Corpus sync: neutrality remakes, note hygiene, datasheet relocation, transformer-split cross-refs
---
title: Laboratory Exhaust and Fume Hoods
…145 unchanged lines
## Design face velocity shall be maintained within the range established by ANSI/ASSP Z9.5-2022, and no individual traverse point shall deviate more than ±20% from the average face velocity.
−## The design face velocity setpoint shall be selected for the specific hood and hazard within the Z9.5 performance range — ANSI/ASSP Z9.5-2022 establishes 80 to 120 fpm (0.4 to 0.6 m/s) average at the sash opening as the working range; 100 fpm is the common default for general chemistry research; lower velocities reduce energy and can perform well on aerodynamically superior hoods; higher velocities do not necessarily improve containment and can induce sash-edge turbulence; the ±20% point-to-point uniformity limit guards against a misleading average produced by hot and dead spots. {note}
+## The design face velocity setpoint shall be selected for the specific hood and hazard within the Z9.5 performance range — ANSI/ASSP Z9.5-2022 establishes 80 to 120 fpm (0.4 to 0.6 m/s) average at the sash opening as the working range; 100 fpm is the common default for general chemistry research; lower velocities reduce energy and can perform well on aerodynamically superior hoods; higher velocities do not necessarily improve containment and can induce sash-edge turbulence; the ±20% point-to-point uniformity limit guards against a misleading average produced by hot and dead spots.
```datasheet
…9 unchanged lines
## Room background ventilation shall be provided at the minimum air change rate established by ANSI/ASSP Z9.5-2022 §5 for the room hazard classification.
−## Room background air change rate is independent of hood exhaust and is set by the room hazard, not the hood count — ANSI/ASSP Z9.5-2022 §5 ranges from 4 ACH for low-hazard and ASHRAE 170 clinical spaces to 10 ACH for high-hazard chemical research; 6 ACH is the common default for general chemistry teaching and research labs; this background rate dilutes fugitive emissions in the room and is in addition to, not satisfied by, the air drawn through the hoods. {note}
−
```datasheet
label: Room Background Ventilation Rate
…6 unchanged lines
```
+## Room background air change rate is independent of hood exhaust and is set by the room hazard, not the hood count — ANSI/ASSP Z9.5-2022 §5 ranges from 4 ACH for low-hazard and ASHRAE 170 clinical spaces to 10 ACH for high-hazard chemical research; 6 ACH is the common default for general chemistry teaching and research labs; this background rate dilutes fugitive emissions in the room and is in addition to, not satisfied by, the air drawn through the hoods. {note}
+
## Laboratory rooms shall be maintained at negative pressure relative to adjacent corridors and non-laboratory spaces.
−## Make-up air shall track exhaust closely enough to preserve the design pressure relationship under all sash and VAV conditions — if exhaust exceeds supply the laboratory goes excessively negative, slamming doors and drawing contaminants in from corridors; if supply leads exhaust the room can go positive and push vapors into adjacent spaces; on VAV systems the supply must respond fast enough to follow hood exhaust swings without inverting the pressure relationship; make-up air coordination is a shared boundary with [[sync/dedicated-outdoor-air-systems]] and, where energy recovery is applied, with [[sync/energy-recovery-ventilators]]. {note}
+## Make-up air shall track exhaust closely enough to preserve the design pressure relationship under all sash and VAV conditions — if exhaust exceeds supply the laboratory goes excessively negative, slamming doors and drawing contaminants in from corridors; if supply leads exhaust the room can go positive and push vapors into adjacent spaces; on VAV systems the supply must respond fast enough to follow hood exhaust swings without inverting the pressure relationship; make-up air coordination is a shared boundary with [[sync/dedicated-outdoor-air-systems]] and, where energy recovery is applied, with [[sync/energy-recovery-ventilators]].
