installation. Advised would have manager put together ther a estimate to replace vent and since would have to open roof would have to get a roofer to seal up flashing once replac