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ChillVentVale

Late summer — the furnace filter that went in at the start of the cooling season, and a portable unit with a few weeks left to run.

The Replacement Calendar

Home climate parts that actually fit, 2026

Start here

I have a furnace filter

It says something like 20x25x1 on the cardboard, and it does not measure that.

Start from the printed number and see what the listings actually state.

Size Finder

Start here

I have an air purifier

The cartridge has a code on the rim — Core 300-RF, FLT4825, something like it.

Seven codes, with genuine parts kept separate from seller compatibility claims.

Every Filter Page, by Size and by Code

Start here

I have a thermostat or a unit

A thermostat that may not suit your system, or a box with two BTU numbers on it.

Six categories, read for what each listing states and what it leaves out.

The Catalog in Six Categories

NOMINAL — PRINTED ON THE BOX20 x 20 x 1 inACTUAL — AS STATED19.50–19.75 x 19.50–19.75 x 0.75 in0.25 to 0.50 in under nominal on the faces
Depth — 0.75 in stated on all three listings, against a nominal 1 in

The whole problem, in one drawing

20x20x1 is a name. It is not a measurement.

Three filters in this catalog are called 20x20x1. One states an actual size of 19.50 inches per face. The other two state 19.75. A quarter of an inch, at the same printed size, and neither of them is mislabeled — nominal sizing describes the slot, and each manufacturer picks its own clearance.

The depth is worse. Every one-inch filter listed here states an actual depth between 0.75 and 0.81 inches. Not one of them is an inch. Put a 0.75 frame in a slot railed for 0.81 and air takes the gap instead of the media.

Read the size decoder

Five furnace sizes, and what they actually measure

Nominal against actual, straight from the listings. Depth is broken out separately because it is the dimension that catches people.

NominalActual, as statedDepthMERV options in this catalogPage
20x25x119.62–19.68 x 24.62–24.68 x 0.78–0.81 in0.78–0.81 in stated, against a nominal 1 inMERV 11, MERV 1220x25x1 Furnace Filters
20x20x119.50–19.75 x 19.50–19.75 x 0.75 in0.75 in stated on all three listings, against a nominal 1 inMERV 5, MERV 820x20x1 Furnace Filters
16x25x115.50–15.68 x 24.50–24.68 x 0.75–0.81 in0.75–0.81 in stated, against a nominal 1 inMERV 8, MERV 1116x25x1 Furnace Filters
20x30x119.81–19.88 x 29.81–29.88 x 0.81 in0.81 in stated on both listings, against a nominal 1 inMERV 5, MERV 820x30x1 Furnace Filters
20x25x419.50 x 24.50 x 3.63 in3.63 in stated, against a nominal 4 inMERV 820x25x4 Furnace Filters

COOLING

Heat has to go somewhere, and air has to move

A portable unit moves heat out of a window. A fan does not remove heat at all. Both get sold as cooling, and only one of them is.

HEATING & HUMIDITY

Adding moisture, and the one that boils water

Cool mist and warm steam are not two flavors of the same appliance. One of them heats water to make steam, and that changes where it can stand.

Or start from the code on the cartridge

Machine names get reused across generations. The code molded into the plastic rim does not.

Tool one of two

Size Finder

Pick what is in your hand and the number on it. Everything it shows is a figure a listing states, not an average of anything.

What are you replacing

The size printed on the frame

20x25x1 — nominal against actual

NOMINAL — PRINTED ON THE BOX20 x 25 x 1 inACTUAL — AS STATED19.62–19.68 x 24.62–24.68 x 0.78–0.81 in0.32 to 0.38 in under nominal on the faces
Depth — 0.78–0.81 in stated, against a nominal 1 in

In this catalog at that sizeMERV 11 and MERV 12

  • Actual sizes differ between manufacturers at the same nominal name, so measure the filter you have and read the stated actual size on the listing you order from.
  • Note which way the airflow arrow points before the old filter leaves the slot.
Open the 20x25x1 page

Actual sizes differ between manufacturers at the same nominal name. Whatever this tool shows, measure the filter you are holding and read the stated actual size on the listing you order from.

5 scales that describe the same shelf and disagree

Scroll sideways. Each one takes a single product and reads it through two competing numbering systems.

Native horizontal scroll with snap points — drag, swipe, or use the arrow keys after focusing a card.

