Advice
How often should I change the filter?
The one maintenance job you should do yourself — and the purchase mistake that does more harm than good.
The simple rule
| Filter type | Frequency |
|---|---|
| Thin disposable (25 mm) | every 1 to 3 months |
| Medium-efficiency pleated | every 3 months |
| Large-surface cabinet filter (100–125 mm) | every 6 to 12 months |
| Washable | clean every 1 to 3 months |
Check it monthly, replace it when it is dirty. The schedule is only a starting point.
What clogs a filter faster
- Pets, especially ones that shed.
- Renovation work — even a single drywall sanding session clogs a filter in days.
- A heat pump, which runs year-round and therefore moves far more air.
- The blower set to run continuously.
- An unfinished basement, or living near a construction site.
The most common buying mistake
Buying the densest filter on the hardware store shelf, thinking it is the better choice.
Past a certain point, a filter restricts airflow. The blower strains, heat builds up inside the equipment in heating mode, and the high-limit safety eventually trips. In cooling, insufficient airflow can freeze the evaporator coil.
Put plainly: a filter too aggressive for your equipment can hurt it more than an ordinary filter changed on schedule.
The right way to get high filtration is not more density in the same space, but more surface area: a 100–125 mm filter cabinet mounted on the return gives markedly better filtration with less restriction, and only needs changing once or twice a year.
How to tell a filter needs changing
- You can no longer see light through it.
- Airflow at the registers has visibly dropped.
- A layer of grey dust covers the surface evenly.
- The equipment cycles shorter than it used to.
What it actually changes
A clean filter means normal airflow. Normal airflow means equipment that reaches its rated performance, cycles properly and does not overheat.
It is also, very concretely, the most common cause of service calls that need never have happened. We would rather tell you here than charge you a call-out to find it.
Read next
- My furnace keeps starting and stopping The possible causes, from the simplest — which you can check yourself in two minutes — to the one that needs a technician.
- Getting your system ready for winter What to check in September so you do not have to call us in January — and what to do if it fails anyway.
Frequently asked questions
Which MERV rating should I choose?
For most homes, a filter between MERV 8 and MERV 11 balances filtration against airflow well. Above that, check your equipment is designed for the extra restriction — a large-surface filter cabinet is almost always a better way to get high filtration.
Which way does the filter go in?
The arrow printed on the frame points in the direction of airflow, meaning toward the equipment. A filter installed backwards filters less well and deforms.
Let’s talk about your project
One of our advisors comes to your home, measures what needs measuring and walks you through your options — free, and with no pressure.
Monday to Friday · 8:30 a.m.–12 p.m. and 1 p.m.–5 p.m. Outside business hours, our 24-hour emergency service is reachable at the same number.