How to replace the HVAC filter and record its size
To replace an HVAC filter, turn the system off, slide out the old filter and copy the size printed on its frame. Put in a new one with the airflow arrow pointing toward the furnace or air handler. Check it monthly. One-inch filters usually last 1 to 3 months, and thicker media filters last longer.
HVAC filter size and MERV rating to buy
Three things decide what you buy: size, thickness and MERV rating. Size is printed on the frame, written as width, height and depth. Buy that exact size. A filter that's too small lets air slip around it.
MERV runs from 1 to 16, and a higher number catches more small particles. EPA says that if you're upgrading, aim for at least MERV 13, or the highest your fan and filter slot can handle. The table shows what that rating means.
| Particle size | A MERV 13 filter captures at least |
|---|---|
| 0.3 to 1.0 microns | 50% |
| 1.0 to 3.0 microns | 85% |
| 3.0 to 10.0 microns | 90% |
Tighter filters can restrict airflow, so don't jump to the top of the range on a guess. Systems with small returns or 1-inch slots can struggle, according to one inspector's guide summarizing EPA advice. Deeper 4- to 5-inch media filters give high efficiency with less airflow resistance. Ask an HVAC technician what your system can handle. True HEPA filters aren't made for most home furnaces.
Where the furnace filter goes and how often to replace it
Look in two places. Many homes have the filter in a slot on the furnace or air handler, near where the return duct joins the cabinet. Others hold it behind a large grille in a wall or ceiling, called a return grille.
The arrow on the frame shows airflow, so it points toward the furnace or air handler. At a return grille, that means it points into the wall or ceiling. If you find a filter already in place, don't assume it was put in correctly. Check the arrow against the direction of airflow.
| Filter type | How often to replace | Replace sooner if |
|---|---|---|
| 1-inch | About every 1 to 3 months; check monthly | You have pets, smokers, allergies, renovation dust or wildfire smoke |
| 4- to 5-inch media | Often every 6 months to a year | Same list, or the maker says so |
ENERGY STAR says to inspect, clean or change filters once a month, and the maker's instructions come first.
How to replace an HVAC filter, step by step
- Turn off the system
Switch it off at the thermostat. If the panel you have to open has a separate power switch, turn that off too.
- Find every filter
Check the furnace or air handler, then each return grille.
- Pull out the old filter
Note which way the arrow was facing, and how dirty it is. A gray or brown filter packed with dust was overdue.
- Copy the size
Take a photo of the frame, or write the size down.
- Install the new filter
Slide it in with the arrow pointing toward the furnace or air handler, and close any access panel fully.
- Date it and restock
Write the date on the frame, set a monthly reminder, and buy spares in the right size.
What it costs to replace an HVAC filter
| Filter | About what it costs (estimates) |
|---|---|
| 1-inch fiberglass | $2 to $6 |
| 1-inch pleated, MERV 8 to 11 | $6 to $20 |
| 1-inch pleated, MERV 13 | $10 to $30 |
| 4- to 5-inch media | $25 to $80 |
| Adding a media cabinet (installed) | $300 to $900 |
A dirty filter costs money as well. ENERGY STAR says it raises energy costs and can damage equipment, leading to early failure.
Mistakes to avoid when you replace an HVAC filter
- Fitting it backward, against the airflow.
- Forcing a filter that's too thick into a slot. A 4-inch filter needs a cabinet built for it.
- Going by the size printed on an old box, not the filter frame.
- Running the system with no filter while you shop.
- Treating a new filter as a cure-all. If airflow stays weak, rooms stay uneven, or the system short-cycles, call a technician. The cause may be a failing blower motor or a blocked return.
What homeowners say about getting the HVAC filter size right
Find every filter location first, since some houses have more than one return or air handler. One owner found the filters hadn't been changed in six years. Also check the fridge water filter size and stock spares of both. (Reddit)
Change the fridge water filter and any other filters on the same day, so every filter has a known start date. Some owners set up a filter subscription that ships every 3 to 4 months, so they never run out. (Reddit)
Write the filter size and change date on the furnace cabinet or return grille, so you never pull the old filter at the store to check the size. (Hacker News)
Where owners disagree: Owners and HVAC techs in several threads push back on the highest-rated filters for ordinary 1-inch slots. They call MERV 8 (some say up to 10) the sweet spot, because dense filters can choke airflow in systems not built for them. One owner's furnace techs said to buy cheap pleated filters and change them monthly. Check what your blower is rated for before buying a bulk pack of MERV 13. (Reddit)
Questions on how to replace an HVAC filter
Which way does the arrow on a furnace filter go?
Toward the furnace or air handler, in the direction air moves. At a return grille, that means pointing into the wall or ceiling.
Is MERV 13 too high for my furnace?
It depends on your system. EPA advises checking that your fan and filter slot can handle it, and asking an HVAC technician if you aren't sure.
How often should I change my HVAC filter?
Check it monthly. One-inch filters usually last 1 to 3 months, thicker media filters last longer, and the manufacturer's instructions come first.
What you’ll need
Sources: EPA, ENERGY STAR, inspectandtest.net
General guidance for U.S. homes, not professional advice. For gas, electrical and structural work, call a licensed pro.