Without knowing just how far away the heat vent is and just how hot it gets around the windowsill, it is hard to say if the forced heat is the problem. Low humidity is overrated as a source of
plant problems. Most of the more common houseplants can and do survive in low humidity, as long as their roots are adeqautely supplied with water.
North light, assuming it is unobstructed and not blocked by blinds or a
tree, is suitable for most houseplants because it is bright indirect light.
Perhaps your
plants are struggling for reasons other than light or low humidity. Unnecessary repotting, poor soil quality,and watering too frequently are the most common reasons for
plant failure.
Pothos, ZZ
plants, snake
plants, peperomias, jades, and Chinese evergreens all thrive in north light and in dry, warm air.