Why Are the Doors Hard to Open When the Exhaust Fans Run?
· 11 min read · Right Away Plumbing and HVAC
Staff usually describe it the same way. The front door is heavy in the morning and nearly impossible to pull open once the kitchen gets busy. A back door slams hard enough to rattle the frame. Cold air pours across the dining room whenever someone walks in, and the hood that used to pull smoke cleanly now lets it drift into the room.
Those are not four unrelated complaints. They are the same complaint. The building is removing more air than it is bringing back in, and the missing air is being pulled through whatever opening it can find — usually a door, a window gap, an elevator shaft or a shared wall. That imbalance is called negative pressure, and in commercial kitchens it almost always traces back to the make-up air system.
This guide explains what negative pressure does to a building, which make-up air failures cause it, why Chicago winters make it worse, what building staff can safely observe, and what a qualified commercial HVAC technician should test. It does not give rooftop, electrical, gas or safety-control instructions — that work belongs to trained technicians with the right equipment and access.
What negative pressure actually is
Every commercial building moves air in two directions. Exhaust fans, kitchen hoods, restroom fans, dishwasher hoods and process exhaust push air out. Make-up air units, rooftop unit outdoor-air sections and intentional ventilation openings bring air back in. When the amount going out is close to the amount coming in, the building sits near neutral and doors behave normally.
When exhaust runs ahead of supply, the inside pressure drops slightly below the outdoor pressure. It does not take much. A difference far smaller than most people would guess is enough to make a 36-inch door feel like it is being held shut from the other side, because that small pressure acts across the entire surface of the door.
The air still gets in. It just arrives on the building's terms instead of the design's: under doors, around loading-dock seals, through the window frames of an older masonry building, down a flue, or out of a neighbouring tenant's space. That uncontrolled air is untempered, unfiltered and, for several months of the Chicago year, extremely cold.
The symptoms operators usually report first
Any one of these can have another cause. A door closer out of adjustment makes a door heavy too, and a badly located thermostat makes a room feel cold without any pressure problem at all. What separates a pressure problem is timing: the symptom appears when exhaust fans run and eases when they stop.
- Entry doors that are hard to pull open, especially during peak kitchen hours
- Doors that slam violently or will not latch
- Constant cold drafts along the floor near entries, even with the heat running
- A hood that no longer captures smoke, steam or grease vapour at the edges
- Backdrafting odours, or odours from an adjacent tenant appearing in your space
- Whistling at doors, windows or ceiling tiles
- Ceiling tiles lifting or sucking down in a suspended grid
- Heating equipment running longer without holding setpoint near the entry
- A dining room or sales floor that feels cold while the back of house feels fine
How to tell if the timing points at pressure
Building staff can gather a surprising amount of useful information without touching equipment. Do this from inside the occupied space only, during normal business operation, with the manager's knowledge.
- Note how the front door feels before the kitchen starts, and again at the busiest point of service
- Note whether the symptom changes when the hood fans are switched between settings by an authorised person
- Note whether it is worse on windy days or on very cold days
- Note whether smoke or steam rolls out of the hood edges instead of being drawn up
- Note which doors are affected and which are not
- Note whether a dishwasher hood, restroom exhaust or process fan runs at the same time
- Write down the time, outdoor temperature and what was running
What staff should not do
Do not go onto the roof, open equipment panels, adjust dampers, move belts, change fan speeds, reset safety controls, or alter gas or electrical components. Rooftop work involves fall exposure, energised equipment, gas-fired heating sections and interlocks that keep the building safe. Those checks belong to a qualified commercial HVAC technician with proper access and fall protection.
Also avoid propping a door open as a permanent fix. It relieves the symptom while creating new ones, and it can make a heating or freeze problem worse in January.
Bring those notes with you when you request commercial heating and rooftop HVAC service.
