Question: My central air filter is inside the air handler in the attic. Due to a recent back injury, i cannot get up to the attic. Would I be able to remove the filter in the air handler and install a filtered return register?
That should not be a problem if you have only one return grill. Of course if you have more than one return grill you will have to install a filter in each.
If the return grill opening is smaller than your current filter size you may find that you have to change the filter more often because it will become dirty faster. You will have the same amount of air moving through a smaller filter and the dust will be collecting on a smaller surface area.
You may have to get somebody to modify the duct behind the return air grill to hold the filter in place but that shouldn’t be difficult. Or you may be able to modify the return grill to hold the filter if you can’t find a new one.
The only real concern that I might have is the impact that a smaller filter will have on air flow if the return air grill is smaller than the current filter size that you are using. This is something that could cause the evaporator to ice up due to reduced air flow and can have a negative impact on air flow. Changing or cleaning the filter regularly will be very important in this case.