Make-Up Air Demystified: Balance Without the Draft

Range hoods move air. That air has to be replaced from somewhere—or the house will find a way to pull it in for you, often through the path of least resistance: a leaky fireplace, a water-heater flue, or a chilly crack around a door. That replacement is make-up air (MUA). It’s the quiet partner of effective kitchen ventilation, and when you ignore it, you pay with drafts, poor capture, whistling doors, and—in the worst cases—backdrafting combustion appliances. This guide explains what make-up air is, when you need it, how to design it so it’s invisible in daily life, and how to retrofit without tearing up your kitchen.

Why Make-Up Air Exists (and Why It Matters)

A range hood creates negative pressure in the room. At low flows and leaky houses, outdoor air sneaks in through incidental cracks. In newer, tighter homes—or whenever you run a powerful hood—the exhaust can outpace those leaks. Pressure drops, doors get hard to close, and cold air pushes in wherever it can. Worse, an orphaned water heater or fireplace can reverse its draft, spilling flue gases into the living space. A well-designed MUA system restores balance: whatever the hood expels, the house receives back in a controlled, tempered, quiet way.

When Do You Need It?

• Code triggers: Many jurisdictions require MUA above a threshold exhaust rating (often 400–600 CFM).
  
• Tight envelopes: New builds, deep energy retrofits, or apartments with window/door upgrades behave “tight” regardless of code.
  
• Stacked loads: Running bath fans, dryers, HRVs/ERVs, and the hood together can push you into negative pressure even with modest hood ratings.
  
• Symptoms: Doors slam or whistle when the hood runs; the fireplace smells odd; capture falls apart unless a window is cracked.
  

If any of the above sounds familiar, you should plan MUA—even if the law doesn’t strictly require it.

Design Goals: Quiet, Invisible, Automatic

Done right, make-up air is something you don’t notice:

1. Arrives gently—no cold blast across the cooktop.
   
2. Is interlocked with the hood—on when needed, off when not.
   
3. Is tempered—warmed (or at least mixed) so winter comfort remains.
   
4. Takes a smart path—enters where it helps capture instead of tearing the plume sideways.
   

The Anatomy of a Good MUA System

• Intake: Exterior wall hood or roof cap sized to duct diameter. Include a bug/rodent screen that won’t clog easily.
  
• Duct run: Full diameter, short, and smooth, just like exhaust. Add backdraft damper to prevent infiltration when off.
  
• Control: A motorized damper wired to the hood (or a pressure sensor). When the fan runs above a set speed, the damper opens.
  
• Tempering/mixing: Options range from simple mixing through a return plenum to inline heaters. In mild climates, mixing is enough; in cold zones, tempering avoids “arctic ankles.”
  
• Diffusion: Deliver make-up air away from the cooktop and above head height when possible, so it doesn’t shear the rising plume.
  

Where to Put the Air (and Where Not To)

Good locations

• A ceiling diffuser a few feet behind the cooktop, aimed gently toward the kitchen zone.
  
• A high sidewall grille on the perimeter of the room, not in a straight line across the cooktop.
  
• Floor registers along exterior walls in open plans, which create a general “fill” without a jet.
  

Bad locations

• A slot that blows across the cooktop (it will knock steam off course).
  
• A diffuser directly above the front burners, aimed down (you’ll feel a breeze).
  
• An inlet tucked low behind toe-kicks that dumps cold air onto feet without mixing.
  

Think of it this way: exhaust shapes the plume; MUA shapes the room.

Sizing: How Much Make-Up Air?

Start with the exhaust you’ll actually use at cruise. If your hood captures well at 300–450 CFM, you don’t need to design for a constant 1000 CFM replacement. Provide a path that comfortably supplies most of your everyday flow and can supplement during brief boost events. Rules of thumb vary, but a practical target is 70–100% of cruise CFM delivered quietly, with the remainder coming from normal leakage and door openings during short spikes. Oversupplying can make the kitchen feel pressurized and drafty.

Tempering Options: From Simple to Luxe

• Passive mixing (most common): Pull in outdoor air and blend it with room air through well-placed diffusers. Cheapest and fine in mild climates.
  
• Hydronic or electric inline heater: Warms MUA during cold snaps. Costs more but protects comfort in cold zones.
  
• HRV/ERV assist: Some projects use existing balanced ventilation to pre-condition a portion of MUA. Coordinate carefully to avoid cross-pressure fights.
  
• Furnace/air-handler integration: Tie the MUA duct into the return side with a control that runs the blower when the hood is on. Add backflow prevention so you don’t push cooking air into bedrooms.
  

Choose the simplest scheme that keeps people comfortable for the few hours per week the hood runs.

Interlocks and Controls

An electrical interlock opens the damper and, if needed, kicks on the air-handler fan when the hood crosses a trigger (e.g., speed 2+). Simpler still, some kits use a pressure switch sensing negative pressure. Either way, the point is automation—no one should have to remember a second switch at 6 p.m.

Ducted vs. Recirculating: Two Different MUA Stories

• Ducted exhaust: You’re removing heat and moisture. Plan MUA so the room doesn’t pull air from places you don’t want (chimneys, garages). The better your hood and duct, the lower the RPM, the less MUA you’ll perceive—comfort improves. To compare canopy depths, capture shelves, and blower options that let you live at mid speed (and thus moderate MUA), browse modern kitchen range ventilation hoods before you finalize the HVAC plan.
  
