The furnace filter is the cheapest part of the heating system and the easiest one to forget. It falls into the gap between your parent's routine and your calendar. Nothing alerts anyone when it gets dirty. You usually find out when rooms run cold, the system runs longer than it used to, or the air feels stale.

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 the direction

Pull the filter and read the dimensions printed on its frame, usually given as length, width, and depth. Note the arrow on the side, which shows the airflow direction. Write those numbers and the arrow on a card and tape it inside the furnace closet door or the cabinet where the filter lives. A replacement that is slightly off can leave gaps where air passes around the filter instead of through it, so match the size exactly.

Check whether the system uses a standard filter or a special one. Some homes have a media cabinet or an electronic air cleaner that needs a different part. Look at the unit's label or ask the installer which one is in the house. Do not buy by memory.

Pick an owner

Decide who swaps the filter. The options are your parent with a reminder, a local helper who visits on a set day, or a handyman you have already checked. Write the name down, along with what that person is allowed to do. A person who swaps filters should not be asked to adjust the thermostat, open the burner panel, or work on the blower. Those belong to the heating technician.

Compare the options by what happens when the owner is away. If the helper is on vacation, who covers? If your parent cannot manage the step on a bad day, who gets the call? A plan with one name and no backup has a single point of failure.

Set the routine

Mark the swap on a calendar at a fixed interval. At each visit, check the filter even when the calendar says it is not due. Hold it up to a light. If you cannot see light through it, it is due. Write the install date on the frame with a marker so the next person can tell how long it has been in use.

Keep one spare on a shelf close to the furnace, stored flat and dry. A spare that sits in the garage in the wrong size does not help on a cold night.

When to call the technician

Call a licensed heating technician if the system turns on and off in short bursts, makes new noises, runs without the fan starting, or if you notice a burning smell or unusual heat near the cabinet. At the annual service, ask the technician to check the filter and the blower and to write the filter size on the service sheet. Ask whether the service visit includes a filter swap and whether the company offers a reminder for the next one.

Tonight

If you can reach the furnace on a visit, pull the filter. If you cannot, ask your parent to read the size off the frame over the phone, and ask them to tell you the date it was last changed. Write the size, the arrow direction, the owner, and the backup on one card. Then check whether the spare is on the shelf. If it is not, add it to the list before you hang up.

If no one on your list lives nearby, the Local Help for a St. Louis Parent page explains how to look for someone who can handle a visit like this.