Makeup Air Requirements for Exhaust
A big range hood, a bath fan, and an atmospheric water heater walk into a tight house. Here is exactly when the code forces you to add makeup air, how much, and why a house that goes negative can kill people.
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In This Guide
Why Makeup Air Exists
Every cubic foot of air you blow out of a building has to be replaced by a cubic foot coming in. In a leaky 1970s ranch, that replacement air sneaks in through gaps around windows, sill plates, and can lights, and nobody notices. In a modern tight house with a sealed envelope, there is no easy path in. When a powerful exhaust fan runs, the building goes to negative pressure and starts pulling air backward down the path of least resistance.
Very often the path of least resistance is the flue of an atmospheric-draft appliance, a standard tank water heater or an 80% furnace that vents by natural buoyancy. Reverse that flow and you spill combustion products, including carbon monoxide, straight into the living space. That is the whole reason makeup air sits in the code: it is a life-safety provision, not a comfort nicety.
The failure mode
A 600 CFM range hood in a tight kitchen can backdraft a nearby natural-draft water heater within seconds. CO climbs, the flame rolls out, and the occupants never hear the fan doing it. Makeup air and combustion air rules exist to keep the combustion appliance zone from going negative enough to spill.
The 400 CFM Kitchen Trigger
The single most-cited residential trigger is the kitchen exhaust threshold. Under IRC M1503.6 (and the parallel language in IMC 505), an exhaust hood system capable of moving air in excess of 400 CFM must be provided with makeup air.
The rule in plain terms
Rated hood capacity > 400 CFM → makeup air required, provided at a rate approximately equal to the amount that exceeds the threshold, and interlocked to start and stop with the hood.
Key word is capable. A hood rated at 600 CFM triggers the requirement even if the homeowner never runs it on high. Rating, not typical usage, is what the inspector reads off the label.
Two important nuances trip techs up:
- Adopted edition matters. The 2018 IRC called for makeup air "approximately equal to the exhaust rate." Later cycles and many state amendments size it to the exhaust in excess of 400 CFM, and some tie the trigger to whether atmospheric appliances are even present. Always verify the edition your AHJ has adopted before you quote a number.
- Downdraft cooktops count. A downdraft vent is still a kitchen exhaust system. If it is rated over 400 CFM, it triggers makeup air the same as an overhead hood.
- Interlock and closure are mandatory. The makeup air system needs a motorized damper that opens automatically with the hood, so you are not leaving a permanent hole in the envelope.
Combustion Air vs. Makeup Air
These two get mixed up constantly. They solve related problems but come from different code sections.
Combustion air
Air a fuel-burning appliance needs to burn cleanly and to vent by draft. Governed by IFGC 304 / IRC G2407. Sized to the total input BTU/hr of the appliances sharing the space.
Makeup air
Air that replaces what mechanical exhaust removes so the building does not go excessively negative. Governed by IRC M1503.6 / IMC 505 & 508. Sized to the exhaust CFM.
The standard indoor-air method for combustion air requires 50 cubic feet of room volume per 1,000 BTU/hr of combined appliance input. When the room is too small, you provide combustion air openings to outdoors. The common two-opening outdoor method uses 1 square inch of free area per 4,000 BTU/hr for each opening (high and low) when the openings communicate directly with the outdoors, or 1 sq in per 2,000 BTU/hr when air travels through vertical or horizontal ducts.
Field reality: Adequate combustion air openings do not guarantee you are safe from exhaust-driven depressurization. Combustion air openings size the supply for burning; they do not cure a house that a 600 CFM hood is dragging to −8 Pa. You can pass combustion air math and still backdraft. That is why big exhaust equipment also needs makeup air.
Tallying Every Exhaust Path
The hood is rarely the only fan. To judge worst-case negative pressure you add up everything that can run at once. Here are the typical rated flows you will meet in the field:
| Exhaust device | Typical rated CFM | Notes |
|---|---|---|
| Bath fan (intermittent) | 50 | 20 CFM if continuous, per ASHRAE 62.2 |
| Standard kitchen hood | 100–400 | No makeup air trigger at/under 400 |
| High-output pro hood | 600–1,200 | Always triggers makeup air |
| Clothes dryer | 100–225 | Gas dryers add their own combustion load |
| Central vacuum (vented outdoors) | 100–150 | Easy to forget in the tally |
| Whole-house / attic fan | 1,000+ | Only balanced when windows are open |
For depressurization analysis the worst case is every fan running with the house buttoned up. Inspectors and energy raters model exactly that scenario.
