The furnace filter is the cheapest job in the house and one of the easiest to lose track of. It does not fail loudly. A clogged one just makes the system work harder, week after week, until something else gives.
Three months is a sensible default for a normal one inch filter in a normal house. The rest of this page is about when to shorten that, and how to stop doing the store-aisle guess.
When to change it sooner
These are ordinary rules of thumb. Your manual and your system's maker have the final say.
Pets that shed
- What tends to happen
- The filter clogs with hair and dander faster
- A sensible change
- Check monthly, change as needed
Someone with allergies
- What tends to happen
- You want the filter working at its best
- A sensible change
- Check monthly
Renovation or heavy dust
- What tends to happen
- It can fill in days
- A sensible change
- Check after the work, change if grey
An empty or seldom-used house
- What tends to happen
- It loads slowly
- A sensible change
- Three months is usually plenty
Heating or cooling all season
- What tends to happen
- It works most of the year
- A sensible change
- Every one to three months
| Situation | What tends to happen | A sensible change |
|---|---|---|
| Pets that shed | The filter clogs with hair and dander faster | Check monthly, change as needed |
| Someone with allergies | You want the filter working at its best | Check monthly |
| Renovation or heavy dust | It can fill in days | Check after the work, change if grey |
| An empty or seldom-used house | It loads slowly | Three months is usually plenty |
| Heating or cooling all season | It works most of the year | Every one to three months |
How to read the size
The three numbers on the edge of the filter are its length, width and depth in inches. A filter marked 16x25x1 is about sixteen inches by twenty-five inches, and one inch thick. Sizes are usually nominal, so the real dimensions can be a fraction of an inch smaller, which is why you buy by the printed number and not by measuring.
Write down the size on the day you change it. Also note the direction of the airflow arrow, and whether the filter slides in from the side or the front. These are the two things you will forget by the next change.
Write these down once
5 things to do
Ticks are for this visit only. Nothing here is saved anywhere.
Where to keep it
The best place is somewhere you will be standing when you need it: on a strip of tape on the furnace, on your phone, or on a card you keep in your wallet. The same goes for the bulb type, the part number for the humidifier pad, and any other fact you look up every time. Some people call this the hardware store line.
For the wider habit, see how often things in your house actually need servicing, and for finding a model number on an appliance, how to find the model number on any appliance.