Booth pressure too positive (pushing out) · Open Face
Booth pressure too positive (pushing out) on Open Face booths
An open-face booth running positive is the most visually obvious version of this symptom across booth types. There is no door, no front seal, nothing to hold back the overspray cloud, when the rear-wall exhaust loads enough that intake exceeds outflow, the cloud rolls forward out of the open face and into the shop. Painters notice immediately because the cloud is in their face. The fix-path is rear-wall exhaust pad replacement; the fresh-media test rules in or out the filter explanation in a single visit, and on an open-face booth that test is unmistakable because the rollback either stops or it doesn't.
Quick answer
Positive pressure in an open-face booth means overspray is rolling forward out of the open intake face into the shop, the most visible filter-replaceable failure mode there is. Loaded rear-wall exhaust pads are the cause in nearly every case; the open-face geometry has no front-door seal to fight against, so any exhaust restriction immediately reverses the airflow at the open face. Replace the rear-wall exhaust pads. Persistent overspray rollback on fresh media routes to professional service for fan and damper diagnosis.
Diagnostic logic for Booth pressure too positive (pushing out) on Open Face
Open-face geometry, restated. Air enters through the open front face of the booth (no door, no front-wall intake panel) at the design face velocity, flows horizontally through the spray zone capturing overspray, and exits through pads mounted in the rear wall. Open-face booths rely entirely on the rear-wall exhaust to maintain inward face velocity at the opening. When the exhaust loads, face velocity drops; when it drops far enough, the airflow reverses at the open face.
Replacement sequence. 1. Rear-wall exhaust pads, typically paper-mesh, accordion-paper, or pocketed-paper. The 25-entry filter media taxonomy on this site distinguishes 12 exhaust types (paper-mesh, accordion-paper, pocketed-paper, fiberglass-arrestor, polyester-arrestor, cube-overspray-arrestor, tower-exhaust-pocket-bag, and others), the verified-fitment kit names the specific media-type slug per the booth make's rear-wall configuration. 2. AMU pre-filter (if the booth has a powered make-up air supply). Many open-face booths run on shop-air ambient draw rather than a dedicated AMU; if your installation includes an AMU, replace its pre-filter as part of the response. 3. Specialty filters where applicable, some open-face booths have a secondary exhaust stage (carbon, HEPA) for VOC or particulate polish; check the verified-fitment kit for any specialty slugs.
Cycle math reminder. Open-face rear-wall exhaust pads run a 7-21 day cycle on production volume, faster than crossdraft because there's no intake-side filter to capture larger particulate before it reaches the exhaust. AMU pre-filter (where present) at 60-90 day cycle.
Regulatory landscape
Open-face booths face the strictest face-velocity scrutiny because the open intake is the containment boundary. OSHA 29 CFR 1910.107 specifies inward face velocity for spray operations; NESHAP Subpart HHHHHH requires the same for area-source surface coaters. AQMD inspections measure face velocity directly with a velometer; positive pressure (outward face velocity) is an immediate notice-of-violation. Filter-replacement cadence on the rear-wall exhaust is the primary maintenance lever; subscription delivery records document the cadence.
Booth pressure too positive (pushing out) on Open Face FAQs
How do I know my open-face is at positive pressure without a velometer?
Hold a tissue or smoke source at the open face during operation. Pulled INWARD means negative face velocity (correct); pushed OUTWARD or hanging stationary means insufficient or reversed face velocity (problem). The visible overspray cloud is the unmistakable confirmation.
Why does my open-face load exhaust pads faster than the closed booth I had before?
No intake-side filter means no first-stage particulate capture — all the overspray hits the rear-wall pads directly. Cycle time per pad face area is shorter than crossdraft or downdraft equivalents at the same throughput.
Can I keep spraying with the cloud rolling forward?
No. The OSHA worker-exposure issue is immediate; you're breathing the overspray. Stop production, replace pads, restore face velocity before resuming.
My open-face doesn't have an AMU — is that a problem?
Many open-face booths don't. They draw make-up air ambient from the shop. The pressure-balance issue is simpler in that case (no AMU oversupply variable), but the rear-wall exhaust loading is still the dominant filter-replaceable cause of positive face condition.
My open-face is a Col-Met / Global / Spray Tech — does the booth-make change this?
No. The fix-path is the same across open-face makes. Specific media-type slug per slot will vary by manufacturer; the verified-fitment kit handles the naming.
What face velocity should I measure on an open-face booth?
OSHA's general spray-finishing guidance and most AQMD permits specify 100 fpm minimum inward at the face for open-face configurations. Some high-VOC operations require 125 fpm. Check your booth's compliance permit for the specific number.
Sources
Primary references cited on this page.
- NESHAP Subpart HHHHHH — Area Source Standardshttps://www.epa.gov/stationary-sources-air-pollution/national-emission-standards-hazardous-air-pollutants-neshap-9
- OSHA 29 CFR 1910.107 — Spray Finishinghttps://www.osha.gov/laws-regs/regulations/standardnumber/1910/1910.107
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