```datasheet
…11 unchanged lines
## The hood type shall be selected for the chemical, thermal, and radiological hazard of the work it serves.
−## Hood type selection trades energy, flexibility, and hazard suitability — CAV hoods exhaust a fixed volume regardless of sash position and are simplest and lowest first cost; VAV hoods modulate exhaust to hold face velocity as the sash moves, saving substantial energy but requiring responsive controls and supply coordination; specialty hoods (perchloric acid, radioisotope, high-heat/digestion, distillation column) address hazards that ordinary hoods cannot; a CAV hood cannot be readily rebalanced to VAV later, so default to VAV in any research lab where future flexibility matters. {note}
−
```datasheet
label: Hood Type
…10 unchanged lines
```
+## Hood type selection trades energy, flexibility, and hazard suitability — CAV hoods exhaust a fixed volume regardless of sash position and are simplest and lowest first cost; VAV hoods modulate exhaust to hold face velocity as the sash moves, saving substantial energy but requiring responsive controls and supply coordination; specialty hoods (perchloric acid, radioisotope, high-heat/digestion, distillation column) address hazards that ordinary hoods cannot; a CAV hood cannot be readily rebalanced to VAV later, so default to VAV in any research lab where future flexibility matters. {note}
+
## The nominal hood opening width shall be selected to suit the bench work and the room layout.
…12 unchanged lines
## Sash configuration shall be selected to suit the work and to limit the open face area at the design face velocity.
−## Sash type governs both ergonomics and exhaust demand — vertical-rising sashes give an unobstructed full-width opening and are the default for general bench work; horizontal-sliding sashes reduce open area and lower exhaust on VAV systems; combination sashes offer both at higher cost and complexity; on VAV systems the chosen configuration directly sets the maximum open area and therefore the maximum exhaust the system must be sized to deliver. {note}
+## Sash type governs both ergonomics and exhaust demand — vertical-rising sashes give an unobstructed full-width opening and are the default for general bench work; horizontal-sliding sashes reduce open area and lower exhaust on VAV systems; combination sashes offer both at higher cost and complexity; on VAV systems the chosen configuration directly sets the maximum open area and therefore the maximum exhaust the system must be sized to deliver.
```datasheet
…9 unchanged lines
## The interior liner material shall be compatible with the chemicals and thermal load the hood will serve.
−## Liner selection is a chemical-compatibility decision, not an aesthetic one — epoxy resin is the general-purpose default, suitable for the majority of organic and dilute-acid chemistry; polypropylene resists strong acids and bases used in trace-metal and digestion work; Type 304 stainless steel gives a coved, easily decontaminated surface for radioisotope work; Type 316 stainless steel is required for perchloric acid because of its oxidizing service; FRP is used where specific corrosion resistance is needed; the liner must be matched to the worst-case chemistry because a mismatched liner degrades and becomes a containment and contamination problem. {note}
+## Liner selection is a chemical-compatibility decision, not an aesthetic one — epoxy resin is the general-purpose default, suitable for the majority of organic and dilute-acid chemistry; polypropylene resists strong acids and bases used in trace-metal and digestion work; Type 304 stainless steel gives a coved, easily decontaminated surface for radioisotope work; Type 316 stainless steel is required for perchloric acid because of its oxidizing service; FRP is used where specific corrosion resistance is needed; the liner must be matched to the worst-case chemistry because a mismatched liner degrades and becomes a containment and contamination problem.
```datasheet
…17 unchanged lines
## The perchloric acid wash-down cycle shall run a minimum 10-minute continuous flush, with hot water preferred.
−## Perchloric acid service demands oxidizer-resistant materials and routine wash-down because perchlorate salts are shock-sensitive — perchloric acid vapor condenses in the hood and duct and deposits perchlorate crystals that become friction- and shock-sensitive explosives when dry; the wash-down system flushes these deposits before they accumulate; a hot-water flush at roughly 60 to 70 °C dissolves them more effectively than cold; the entire wetted path must be Type 316 stainless steel since FRP and ordinary stainless are incompatible with the oxidizing service; the wash-down drain must connect to the acid waste system and that connection must appear on the plumbing and structural drawings early, because it is frequently missed until the hood arrives. {note}
+## Perchloric acid service demands oxidizer-resistant materials and routine wash-down because perchlorate salts are shock-sensitive — perchloric acid vapor condenses in the hood and duct and deposits perchlorate crystals that become friction- and shock-sensitive explosives when dry; the wash-down system flushes these deposits before they accumulate; a hot-water flush at roughly 60 to 70 °C dissolves them more effectively than cold; the entire wetted path must be Type 316 stainless steel since FRP and ordinary stainless are incompatible with the oxidizing service; the wash-down drain must connect to the acid waste system and that connection must appear on the plumbing and structural drawings early, because it is frequently missed until the hood arrives.
