How Often Should You Really Change Your Furnace Filter?

The box says ninety days. Your house has two dogs, a woodstove, and somebody sanding drywall in the spare room. Ninety days is not your number.

More furnace and air handler failures trace back to a neglected filter than to anything else I see. It is a five dollar part that protects a five thousand dollar machine, and it gets ignored because nothing dramatic happens when you skip it. Right up until something dramatic happens.

What the filter is actually for

Most people assume it is there to clean the air they breathe. That is a secondary benefit at best on a standard filter.

Its primary job is protecting the equipment. It keeps dust off the blower wheel and, more importantly, off the evaporator coil. A coil that loads up with dust loses its ability to transfer heat, and airflow through it drops. In cooling season that means a coil that runs colder than it should and eventually freezes into a block of ice. In heating season, restricted airflow means the heat exchanger runs hotter than designed, which is how heat exchangers crack.

Cracked heat exchanger is a condemned furnace and, on a gas system, a combustion safety issue.

Real intervals

Start at one to three months for a standard one inch filter and adjust based on what is actually true about your house.

Change more often if you have:

• Pets, especially shedding breeds. This is the biggest single factor.
• Anyone in the house with allergies or asthma
• A woodstove or fireplace in use
• Construction or renovation going on, even in one room
• A dirt floor or damp basement with the air handler in it
• A system that runs nearly continuously, which describes most heat pumps here in winter
• Smokers

You can stretch it if:

• No pets
• Nobody home most of the day
• The system runs seasonally rather than constantly
• Newer, tighter house

The honest method is to look at it. Pull it out once a month for a while and hold it up to a light. If you cannot see light through it, it is done, and the calendar does not matter. After a couple of cycles you will know your house’s actual interval and can stop checking.

Thicker is generally better

If your system has a media cabinet that takes a four or five inch filter, use it. More surface area means more dust capacity and, counterintuitively, less airflow restriction than a one inch filter of the same rating. Those often go six months to a year.

If you only have a one inch slot, do not try to jam a thicker one in. That is not how it works and you will get air bypassing around the frame, which is worse than a dirty filter because the dust goes straight to the coil.

The MERV trap

MERV is the filtration rating. Higher catches smaller particles.

Higher is not automatically better, and this is where people get themselves in trouble. A high MERV filter restricts airflow more. Residential systems are designed around a certain static pressure, and dropping a very high MERV one inch filter into a system that was not designed for it can choke airflow badly enough to cause the exact problems the filter was supposed to prevent.

For most residential systems, MERV 8 to 11 in a one inch slot is the sweet spot. Real filtration, airflow the equipment can live with. If you want serious filtration for allergies, the right answer is a proper media cabinet or an electronic air cleaner sized for your system, not a restrictive filter crammed into a slot built for a cheap one.

The very cheap spun fiberglass filters are the other extreme. They protect the equipment marginally and filter almost nothing. They are better than no filter and that is about the strongest thing I will say for them.

Getting the direction right

There is an arrow on the frame. It points in the direction of airflow, which is toward the blower, away from the return duct. Backwards, the filter media collapses and does not work properly. I find backwards filters constantly.

Write the date on the edge of the frame in marker when you install it. Takes two seconds and settles the argument about when it was last done.

What a neglected filter costs

Higher bills, because restricted airflow makes the system run longer for the same result. Frozen coils in summer. Cracked heat exchangers in winter. Blower motors that fail early from running against high static pressure. Dirty coils that need a professional cleaning that costs many times a box of filters.

Set a reminder on your phone. It is the cheapest maintenance in the building.