Installing harman xxv and my horizontal vent is about 18" to pvc rail on my deck should I vent up or see how it goes.Also how should I silicone the fllange on the rear of the stove. There is a fiber gasket and metal flange.should I silicone the gasket to the stove and silicone the flange to the gasket.And should I silicone the 3" pipe inside the flange.