```datasheet
…9 unchanged lines
## Radioisotope hoods shall have a Type 304 stainless steel liner with coved interior corners and a continuous air monitoring port.
−## Radioisotope hood interiors must be fully decontaminable — coved corners and a seamless stainless liner leave no crevice where contamination can lodge, so the interior can be wiped down and surveyed to clearance; a dedicated monitoring port allows continuous air sampling of the exhaust for released activity; these features distinguish a radioisotope hood from a general chemical hood and are not optional retrofits. {note}
+## Radioisotope hood interiors must be fully decontaminable — coved corners and a seamless stainless liner leave no crevice where contamination can lodge, so the interior can be wiped down and surveyed to clearance; a dedicated monitoring port allows continuous air sampling of the exhaust for released activity; these features distinguish a radioisotope hood from a general chemical hood and are not optional retrofits.
## High-heat/digestion and distillation column hoods shall provide increased interior height and high-temperature-rated liner and ductwork suitable for the thermal load.
…3 unchanged lines
## Ductless/recirculating hoods shall be limited to low-toxicity volatile organics whose breakthrough behavior on the specified filter media is confirmed.
−## Ductless hoods are filtration devices and are inappropriate for chemicals that break through carbon — a ductless hood draws air through activated carbon and HEPA media and returns it to the room; carbon does not capture formaldehyde, strong acids, strong bases, or many common solvents; a saturated filter releases captured vapor back into the room with no warning; filter compatibility must be confirmed against the specific chemicals in use before a ductless hood is selected, and ductless hoods are never a substitute for a ducted hood on hazardous chemistry. {note}
+## Ductless hoods are filtration devices and are inappropriate for chemicals that break through carbon — a ductless hood draws air through activated carbon and HEPA media and returns it to the room; carbon does not capture formaldehyde, strong acids, strong bases, or many common solvents; a saturated filter releases captured vapor back into the room with no warning; filter compatibility must be confirmed against the specific chemicals in use before a ductless hood is selected, and ductless hoods are never a substitute for a ducted hood on hazardous chemistry.
# Exhaust Volume and Sizing {toc}
## Each VAV hood shall maintain a specified minimum exhaust volume at the minimum (closed) sash position sufficient to hold a safe face velocity per ANSI/ASSP Z9.5-2022.
−## A VAV hood must never be allowed to fall to zero exhaust at a closed sash — even with the sash down a hood must continue to capture fugitive emission from spills and apparatus left inside; ANSI/ASSP Z9.5-2022 requires a minimum exhaust that preserves a safe face velocity at the minimum sash opening; a minimum near 150 to 250 cfm is typical for a 4-ft hood but must be set explicitly; omitting the minimum cfm setpoint is a common VAV controls error that leaves the hood unsafe at rest. {note}
−
```datasheet
label: VAV Hood Minimum Exhaust (4-ft hood, closed sash)
…6 unchanged lines
```
+## A VAV hood must never be allowed to fall to zero exhaust at a closed sash — even with the sash down a hood must continue to capture fugitive emission from spills and apparatus left inside; ANSI/ASSP Z9.5-2022 requires a minimum exhaust that preserves a safe face velocity at the minimum sash opening; a minimum near 150 to 250 cfm is typical for a 4-ft hood but must be set explicitly; omitting the minimum cfm setpoint is a common VAV controls error that leaves the hood unsafe at rest. {note}
+
## Maximum hood exhaust shall be calculated from the design face velocity and the maximum open face area for the selected hood width and sash.
−## Maximum exhaust scales with hood width and open face area — at a given face velocity the exhaust a fully open hood draws is set by its open area; typical maxima are 400 to 600 cfm for a 4-ft hood, 600 to 900 cfm for a 6-ft hood, and 800 to 1,200 cfm for an 8-ft hood; these figures size the branch duct and the connected load on a manifold; the actual value follows directly from the chosen face velocity and sash configuration. {note}
−
```datasheet
label: Hood Maximum Exhaust (full open, per hood)
…6 unchanged lines
```
+## Maximum exhaust scales with hood width and open face area — at a given face velocity the exhaust a fully open hood draws is set by its open area; typical maxima are 400 to 600 cfm for a 4-ft hood, 600 to 900 cfm for a 6-ft hood, and 800 to 1,200 cfm for an 8-ft hood; these figures size the branch duct and the connected load on a manifold; the actual value follows directly from the chosen face velocity and sash configuration. {note}
+
## A diversity factor shall be applied to the sum of connected hood maximum exhaust when sizing a manifolded exhaust fan.