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Worked Example: Sizing Makeup Air
Problem
A new tight-envelope home has a 600 CFM island hood, a natural-draft 40-gallon gas water heater in the mechanical closet, and a 90%+ sealed-combustion furnace. The AHJ is on the 2018 IRC. How much makeup air do you provide, and what has to interlock?
Step 1: Check the trigger
Hood rated 600 CFM > 400 CFM → makeup air is required.
Step 2: Size to the exhaust in excess of 400 CFM
MUA = 600 − 400 = 200 CFM minimum
Some jurisdictions on amended language require makeup air approximately equal to the full 600 CFM. When atmospheric appliances share the space, sizing to the full exhaust is the safer call.
Step 3: Interlock and damper
The makeup air duct gets a motorized damper wired to open and its inline fan (if used) to energize only when the hood runs. Add tempering (a duct heater or a conditioned-air tie-in) so 200 CFM of winter air is not dumped raw into the kitchen.
Step 4: Protect the water heater
The natural-draft water heater is the vulnerable appliance. The sealed-combustion furnace pulls its own outside air and is largely immune. Verify the makeup air actually holds the closet near neutral with a manometer, not just on paper.
Result
Provide at least 200 CFM of tempered, interlocked makeup air; consider the full 600 CFM given the atmospheric water heater. Confirm the combustion appliance zone stays inside its depressurization limit with the hood on high and all other fans running.
Depressurization Limits & Testing
The number that ultimately matters is how negative the combustion appliance zone (CAZ) gets. A worst-case depressurization test closes the house, turns on every exhaust device, sets the air handler to its most demanding condition, and reads CAZ pressure with a digital manometer referenced to outdoors. Widely used field limits by appliance type:
| Appliance / vent type | Typical worst-case limit |
|---|---|
| Natural-draft (atmospheric) gas | −5 Pa |
| Induced-draft / fan-assisted | −15 Pa |
| Oil-fired | −5 Pa |
| Direct-vent / sealed combustion | −50 Pa |
Those are commonly referenced BPI worst-case values; your program or AHJ may specify their own. The practical takeaway is that a natural-draft appliance tolerates almost no negative pressure, roughly the weight of a sheet of paper, while a sealed-combustion unit shrugs off ten times as much. When a house fails the test, the fixes are the same order every time: add or increase makeup air, seal duct leakage in the CAZ, or replace the vulnerable atmospheric appliance with a sealed-combustion or power-vented model.
Manometer, every time
Do not eyeball this. A $150 digital manometer referenced to outside tells you the actual CAZ pressure with the fans loaded up. A smoke pencil or a mirror at the draft hood confirms the flue is drafting the right direction under worst case. Paper compliance is not proof of safe draft.
Commercial Kitchens Are Different
Type I and Type II commercial hoods live under IMC 507 and 508, not the residential 400 CFM rule. Here makeup air is never optional, and the design is about keeping the space close to neutral so the hood captures grease-laden vapor instead of dumping it into the dining room.
- Replacement air is typically engineered at roughly 80% to 90% of the exhaust rate, with the balance drawn from the conditioned space to hold the kitchen slightly negative relative to the dining area.
- Makeup air must be interlocked with the exhaust fan so they run together, and it usually needs tempering to prevent dumping cold air over the cooking line.
- Type I (grease) hood exhaust duct construction, clearances, and cleanouts are governed separately under IMC 506 and carry their own inspection points.
If you jumped from residential change-outs into a restaurant job, do not carry the 400 CFM mindset with you. The commercial math is a balanced-airflow design problem, and it is usually stamped by an engineer.
Field Checklist
- ✅ Read the hood rating plate. Over 400 CFM means makeup air, period.
- ✅ Confirm which code edition the AHJ has adopted before you quote the sizing method.
- ✅ Inventory every exhaust device in the house, not just the hood.
- ✅ Identify atmospheric appliances. They set the depressurization limit at about −5 Pa.
- ✅ Interlock makeup air with the exhaust and install a motorized damper.
- ✅ Temper winter makeup air so you do not create a comfort or freeze complaint.
- ✅ Verify combustion air openings separately, then confirm draft under worst case with a manometer.
- ✅ When in doubt, spec sealed-combustion or power-vented appliances and remove the risk entirely.
Makeup air is one of those requirements that is easy to design around and dangerous to skip. A tight envelope plus a big fan plus a natural-draft flue is a CO event waiting for a cold, closed-up night. Do the tally, size the air, interlock the damper, and prove the draft.
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