Exhaust + Ventilation Reference
Why this matters
Exhaust systems remove moisture, odors, smoke, combustion products from homes. Bath fans, range hoods, dryer vents, attic vents, fireplace flues - every service trade touches them. The tech who installs + diagnoses correctly delivers healthy indoor air + prevents structural damage. This is the field card.
Bath exhaust (covered in ventilation fan ratings)
Quick refresher: 1 CFM per sq ft + 50 CFM per fixture; ≤ 1 sone preferred; vent OUT of home (not attic OR soffit).
Kitchen range hood + dryer vent
Range hood sizing: 100 CFM per 12" of cooktop OR 1 CFM per 100 BTU gas. Typical: electric 30" 250 - 300 CFM; gas 30" 300 - 600 CFM (per BTU); pro 48"+ 600 - 1,200+ CFM. Make-up air required > 400 CFM (IRC M1503.4). Mount 24 - 30" above electric; 30 - 36" gas. Duct 6/8/10" round per CFM. Termination roof OR wall (never attic). Sone 5 - 8 typical, quieter premium.
Dryer vent: 4" rigid metal duct preferred (smooth, fire-safe); flex only for transition (max 6 ft per IRC). Termination: louvered cap on exterior wall OR roof; never attic/crawl. Length max 35 ft total equivalent per IRC (subtract 5 ft per 90°; 2.5 ft per 45°). Backflow damper prevents reverse flow. Annual cleaning critical - lint accumulates + causes thousands of fires per year.
Combustion appliance venting
Atmospheric vent (B-vent): gravity-vented furnace / water heater; double-wall metal pipe through roof.
Power vent: blower-driven; PVC OR stainless; horizontal OR vertical.
Direct vent (sealed combustion): outside air for combustion; exhaust + intake separated.
Common locations:
- B-vent through roof: traditional
- Side-wall power vent: modern; saves roof penetration
- Direct vent + intake / exhaust pair: sealed combustion equipment
Required clearances: per manufacturer + NFPA 211 (chimneys); typical:
- 12" from window OR door
- 36" from fresh air intake
- 12" from grade
- Plus specific cap height above roof
Attic ventilation
Total NFA (net free area): 1 sq ft per 300 sq ft attic (with vapor retarder) OR 1 per 150 (without).
50/50 intake / exhaust split: half soffit, half ridge / gable / roof.
Power attic vent: 600 - 1500 CFM; use only if passive insufficient; can pull conditioned air from house.
Solar attic fan: lower-cost alternative to electric powered.
Avoid: gable vents + ridge vents together (short-circuit air flow).
Crawlspace ventilation
Vented crawl (older code): 1 sq ft NFA per 150 sq ft floor.
Sealed crawl (modern; IRC R408.3): no venting; encapsulated; conditioned. Better moisture control.
Fireplace + chimney venting
Masonry chimney: brick + flue tile; per NFPA 211.
Class A chimney pipe: prefab; insulated double + triple-wall.
Catalytic OR non-catalytic wood stove: specific venting per manufacturer.
Gas fireplace: direct-vent (B-vent OR concentric) OR vent-free (no chimney; CO concern).
Annual sweep: chimney cleaning + inspection.
Roof ventilation
Soffit vents: continuous OR individual; intake air for attic.
Ridge vent: along peak; exhaust attic air.
Gable vent: end-wall louvered; exhaust.
Roof vent (turbine OR static): individual penetrations.
Boot OR roof penetration: vent stack flashing.
Specific duct considerations
Smooth metal duct: lowest friction; preferred for all exhaust.
Flex (insulated): only short runs; many bends OK but each loses CFM.
Termination cap with damper: prevents reverse flow + bug intrusion.
Insulation: cold-climate exhaust ducts (heat loss prevention + condensation reduction).
Common exhaust ventilation problems
Bath fan loud + ineffective: undersized OR crushed duct OR soffit termination.
Dryer slow + hot: lint clogged duct.
Kitchen smoke fills house: range hood undersized OR clogged filter OR not vented outside (recirculating).
Smoke smell coming back: chimney backdraft; depressurized house; cold flue.
CO in living space: blocked OR cracked flue; cracked heat exchanger; depressurized house.
Attic too hot in summer: insufficient ventilation; install ridge + soffit vent.
Moisture in attic + mold growth: bath fan venting into attic; soffit termination; ridge vent inadequate.
Make-up air
Required when: exhaust > 400 CFM (range hood).
Passive: fresh-air intake through wall OR ducting.
Powered: active make-up air system.
Critical: without make-up air, house depressurizes; pulls flue gas back through atmospheric appliances.
Common service-call diagnostics
Test fan: power applied + measure CFM at register.
Test duct: smoke pencil; verify outdoor air movement.
Test backdraft: water heater backdrafting? close house up + run all exhaust; observe.
Test combustion: combustion analyzer (chimney pressure, CO, draft).
Dryer vent cleaning
Tools: rotary brush + vacuum.
Process: from inside OR outside; brush + vacuum lint.
Frequency: annually for typical household; more for high-use.
Common exhaust mistakes
Soffit termination (moisture returns to attic). Attic termination (moisture damage + structural rot). Flex duct used for long runs (friction crushes CFM). Wrong size duct (undersized = slow + restricted). Missing backflow damper. Termination too close to door / window / fresh-air intake (recirculation). Powered attic vent without sealing house (pulls AC air into attic; wastes energy).
Code requirements
IRC M1503: range hoods + kitchen ventilation.
IRC M1505: dryer venting.
IRC M1506: bath ventilation.
ASHRAE 62.2: whole-house ventilation.
Local + state: may exceed national.
Truck stock
Bath fan (Panasonic, Broan; 80 + 110 CFM common), range hood vent connector, dryer vent rotary brush, dryer vent cleaning vacuum, foil tape (181A), flex duct, mastic + brush, anemometer (CFM measurement), smoke pen (leak / draft test), assorted duct fittings (4", 6", 8", 10").
Customer education
Annual maintenance: bath fan + dryer vent cleaning.
Don't run bath fan without supply: pressure issues (rare residential).
Range hood at high while cooking: not just brief use.
Open window when fireplace lit: combustion air for traditional + airflow.
Test smoke + CO detector monthly: covered separately.
The single most-impactful exhaust-vent habit is the BACKDRAFT TEST during furnace + water heater service. Close all windows + doors + run bath fans + dryer + kitchen hood at high. Then check water heater + furnace flue with smoke pen at vent hood - if smoke pulls AWAY from flue, draft is good; if smoke goes INTO living space (backdraft), the house is pulling flue gases back inside. Solutions: better venting, sealed combustion equipment, makeup air. Customer's family is potentially breathing CO without knowing; the 2-minute test reveals the issue. Especially important for atmospheric water heaters + furnaces in basement.
References
- IRC M1503, M1505, M1506 (vent code requirements)
- ASHRAE 62.2 (Indoor Air Quality)
- NFPA 211 (Chimneys, Fireplaces, + Solid Fuel-Burning Appliances)
- UL 181 (HVAC Duct Closure)
- Manuall internal: Ventilation Fan Ratings, Duct Sealing Methods