Short answer
Every cubic foot of air a store exhausts must be replaced. Make-up air systems bring that air back in deliberately, so the building does not pull it through doors, gaps, and plumbing vents instead.
Why does exhaust need to be balanced?
A building is a box with a fixed volume. When exhaust fans push air out, an equal amount of air has to come in.
If the HVAC system does not supply that air deliberately, the building pulls it in wherever it can: through entry doors, dock doors, gaps in the envelope, and sometimes backward through flues and plumbing vents. That condition is called negative pressure, and it causes problems that are frequently misdiagnosed as HVAC equipment faults.
Glossary: make-up air unit and test and balance.
Where does exhaust come from in a retail or service building?
Exhaust in retail and service buildings comes from kitchen hoods, restrooms, grooming and kennel areas, process equipment, and the relief or powered exhaust on rooftop units.
- Kitchen hoods in restaurants, cafés, and in-store food service, which can move large volumes of air.
- Restroom and janitorial exhaust, small individually but continuous.
- Grooming, bathing, and kennel exhaust in pet retail and veterinary facilities, where odor and humidity control matter.
- Process exhaust such as dryers, photo or print equipment, and lab or procedure-room exhaust in healthcare.
- RTU relief and powered exhaust, which operate when economizers bring in extra outside air.
What does negative pressure look like in a store?
The symptoms tend to show up as complaints from store staff rather than equipment alarms:
- Entry doors that are hard to open, pull shut, or whistle at the gaps.
- Hot, dusty air at the entrance in summer, and cold drafts in winter.
- Kitchen or restroom odors drifting into sales or waiting areas.
- Hoods that fail to capture smoke or steam, leaving residue on nearby surfaces.
- RTUs that seem undersized near the entrance, because they are conditioning infiltrating outdoor air they were not designed for.
- Gas appliances or water heaters with drafting problems, which is a safety concern and should be checked by a qualified technician without delay.
Positive pressure, the opposite problem, is less common but also matters: doors that blow open, and conditioned air escaping through the envelope.
How is make-up air delivered?
Make-up air is delivered in one of two common ways. A dedicated make-up air unit brings in outdoor air, often tempers it (heating, sometimes cooling), and delivers it near the exhaust source, typically interlocked to run with the hood.
Alternatively, rooftop units serving the space can be set to bring in more outside air to offset exhaust, which works for smaller and steadier exhaust loads but adds to the cooling and heating load on those units.
In either case, the system has to be designed and balanced as a whole. Untempered make-up air delivered near a cooking line in winter, for example, creates comfort problems for staff and can upset hood capture.
Why does balance drift over time?
Few stores stay as they were designed. Common causes of drift include:
- Exhaust fans added during a remodel without matching make-up air.
- Make-up air units with failed motors, slipping belts, or clogged filters, so the exhaust runs while the supply does not.
- Interlocks that were bypassed or failed, so make-up air no longer starts with the hood.
- RTU outside-air dampers that were closed or reset during unrelated repairs.
- Economizer relief dampers stuck closed. See economizers on retail rooftop units, explained.
How is balance checked and restored?
A technician measures exhaust airflows, make-up and outside airflows, and building pressure at representative points, then compares them to the design intent.
Correcting the balance may be as simple as replacing a belt, repairing an interlock, or resetting dampers, or it may require replacing an undersized or failed make-up air unit. Formal testing and balancing by a certified balancing contractor is sometimes required for new work or permits.
REE Mechanical Inc. can scope make-up air and exhaust installation and replacement alongside the rooftop units they interact with, because the two cannot be evaluated separately.
How does this affect RTU sizing and replacement?
Outside air is one of the largest loads on a rooftop unit. When exhaust and make-up air change, the load the RTUs see changes too, and a unit that was correctly sized can begin to fall short.
If a store has added exhaust since the RTUs were installed, include air balance in any replacement review. The RTU tonnage article explains how outside air enters the sizing calculation, and how a commercial rooftop unit works covers the outside-air path through the unit.
When a unit already falls short because exhaust has changed, weigh the fix against the unit's age and condition using repair or replace a rooftop unit: the decision factors.
What should be checked during a remodel or tenant change?
Remodels and tenant changes are where balance most often goes wrong. Before work begins, list every exhaust and make-up air device that will be added, removed, or relocated, and confirm how the RTUs serving the space will be affected.
After work is complete, verify that interlocks operate, that make-up air starts with the exhaust it serves, and that building pressure is reasonable at entrances. Include these checks in the project closeout rather than leaving them to the first maintenance visit.
A remodeled or newly acquired store should go through the same baseline steps as any new site; see onboarding a new store into a maintenance program.
What should a facility team record?
For each site, list every exhaust fan and make-up air unit, what it serves, its rated airflow, and how it is controlled or interlocked. That record makes a remodel review straightforward and lets technicians spot changes on arrival.
Methodology: Written by REE Mechanical's team from field experience on multi-site commercial portfolios; standards cited link to the issuing body.