By the dispatch desk · 2026-02-12 · Read time ≈ 7 min
Half the makeup-air conversations we have with new clients start with the same complaint: my dining room is always too cold in winter and too humid in summer, even though my AC is the right size for the building. The AC is the right size for the building. The problem is that the building is being pulled into negative pressure by the kitchen hood, and the AC is having a conversation with the outside air, not the dining room. Below is the part of ASHRAE 154 that explains this, written for a chef.
What the hood actually does
A commercial kitchen exhaust hood pulls air out of the building. A typical Type I hood over a moderate-volume cookline pulls between 1,800 and 4,000 cubic feet per minute (CFM). For context, a typical 80-seat restaurant has an interior air volume of around 24,000 cubic feet. A 3,000 CFM hood is replacing the entire interior air of the restaurant every eight minutes. That replacement air has to come from somewhere.
Where the replacement air should come from
It should come from your makeup-air unit (MUA). The MUA is a piece of HVAC equipment that sits on the roof or in a mechanical closet, draws outside air, conditions it (heats it in winter, cools and dehumidifies it in summer), and delivers it to the kitchen — usually through a perforated plenum near the hood, or through a transfer grille from the dining room. A correctly sized MUA matches 80–90% of the hood's exhaust CFM directly to the kitchen, with the remaining 10–20% drawn from the dining room as transfer air.
What happens when the MUA is undersized or broken
The hood still pulls 3,000 CFM. The MUA is delivering, say, 1,200 CFM. The remaining 1,800 CFM has to come from somewhere — and the only places it can come from are the dining-room transfer grille (designed for 600 CFM), the dining-room HVAC supply (designed for the dining room, not the kitchen), and unintentional infiltration through the front door, the kitchen back door, the bathroom exhaust, and the gaps around the windows. The result: the dining room is being pulled into negative pressure relative to outside.
What the readings should look like
At the hood plane: the design target is mildly negative, around −0.01 to −0.03 inch-water-column. This is the negative pressure that pulls grease-laden vapor up into the hood and prevents it from spilling into the kitchen. This is a feature.
At the dining-room boundary or the front door: the design target is near-neutral, between −0.005 and +0.005 inch-water-column. The dining room should be neither pulling outside air in nor pushing inside air out at any meaningful rate. This is what a balanced kitchen looks like.
What we adjust
Three things, in order of frequency. The MUA's belt and sheave. Belts wear and slip. A 12-month-old belt typically delivers 90% of its rated CFM. A 24-month-old belt typically delivers 75%. The fix is a $12 belt and ten minutes on the roof. The MUA's outdoor damper. Coastal humidity rusts the linkage; the damper sticks at 60–70% closed. The fix is lubrication and re-zeroing. The exhaust-side balance damper on the rooftop fan. Surprisingly often set wrong by the original installer.
What we cannot fix mid-route
An undersized MUA. If the unit was specified at 1,200 CFM for a 3,000 CFM hood — a real condition we have seen on retrofit kitchens that expanded their cookline since the original install — no amount of belt-tightening will fix it. The fix is a new MUA, which is a project quote, not a quarterly visit. We have been on the phone with five different operators about this in 2026 already.
How to know if this is your problem
Three signs. The dining room is running 4°F or more off the thermostat target despite a recently-serviced HVAC. The front door is hard to open from outside. Customers in the dining room have been mentioning a draft or humidity by name. Any one of these is a yellow flag. All three at once is a red flag, and you should ask your cleaning shop for a balancing visit on the next route.
What it costs
A balancing visit at Updraft is $320 for a single-hood operator and runs about ninety minutes on site. A new MUA is much more — typically $7,000–$14,000 for a small commercial unit installed and commissioned. Most of our clients do not need a new MUA. Most of them need a $12 belt.