Manifolded lab exhaust systems

Older laboratory buildings often have a separate exhaust fan on the roof for every fume hood. Each fan runs whenever its hood is in use, and the roof becomes a forest of small stacks. Modern lab buildings more commonly use a manifolded exhaust system, in which many hoods and general exhaust points connect to a common duct served by a group of large fans.

Manifolding offers redundancy, energy savings and better plume dispersion, but it also creates a shared system that must be controlled carefully. This guide explains how it works and what to watch for.

How a manifolded system works

Each fume hood and each lab's general exhaust connects through its own air valve to a branch duct. Branches join a main exhaust manifold, which runs to a fan plenum on the roof or in a penthouse. Two or more exhaust fans draw from the plenum and discharge through high-velocity stacks.

Key components include:

• Individual air valves controlling each hood and general exhaust point.
• A manifold and branch ductwork sized for combined flow.
• A static pressure sensor in the manifold.
• Exhaust fans, commonly in an N+1 arrangement.
• A bypass or outdoor air damper at the fan plenum.
• Stacks or high-plume induced-flow fans.
• Optional heat recovery coils.

Advantages over individual hood fans

Diversity is a significant benefit. Not every sash is fully open at once, so the combined peak is usually lower than the sum of individual maximums. Choosing a diversity factor needs care and an understanding of how the labs will be used; aggressive assumptions leave no room for growth.

Manifold static pressure control

Every air valve on the system needs a minimum pressure drop to stay pressure independent. The fan system therefore maintains a negative static pressure in the manifold strong enough that the most remote valve has its minimum drop at peak flow.

A typical control approach:

1. A static pressure sensor in the manifold, or several at critical points, reports to the fan controller.
2. The controller varies fan speed, the bypass damper position, or both, to hold the setpoint.
3. Setpoint may be reset based on valve position feedback, so the manifold runs no more negative than needed.
4. Alarms notify operators if static pressure is lost.

Static pressure reset using valve feedback can save significant fan energy, since lab exhaust systems run continuously. The principle is the same as in supply systems, covered in static pressure reset with trim and respond.

Stack discharge and bypass air

Lab exhaust must leave the building with enough velocity and height that it disperses and does not re-enter through air intakes or reach people on the roof. ANSI/ASSP Z9.5 has long recommended a minimum stack discharge velocity of about 15 m/s (3,000 fpm) and a stack height of at least 3 m (10 ft) above the adjacent roof line, with dispersion analysis or wind tunnel studies for sensitive sites.

As hood demand drops, exhaust flow from the labs falls. To keep stack velocity up, systems typically do one of the following:

• Bypass air: an outdoor air damper at the fan plenum opens to add air, keeping fan flow and stack velocity constant even as lab exhaust varies.
• Variable stack geometry: some fans use variable-geometry nozzles to maintain discharge velocity at lower flow.
• Staged fans: fans turn off as demand falls, so remaining fans run near design flow.
• Wind-responsive control: with a site wind study, systems can reduce stack velocity when wind conditions allow safe dispersion at lower flow.

Bypass air is simple and reliable but wastes fan energy. Combining staged fans, variable-geometry nozzles and wind-responsive control can reduce energy significantly where a dispersion study supports it. See odour control and exhaust re-entrainment for related dispersion issues.

Redundancy and reliability

Lab exhaust is a life safety system. Typical reliability features:

• N+1 fans, so the failure of one fan does not reduce capacity.
• Fan isolation dampers so a failed fan can be removed from service.
• Emergency power for exhaust fans, as determined by the owner and code.
• Automatic standby fan start on failure, with alarms.
• Redundant static pressure sensors to avoid control failure from one bad sensor.

Chemical compatibility and exclusions

Manifolding combines exhaust from many hoods, so chemical compatibility must be reviewed. NFPA 45 and the owner's safety program guide which hoods may share a system. Typical exclusions include:

1. Perchloric acid hoods, which need dedicated exhaust with washdown.
2. Radioisotope hoods with specific licensing requirements.
3. Hoods handling materials that may react dangerously when combined.
4. Biological safety cabinets with specific exhaust connections, depending on type.

Duct and valve materials must suit the combined exhaust. See air valve materials and coatings for corrosive exhaust.

Energy recovery

A manifold concentrates exhaust in one location, which makes heat recovery practical. In Canadian climates, recovering heat from lab exhaust can substantially reduce the energy needed to heat 100 percent outdoor make-up air.

Common approaches include run-around glycol loops, which keep exhaust and supply streams completely separate, and heat pipes. Energy recovery wheels are used less often in fume hood exhaust because of cross-contamination concerns. Coils in exhaust must suit corrosive service and include provisions for cleaning and frost management.

Commissioning and operation

Commissioning a manifolded system should confirm:

• Manifold static pressure control is stable across the full range of demand.
• The most remote valve has its minimum pressure drop at peak flow.
• Standby fan starts on failure without loss of hood flow.
• Stack velocity is maintained at all demand levels.
• Bypass damper and fan staging sequences work as intended.
• Alarms report to the BAS.

Ongoing checks during fume hood certification confirm hoods still achieve face velocity as the building's use changes.

Key takeaways

• Manifolded exhaust connects many hoods and exhaust points to shared fans, offering redundancy, diversity and better dispersion.
• Manifold static pressure must keep every air valve above its minimum pressure drop.
• Stack discharge velocity and height protect against re-entrainment; bypass air, staging and variable nozzles maintain it at low flow.
• Chemical compatibility reviews decide which hoods may share the system.
• Concentrated exhaust makes heat recovery practical, using methods that avoid cross-contamination.