Whole House Fan CFM: Sizing Whole-House Fans, HRVs & ERVs
A whole house fan CFM calculator for the two machines people confuse: the big seasonal flush fan (2–3 CFM per sq ft, or one air change every 3–5 minutes) and the quiet HRV/ERV air exchanger that runs all year — sized here from occupants and the ASHRAE 62.2 fresh-air baseline.
Whole-house fan & fresh-air sizing dashboard
Step 1 — describe the house
Step 2 — pick the climate band
Works hard in spring and fall, as a night-flush in summer shoulder months, and rests during humid spells.
Whole-house fan (seasonal flush)
Safe fitBuy this size (larger method, stocked)
4,000
CFM · needs ≥ 5.0 sq ft net-free attic vent
HRV / ERV (continuous fresh air)
Year-roundContinuous fresh-air target
75
CFM · ≈ 0.38 air changes per hour
Shows how the chosen method compares to the ASHRAE 62.2 requirement.
Checkpoint: the two whole-house methods should land within ~25% of each other. If the area rule wins by a wide margin, the home has a low ceiling or a hot-dry climate — both point to the bigger fan.
Five checkpoints from floor plan to fan model
The dashboard above computes instantly, but the order of operations matters — each checkpoint below catches one classic sizing mistake before it becomes a return-shipment. Work them in order:
Measure the house once
Floor area and average ceiling height — the same two numbers the dashboard asks for. For split-level homes, use conditioned floor area and a weighted-average ceiling height.
Volume = area × height (a 1,500 sq ft × 8 ft home is 12,000 ft³).
Run both sizing methods
The area rule (2–3 CFM/sq ft by climate) and the air-change rule (volume every 3–5 minutes). They are two views of the same physics; the larger result is the honest one.
Both results within ~25% of each other means the inputs are sane.
Round up to a stocked fan
Whole-house fans come in 1,000–6,500 CFM steps. Always round up — a fan 20% over target runs quieter and reaches temperature faster than one 20% under.
Never round down: the shortfall shows up as minutes added to every flush.
Budget the attic exhaust
Every CFM the fan pushes into the attic needs a way out: 1 sq ft of net free vent area per 750 CFM. Screens and louvers cut gross opening by 50–75%.
3,000 CFM fan → ≥ 4 sq ft net free → typically 8+ sq ft gross venting.
Pair with continuous fresh air
The whole-house fan is seasonal and switched; the HRV/ERV is continuous and quiet. Size the air exchanger by ASHRAE 62.2 and occupants — the dashboard prints both from the same inputs.
One machine cools evenings, the other keeps air fresh all year.
The 2–3 CFM per square foot rule, house by house
Every whole-house fan sizing conversation starts with the same rule of thumb — and the honest version has a climate lever built in. At 2 CFM/sq ft a home flushes in roughly 4–5 minutes with average ceilings; at 3 CFM/sq ft it flushes in about 3 and pre-cools thermal mass faster, which is what desert climates want. The table applies the band to common floor areas:
| Floor area | Volume (8 ft ceil.) | Cool climate (2.0) | Mixed (2.5) | Hot-dry (3.0) | Buy (stocked) |
|---|---|---|---|---|---|
| 800 sq ft | 6,400 ft³ | 1,600 | 2,000 | 2,400 | 2,000 |
| 1,200 sq ft | 9,600 ft³ | 2,400 | 3,000 | 3,600 | 3,000 |
| 1,500 sq ft | 12,000 ft³ | 3,000 | 3,750 | 4,500 | 4,000 |
| 2,000 sq ft | 16,000 ft³ | 4,000 | 5,000 | 6,000 | 5,000 |
| 2,500 sq ft | 20,000 ft³ | 5,000 | 6,250 | 7,500 | 6,500 |
| 3,000 sq ft | 24,000 ft³ | 6,000 | 7,500 | 9,000 | 6,500 |
Why rounding up is the cheap insurance
Fan cost rises far more slowly than CFM across the stocked range — the jump from 3,000 to 3,500 CFM is typically 10–15% of price — while the cost of an undersized fan is paid every single evening in extra flush minutes and a half-hearted draft. Between two sizes, take the bigger one; the noise difference at low speed is usually smaller than the patience cost of a slow flush.
