Static pressure too high after a thick filter
A thicker furnace filter can restrict airflow when the HVAC system or filter slot was not designed for it. Check the filter fit, airflow…
Troubleshooting
Help choosing furnace filter size and MERV rating without starving the furnace for air.
MERV & Airflow
Troubleshooting
Allergy season filling a filter in two weeks
A furnace filter that loads up in two weeks during allergy season may be catching an unusually high amount of pollen and dust—but it can…
Collapsing cardboard frame on a cheap filter
A collapsed cardboard furnace-filter frame usually means the filter is too weak for the airflow, installed incorrectly, wet, clogged, or…
Filter collapsing in the slot
A furnace filter that collapses in its slot usually has a fit, airflow, installation, or excessive-resistance problem. Check the filter…
Filter wet from an AC coil
A wet filter should not be left in place. Turn off cooling, replace the filter after finding the source of the moisture, and check for a…
Furnace filter clogged after a month
A furnace filter that clogs after only a month may be catching an unusual amount of dust—or restricting airflow because of the wrong…
Furnace whistling after a new filter
If your furnace whistles after installing a new filter, the most likely causes are a restrictive filter, incorrect size, poor fit, or an…
High MERV filter tripping the furnace limit
If a high-MERV filter is tripping your furnace limit switch, restricted airflow is a likely cause. Check the filter, return and supply…
Wrong size filter leaving gaps: how to fix it
Gaps around a furnace filter let unfiltered air bypass the media. Learn how to measure the opening, identify the correct nominal size, and…
Dust Still on Furniture After Filter Changes
A new furnace filter can reduce airborne particles, but it cannot prevent all dust from settling. Check the filter’s size and seal first,…
Foam Cut-to-Fit Filters in a Return Grille: What to Know
Foam cut-to-fit media is not automatically suitable for a furnace return grille. Use it only if the HVAC or grille manufacturer approves it…
Setup
Sizing
Bypass around an ill-fitting filter
Air bypass around a furnace filter usually comes from a size mismatch, gaps in the filter slot, or a filter that is bent or installed…
Buying the Next Thickness Without a Cabinet Change
A thicker furnace filter is only an option if the filter cabinet or rack is built to accept it. Check the opening, filter dimensions, and…
Return grille filter versus a slot filter confusion
A filter may go behind a return grille or in a slot near the furnace or air handler. Find the system’s intended filter location before…
