By Derim “Mike” Vinca, Owner of A-1 Milwaukee Heating & Cooling · 17+ years in the trade · Updated October 2026 · More HVAC guides
The filter is the cheapest part of your heating and cooling system and the cause of more service calls than any other part. Here is how to choose one and how often to change it. Questions? Call or text 414-595-5175.
How often to change it
- 1-inch filters: every 1 to 3 months, and monthly if you have pets, allergies, a smoker or a renovation going on.
- 4- to 5-inch media filters in a cabinet next to the furnace: every 6 to 12 months.
- Washable or electronic filters: clean them on the schedule in the owner’s manual.
The simple test: pull the filter out and hold it up to a light. If you cannot see light through it, change it. In Milwaukee, check it at the start of the heating season and again in midwinter, when the furnace runs the most.
Which MERV rating to buy
MERV tells you how fine a particle a filter catches. For most homes, MERV 8 to 11 is the sweet spot: it catches dust, pollen and pet dander without choking the airflow. Higher ratings catch more, but a 1-inch MERV 13 filter can restrict airflow on some systems, which makes the furnace overheat and the air conditioner coil freeze. If you want MERV 13 filtration, a 4- or 5-inch media cabinet gets you there without the restriction. We can measure your system’s static pressure and tell you what it can handle.
Find the right size
The size is printed on the edge of the old filter, for example 16x25x1. That is a nominal size and the real filter is slightly smaller, so buy the size printed on the frame. If the old filter is missing, measure the slot and we can help you match it.
Put it in the right way
Every filter has an arrow on the frame. It points the way the air moves: toward the furnace and away from the return duct.
Where to find it
Usually in a slot between the return duct and the furnace, sometimes behind a door on the furnace itself, and sometimes behind a large return grille in a wall or ceiling. If you find filters in two places, ask us before you buy two; stacking filters can choke the airflow.
What a dirty filter costs you
- Higher gas and electric bills, because the system runs longer to move the same air
- A furnace that shuts off on its high-limit switch and short cycles (see furnace short cycling)
- A frozen air conditioner coil in summer (see AC not cooling?)
- More dust in the house
- Years of overheating, which is hard on the heat exchanger
Let us check it for you
If you are not sure which filter your system takes, or why it clogs so fast, we check the filter and the airflow on maintenance visits. Our Comfort Club maintenance plan includes a yearly visit for each system, and repairs are guaranteed for one year, parts and labor.
Frequently asked questions
Does a more expensive filter mean cleaner air?
Up to a point. MERV 8 to 11 is right for most homes. Past that, a 1-inch filter can restrict airflow more than the system was designed for; a 4- or 5-inch media filter is the better way to get finer filtration.
Can a dirty filter cause no heat?
Yes. It makes the furnace overheat until a safety switch shuts the burners off. Replace the filter, then turn the furnace switch off for 30 seconds and back on. If it still will not heat, see our guide Furnace not turning on.
Should I change the filter in summer too?
Yes. The same filter protects the air conditioner’s indoor coil, and a clogged filter is one of the most common reasons an AC freezes up.