Why the kitchen hood suffers first
A commercial kitchen hood is designed around a supply and exhaust pair. The exhaust fan removes a defined volume of air from over the cooking line, and a make-up air unit replaces most of that volume with tempered outdoor air delivered near the hood. Capture depends on both halves working.
When make-up air fails, the exhaust fan still runs, but the air it removes has to come from somewhere. It is now arriving from across the dining room, through the front door and around the building envelope. That cross-room current sweeps past the cooking line sideways and drags smoke and grease vapour out from under the hood before it can be captured.
Operators often respond by asking for a bigger exhaust fan. That usually makes capture worse, not better, because it deepens the imbalance that caused the problem. The useful question is whether the supply side is delivering the air the hood was designed to receive.
Make-up air failures that cause negative pressure
A make-up air unit is a fairly simple machine with several parts that all have to cooperate. Any one of them can take the supply side out of service while the exhaust side keeps running normally.
- Supply fan not running: a failed motor, a tripped overload, a control fault or a unit left in an off state
- Belt problems: a broken, slipping, glazed or badly tensioned belt moves far less air than it should while the motor still turns
- Loaded filters: a make-up air unit in a restaurant or industrial setting loads filters faster than an office unit, and a heavily loaded bank strangles airflow
- Intake damper stuck closed or barely open: a failed actuator, linkage or control signal leaves the unit blowing against a closed door
- Bird screens, hoods and intakes blocked by debris, nesting material, snow or ice
- Heat section locked out: a gas-fired or electric heating section that will not prove can shut the whole unit down rather than deliver cold air
- Freeze-protection control tripped: a low-temperature device that opens on cold intake air, correctly protecting equipment, but leaving the building with no supply
- Interlock not made: the make-up air unit is supposed to start with the exhaust fan, and a failed interlock relay, switch or control point breaks that link
- Building-control scheduling that no longer matches the kitchen's operating hours
- Distribution problems downstream: a disconnected duct section, a closed manual damper, or supply delivered to the wrong area
The interlock deserves special attention
The interlock is the single most common reason an operator says “the exhaust fan runs but the make-up air unit stays off.” It is the control link that starts supply whenever exhaust starts. When it is broken, bypassed during a past repair, wired around during a remodel or lost after a controls change, the building can run in deep negative pressure every single service without anything looking obviously broken.
It is also frequently disabled on purpose. If the heating section was failing, someone may have shut the unit off to stop nuisance alarms, intending it as a temporary measure. Months later, nobody remembers.
What Chicago winter does to the problem
Cold weather turns a nuisance into an operating problem. Air that leaks in through a door in July is merely uncomfortable. The same leak in January arrives near or below freezing, lands on the floor, and the heating equipment nearest the entry runs continuously trying to cover it.
Stack effect adds to it. Warm indoor air rises and escapes at the top of a building, pulling replacement air in near the bottom, and the temperature difference driving that is at its greatest in deep winter. In a multi-storey Chicago building, the ground-floor entrance is where all of that shows up.
Timing matters for planning. At O'Hare, the normal first freeze is October 19, based on the 1991–2020 National Weather Service climate normals. That is a normal, not a forecast — any individual year can arrive earlier or later. It is still a reasonable date to plan backwards from, because make-up air heating sections, freeze controls and dampers are best tested while the weather is mild enough that a lockout does not shut the kitchen down.
- Frozen or ice-blocked intakes and hoods after a wet cold snap
- Freeze-protection controls tripping repeatedly on very cold mornings
- Heating sections failing to prove, so the unit shuts down rather than supply cold air
- Water piping near entries and exterior walls exposed to a steady stream of cold infiltration
- Condensation and frost forming on entry glass, frames and nearby surfaces
Fall is also the right time to run through the commercial rooftop heating checklist, since the same visit can cover the make-up air unit and the rooftop units serving the space.
How a technician diagnoses it properly
Pressure problems are measured, not guessed. A qualified commercial HVAC technician approaches the building as a system rather than as one complaining door.
