What is make-up air, and when does a kitchen range hood need it in Minnesota?
Make-up air is outdoor air brought into the house to replace what exhaust fans blow out. Minnesota's mechanical code says a kitchen hood ducted outdoors needs it according to Minnesota Rules 1346.0501. In homes built before 1999, installing a hood rated over 300 cfm triggers it; newer homes use a calculation based on all exhaust.
Key points
- Make-up air replaces the air a range hood, dryer and bath fans push out, so the house doesn't pull air backward down a chimney.
- Minnesota Rules 1346.0505 sends ducted kitchen hoods to the make-up air rules in 1346.0501.
- In existing homes, an exhaust system rated over 300 cfm is the common trigger. Homes built after 1999 use the full calculation whenever an exhaust system is installed or replaced.
- Passive openings over 8 inches need a motorized damper tied to the exhaust; over 11 inches, powered make-up air is required.
- The risk is backdrafting an atmospherically vented water heater or furnace, which can spill carbon monoxide.
Make-up air is outdoor air brought into the house on purpose to replace the air that exhaust fans push out. A kitchen range hood needs it in Minnesota when the state's mechanical code says the hood's exhaust is more than the house can safely make up through its own small leaks, which for homes built before 1999 means installing a hood rated over 300 cubic feet per minute (cfm).
Without it, a big hood in a tight house pulls replacement air from wherever it can, and one of the easiest paths is backward down the chimney that vents the water heater or furnace.
Why does a range hood need make-up air?
Every cubic foot of air a fan blows outside has to be replaced by a cubic foot coming in. In a drafty 1920s house, it leaks in around windows and through the attic floor. In a tighter house, especially one built or remodeled under newer energy codes, there aren't many easy leaks. The hood lowers the pressure inside the house until air comes in somewhere.
That "somewhere" can be:
- The water heater's draft hood or the furnace chimney. An atmospherically vented gas water heater or 80% furnace relies on warm flue gases rising up the chimney. When the house is under negative pressure, air can push down that chimney instead, and combustion gases spill into the basement. That's how a kitchen fan becomes a carbon monoxide problem.
- A fireplace. Smoke and ash drawn back into the room.
- Nowhere useful. The hood simply moves less air than its rating, and cooking smoke hangs in the kitchen.
Houses with a sealed-combustion condensing furnace and a power-vent or direct-vent water heater are less exposed, because those appliances bring in their own combustion air through pipes. The code accounts for that difference, as the next section shows. Our AFUE post explains how the two furnace types vent.
What Minnesota's code says
We read the current rules on revisor.mn.gov. Minnesota's mechanical code is Minnesota Rules chapter 1346, which amends the International Mechanical Code.
Kitchen hoods. Minnesota Rules 1346.0505 says domestic kitchen exhaust hoods ducted to the outdoors must have make-up air provided according to part 1346.0501.
When it's required. Minnesota Rules 1346.0501 sets the triggers by when the house was built:
| House | What triggers make-up air |
|---|---|
| New dwelling | Always calculated with the code's Table 501.4.1 |
| Built after 2003 under these make-up air provisions | Installing or replacing a vented combustion appliance or an exhaust system |
| Built after 1999 under the Minnesota Energy Code | Installing or replacing a vented combustion appliance or an exhaust system |
| Built during or after 1994 under the earlier energy code | Installing an exhaust system rated greater than 300 cfm, or a solid fuel appliance |
| Built before 1994 | Installing an exhaust system rated greater than 300 cfm, or a solid fuel appliance |
The rule also has exceptions. Extra make-up air isn't needed if a powered make-up air system is electrically interlocked with the exhaust and matched to its airflow. It also isn't needed if a depressurization test (the code names ASTM E1998-02) or a test the building official approves shows the vented combustion appliances still operate properly.
How much. For new homes, Table 501.4.1 estimates how much air the house leaks on its own, from its floor area and a factor that depends on how the appliances vent. The factor is highest for homes with only power-vent or direct-vent appliances and lowest for homes with several atmospherically vented ones. It then adds 135 cfm for the clothes dryer and 80% of the largest exhaust fan's rating (plus 80% of the next largest in homes with fan-assisted or atmospheric appliances). Subtract the leakage from the exhaust, and what's left is the make-up air required. If the answer is negative, none is needed.
