Introduction
Ventilation questions tend to cluster around the same dozen topics: how the ACH formula works, which CFM a bathroom or kitchen needs, what ASHRAE 62.2 requires, and whether a given fan is big — or quiet — enough. This page collects the most asked questions about ventilation sizing in one place, with direct answers and links to the in-depth guides where a topic needs more room.
If you haven’t run your numbers yet, the Ventilation Calculator converts room dimensions and a target air-change rate into required CFM and m³/h — the starting point for nearly every answer below.
Sizing and Formulas
How do I calculate the CFM I need for a room?
Measure the room’s volume (length × width × ceiling height, in cubic feet), multiply by the target air changes per hour (ACH), and divide by 60: CFM = ACH × Volume (ft³) ÷ 60. In metric: m³/h = ACH × Volume (m³). A 12 × 10 ft bedroom with 8 ft ceilings is 960 ft³; at 5 ACH that’s 5 × 960 ÷ 60 = 80 CFM (≈ 136 m³/h). The full derivation is in the formula-explained guide.
What is a good ACH for a bedroom or living room?
5–6 ACH is the standard planning range for bedrooms, living rooms, and home offices — enough for occupant comfort and pollutant dilution without excessive energy loss. Finished basements typically use 4–5 ACH. These are planning values; confirm the governing requirement with your local code.
How many CFM does a bathroom exhaust fan need?
The widely used baseline is 50 CFM for intermittent (switched/timer) operation or 20 CFM for continuous operation. Larger bathrooms (over ~100 ft² / 9.3 m²) or heavy shower use justify sizing up — many designers add roughly 1 CFM per square foot of floor area as a planning check for big baths.
How do I size a kitchen range hood?
By cooktop output, not kitchen volume: 100 CFM per 10,000 BTU of gas cooktop rating. A 40,000 BTU range suggests ~400 CFM. For electric or induction, a common planning rule is ~100 CFM per linear foot of cooktop width (a 30-in / 76 cm cooktop → ~250 CFM). Then step up for long duct runs.
What does ASHRAE 62.2 require for whole-house ventilation?
The standard’s formula is Q = 0.03 × conditioned floor area (ft²) + 7.5 × (bedrooms + 1), in CFM of continuous ventilation. A 2,000 ft², 3-bedroom home needs 0.03 × 2,000 + 7.5 × 4 = 90 CFM (≈ 153 m³/h). Local codes may adopt this with modifications — the code-compliance guide explains how to confirm your jurisdiction’s version.
How do I convert CFM to m³/h?
Multiply by 1.699 (planning shortcut: × 1.7). So 100 CFM ≈ 170 m³/h, and 80 CFM ≈ 136 m³/h. Divide by the same factor to go the other direction. Always match the calculator’s output unit to the unit on the fan nameplate.
Equipment Selection
Should I buy a larger fan than the calculation says?
One standard size up — yes. Rated CFM is measured at a standard test pressure, and your duct run adds resistance that reduces delivered airflow. A speed control lets you trim an oversized fan down; nothing lets you upsize an installed one. Don’t oversize wildly, though: excess exhaust wastes conditioned air and can depressurize the house.
What sone rating should I look for?
1.5 sones or less for bathrooms and bedrooms; 1.0 or less for continuous operation. Above ~3 sones most people switch the fan off, which defeats the purpose entirely. Noise is a performance specification — see the equipment guide for how sone ratings are measured.
What’s the difference between an ERV and an HRV?
Both are whole-house balanced ventilation systems that exchange stale indoor air for fresh outdoor air while recovering energy. An HRV (heat recovery ventilator) transfers heat only; an ERV (energy recovery ventilator) transfers heat and moisture. ERVs suit humid climates (they reduce incoming humidity in summer); HRVs suit cold, dry climates. Either satisfies ASHRAE 62.2 whole-house requirements when sized correctly.
Do I need a humidity-sensing bathroom fan?
Not required, but valuable in households that forget timers — the fan runs automatically when shower steam spikes humidity, then shuts off. It costs roughly $30–$80 more than a basic switched model and is the most reliable way to guarantee the fan actually runs.
Installation and Ductwork
Can I vent a bathroom fan into the attic?
No. Exhaust must terminate outdoors through a roof or wall cap. Venting into an attic dumps shower moisture into the roof assembly, causing mold and rot, and most codes explicitly prohibit it. Correcting an attic-vented fan should outrank any other ventilation upgrade.
How long can the duct run be?
As short and straight as possible. Every 90° elbow adds the resistance of several feet of straight duct; flexible duct roughly doubles resistance per foot versus smooth rigid duct. Two elbows maximum is a good planning rule for bath fans, and ducts in unconditioned spaces should be insulated to prevent condensation.
Do I need make-up air?
For small bath fans in typical houses, infiltration usually supplies it. Above roughly 400 CFM of intermittent exhaust (large range hoods), most codes require dedicated make-up air — a dampered intake that opens with the hood. Without it, a big hood can backdraft combustion appliances, which is a safety issue. When in doubt, ask a licensed HVAC professional.
Where should the fan or grille go?
At the pollutant source: bathroom fans over or near the shower, range hoods directly over the cooktop (hood width ≥ cooktop width, 24–30 in / 61–76 cm above a gas cooktop per manufacturer specs). For whole-house supply air, put supply grilles in bedrooms and living areas and exhaust from baths, kitchens, and laundries.
Operation and Costs
How long should a bathroom fan run after a shower?
20–30 minutes is the standard recommendation. A countdown timer ($20–$40) automates this and is the cheapest performance upgrade in ventilation — it guarantees the runtime that memory alone won’t deliver.
How much does a bathroom fan cost to run?
The motor itself is cheap — a 100 CFM fan drawing ~25 W run an hour a day costs a few dollars per year. The bigger cost is the conditioned air it exhausts: continuous high-CFM exhaust can add $100–$300 per year in heating/cooling energy depending on climate. ERVs recover 60–80% of that energy. See the cost guide for full project budgets.
Can I install a bathroom fan myself?
A like-for-like swap on existing wiring and duct is a reasonable DIY project (materials $30–$150, a few hours). New duct penetrations, roof work, and new circuits are where DIY gets risky — that’s electrician and roofer territory. The DIY vs pro guide draws the line in detail.
How often should ventilation equipment be maintained?
Vacuum grilles and wash range-hood grease filters quarterly; check exterior terminations for blockages semi-annually; clean or replace ERV/HRV filters and cores annually per manufacturer schedule. Budget fan motor replacement every 10–15 years.
Related Guides
- Ventilation Complete Guide — the full overview with a worked example.
- Formula Explained — derive and hand-check every calculation.
- Ventilation Estimating Guide — step-by-step room-by-room workflow.
- Ventilation Equipment Guide — fans, hoods, ERVs, and nameplates.
- Common Mistakes — errors that distort results and how to fix them.
- Ventilation Checklist — everything to verify before you buy.
Still Have Questions? Run the Numbers
Most ventilation questions resolve into the same workflow: measure the room, pick a target rate, apply the ACH formula, and select equipment one size up with a quiet sone rating. The Ventilation Calculator handles the arithmetic — then confirm the governing requirements with your local building authority before you buy or install.
Disclaimer: Answers are general planning guidance based on widely used industry values. Actual requirements are set by your local building code and vary by jurisdiction, climate, and construction. Verify targets with your local building authority (AHJ) and consult a licensed HVAC professional for system design and installation decisions.
