You said you have a fan in the 8-inch duct, and also on the stove. That's good because I wonder if the stove fan itself would move enough air. You said you would also capture the output from one of the blower outlets on the stove and get that into the hood as well. Why not capture both outlets instead of just the one? Is it a finished cellar and do you need the 70 degree heat in the cellar? If not, route it all upstairs, not just half of it. You will get a great deal of heat in the cellar just from radiation off the stove and the plenum and the ducts.
I think the reason people suggest a cold air return duct, is so that you will be losing less heat to the cellar if you channel that upstairs air directly back to the stove, rather than let it flow all over the cellar. It looks like you have an uninsulated concrete-wall cellar, which will suck up huge amounts of heat just trying to warm the concrete. By the same token, maybe you could extend your hood down around the sides of the stove to capture that heat as well, rather than let it warm the cellar. The Mark II puts out lots more heat from its sides than from the top, so a lot of it is getting away from you!
I don't know how wedded you are to the Mark II, but there are stoves more suited to serving as a hot-air furnace.
Last edited by rberq
on Sun Feb 08, 2009 8:13 pm, edited 2 times in total.