As an illustration of the math only: a 2,000-square-foot house with only direct-vent and power-vent appliances uses a factor of 0.15, for 300 cfm of estimated leakage. Add a 600 cfm hood: 135 + 480 = 615 cfm of exhaust capacity, minus 300, leaves 315 cfm of make-up air. Change the appliance types and the answer changes a lot.
What are the ways to supply kitchen make-up air?
1346.0501 lists three methods and the rules for each:
- Passive opening. A duct from outdoors, sized by the code's Table 501.4.2. Barometric dampers aren't allowed in passive openings when any atmospherically vented appliance is in the house, and a single passive opening larger than 8 inches in diameter needs a motorized damper electrically interlocked with the largest exhaust system.
- Powered make-up air. Required once the passive opening would exceed 11 inches in diameter. It must be interlocked with the largest exhaust system and matched to its airflow.
- A combination of passive and powered.
Other details from the same rule: make-up air of 175 cfm or more has to enter the space with the combustion appliances, the space with the exhaust, or a connected space the building official approves. The intake must be at least 12 inches above grade with a screen no finer than 1/4-inch mesh, and located away from exhaust outlets. It can't dump into a furnace's return plenum unless the furnace maker's instructions allow it. Make-up air and combustion air need separate openings unless the official approves a combined system.
Does make-up air need to be heated in a Minnesota winter?
The code also says make-up air must not hurt the heating or cooling capability of the home's equipment. In January, a few hundred cfm of subzero air pouring into a kitchen or basement is a real load. That's why larger systems often temper the air, heating it with an electric or hot-water duct heater, or introduce it where the furnace can mix it, following the furnace maker's instructions. A motorized damper that opens only when the hood runs keeps the opening closed the rest of the time. For continuous fresh air, a separate ventilator does a different job, which our ERV vs HRV post covers, and central AC doesn't bring in outside air at all, as our outside air post explains.
What you can check, and when to call
- Know your appliances. Look at the water heater and furnace. A metal flue into a chimney means atmospheric or fan-assisted venting. Two white PVC pipes mean sealed combustion.
- Check the hood's rating in cfm on its label or spec sheet before you buy.
- Keep CO alarms working. Minnesota law requires one within 10 feet of every sleeping room, and one near the fuel-burning appliances is a good idea. Our CO alarm guide covers placement.
Make-up air is mechanical work that needs a permit from the city; our permit guide covers how that works in Minneapolis and Saint Paul. We install ventilation and make-up air as part of our indoor air quality work and price it as a written, flat-rate quote after we see the kitchen, the appliances and the duct route. Call 612-429-3465, open 7 days, 7am–9pm, ideally before the cabinets go in. If you'd like us to check your existing appliances first, a service call is a $79 trip charge, waived when you approve the repair.
Questions
Does a 300 cfm range hood need make-up air in Minnesota?
In a home built before 1999, Minnesota Rules 1346.0501 triggers make-up air when an exhaust system rated greater than 300 cfm is installed, so a hood rated at exactly 300 cfm does not trip that trigger by itself. Homes built after 1999 under the Minnesota Energy Code use a calculation instead, which can require make-up air for smaller exhaust. Your city's building official decides how it applies to your house.
Can I just crack a window when the hood runs?
It helps the hood pull, but it isn't how the code provides make-up air, and in a Minnesota winter nobody leaves a window open for long. The code calls for a sized opening from outdoors, a motorized damper on larger passive openings, or a powered system interlocked with the hood so it runs whenever the hood runs. Ask the building official before counting on anything else.
Who installs make-up air for a range hood?
It's mechanical work, so a heating and ventilation contractor normally designs and installs it, with a mechanical permit from the city. It needs an outdoor intake, ductwork, a damper and often wiring to interlock with the hood, plus a plan for heating the incoming air in winter. Bring the contractor in before the kitchen layout is final, because the duct route is easier to plan than to retrofit.