- Measure the pressure difference between the space and outdoors, and between key interior zones
- Confirm which exhaust fans are running and at what settings during the test
- Verify that the make-up air unit starts when exhaust starts, and that the interlock is intact
- Measure supply airflow and compare it against the exhaust the building is actually removing
- Inspect filters, belts, drives, motor operation and intake condition
- Confirm damper command versus actual damper position
- Test the heating section, freeze control and safety sequence through a complete cycle
- Check that supply air reaches the area it was designed to serve
- Review controls, schedules and any overrides left in place from earlier work
- Document findings with readings, photographs and equipment information
Why one number is not enough
A single pressure reading taken at one moment can be misleading. The building behaves differently at open, at peak service, on a windy afternoon and on a still cold night. Readings taken under the conditions that produce the complaint are the ones that lead to a correct diagnosis.
It also matters which zones are compared. Several buildings we see are roughly neutral overall while the dining room is starved and the kitchen is fine, because supply is arriving in the wrong place rather than being absent.
Should the kitchen and dining room be at the same pressure?
Generally no, and the difference is intentional. A commercial kitchen is usually kept slightly negative relative to the dining room so that cooking odours, heat and grease-laden vapour move toward the hood rather than out into seating. The building as a whole is usually kept close to neutral or slightly positive relative to outdoors so that doors behave and outdoor air is not pulled in uncontrolled.
Problems appear when that small, deliberate difference becomes a large, accidental one. Then the dining room becomes the make-up air path for the kitchen, and the front door becomes the intake.
The correct target values depend on the building, the exhaust equipment, local code and the design of the ventilation system. That is a design question for the mechanical professional reviewing the property, not a number to copy from an article.
Repair, rebalance or replace
Once the system is measured, the fix normally falls into one of three categories, and it is worth understanding the difference before approving work.
Repair covers a specific failed component: a belt, a motor, an actuator, a control relay, a heating-section part, a failed interlock or a blocked intake. If the system was designed correctly and worked in the past, this is the most common answer and the least disruptive.
Rebalancing addresses a system where the components work but the quantities no longer match. Kitchens change. A new fryer bank, a larger hood, an added dishwasher or a remodel that moved a wall can leave a perfectly healthy make-up air unit undersized for the exhaust now installed. Rebalancing may involve adjusting fan speeds and dampers, correcting distribution, or modifying controls.
Replacement or a design change comes up when the unit is at the end of its service life, when parts are no longer practical to obtain, when the heating section is unsafe, or when the exhaust demand has grown well past what the supply equipment can ever deliver. Age alone does not decide this; condition, repair history, parts availability, controls and the building's plans all matter.
We do not promise a specific energy or cost reduction from any of these. A building that stops heating a stream of outdoor air pulled through the front door is usually easier to keep comfortable, but the actual result depends on the building, the equipment, the weather, occupancy and how the system is operated.
Our commercial HVAC and mechanical services cover make-up air units, rooftop units, unit heaters and commercial boilers together, so one visit can look at the whole picture.
When the same complaint keeps returning across a property or a portfolio, our commercial HVAC and mechanical consulting can help plan the correction instead of repeating the service call.
Before a purchase, lease, renovation or budget decision, a plumbing and HVAC property inspection can document equipment condition, photographs and priorities. An inspection reduces uncertainty but cannot reveal every concealed or intermittent condition.
What to have ready when you call
If occupants report a gas odour, visible smoke, soot, unusual combustion noise or a carbon-monoxide alarm, stop and follow the property's safety and utility procedures immediately. Keep people away from the affected area and request service rather than investigating it yourself.
- Property address, on-site contact and business hours
- Which doors are affected and when the symptom is worst
- Which exhaust fans run, and whether the symptom follows them
- Whether the make-up air unit is known to run, and whether it has been shut off
- Any alarm, fault code, lockout or recent reset
- Recent kitchen, hood, tenant or remodel changes
- Roof-access instructions, keys, hatches and required authorisation
- Past service records, drawings or equipment photographs if available
You can send those details through our contact page or call the number for your location.
