Ducts & air
MERV filters: filtration vs airflow, and the replacement schedule
A better filter catches more and restricts more. The answer is rarely the cheapest or the highest MERV; it is the right rating in a filter deep enough for the system.
What a filter is for
The filter in a furnace or air handler was originally there to protect the equipment: to keep dust off the blower and the cooling coil, where it cuts airflow and heat transfer. It also cleans the air in the house, to a degree that depends on its rating and on how much of the time the system runs. A filter that catches more particles does more for the air and the coil, but it also resists airflow more, and airflow is what the whole system depends on.
The MERV scale
MERV, minimum efficiency reporting value, comes from ASHRAE Standard 52.2. A filter is tested against particles in three size ranges and given a rating from 1 to 16 according to the lowest efficiency it reaches in each range over the test:
| MERV | Particles 3 to 10 µm (pollen, dust mite debris, mould spores) | Particles 1 to 3 µm (fine dust, some bacteria) | Particles 0.3 to 1 µm (smoke, fine combustion particles) |
|---|---|---|---|
| 1 to 4 | Under 20% | Not rated | Not rated |
| 8 | At least 70% | Not rated | Not rated |
| 11 | At least 85% | At least 65% | At least 20% |
| 13 | At least 90% | At least 85% | At least 50% |
A cheap fibreglass panel at MERV 1 to 4 protects the equipment from lint and little else. MERV 8 is the usual standard for a house. MERV 11 to 13 catches a useful share of fine particles, which helps with allergies, and the EPA suggests MERV 13, or the highest rating the system can take, for wildfire smoke.
The trade-off: pressure drop
Every filter takes some of the blower’s available pressure, and the more it catches, the more it takes. A 1-inch filter has a small face area of filter media folded into a thin frame; at a typical 300 FPM face velocity, a 1-inch MERV 8 pleated filter might take around 0.10 in wc when clean, and a MERV 13 considerably more. A 4- or 5-inch media filter of the same rating has many times the media area, so the air moves through it more slowly and it takes much less, around 0.05 in wc in the example on the duct equivalent length page.
On a system with a 0.5 in wc blower, every tenth of an inch matters. The guide to duct sizing basics shows how a 0.5 in wc blower can be left with only 0.14 in wc for all the ducts after the coil and filter. A dense filter can take that to almost nothing, and the airflow falls.
Signs a filter is too restrictive
- A 1-inch filter that bows inwards or sucks into the grille.
- Whistling at the return grille, or a big jump in noise when the filter is new.
- A cooling coil that freezes, or a furnace that trips its high-limit switch, after changing to a higher-MERV filter.
- A technician measuring high total external static pressure.
If the system cannot take MERV 11 or 13 in a 1-inch slot, the fix is more filter area, not a lower rating. A return grille with a filter in it, sized with the return air grille size calculator at 300 FPM, or a 4- or 5-inch media cabinet at the air handler, gives a high rating at a low pressure drop. The guide to return air covers the related problem of an undersized return.
How often to change it
A filter loads up as it works and its pressure drop rises, so the interval matters as much as the rating. The filter cost calculator starts from typical maker guidance by thickness and shortens it for pets, allergies and dusty conditions:
| Thickness | Typical interval | Typical price, MERV 8 |
|---|---|---|
| 1 in | 90 days | $12 |
| 2 in | 120 days | $20 |
| 4 in | 180 days | $35 |
| 5 in | 270 days | $50 |
Two households
A 1-inch MERV 8 filter, no pets, system running all year:
- Interval: 90 days; filters a year: 365 ÷ 90 = 4.06, so 5
- Cost: 5 × $12 = $60 a year
A 1-inch MERV 11 filter with two pets and allergies:
- Interval: 90 × 0.5 × 0.75 = 34 days
- Filters a year: 365 ÷ 34 = 10.7, so 11
- Cost: 11 × $12 = $132 a year
Check a 1-inch filter monthly in the cooling and heating seasons until you know how fast it loads. A deeper media filter usually costs less a year, despite its higher price, because it lasts several times longer.
How fast air moves
Filter face velocity is the airflow divided by the filter’s face area. The duct velocity calculator works it out for any opening: 1,200 CFM through a single 20 × 25 in filter is 1,200 × 144 ÷ 500 = 346 FPM. Most 1-inch filters are tested at around 300 FPM; above that their pressure drop rises quickly.
Filters and room air purifiers
A central filter cleans the air only while the blower runs. Set the fan to run for part of each hour, or use a portable purifier in the rooms that matter. The air purifier CADR calculator sizes one by the AHAM rule: a clean air delivery rate of two thirds of the room’s floor area, so 112 CFM for a 168 sq ft bedroom. Running the central fan continuously in humid weather has a cost, which the guide to humidity and comfort explains: it can blow moisture from the coil back into the house.