Joe, I've looked into it in some detail...
and have decided to go with a Wallas diesel forced air furnace because of its low power consumption and reliability. We were advised not to use the existing heat pump ducting on our P42 because of the higher operating temperatures. My plan is to install the furnace unit in the steering quadrant compartment on the starboard side. This location should give easy access for fuel lines and exhaust ducting.Wallas uses a three inch duct system. I've scoped out a duct route on the port side with one register under the aft cabin setee, another under the port setee in the main cabin and a third under the dressing table in the forward cabin.The furnace would require a solenoid shut off valve since it would be below the fuel tank. The 40D should produce enough BTUs to keep a P42 cozy.TerryP.S. where have you located your future P42? Boat name? We keep ours at Semiahmoo.