48

listings, every one with the figures its own listing states

6

catalog categories, each with a stated job it is sorted around

12

compatibility pages — five sizes and seven cartridge codes

2026

the year these pages were last revised

The room decides the filter, the fan and the humidifier together

Which is why these pages run cooling down one side and heating and humidity down the other, with a list of things to skip at the bottom.

Tool two of two

BTU Check

Two capacity numbers, one machine. This works out which one to read and prints the arithmetic underneath the answer.

Floor area of the room

Direct afternoon sun

A window the exhaust hose can reach

Which number to read on the box

5,000 to 6,500 BTU SACC

Compare that against the smaller figure in the specifications, not the larger one in the product name. The two describe the same machine measured two different ways.

The arithmetic came out below the band shown, and the band has been held at the bottom of what this catalog stocks: 5,000 BTU SACC is the smallest SACC figure any listing here states.

Stated SACC figures in this catalog near that rangeBLACK+DECKER Portable AC, 9,000 BTU at 5,300 BTU SACC; Midea Portable AC, 8,500 BTU ASHRAE at 5,000 BTU SACC.

The arithmetic, in full

  1. Base load = floor area in square feet × 20 BTU/h per square foot, read as a SACC figure.
  2. If the room takes direct afternoon sun, add 10 percent to the base load.
  3. Round the result to the nearest 500 BTU.
  4. Open a band 1,500 BTU wide, starting at that figure.
  5. Hold the whole band inside the 5,000 to 12,000 BTU SACC span the listings in this catalog state — both ends of that span are read off the data rather than typed in.

Worked for this selection: 200 sq ft × 20 = 4,000 BTU, with no sun uplift applied, rounded to 4,000 BTU.

What it does not account for: ceiling height, insulation, how many people are in the room, heat given off by equipment, how well the windows seal, or what the sun does to the wall rather than through the glass. It is a starting figure for reading a spec sheet, not a load calculation.

Six questions, answered without hedging

No named experts, no quotes from anybody. These are the answers the rest of the site works through in detail.

Why does my 20x20x1 filter measure 19.5 inches?
Because nominal size is a naming convention for the slot, not a measurement of the filter. Manufacturers build under the nominal figure so it slides in, and each one decides how far under. In this catalog the gap on a 20-inch face runs from 0.12 to 0.50 in.
Should I just buy the highest MERV that fits?
No. Finer media resists airflow more, and the equipment manufacturer documentation states the rating and pressure drop the system was designed around. That document decides. A clean coarse filter is easier on a system than a loaded fine one.
Which BTU number is the real one?
Both are real, measured by different procedures. The larger ASHRAE figure goes in the product name; the smaller SACC figure comes from the Department of Energy procedure written for this appliance and is the one to size a room against.
Is a fan going to cool the room down?
It moves air. Moving air feels cooler on skin, but the air temperature in the room does not fall. Only a machine that sends heat out through a hose or a wall actually removes it.
Cool mist or warm steam humidifier?
A warm steam vaporizer boils water, and both the steam and the water in it can scald. That rules it out of any room a child can reach. Cool mist units have no heated water and no hot surfaces, which decides it before any feature does.
Will this thermostat work with my system?
Only the compatibility checker the manufacturer publishes for that specific model can say. Some systems — line voltage, millivolt, certain multi-stage and heat pump setups — are not compatible at all, and connecting one can damage equipment.

6 listings, one from each of the 6 categories

The listing that answers its category stated job most directly. Everything attached to each card comes from the listing itself.

Amazon links across this site are affiliate links, and a purchase made through one may earn a commission at no additional cost to you. No product here was tested by this site, and no placement was paid for.

  • Actual size stated
    Filterbuy 20x20x1 MERV 8, 4-pack

    Filterbuy 20x20x1 MERV 8, 4-pack

    • Nominal 20x20x1
    • Actual 19.50 x 19.50 x 0.75 in
    • MERV 8
    • 4 per box

    Nominal 20x20x1, and the listing states an actual size of 19.50 x 19.50 x 0.75 in — a full half inch under the printed number on both faces. That is the widest gap of any 20x20x1 listing in this catalog, which is precisely why the actual figure being published matters. MERV 8, electrostatic pleated, four to a box, made in USA per the listing.

    Best forA 20x20x1 slot where the difference between nominal and actual has already caused a problem once.

