What the swap really is
Swapping a furnace filter is a short job. The filter sits in a slot near the unit, often in a return grille or a cabinet beside the furnace, and it slides in and out. The hard part is not the work. It is remembering to do it, and making sure the new one goes in the right way. The rest of the furnace, the burners, the blower, and the safety controls, belongs to a technician.
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.
Do it yourself if
He can do it himself if the filter slot is easy to reach, he can lift the filter without straining his back, and the size printed on the filter's edge matches the size on the unit or in the manual. Look for the airflow arrow on the frame and point it the way the air moves, which is toward the furnace. Hold the old filter up to a light to see how clogged it is. A gray filter with a thick coat of dust is past due.
Buy the replacement in the size printed on the frame, not the size you remember. Keep a spare in the basement or a closet so the swap does not depend on a store trip in the cold.
Hire it out if
Hire the visit if the slot sits behind a panel that needs tools to open, if the unit has not been looked at in a long time, or if anything about it worries you. If you smell gas at any point, leave the house and call the gas company from outside. Do not wait to see if it passes.
A technician will check the blower, the burner flame, the thermostat, and the safety switches, and should tell you in writing what they found. Ask three things before they start: what they will check beyond the filter, whether they will give you a written report with what they found and what they recommend, and how they will tell you when the next visit is due. Compare two companies on those answers, not on how friendly the phone call sounded.
What to do tonight
Find the filter size on the frame, check the airflow arrow, and write both on a card by the thermostat. Ask your father whether he wants to do the swap himself, have you do it, or book a visit. Each of those is a fair answer. Whichever he picks, set a reminder on the phone that goes off the same day each month during the heating season.
If he is in the St. Louis area, Local Help for a Widowed Father in St. Louis points to places to start. Make the call on a weekday, when offices can answer questions and send someone out with less of a wait.