One big fan, one fast evening flushPhoto: UnsplashThe air-change method: the whole house every 3–5 minutes
Installers who think in flush times use the volume method instead of the area rule, and it is worth learning because it explains why the 2–3 CFM/sq ft band exists. The formula a whole-house fan designer uses:
CFM = (floor area × ceiling height) × 60 ÷ (flush minutes × 60)
…which collapses to volume ÷ flush minutes — the same air-change arithmetic the ACH table on the homepage uses, just at a 12–20 ACH target.
| 1. Home volume | 1,800 sq ft × 9 ft ceilings | 16,200 ft³ |
| 2. Target flush (mid-band) | every 4 minutes | 15 ACH |
| 3. Required airflow | 16,200 ÷ 4 | 4,050 CFM |
| 4. Area-rule cross-check | 1,800 × 2.5 | 4,500 CFM |
| 5. Buy decision | larger, stocked | 4,500–5,000 CFM |
💡 3 minutes vs 5 minutes
The band is a lifestyle dial, not a physics constant. A 3-minute flush (20 ACH) pre-cools furniture and drywall before bed — right for a house that cooks hot all day. A 5-minute flush (12 ACH) is quieter and cheaper — right for mild climates where nights do half the work.
❌ Don't: size from the attic opening
The 14"×22" ceiling cutout is a frame size, not a flow spec — it happens to fit fans from 1,000 to 4,500 CFM. Sizing is decided by house volume and climate; the ceiling opening and the attic venting are then matched to the fan, never the reverse.
Climate decides whether the fan helps or costs you
A whole-house fan is an air-swap machine: it only cools when the outside air is genuinely cooler and drier than what it replaces. The dashboard's climate selector encodes this; the honest breakdown:
Hot-dry climates
SafeNights 20–30°F below indoor temps with dewpoints in the 40s. The fan pre-cools thermal mass so the AC barely runs before noon. Size at 3 CFM/sq ft and enjoy the fastest payback in ventilation.
Mixed / cool nights
SafeFree cooling in spring, fall, and summer evenings after cold fronts. Size at 2–2.5 CFM/sq ft; humid weeks mean switching it off and letting the AC dehumidify.
Hot-humid climates
Humid riskNight air with a 72–76°F dewpoint imports moisture the AC must wring out the next day — often 0.3–0.5 kWh of extra compressor time per flush hour. An ERV is usually the better air exchanger here.
Attic venting: the silent half of the installation
The fan pushes the house's entire air volume into the attic, and the attic must exhale it freely or the fan suffocates. The planning rule:
1 sq ft net free vent area per 750 CFM of fan
3,000 CFM fan
needs 4 sq ft net free
After screens (−50%)
8 sq ft gross opening
Plus louvers (−25%)
≈ 11 sq ft gross venting
HRV & ERV sizing: the 7.5–15 CFM-per-person method and ASHRAE 62.2
Where the whole-house fan is a firehose, an air exchanger (HRV or ERV) is an IV drip: 30–150 CFM of filtered, conditioned outside air running continuously. An HRV cfm calculator question is really two questions, and both appear on the dashboard above:
Method 1 — occupants
CFM = people × 7.5 (base) … people × 15 (active)
Four occupants: 30 CFM at the base, 60 CFM for a household that cooks hard, exercises indoors, or keeps large pets. This method matches how fresh air is actually consumed — by lungs — and it is the minimum we recommend for occupant health.