−## Sizing a manifold fan for 100% of connected hoods is wasteful and is an avoidable RFI — not every hood on a manifold runs at full open simultaneously; ANSI/ASSP Z9.5 and ASHRAE Chapter 16 permit a diversity factor, typically 0.50 to 0.75 of the summed connected maximum, with 0.65 a common default; sizing the fan to the full undiversified total oversizes the fan, the VFD, and the make-up air, and is regularly flagged in design review; the factor selected must be justified by the expected simultaneous-use pattern of the served labs. {note}
−
```datasheet
label: Manifold Exhaust Diversity Factor
…5 unchanged lines
```
+## Sizing a manifold fan for 100% of connected hoods is wasteful and is an avoidable RFI — not every hood on a manifold runs at full open simultaneously; ANSI/ASSP Z9.5 and ASHRAE Chapter 16 permit a diversity factor, typically 0.50 to 0.75 of the summed connected maximum, with 0.65 a common default; sizing the fan to the full undiversified total oversizes the fan, the VFD, and the make-up air, and is regularly flagged in design review; the factor selected must be justified by the expected simultaneous-use pattern of the served labs. {note}
+
# Exhaust Fan and Stack {toc}
…2 unchanged lines
## Dedicated versus manifolded topology drives fan selection — a dedicated single-hood fan isolates one hood (appropriate for incompatible or especially hazardous service) but multiplies roof penetrations and fans; a manifolded system collects several hoods on one riser to a single or redundant high-plume fan, which is more economical and allows dilution of any one hood's effluent in the combined stream; fan construction (FRP or stainless centrifugal, vane-axial, or high-plume dilution) follows from the corrosivity of the effluent and the dispersion requirement; high-plume dilution fans induce ambient air to raise stack exit velocity and lift the plume clear of the building. {note}
+## The exhaust system topology and fan construction shall be specified based on the chemical service, effluent corrosivity, and required containment continuity of each hood.
+
```datasheet
label: Exhaust System Topology
…19 unchanged lines
## Exhaust fan redundancy shall be provided where continuity of containment is required during fan maintenance or failure.
−## Redundancy is a continuity-of-containment decision — an N arrangement has no spare; a failed fan stops exhaust and the hoods become unsafe; N+1 provides a standby fan that takes over on failure or for maintenance, which is the common choice for research manifolds; 2N provides full duplication for the most critical facilities; automatic switchover and its sequencing with the bypass damper and VFD must be specified, not left to the controls trade to infer. {note}
+## Redundancy is a continuity-of-containment decision — an N arrangement has no spare; a failed fan stops exhaust and the hoods become unsafe; N+1 provides a standby fan that takes over on failure or for maintenance, which is the common choice for research manifolds; 2N provides full duplication for the most critical facilities; automatic switchover and its sequencing with the bypass damper and VFD must be specified, not left to the controls trade to infer.
```datasheet
…21 unchanged lines
## The exhaust stack shall terminate a minimum of 10 ft above the roofline.
−## Dispersion modeling shall be performed where any stack is within 50 ft of an air intake.
−
−## Stack height and intake separation must be resolved in early design, not after the roof is set — a stack discharging too low or too close to an intake re-entrains its own effluent into the building; 10 ft above the roofline is a practical minimum; ASHRAE Chapter 16 dispersion modeling is warranted whenever a stack falls within 50 ft of any intake; discovering a re-entrainment problem after the roof is built forces expensive remediation or a high-plume fan upgrade; the stack location relative to intakes is an arrangement that belongs on the drawings: [[drawing: exhaust stack and intake locations]]. {note}
−
```datasheet
label: Minimum Stack Height Above Roofline
…6 unchanged lines
```
+## Dispersion modeling shall be performed where any stack is within 50 ft of an air intake.
