A furnace filter is the cheapest maintenance task in the house and the one most likely to slide. Nobody owns it, so nobody notices until the furnace runs longer, rooms feel uneven, or the air gets dusty. Dad is not going to book a service visit for a filter, and he may not want anyone poking around the furnace closet. That makes it a family job with a name on it.
Local option: New Plan Care gives families in the St. Louis area in-home help so a parent can stay home: companionship, help with bathing and dressing, meals, and overnight care. Call (314) 405-0887.
Read the filter before you buy one
Pull the filter out tonight and read the printed size on the cardboard frame. Write it down in the order the label gives. Look for the airflow arrow on the frame. It should point in the direction the air travels, toward the furnace or blower. Compare the printed size with the slot or the return grille where the filter sits, because some homes take a filter that is not the size on the box. If the number on the filter does not match what the installer used, call the company that installed the system before you buy a different size.
Name the owner in three parts
A filter has three jobs that different people can do: buying, swapping, and checking. Write each one on paper and put a name beside it. Buying can be you or a store run. Swapping goes to the person who can get into the furnace area without a fight. Checking goes to someone who looks at the filter on each visit and notes whether it is gray or clogged. If one person is doing all three, write that down too, because that is usually where the job fails.
Tie the check to a visit, not a calendar Dad ignores
Dad will ignore a reminder about his own house. He will not refuse you standing in the hallway with a flashlight. Attach the check to a visit you already make, such as a grocery run or a drop-off. Pull the filter, compare it with the spare, and swap it if it is loaded. How often to change it depends on the filter type, how many people live in the house, pets, and cooking, so use the label on the filter and the installer's advice rather than a number from a website. Keep at least one spare of the right size in the same place each time so the swap does not depend on a store trip.
When the furnace needs more than a filter
A furnace that is cycling on and off quickly, making new noises, running much longer than before, or leaving rooms cold needs a service visit, not a new filter. Frame the visit around the system, not around Dad. Look for the last service record on the unit or in the paperwork, and if none exists, add that to the list for the installer. Keep the installer's name and number on the fridge where anyone can find them.
Next move tonight
Pull the filter, read the size and the arrow, and write the three names on the same sheet of paper. If you cannot find the size or the arrow, photograph the filter frame and the furnace door and bring both to the store counter or the installer. If you need a name for the service call, the St. Louis helpers listed at Local Help in St. Louis are a place to start.