    Check it on Amazon
  • Code on the cartridge
    Levoit Core 300-RF Filter

    Levoit Core 300-RF Filter

    Core 300-RF is the code printed on the cartridge you are pulling out, and this is the genuine Levoit part carrying it. The listing states it is for the Core 300-P, that it is AHAM Verifide, and that it includes activated carbon. Matching the code beats matching the description of the machine, because Levoit ships several Core models with cartridges that look alike.

    Best forA Core 300-P where you would rather not think about whether a filter fits.

    Check it on Amazon
  • States what it controls
    Honeywell Home X2P Programmable Thermostat

    Honeywell Home X2P Programmable Thermostat

    This listing does the thing the others mostly leave to a compatibility tool: it states what system the thermostat controls. Single-stage, 1 heat and 1 cool, with a 5-2 day schedule. If your system is not single-stage, that sentence has already answered the question. Turn the power off at the panel before any old thermostat comes off the wall, and check compatibility with the manufacturer tool before ordering.

    Best forA single-stage system, confirmed with the manufacturer compatibility tool first.

    Check it on Amazon
  • Both BTU numbers stated
    DREO 515S Portable AC, 12,000 BTU ASHRAE

    DREO 515S Portable AC, 12,000 BTU ASHRAE

    • 12,000 BTU ASHRAE
    • 8,000 BTU SACC

    This is the unit that puts both numbers where you can see them: the listing states 12,000 BTU ASHRAE in the name and 8,000 BTU DOE in the details. Those are the same machine measured two ways, and the smaller one is the figure that gets compared against room area. The listing also states drainage-free cooling plus fan and dehumidifier modes.

    Best forSomeone comparing boxes and trying to work out which BTU number to trust.

    Check it on Amazon
  • Airflow given in CFM
    Levoit 2-in-1 Pedestal and Tabletop Fan

    Levoit 2-in-1 Pedestal and Tabletop Fan

    The only fan here whose listing states airflow as a number: 1024 CFM. Everything else in this group describes throw distance or speed count, which are not measurements of how much air moves. The listing also states 20 dB, 90 degree oscillation, a DC motor, 5 speeds and a 12 hour timer, and the unit converts between pedestal and tabletop.

    Best forA room where the airflow number matters and the fan has to work at two heights.

    Check it on Amazon
  • Cool mist, top fill
    Levoit Top Fill Humidifier, 2.5 L

    Levoit Top Fill Humidifier, 2.5 L

    Cool mist with a stated top fill, which is the difference that decides whether the tank actually gets cleaned. A tank you carry to the sink upside down gets rinsed less often than one you fill from the top and can reach into. The listing states 28 dB, up to 25 hours of runtime, and auto shut-off. Nothing here boils water.

    Best forA bedroom where the tank has to be easy enough to clean that it actually gets cleaned.

    Check it on Amazon

Six cards on a snapping track. No script runs this — it is the browser own overflow scrolling.

Three things this site will not soften

Warm steam and children

A warm steam vaporizer boils water. The steam and the water in the tank are hot enough to scald. Out of reach, on a stable level surface, never on the floor of a room where a child plays, and never on or in a bed.

CPSC

Thermostats and the service panel

Cut power at the service panel before an installed thermostat comes off the wall. Run the manufacturer compatibility checker first. Some systems are not compatible at all. This site publishes no wiring guidance, and unclear jobs go to a licensed HVAC professional.

U.S. Department of Energy

MERV and airflow resistance

Finer filter media resists airflow more. The equipment manufacturer documentation states the rating and pressure drop the system was built around, and that document is what decides. Nobody here is in a position to recommend the highest number that fits.

ASHRAEEPA

How this site is written

  1. Every product characteristic comes from that product listing, and is written as what the listing states. Nothing is inferred from a similar product.
  2. Compatibility lists contain only the machines a listing names. A cartridge is never assumed to fit a machine because the model numbers look related.
  3. Seller language about capture percentages, contaminant categories, or relief from symptoms is quoted as seller language. It is never restated as a result, and there are no health claims anywhere on this site.
  4. No product here was tested, measured, or used by anybody writing about it, and no page says otherwise.
  5. Standards, test methods and safety statements come from published documents — EPA, ASHRAE, the Department of Energy, CPSC, CDC, NIST — with the link included so the source can be read directly.
  6. Where a listing is silent, the page says it is silent. That costs some completeness and it is the right trade.

To repeat what is said at the top of the page: the Amazon links here are affiliate links and may earn this site a commission. Ordering, badges, and every claim on the page were decided from what the listings state, not from any commercial arrangement.