+
+## Stack height and intake separation must be resolved in early design, not after the roof is set — a stack discharging too low or too close to an intake re-entrains its own effluent into the building; 10 ft above the roofline is a practical minimum; ASHRAE Chapter 16 dispersion modeling is warranted whenever a stack falls within 50 ft of any intake; discovering a re-entrainment problem after the roof is built forces expensive remediation or a high-plume fan upgrade; the stack location relative to intakes is an arrangement that belongs on the drawings: [[drawing: exhaust stack and intake locations]]. {note}
+
# Airflow Monitoring and Controls {toc}
…6 unchanged lines
## The airflow monitor is the operator's only real-time indication that the hood is safe — without a monitor an operator cannot know that exhaust has failed or fallen below a safe face velocity; ANSI/ASSP Z9.5-2022 requires a monitor with an audible and visual alarm; a setpoint roughly 20% below design (for example, an 80 fpm alarm against a 100 fpm design) warns before containment is actually lost; the sensing technology (paddle/differential-pressure, hot-wire anemometer, or averaging pitot array) is selected for accuracy and maintainability. {note}
+## The airflow monitor sensing technology shall be specified for each hood based on accuracy and maintainability requirements.
+
```datasheet
label: Airflow Monitor Sensor Type
…18 unchanged lines
## VAV hood controllers shall maintain the design face velocity across the full sash travel and shall integrate with the building automation system where required.
−## VAV controls hold face velocity as the sash moves; supply must follow — a VAV hood controller reads sash position and modulates the exhaust valve to keep face velocity constant; the sash sensor may be ultrasonic, infrared, or mechanical; whether the controller is stand-alone or BAS/DDC-integrated, the room supply must be able to respond fast enough to follow the exhaust swing or the room pressure inverts; specifying VAV hoods without confirming the supply system's response speed is a common coordination failure. {note}
+## VAV controls hold face velocity as the sash moves; supply must follow — a VAV hood controller reads sash position and modulates the exhaust valve to keep face velocity constant; the sash sensor may be ultrasonic, infrared, or mechanical; whether the controller is stand-alone or BAS/DDC-integrated, the room supply must be able to respond fast enough to follow the exhaust swing or the room pressure inverts; specifying VAV hoods without confirming the supply system's response speed is a common coordination failure.
```datasheet
…24 unchanged lines
## Exhaust ductwork material shall be selected for the corrosivity of the effluent it conveys.
−## Duct material is a chemical-service decision — PVC-lined galvanized duct suits many general chemical exhausts; FRP resists a broad range of corrosives; stainless steel handles high temperature and is mandatory for perchloric acid; FRP shall never be used for perchloric acid service, where its organic resin is incompatible with the oxidizing condition and Type 316 stainless steel is required; round duct is generally preferred over rectangular for the negative pressures and cleanability of lab exhaust. {note}
+## Duct material is a chemical-service decision — PVC-lined galvanized duct suits many general chemical exhausts; FRP resists a broad range of corrosives; stainless steel handles high temperature and is mandatory for perchloric acid; FRP shall never be used for perchloric acid service, where its organic resin is incompatible with the oxidizing condition and Type 316 stainless steel is required; round duct is generally preferred over rectangular for the negative pressures and cleanability of lab exhaust.
```datasheet
…19 unchanged lines
## Exhaust ductwork shall meet a minimum duct leakage class appropriate to the hazard of the conveyed effluent.
−## Leakage class scales with the toxicity of the effluent — because lab exhaust runs at negative pressure, leakage draws room air inward and is less hazardous than positive-pressure leakage, but it still degrades capture and balance; SMACNA Leakage Class 3 (3 cfm/100 sf at 1 in. w.g.) is the minimum for standard lab exhaust; perchloric acid and highly toxic chemical service require a tighter sealed/pressure-tested construction (Leakage Class 6 or pressure-tested) to ensure nothing escapes the duct path. {note}
−
```datasheet
label: Duct Leakage Class
…6 unchanged lines
```
+## Leakage class scales with the toxicity of the effluent — because lab exhaust runs at negative pressure, leakage draws room air inward and is less hazardous than positive-pressure leakage, but it still degrades capture and balance; SMACNA Leakage Class 3 (3 cfm/100 sf at 1 in. w.g.) is the minimum for standard lab exhaust; perchloric acid and highly toxic chemical service require a tighter sealed/pressure-tested construction (Leakage Class 6 or pressure-tested) to ensure nothing escapes the duct path. {note}
+
## Manifolded exhaust risers shall be designed for the specified header static pressure.
…16 unchanged lines
## The make-up air strategy shall be selected to balance the exhaust without compromising containment or comfort at the hood face.
