The air filter in your furnace or air handler protects the equipment from dust and can improve indoor air quality. Choosing one involves two numbers: the size and the MERV rating.
Key Takeaways
- Find the filter size printed on the existing filter’s frame, and match it exactly.
- MERV ratings run from 1 to 16 for residential and commercial filters; higher numbers capture smaller particles.
- MERV 8 to 13 is a common range for homes, but check what your system can handle.
- Check filters monthly and replace them according to the filter type and conditions.
Finding the right size
Look at the edge of your current filter for the nominal size, such as 16x25x1. The actual dimensions are often slightly smaller. Buy the same size; a filter that is too small lets air bypass it.
Note the airflow arrow on the frame, which should point toward the furnace or air handler.
What MERV means
MERV, minimum efficiency reporting value, is a standard rating from ASHRAE for how well a filter captures particles of various sizes.
- MERV 1 to 4: basic fiberglass filters that protect equipment from large debris.
- MERV 5 to 8: common pleated filters that capture dust, pollen and mould spores.
- MERV 9 to 12: better capture of finer particles.
- MERV 13 to 16: captures many fine particles, including some smoke and bacteria-sized particles.
Is higher always better?
Not necessarily. Denser filters can restrict airflow, especially in systems not designed for them or when the filter is thick with dust. Restricted airflow can reduce efficiency and comfort. Check your HVAC manufacturer’s recommendations, or ask your technician, before moving to a high MERV filter. Thicker media filters of 4 or 5 inches can offer higher filtration with less airflow restriction, but they require a compatible cabinet.
How often to change it
Common guidance is every one to three months for 1-inch filters, and longer for thicker media filters. Change it more often if:
- You have pets
- Someone in the home has allergies or asthma
- There is construction or renovation dust
- It is wildfire smoke season
- The system runs heavily in peak heating or cooling
Check monthly. If the filter looks grey and clogged, replace it.
Signs of a clogged filter
- Reduced airflow from vents
- The system running longer
- More dust on surfaces
- Higher energy bills
Write it down
Write the installation date on the filter frame and set a reminder. Buying a multi-pack saves trips.
Other air quality steps
- Run bathroom and kitchen exhaust fans.
- Vacuum with a HEPA vacuum.
- Use portable air cleaners for specific rooms if needed.
Frequently Asked Questions
Can I wash and reuse filters? Only filters labelled as washable. Most pleated filters are disposable.
Does a better filter lower energy bills? A clean filter helps efficiency. A very dense filter can raise energy use if it restricts airflow.
What about electrostatic filters? They can work well; check their MERV rating and maintenance instructions.
The bottom line
Match the size exactly, choose a MERV rating your system can handle, and check the filter every month.
