Installing a 1-inch furnace filter is quick — if the size matches and the airflow arrow points the right way. This illustrated guide walks through both. Watch the short walkthrough below, then use the diagram for the airflow arrow.
\n\nThe one diagram that prevents the #1 mistake
Air moves from the dirty air side (return) through the filter to the clean air side (toward the blower). The black arrow on the frame — the airflow indicator — must point toward the clean air / blower side. Memory trick: arrow flies toward the fan.
Step 1 — Size first
Read the printed nominal size on the old frame (for example 16x25x1 or 20x25x1). Order that size. Full walkthrough: how to measure a furnace filter.
Step 2 — Turn the system off
Switch the HVAC off at the thermostat before you open the slot or return grille.
Step 3 — Find the slot and remove the old filter
Filter slot is usually at the furnace / air handler or behind a wall/ceiling return grille. Slide the old filter out. Optional: photo the arrow direction before you pull it.
Step 4 — Install with the arrow toward the blower
Unwrap the new pack. Find the airflow indicator on the cardboard frame. Point it toward the furnace blower (clean air side in the diagram above). Seat the frame flat — no crushed edges, no big gaps.
Step 5 — Close up and mark the date
Replace the cover. Write the install date on the frame. Many homes swap about every 90 days.
MERV note
Pick MERV 11 or MERV 13 from what your furnace manual allows — not a medical upgrade. See MERV 11 vs MERV 13.
More from Caliza
Text install guide · Filter FAQ · 16x25x1 · 20x25x1
Caliza Home is a small Southern California shop. HVAC packs use the Aerostar line with manufacturing tied to Orlando, Florida factory operations for this line. Not a medical device. No softener or 99.9% claims.