Method 2 — ASHRAE 62.2
CFM = 0.03 × floor area + 7.5 × (bedrooms + 1)
The residential ventilation standard's baseline: an area term that scales with building materials and a bedroom term that scales with occupancy. This is the number code officials and HVAC designers will check against — treat it as the floor, not the target.
| 1. The home | 2,400 sq ft · 4 bedrooms · 4 people | — |
| 2. Occupant method | 4 × 7.5 (base) / 4 × 15 (active) | 30–60 CFM |
| 3. ASHRAE 62.2 | 0.03 × 2,400 + 7.5 × 5 | 110 CFM |
| 4. Size the unit | larger result, mid-speed on a 130-CFM ERV | 110 CFM |
| 5. In air changes | 110 × 60 ÷ 21,600 ft³ | ≈ 0.31 ACH |
HRV or ERV — which air exchanger?
Both move the same fresh-air CFM; they differ in what they hand back from the exhaust stream:
HRV — heat recovery
Recovers heat only (typical core effectiveness 60–85%). The choice for cold or dry climates where winter air is already parched and the house needs humidity kept out of the equation — Minneapolis, Denver, most of Canada.
ERV — enthalpy recovery
Recovers heat and moisture (typical total effectiveness 50–75%). The choice for humid and mixed climates: summer incoming air leaves most of its water on the exhaust stream, and winter the reverse. Gulf states, the Southeast, coastal California — ERV territory.
Fresh air & outside air CFM: the worked answers
“How to calculate fresh air CFM” and “how to calculate outside air CFM” are the same calculation wearing two names — deliberate, mechanical outdoor air, distinct from leaky infiltration. The decision table by room:
| Space | Fresh-air basis | Typical result | Delivered by |
|---|---|---|---|
| Whole house (continuous) | ASHRAE 62.2 | 50–150 CFM | ERV / HRV on low |
| Whole house (seasonal flush) | 2–3 CFM/sq ft | 2,000–6,000 CFM | Whole-house fan |
| Bathroom (intermittent) | 50 CFM or 8 ACH | 50–110 CFM | Bath exhaust fan |
| Kitchen (capture) | 100 CFM per linear ft of range | 150–300 CFM | Range hood |
| Workshop / hobby room | 5–10 ACH | 100–400 CFM | Exhaust fan + intake |
A note on the boundary with the homepage: the homepage's continuous-ventilation table keeps the ASHRAE 62.2 room-by-room baseline for quick reference. This page stays one level up — choosing and sizing the machines (whole-house fan, HRV, ERV) — and links back down whenever the raw baseline is what you need. For a second opinion on room-level ACH before buying anything, the homepage CFM calculator sizes any single room from its air changes per hour.
💡 The two-machine checklist
Whole-house fan sized by flush speed (thousands of CFM, seasonal). Air exchanger sized by occupants and 62.2 (tens of CFM, continuous). Bath and kitchen exhaust sized by spot rates. Three separate purchases, three separate calculators — and a home where the air is fresh at 2 p.m. and cool at 9 p.m. without either machine fighting the other.
Whole-house fan & fresh-air sizing FAQ
TopHow do I use a whole house fan CFM calculator?▼
Enter the home's floor area and ceiling height, then read the fan size two ways: the area rule (2–3 CFM per square foot) and the air-change rule (the home's volume every 3–5 minutes). Buy the larger of the two, rounded up to a stocked fan size. Our sizing dashboard above runs both methods at once — a 1,500 sq ft, 8 ft-ceiling home lands at 3,000–4,500 CFM by area and 3,600 CFM by a 4-minute air change.
How many CFM does a whole house fan need?▼
A practical band is 2–3 CFM per square foot of floor area. A 1,200 sq ft home needs 2,400–3,600 CFM; a 2,000 sq ft home needs 4,000–6,000 CFM. The air-change check should agree: dividing the home's volume by a 3–5 minute target gives the same order of magnitude. Anything below about 1.5 CFM/sq ft will feel like a weak draft rather than a cooling flush.
