Answer: Unfortunately, based on your description, I am not certain what is happening to your plants. It may be that the heat vent is drying out the tops of the plants as most plants will not tolerate a direct draft of hot drying air. It may be that your watering practices are allowing the plants to become overly dry or overly wet. It may be that there is an infestation of some sort. It may be that your fertilization program is lacking in some way. It may be that the air quality is poor or there is inadequate air circulation or there is too much or too little sunlight. It is probably a combination of factors. It is very difficult to diagnose these types of problems long distance.
I would strongly suggest you work with your local county extension to try to obtain a specific diagnosis and then work from there. Another option would be to hire a professionally trained horticulturist with extensive interiorscape (interior plants in commercial settings) experience to look at the plants and their growing conditions and advise you. I'm sorry you are having trouble.
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