Quote Originally Posted by Bob-D659 View Post
...Even if the tank and furnace are partitioned off, there has to be a good sized vent grill to the open area of the basement/house.
Yes, and even that relies on the house being leaky enough so outside air can enter. A space that's sealed from the rest of the house but communicates directly with the outside would be better. Heating appliance manufacturers specify the free vent area in their Installation and Operation manuals.

It's unfortunate that so much of this continent heats with "scorched air." If the OP's house used a sealed combustion boiler and hot water space heating, his comfort level would be higher and an indirect tank could be installed for domestic hot water too. Then there would be no need to consider these venting matters when planning his darkroom.