We have the Wallas 30D in our 29. This is our first year with the Ranger and we are in Alaska. It was not a great year weather wise for fishing or boating this year so we spent many of nights at the dock just enjoying the boat life. We had the heater on pretty much all the time and it worked good.
We have three vents. One at the main cabin door, one just before you enter the main bedroom, and one in the main bedroom. The one in the bedroom is the farthest away from the furnace and has very little air coming out of it.
The furnace is mounted in the starboard hatch with the generator. The vent at the main cabin door has the most heat and everyone that has stayed aboard with us has asked to turn the head down at night. The vent blows right under the glass door and makes the back room toasty. If there is a way to close one vent down and keep the others open I haven't figured it out yet. I really haven't needed to because we have kept very comfortable even in some really windy rainy nights. This weekend we had over 80 mile an hour winds and better than 10 inches of rain. We were quite comfortable (again - tied to the dock in Seward).
We don't have AC, so I don't know if it uses the same ducts or not. It does not use the same duct as the engine heater.
I didn't keep track of how often we had it on, but I only put 9 gallons of diesel in the auxiliary tank a couple weeks ago. I would guess we've had in on all night long for 20+ nights set at half way or better. I'm really pleased with the heat and the diesel consumption.