Furnace Filter Changes: Arrow Direction, MERV Ratings and Timing

A tradesperson who says he has worked on furnaces for 30 years lists five filter mistakes that can cost homeowners money, and the questions he sets out to answer are whether cheap or expensive filters are better for air quality, and whether expensive ones raise electricity use.

Removing the old filter and cutting power

A filter that is hard to grip can be pulled with pliers. Without pliers, a wood screw or drywall screw turned into the filter gives a handle to pull it out.

Running the furnace during a change is a mistake: the pressure against the dirty filter makes it hard to pull out, and it can tear into pieces. A piece of plastic or paper left in the return air since new construction could be sucked into the blower once the filter is out, which can be costly.

Power should be off at the switch near the furnace or at the breaker. The furnace needs time to cool first: turn the thermostat down, wait 60 seconds, then cut power. Switching off immediately can overheat the furnace and lead to a service call. He says nine times out of 10 it is fine, but once in a while it costs money.

Direction of the filter

A backward filter is more restrictive, and filter material can be pulled into the furnace and cause damage. The arrows on the filter should point toward the furnace. With the furnace in the basement, they point toward the equipment. With the furnace on the main floor and ducting in the crawl space, the furnace sits on top and the arrows point down. A Sharpie arrow drawn on the ducting toward the furnace works as a reminder.

How often to change it

The industry standard is 3 months. If the filter looks like the dirty one shown at 3 months, change it every month or two; if it looks cleaner, it can stay another month or two. A filter that dirty makes the furnace run hot and costs more in gas and about 25 to 30% more in electricity. A phone reminder every 3 months helps, and at the very least the filter should be changed twice a year.

Not overspending

For most people with a 1-inch filter, a cheap fiberglass filter is recommended. With restrictions such as an air conditioning coil or poor ducting, a MERV 13 or MERV 14 filter is restrictive, almost like a dirty one, and costs more in electricity. A MERV 8 is called a great option.

For those concerned about dust and air quality, MERV 11, 12 or 13 filters are options, but they are more restrictive, need replacing more often, cost more in electricity and gas, and call for the fan running 24/7. For allergies or asthma, a whole house air cleaner is something different, and a local HVAC professional should be contacted. Filter sizes differ between furnaces, so the size should be checked before ordering.