How do you size a fan to cool a house in 3–5 minutes?▼
Compute the home's air volume (floor area × ceiling height), multiply by 60 to get air changes per hour at one change per minute, then divide by the target minutes. A 1,500 sq ft × 8 ft home holds 12,000 ft³; a 4-minute flush needs 12,000 × 60 ÷ 4 = 180,000 ft³/h = 3,000 CFM. A 3-minute target on the same home is 4,000 CFM.
How is an HRV CFM calculator used to size an air exchanger?▼
Two inputs: people and floor area. The occupant method sizes 7.5–15 CFM per person (7.5 is the ASHRAE base; 15 suits active households or 2+ pets). The ASHRAE 62.2 method sizes 0.03 CFM/sq ft + 7.5 CFM × (bedrooms + 1). For a 3-bedroom, 1,800 sq ft home with 4 occupants: occupants → 30 CFM; 62.2 → 54 + 30 = 84 CFM. Size the HRV/ERV to the larger figure, on low speed.
How do you calculate fresh air CFM for a house?▼
Continuous fresh air uses the ASHRAE 62.2 formula: 0.03 CFM per square foot of floor area plus 7.5 CFM per bedroom plus one. A 2,400 sq ft, 4-bedroom home: 0.03 × 2,400 = 72, plus 7.5 × 5 = 37.5 → about 110 CFM of continuous outside air. The per-person shortcut (7.5–15 CFM × occupants) is the quick sanity check — use whichever result is larger.
How do you calculate outside air CFM for dilution?▼
For whole-house dilution, use the same ASHRAE 62.2 baseline as the floor. For a single polluting source (a workshop, a paint room), the spot-ventilation method applies: 100 CFM intermittent per bathroom, 5–10 air changes per hour for the room in question. For cooking capture, hoods are sized by the range, not by the house. Outside air CFM always means air brought in deliberately — infiltration alone is not a strategy.
What is the 'all house fan' CFM rule of thumb per square foot?▼
2–3 CFM per square foot, with the climate deciding where in the band you land: hot-dry climates toward 3 (fast pre-cooling before bed), temperate and coastal toward 2 (cool air arrives quickly). This is the same band professional installers quote, and the dashboard above converts it into a buyable fan size for your exact floor area.
Do whole-house fans work in humid climates?▼
Poorly, and only with discipline. A whole-house fan trades indoor air for outdoor air; in a Gulf-coast night with a 74°F dewpoint, that trade imports pounds of water vapor that the air conditioner must remove the next day at 300–600 BTU per pound of condensate. Hot-dry climates are the fan's natural habitat; in humid ones an ERV, which recovers dryness from the exhaust stream, is usually the better air exchanger.
How much attic vent area does a whole-house fan need?▼
The industry rule is 1 square foot of net free vent area per 750 CFM of fan flow — and net free means after screen and louver deductions (screens eat ~50%, louvers ~25%). A 3,000 CFM fan therefore needs about 4 sq ft of net free vent area, which can mean 8–16 sq ft of gross vent opening once deductions land. Starved attic vents throttle the fan exactly like a blocked exhaust.
Should I size the whole-house fan or the HRV first?▼
They are different machines for different jobs, so both: the whole-house fan is a seasonal fast-flush device (thousands of CFM, switched on in the evening), while the HRV/ERV is a year-round continuous air exchanger (tens to low hundreds of CFM, always on). Size the whole-house fan by air-change speed, the HRV/ERV by occupants and ASHRAE 62.2 — the dashboard above sizes both from the same inputs.
Can one fan do both whole-house cooling and fresh-air ventilation?▼
No — their operating profiles barely overlap. A whole-house fan at 3,000 CFM delivers 20+ air changes per hour, which is superb for an evening cool-down but would hammer humidity, dust, and energy bills if run continuously. An ERV at 100 CFM gives about 4 air changes per day — invisible, steady, and efficient, but useless for cooling. The sizing dashboard prints both numbers precisely so neither machine is asked to do the other's job.
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Size both machines before you buy either
One dashboard gives you the evening flush fan and the all-day fresh-air exchanger — enter the house once, read both shopping-list numbers, copy them to your notes.