The furnace filter is the least noticed part of the house, and it is one of the easiest to let slide. Nobody thinks about it until the air feels heavy, the blower sounds strained, or a helper mentions dust on a vent. When your mother is staying in her own house, a dirty filter is not an emergency, but it is a small, repeating job that someone has to own. Without an owner, it quietly stops happening.
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.
Find the size and write it down
Pull the filter out of its slot and read the dimensions printed on the frame, usually three numbers for length, width, and depth. If the numbers do not match the box you were planning to buy, measure the slot itself, since older furnaces are not always sized the way the label suggests. Also note the airflow arrow on the frame. It shows which way air should move, which is toward the furnace. Write the size and the arrow direction on a card and keep it with the house papers, or with the thermostat notes. Whoever buys the next filter should not have to pull the old one out and guess.
Set the cadence from the label and the house
The filter packaging or the furnace manual usually gives a replacement interval. Use that as the starting point, then adjust it to the house. A home with a shedding dog, or a lot of foot traffic from helpers and visitors, will fill a filter faster than a quiet one. For the first few months, check the filter once a month and note how it looks each time. Hold it up to a light. If you cannot see light through the pleats, it is due. That check takes less than a minute, and after a few months you will know the real interval for this house.
Name the owner in three parts
A filter job has three pieces, and each one needs a name beside it. The first person checks the filter on the set day. The second person buys the right size, which may be the same person or someone who keeps a spare on hand. The third person swaps it in and marks the change on the card. Families often assume the check happens on its own, then discover at a service visit that the filter has been in place far longer than it should. Writing down three names, even if one person holds all three jobs, is the difference between a habit and a hope.
Be specific with any helper or handyman
If you bring in a non-medical helper or a handyman for this, be exact about the furnace. A helper can be asked to check the filter, slide in a new one of the right size, and note the swap on the card. Ask whether the helper will work only at the return grille or will also open the furnace cabinet, and whether that cabinet has a latch or panel that needs a tool. Do not let a filter swap grow into a request to adjust the furnace, service the blower, or open panels. Those are repair questions for the company that installed or services the system. When you compare helpers, ask the same three questions you would ask about any household task: who does it, who checks that it was done, and who gets called when something looks wrong.
When the filter stops being the question
Bring in the care team or a service company when the furnace makes new sounds, runs without stopping, leaves one room much colder or warmer than the rest, or when a fresh filter is dirty again within a short time. Those are signs the system is working harder than it should, and a new filter cannot fix that. The filter is the easy part. The real question is whether the furnace is still keeping her house comfortable enough that she can stay in her own chair. If you are not sure who to call near you, the local help page is a good place to start.