That heating/water installation is really incredibly huge! Or compared to here (Holland) anyway, but we only get down to minus 15C or so in the Winter - and that for only a few weeks.

As the burner(s) is apparently drawing combustion-air from it's surroundings I wouldn't put myself down there. Like others have said, can you partition the basement and give the burner it's own inlet and outlet? Then a filtered positive-pressure ventilator for the darkroom can work completely separately from your heating, perhaps with heated air from another part of the house in the Winter, so that you will be pushing clean air into your space instead of sucking in dust all the time.