Building: restaurant, commercial suite, Richardson, TX. Equipment: rooftop make-up air unit, kitchen exhaust hood, a fuel-fired cooking appliance with its own flue. On site: 1 hour 13 minutes.
What the owner reported
Two complaints that turned out to be one problem. The kitchen smelled like a drain whenever the hood was running, and the flue on a cooking appliance was backdrafting at the same time.
What we measured
We put a manometer on the kitchen against the outdoors with the hood on. The first reading was -0.04 in. w.c. About twelve minutes later it was -0.08 in. w.c., with the outdoor reference at +0.01. The second reading was twice as negative as the first.
What we found on the roof
The make-up air unit, a CaptiveAire rated at 2,000 CFM on its nameplate, wasn’t running. There was no power to it. Its intake filters had been pulled off and were lying on the roof next to it, loaded with dust and lint. Nothing at the panel or the disconnects was labeled, so we couldn’t tell which breaker fed the unit.
Why that explains the smell and the flue
A hood moves air out of the building. If no make-up air comes in, the building finds replacement air wherever it can: floor drains, door gaps, and appliance flues. The drain smell and the backdraft were the same symptom.
What we did and didn’t do
This was a diagnostic visit. We didn’t repair or adjust the make-up air unit, and we didn’t balance the hood, because you can’t balance a system with half of it dead. We left the customer with a written report and these steps:
- Have an electrician trace and label the circuit and restore power to the unit.
- Clean or replace the intake filters before it runs.
- Re-test kitchen pressure with the unit running.
- If the kitchen is still negative, compare the make-up air capacity to the hood’s exhaust volume and to the appliance manufacturer’s fresh-air requirement.
We also recommended labeling every rooftop disconnect, so the next technician can find the right breaker without guessing.
The takeaway
Don’t run the hood with the make-up air off. In a busy kitchen, check those intake filters monthly.
More on this kind of work: commercial kitchen ventilation. All case studies.