• Recirculating (ductless) exhaust: You’re not ejecting air outdoors, so you generally don’t require dedicated MUA. Moisture stays in the envelope, so focus on longer run-on and high-capacity carbon. If exterior venting isn’t allowed (condos, heritage façades, interior kitchens), shortlist quality ductless range hoods with usage-based reminders so odor control stays consistent.
  

Open-Plan and Island Kitchens

Islands are already geometry-challenged; don’t sabotage them with an updraft of cold air. In large rooms, place MUA behind the cook zone or higher up in the ceiling grid so incoming air descends gently. If you must bring air from the floor, split it across multiple low-velocity registers along the room’s perimeter. Avoid a single “firehose” grille that aims straight at the cooktop.

Gas vs. Induction Loads

• Gas: More spike events (sear, drippings smoke) and combustion byproducts. You’ll have brief forays into higher exhaust flows. Design MUA that responds quickly (fast-open damper) and is well diffused so it doesn’t chill the cook.
  
• Induction: Less smoke, more steam. Exhaust flows are steadier and lower; MUA can be modest and gentle. The most important comfort tool is the run-on timer that clears humidity without boosting.
  

Retrofitting MUA in Existing Homes

• Easiest: A motorized damper feeding a short duct to a high wall or ceiling diffuser near the kitchen, interlocked with the hood.
  
• Leverage existing chases: Unused chimney flues (lined and repurposed), pantry ceilings, or dropped soffits can hide short, straight runs.
  
• Return-plenum tie-in: If an air handler lives near the kitchen, a small MUA duct to the return side with an interlock is often cost-effective.
  
• Door sensor hack (temporary): In some mild climates, interlock the hood to a patio door contact so boost only works when the door is cracked—crude but useful until a proper MUA line is installed.
  

Comfort Pitfalls (and Fixes)

• Cold ankles at the island: The diffuser is dumping low and fast. Fix with a higher discharge and two smaller registers instead of one big blast.
  
• Plume breakup: MUA grille is aimed at the cooktop. Re-aim or relocate so flow arrives from behind or above, not across.
  
• Whistling or rattling: Undersized intake or sticky backdraft damper. Upsize and use a free-swing damper with a soft stop.
  
• Backdraft alarms in winter: Exhaust is strong, MUA absent. Even a temporary cracked-window protocol during boost is safer than starving the room.
  

Noise: The Hidden Benefit of Balanced Air

When MUA is right, the hood can run slower for the same capture because negative pressure isn’t fighting the blower. Slower RPM = less noise. It’s one reason well-balanced systems feel calmer at the same “spec CFM” than unbalanced ones that howl yet underperform.

A Simple Specification You Can Copy

• MUA Capacity: Provide make-up air at ~80% of hood cruise CFM via a dedicated outside air duct.
  
• Damper: Motorized, normally closed, interlocked with hood fan speed ≥ 2.
  
• Duct: Full diameter, smooth metal, shortest practical route; seal with UL 181 foil tape.
  
• Diffuser: Ceiling or high sidewall, located behind cook zone; throw aimed toward kitchen but not across the cooktop.
  
• Temper: Mix via air-handler on interlock; add inline tempering coil in climate zones with winter design temps < 20°F (-6°C).
  
• Backdraft Protection: Spring-assisted damper or gravity damper with soft stop at intake and discharge.
  
• Commissioning: Verify damper open/close, measure pressure at door crack (qualitative), confirm no backdrafting of atmospherically vented appliances.
  

Commissioning Checklist (15 Minutes)

1. Turn hood to cruise. Confirm damper opens fully.
   
2. Hold a tissue at a nearby door—minimal suction indicates balance.
   
3. Light a match near a water-heater draft hood (if atmospherically vented) and extinguish; smoke should rise into the flue, not spill.
   
4. Walk the room: feel for cold jets; redirect diffusers if needed.
   
5. Switch to boost for 60 seconds; ensure comfort remains acceptable and capture improves.
   
6. Return to cruise and set run-on for 10 minutes.
   

Cost and Payback

• Hardware (damper, duct, grille): $250–$600
  
• Interlock/control: $80–$300
  
• Labor: Half-day to a day depending on access
  
• Optional tempering: $300–$1,200 installed, climate-dependent
  

The “payback” shows up as comfort (no drafts), safety (no backdraft), quieter operation, and better capture at mid speed.

Case Notes

The Tight Townhome: 36″ wall hood performed poorly unless a window was cracked. Added a motorized damper to a short ceiling run and tied it to speed 2+. Ceiling diffuser aimed gently toward the cook zone. Result: no window rituals, mid-speed capture, quieter evenings.

The Great-Room Island: Long duct, inline blower, and winter discomfort when boosting. Fix: MUA via two high sidewall diffusers behind the island, linked to the hood with return-fan assist for mixing. Result: no cold ankles, no plume breakup, same capture at lower RPM.

The Bottom Line

Make-up air isn’t a luxury add-on; it’s how you close the loop. Provide a gentle, interlocked path for outdoor air, place it where it helps instead of hurts, and temper or mix it enough that nobody notices except in the best way: calmer voices, better capture, and a kitchen that feels like a living room—even on boost. Design for balance, and everything else—noise, odor, moisture—gets easier.