The fwd heat exchanger has two independant fans and for the v-berth zone a 3" duct was more suitable for the space available. Unfortunately they didn't sell a plate that has one 3" and one 4" so ai built an adapter to accept a 3" duct transition that screws into the hole pattern of the 4" duct transition. This makes the run to the V berth less distructive to run and saves space in the fwd setee storage box where the ducting runs.