−## Make-up air strategy trades energy against capture stability — untempered transfer air from adjacent spaces is the lowest energy option but introduces uncontrolled temperature and humidity at the hood; room DOAS supply tempers and distributes make-up through the room (the common modern approach), coordinated with [[sync/dedicated-outdoor-air-systems]]; a tempered auxiliary (compensating) air device delivers make-up directly at the hood face but is energy-intensive and rarely specified for new work; whatever the strategy, exhaust and supply must be balanced or the lab pressure relationship fails. {note}
+## Make-up air strategy trades energy against capture stability — untempered transfer air from adjacent spaces is the lowest energy option but introduces uncontrolled temperature and humidity at the hood; room DOAS supply tempers and distributes make-up through the room (the common modern approach), coordinated with [[sync/dedicated-outdoor-air-systems]]; a tempered auxiliary (compensating) air device delivers make-up directly at the hood face but is energy-intensive and rarely specified for new work; whatever the strategy, exhaust and supply must be balanced or the lab pressure relationship fails.
```datasheet
…13 unchanged lines
## Bench area per hood is limited by NFPA 45 fire hazard class — NFPA 45-2019 limits the maximum bench area served per hood as a function of the laboratory's flammable-liquid quantity classification (for example, 50 ft² per hood for the highest-quantity Class A labs); the limit constrains how much hazardous bench work a single hood may serve and must be coordinated with the fire protection engineer during layout, because it can drive the number of hoods required. {note}
+## The laboratory fire hazard class per NFPA 45 shall be specified for each hood location, based on the flammable-liquid quantity classification.
+
```datasheet
label: Laboratory Fire Hazard Class (NFPA 45)
…15 unchanged lines
## The ASHRAE 110 field test shall meet the specified control level (AM/AI rating designation) for tracer-gas containment.
−## The tracer-gas control level is the contractual acceptance criterion — the ASHRAE 110 designation reports the tracer-gas concentration at the breathing zone at a stated release rate (4 AM 0.05 means under 0.05 ppm SF6, the criterion for standard labs; some owners accept 4 AM 0.10 for lower-hazard applications); the field AI test must meet the specified level; without this language in the specification, commissioning agents frequently omit the tracer-gas test, leaving the owner with no performance baseline. {note}
−
```datasheet
label: ASHRAE 110 Tracer-Gas Acceptance Criterion
…5 unchanged lines
```
+## The tracer-gas control level is the contractual acceptance criterion — the ASHRAE 110 designation reports the tracer-gas concentration at the breathing zone at a stated release rate (4 AM 0.05 means under 0.05 ppm SF6, the criterion for standard labs; some owners accept 4 AM 0.10 for lower-hazard applications); the field AI test must meet the specified level; without this language in the specification, commissioning agents frequently omit the tracer-gas test, leaving the owner with no performance baseline. {note}
+
## The exhaust system shall be air balanced and each hood's measured face velocity and exhaust volume documented in the balance report.
…14 unchanged lines
## The acid waste drain for perchloric acid hoods shall be installed and connected to the acid waste system before the hood is set.
−## Service connections must be located before the hood arrives — the hood's service fixtures, electrical, exhaust collar, and (for perchloric hoods) the wash-down acid waste drain all rely on rough-in placed during construction; the acid waste drain in particular is frequently missed because it must appear on the plumbing and structural drawings and be installed before the hood is set; coordinating these connections late forces wall and floor demolition. {note}
+## Service connections must be located before the hood arrives — the hood's service fixtures, electrical, exhaust collar, and (for perchloric hoods) the wash-down acid waste drain all rely on rough-in placed during construction; the acid waste drain in particular is frequently missed because it must appear on the plumbing and structural drawings and be installed before the hood is set; coordinating these connections late forces wall and floor demolition.
# Delivery, Storage, and Handling {toc}
…36 unchanged lines
- Hood controller spare
```