Service across Chicago and the suburbs
We work on make-up air units, kitchen ventilation support equipment, rooftop units and commercial heating systems for restaurants, retail spaces, offices, warehouses and mixed-use buildings throughout Chicago and the surrounding Cook and DuPage County communities.
For a Chicago address, call 773-250-6060.
For Des Plaines and the surrounding suburbs, call 847-550-3322. Our shop is at 2093 Rand Rd, Des Plaines, IL 60016.
Every community we cover is listed on our service areas page.
Frequently asked questions
Why is our restaurant door hard to open when the kitchen hood runs?
The building is most likely under negative pressure. The hood exhaust is removing more air than the make-up air system is replacing, so outdoor air is pulled in through the door. That pressure difference acts across the whole door surface and makes it feel like it is being held shut. A door closer out of adjustment can feel similar, so the giveaway is whether the problem appears and disappears with the exhaust fans.
Can a make-up air problem make a commercial building feel cold?
Yes. When supply air is missing, the building pulls untempered outdoor air in through doors, dock seals and envelope gaps. In a Chicago winter that air arrives near or below freezing, settles at floor level, and the heating equipment near the entry runs longer without holding setpoint. The heating unit can be working correctly while the space still feels cold.
Why does the exhaust fan run but the make-up air unit stay off?
Common reasons include a broken or bypassed interlock between the exhaust and supply fans, a heating-section lockout, a tripped freeze-protection control, a failed motor or belt, a stuck intake damper, a control-schedule problem, or the unit having been switched off during an earlier repair and never restored. A technician should confirm which of these applies before anything is changed.
Should the kitchen and dining room have the same air pressure?
Usually not. A commercial kitchen is normally kept slightly negative relative to the dining room so cooking odours and vapour move toward the hood rather than into seating, while the building overall is kept close to neutral relative to outdoors. Problems start when that small intentional difference becomes a large accidental one. The correct values depend on the building, the equipment and applicable code.
Can we fix negative pressure by opening a door or window?
It may relieve the symptom briefly, but it is not a fix. Propping an opening admits unfiltered, untempered air, can make heating and freeze exposure worse in winter, may affect hood capture, and can conflict with security, code and food-safety requirements. The imbalance itself still needs to be measured and corrected.
Who should test a commercial make-up air system?
A qualified commercial HVAC technician, or a testing and balancing professional where the scope calls for it. The work involves rooftop access, fall exposure, energised equipment, gas-fired heating sections and safety interlocks, plus instruments for measuring pressure and airflow. Building staff should limit themselves to observing and documenting symptoms from inside the occupied space.
When should a Chicago property test make-up air heating?
Test it in early fall, while the weather is still mild enough that a lockout will not shut the kitchen down. At O'Hare the normal first freeze is October 19 based on the 1991–2020 National Weather Service normals, which is a reasonable date to plan backwards from. The appropriate maintenance frequency depends on the equipment manufacturer's requirements, operating hours and environment.
Talk to Right Away
Right Away Plumbing and HVAC services make-up air units, kitchen ventilation support equipment, rooftop units, unit heaters and commercial boilers for restaurants, retail properties, offices, warehouses and mixed-use buildings across Chicago and the surrounding suburbs.
We can measure building pressure and airflow, find why the supply side is not keeping up with exhaust, document the findings and help plan the repair, rebalance or replacement that fits the property.
For a Chicago address, call 773-250-6060. For Des Plaines and the surrounding suburbs, call 847-550-3322 or request service online.
Right Away Plumbing and HVAC is based at 2093 Rand Rd, Des Plaines, IL 60016 and is licensed, bonded and insured.
You can read more about our plumbing services, check the communities we serve, or start with plumbing in Des Plaines and Park Ridge. Planning a remodel or weighing a big replacement? Our consulting service is built for that conversation.