Short answer
Provide make-up air by adding dedicated outdoor air pathways sized to match your exhaust flow. In homes without central HVAC, this is typically done with: (1) wall-mounted passive air inlets or transfer grilles for small bath fans; (2) a dedicated make-up air (MUA) duct with a motorized damper and optional inline fan for large kitchen hoods; or (3) a small through‑wall ERV/HRV for balanced, filtered air. Interlock MUA with high‑CFM exhausts and ensure air is filtered, insulated, and brought in where it won’t cause drafts.
Why make-up air matters
Exhaust fans remove air. Without an equal way for air to enter, the house can go negative, leading to backdrafting of combustion appliances, whistling doors, poor exhaust performance, cold infiltration, and moisture problems. Many jurisdictions require dedicated MUA for kitchen hoods over 400 CFM (IRC M1503.6). Tight homes or stacked exhausts (bath + kitchen + dryer) benefit from planned MUA even below that threshold.
Plan and size your solution
1) Add up your exhaust loads:
- Bathroom fan(s): 50–110 CFM each
- Range hood: 200–1200+ CFM (check spec label)
- Dryer: 150–200 CFM (not typically balanced, but affects whole-house pressure)
2) Choose an approach:
- Small loads (≤80–150 CFM): Passive inlets, door undercuts, or transfer grilles can work.
- Large loads (≥300–400 CFM): Dedicated MUA duct with motorized damper; consider an inline supply fan and tempering.
- Whole-home balance: A compact through‑wall ERV/HRV provides balanced supply/exhaust with heat recovery.
Targets:
- Keep house pressure within ~±3 Pa relative to outdoors during exhaust operation.
- For range hoods ≥400 CFM: provide interlocked MUA sized roughly equal to hood CFM.
- Passive inlets: use manufacturer airflow charts; a single 6" round wall inlet often supplies ~50–150 CFM depending on pressure.
Options and step-by-step
A) Passive wall inlets (good for bath fans and modest loads)
What you’ll do:
- Install one or more wall-mounted passive air inlets near the room served, high on the wall or ceiling to reduce drafts.
- Provide interior transfer paths (door undercut 3/4", or a 6x10" transfer grille) so air can reach the exhaust location.
Tools/materials:
- 4–6" wall inlet with backdraft damper, insect screen, MERV 8–11 filter
- 4–6" hole saw or core drill, drill/driver, jigsaw
- Exterior hood/cowl, flashing, silicone/urethane sealant, low-expansion foam
- Insulated duct (if offset needed), tape/mastic
Steps:
1. Choose a location away from pollution sources (not near garage, dryer vents, or chimneys) and at least 10 ft from exhaust terminations.
2. Cut the opening, slope slightly to exterior for drainage. Install sleeve, damper, and exterior hood with flashing.
3. Seal and insulate any ducted section in unconditioned areas to prevent condensation.
4. Add a washable filter or filter box for cleanliness.
5. Confirm airflow with a tissue test at the inlet while the exhaust runs; adjust quantity/size as needed.
Time/cost: 2–4 hours per inlet; $50–$200 in materials each.
B) Dedicated make-up air kit for range hoods (best for ≥400 CFM)
What you’ll do:
- Run a 6–10" insulated duct from outside to a diffuser in the kitchen (not blowing directly on the cooktop).
- Add a motorized damper interlocked with the hood; for big hoods, add an inline supply fan.
- Optional: temper the air in cold climates with an electric duct heater or hydronic coil.
Tools/materials:
- 6–10" round metal duct, R-8 insulated flex for long runs
- Motorized MUA damper (24V), pressure switch or hood control relay, low-voltage wire
- Inline supply fan (sized to hood CFM as needed)
- Diffuser/register, filter box, backdraft damper, hanger straps
- Electrical tools for interlock; sealants, mastic, firestop where required
Steps:
1. Size the duct to match hood CFM with low pressure drop (e.g., 8" for ~400–600 CFM, 10" for higher). Avoid sharp turns.
2. Place the supply diffuser near the kitchen but not over burners; aim for mixing (e.g., toe‑kick grille or ceiling diffuser).
3. Wire the damper (and fan if used) to open/run when the hood is on. Many hoods offer an interlock output; otherwise use a current sensing switch.
4. Include a filter and a backdraft damper to prevent winter drafts.
5. In cold climates, consider a duct heater interlocked with a discharge temperature sensor.
Time/cost: $300–$1,000+ in materials; 1–2 days DIY. Electrician for interlock/heater: $200–$600.
C) Through‑wall ERV/HRV (balanced ventilation without central HVAC)
- Examples: small paired through‑wall ERVs (e.g., 15–30 CFM each) or compact bath/kitchen ERVs. Install one or more to offset steady exhaust and improve indoor air quality.
- Benefits: tempered, filtered air with sound control; drawbacks: limited CFM for large hoods.
Safety checks
- Combustion safety: If you have an atmospherically vented water heater or furnace, test for backdrafting under worst‑case exhaust. Install CO alarms.
- Electrical: Follow local code, GFCI/AFCI where required, proper gauge wiring, and listed components.
- Building envelope: Seal and flash penetrations to prevent water intrusion; insulate ducts in unconditioned spaces.
Tips for best results
- Provide transfer pathways inside the home so replacement air can reach the exhaust room.
- Use sound‑attenuating flex sections and quality diffusers to reduce noise and drafts.
- Locate inlets away from driveways, dryer vents, and chimneys; maintain minimum separations.
- Keep filters accessible; mark a maintenance schedule.
- Aim for balanced flows rather than relying on leaky windows.
Common mistakes
- Undersizing MUA or using a single small inlet for a 600 CFM hood.
- No interlock, leaving the damper closed while the hood runs.
- Placing the intake too close to an exhaust outlet or contaminants.
- Skipping filtration or insulation, leading to dust and condensation.
- Relying solely on door undercuts for high‑CFM exhausts.
When to call a pro
- You have large hoods (≥600 CFM), multiple exhausts, or combustion appliances.
- You need core drilling through masonry or structural assessment.
- You plan to add electric duct heat or complex interlocks.
- You want pressure measurements and commissioning (manometer, smoke tests) to verify performance.
With a clear plan—matching MUA to exhaust, interlocking where needed, and managing comfort and filtration—you can run exhaust fans effectively without a central HVAC system.