One of our furnace and air handler units is located in the crawl space of our house. It is very difficult at our age to reach the unit and change the air filter every 90 days.
A friend of ours in the HVAC business said that if we remove the air filter from the air handler, we can install a filter on the return vent which is located inside the house. Is this an appropriate solution to our problem?
Yes. As long as there is a filter in the return before the air handler somewhere. All four of mine are inside my house.
They are the kind that you can backwash clean with a water hose, and I do it faithfully every first of the month.
If I were you, I would check/change them monthly for more energy efficiency.