Furnace Filters:
Dirty, Backwards, Doubled Up, Missing or the Wrong Size
The filter is the cheapest part of a heating system. It’s also the part a furnace depends on every hour it runs.
A furnace filter installed backwards in Vancouver. Two dirty filters stacked in a Portland furnace. A 16 by 20 inch filter sitting in a 16 by 25 inch slot in Springfield, with a five-inch gap for air to go around it. A Woodburn furnace with no filter housing at all and the filter lying on the blower motor. In Russ’s recent reports, 75 of 269 houses had an HVAC filter finding, and most of those were simply dirty.
It’s a five-minute fix. It’s also the finding that tells you how the system has been cared for.
What a dirty filter does
A furnace or heat pump moves air across a heat exchanger or a coil. The filter is on the return side, catching dust before it reaches those parts. As it loads up, the blower has to pull harder to move the same air.
Russ’s report wording covers it: a dirty air filter restricts airflow, which can make the system work harder and reduce its efficiency. Low airflow can also make a furnace run hot and trip its high-limit safety, and make an air conditioner coil freeze in summer. The post on leaky and crushed ducts covers the other way airflow gets lost.
Filters on a new build fill up fast. In a Tigard new construction house, the main return filter and the fresh air intake filter were already caked with construction dust.
Real Inspection Photos
Filter Findings From Russ's Inspections
Dirty, doubled, wrong-size and loose filters from Russ's recent reports.
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Two filters installed at once, both dirty. Doubling up restricts airflow. -
A 16 by 20 inch filter in a 16 by 25 inch housing, leaving room for air to bypass it. -
No filter housing; the filter was found lying on top of the blower motor. -
A heavily clogged filter, not seated in its housing, so air could go around it. -
A fresh air intake filter bunched up in its housing instead of lying flat.
Direction, size and fit
Every filter has an arrow on the frame. It points the way the air flows: toward the furnace blower. Filter makers build the media to face the airflow one way, and backwards, the side meant to catch dust faces the wrong direction.
Size counts too. A filter smaller than its slot lets air go around it, which defeats the point. Russ found that in Springfield, and filters displaced or not seated in their racks in Portland and Lake Oswego, so return air was bypassing them. A Beaverton furnace had no proper filter rack, and the filter had collapsed into the return duct. In Battle Ground and Vancouver, the filter grille or cover didn’t sit flush, and a Beaverton media filter housing door was held shut with tape.
Stacking two filters is a common idea that backfires. In Portland, Russ found two dirty filters in the same furnace. Doubling up restricts airflow far more than one good filter.
How often to change it
Filter makers commonly suggest every 1 to 3 months for a standard 1-inch filter. Thicker 4- or 5-inch media filters in a cabinet next to the furnace last longer, often 6 months to a year. Pets, a remodel, wildfire smoke and a house full of people shorten all of those.
The best schedule is the one in the furnace or filter cabinet manual. Write the date on the filter frame when you put it in. Next time you’ll know.
Higher MERV ratings catch finer particles. Some systems handle a high-MERV filter fine and some don’t, because a denser filter restricts airflow. Russ doesn’t recommend a rating. An HVAC technician can tell you what your system was designed for.
Electronic air cleaners and abandoned upgrades
Some older furnaces have an electronic air cleaner, a cabinet with metal cells that need washing. In Oregon City, Russ found one that wasn’t operating. These are often left in place after the owner switches to ordinary pleated filters. That works, as long as a filter still sits somewhere in the airflow.
The other filters in a newer house
Newer houses, especially in Clark County, often have more filters than the furnace:
- Fresh air intake filters. Many newer homes bring in outdoor air through a duct to the return, with its own filter. Russ found these dirty, missing, bunched up or loose in Ridgefield, Brush Prairie, Vancouver and Gresham, and in one Ridgefield house the refrigerant lines blocked the access door.
- ERVs and HRVs. Whole-house ventilators have filters too. A Portland ERV was switched off with a dusty filter.
- Heat pump water heaters. These pull air across a coil and have an intake filter that needs cleaning. Russ found dirty ones in Ridgefield, Vancouver and Brush Prairie, and two in Vancouver blocked by water lines so the filter couldn’t come out.
The post on heat pumps and ductless mini-splits covers the washable filters in each indoor head.
What Russ checks
He finds the filter, pulls it when it’s accessible, and notes its condition, size, fit and direction. He looks for fresh air intake filters, ventilator filters and heat pump water heater filters where those systems exist. He doesn’t test airflow or rate filtration. A dirty filter is a maintenance item; a missing rack, a filter that can’t be removed or a blocked access goes to a licensed HVAC contractor.
- Find the filter slot at the furnace or at a return grille in a wall or ceiling
- Check the arrow points toward the furnace
- Make sure the filter fills the slot with no gap around it
- Look for a date written on the frame
- Ask whether the house has an ERV, HRV, fresh air intake or heat pump water heater with its own filter
A dirty filter was the most common HVAC finding in Russ’s reports, and one of the cheapest to fix. Backwards, undersized, doubled-up and missing filters came up too. Newer homes often have filters in more than one place.
For the rest of the heating and cooling system, the guide to what to look at in the HVAC before you make an offer covers age and service history. To set up an inspection, you can book online.
Furnace Filter FAQ
