Changing an HVAC filter can be a two-minute job. The reasons to have it done properly in a Daly City home are the ones you can’t see from the hallway: a filter rated higher than the blower can push air through, a filter that is the wrong size for its slot, a second filter nobody knew about, or a slot cover at the furnace that went missing years ago and lets the blower pull unfiltered garage air.
MERV in plain numbers
MERV (Minimum Efficiency Reporting Value) rates how much of three particle-size bands a filter captures in a standard lab test. The higher the number, the smaller the particles it catches well.
| MERV | What it catches well | Where it fits |
|---|---|---|
| 1–4 | Lint, hair, large dust | Protects the equipment; does little for fine dust |
| 5–8 | Household dust, most pollen, mold spores | Everyday filtration for most homes |
| 9–12 | More of the finer dust, including more pet dander | A step up where pets or dust are the complaint |
| 13–16 | At least half of the finest test particles (0.3–1 micron), the size range of much wildfire smoke | Smoke days and finer filtration, if the system can move air through it |
Why a higher MERV isn’t automatically better
A denser filter resists airflow. On a system built for it, that’s fine. On a furnace with a 1-inch slot and a blower already near its limit, a high-MERV filter can cut airflow enough to cause trouble: the heat exchanger runs hotter and the high-limit switch shuts the burners off, and on a system with air conditioning or a heat pump the indoor coil can freeze. Before recommending an upgrade we measure static pressure, the resistance the blower is working against, with the filter in place, and compare it with what the blower is rated for.
If the numbers are tight, the usual answer is a deeper filter rather than a lower rating. A 4- or 5-inch media filter has several times the pleated surface of a 1-inch one, so it can reach MERV 11 or 13 with less resistance and lasts longer between changes. Adding a cabinet for one is covered on our air purification page.
Where your filter actually is
In Daly City’s mid-century homes, the living space often sits over a ground-floor garage with the furnace in it. The filter can be in one of three places:
- In a slot at the furnace, usually behind a thin sheet-metal cover that is easy to lose. Without the cover, the blower draws garage air straight past the filter.
- Behind a return grille in a hallway wall or ceiling, held by two latches or screws.
- In both places. Two dense filters in a row double the resistance. Usually one location should hold the real filter, and the other should stay empty with its cover closed, or hold a low-resistance screen.
Condos and newer homes often have an air handler in a closet, with the filter inside the unit itself.
How often to change it
- 1-inch pleated filter: check monthly; change every one to three months.
- 4- or 5-inch media filter: commonly every six months to a year.
- Pets or a full household: toward the short end of either range.
- During renovation or construction: check weekly, and switch the system off during sanding and demolition. Change the filter once the dusty work is done.
- After smoke days: check it as soon as the air clears; a filter that has caught smoke darkens fast.
- Before spring pollen: start the season with a fresh filter.
Because fog-belt homes run the heat in most months, a filter here tends to load steadily through the year rather than in one short winter. A calendar reminder works better than waiting for the first cold snap.
What the visit includes
Most visits take 15 to 30 minutes. We find every filter location and check the condition of the old filter, its fit and any gaps around it. We measure the slot itself, since the size printed on a filter frame is a rounded nominal size, and make sure the airflow arrow points toward the blower. If you want an upgrade, we work out which MERV rating your system can actually handle. If the blower wheel or coil is coated with dust that got past an old filter, we tell you; that cleaning is a separate job (coil cleaning). Before we leave, you know the exact size and rating to buy and how often to change it.
What the service costs
Filter replacement runs $50 to $150. The filter itself is the main variable: a standard 1-inch pleated filter sits at the low end, while a deep media filter, or a system with two filter locations, costs more. You see the price in writing before we start. Many owners pair it with furnace maintenance or include it in a maintenance plan.