I think you stated what the problem is, the heating 3 feet away. Zone 7 is too cold to keep it outside. My advice, move further south so you can keep the plant outside year long. Anyway... You need to correct a few things. First move it away from the vent. A breeze is good, a heating/cooling vent is bad. The water fall, honestly, is not doing a whole lot. The humidity the fall makes disfuses through the room and probably only raises the humidity about 0.25 percent, if that. That would be my guess. Ficus can grow great inside. I have done it for years living up north. But soon as it was warm enough to move the plant outside, I did. I had a grow light with about 10-15 plants packed around it for the winter. I kept them watered, but not wet. Let it dry out all the way before watering. If you have a damp soil before you water, your watering too soon. The reason why I say this is because leaves falling off is indicitive of a root rot fungus. This is indirectly caused by watering too much. I rambled on here. In short. Move the plant away from the vent. Let it dry out all the way between waterings. Get a grow light for the winter. Move it outside soon as the last frost is past. Some one else can post and clear up my rambling on.

That looks like spider mites. I get them sometimes too when its gets very hot. I cut off all badly spotted leafs (they fall off anyways and look very bad) and then spray all the plant and especially under the leafs with soap and water with a spray bottle. Do this in the evening not in full sun. I spray and then wait for it to dry then I spray again and wait then I spray with the hose and wash the plant good and it gets rid of them. If it usually gets that hot in your area you should have yours in atleast morning sun and afternoon shade and I would spray it with the hose every few days just to keep any pests away.

Throwing carbaryl around without knowing why is a waste of time, energy, and money, as well as contributing to the pollution of our planet. What is the soil these pepers are growing in like? How much organic matter is in the soil? What does the soil smell like? How well does this soil drain? How many earthworms do you see in each shovelful of that soil?

A light reduction, either in intensity or duration, can cause leaves to fall. In most plant species, any given leaf will only be able to function efficiently if the amount of light it receives remains within a fairly narrow range. IOW, if we graduate light intensity on a scale of 1-10, leaves accustomed to a light level of 6, might only be able to function at levels between 4-8. Exposed to light outside of those parameters, the plant might shed the leaves and replace them with new foliage that comes online accustomed to current light levels and able to function efficiently at those levels. Individual leaves are usually quite a bit better at tolerating a change in light that finds them getting MORE light than at tolerating LESS light; so, that 6-rated leaf might better be described as being able to tolerate a level of 4-9 in most species of plants. Leaf abscission also occurs as a drought response, which can be caused by under or over-watering, and root congestion tends to cause leaf loss proximal to (toward the root to shoot transition) the growing tips of branches, leaving the plant looking a little poodleish, with most of the growth concentrated in poms of growth at branch/stem tips. The issues most important to growing this plant well are favorable light/ temps and a soil you can water to beyond saturation, whenever you want or need to, w/o having to worry the soil will remain soggy so long it compromises the health/ function of the root system. Maintaining a healthy root system is a prerequisite if you are hoping for a healthy plant. Al