It would be a very good idea to use positive ventilation - knock a hole in an internal wall and install a fan to push the air into the room instead of pulling it out. This is better for dust too because you can filter the air going into the room from the rest of the house.
What would be the disadvantages of positive pressure ventilation?
One could be that lab air is pressed into other rooms. Any more?
Without it, the flue gases from the existing hot water tank would be drawn into the room by the exhaust fan instead of going up the chimney. All that is needed is a fan of the same cfm rating as the exhaust fan, and they both should be on the same power switch.
Balanced ventilation of the space is definitely necessary to avoid problems with water heater exhaust gas. I still wouldn't do this without installing a fast-acting, low-level carbon monoxide detector. Better yet, partition off the furnace and water heater as suggested in post #9. I'd rather see you go digital than die in a darkroom. :blink:
Steve, that style of hot water tank has been the norm in the US and Canada for decades. Easily identifiable from the photo as Sal wrote.
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.
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.