Shut-off and two-position air valves

Not every critical space needs fully modulating airflow. Many rooms operate in just two states: occupied and unoccupied, normal and isolation, operating and decontaminating. For these, a two-position air valve offers the accuracy and pressure independence of a venturi valve with simpler controls. When a room must be sealed off entirely, a shut-off valve adds the ability to close tightly.

This guide explains how these valves work, where they fit, and what to test.

Two-position air valves

How they work

A two-position valve is a pressure-independent air valve, often a venturi type, with an actuator that moves between two fixed positions. Each position corresponds to a calibrated flow. Because the valve is pressure independent, each flow is held steady across the valve's rated static pressure range.

The actuator may be electric, using a simple open or closed signal, or pneumatic, using a solenoid to apply or vent air pressure. The two flows are set at the factory or in the field during setup, and position switches can confirm which state the valve is in.

Typical applications

• Occupied and unoccupied setback: a lab or classroom lab drops to a lower air change rate at night.
• Constant volume hoods with setback: a hood exhaust that drops to a reduced flow when the sash is closed or when the lab is unoccupied, where the hazard assessment supports it.
• Convertible isolation rooms: a hospital room that switches between normal and negative pressure modes.
• Biosafety cabinets and equipment exhaust: switching between an operating and a standby flow.
• Vivarium rooms: changing flows between animal holding and procedure modes.

Advantages and limits

Two-position valves cannot track continuously changing demand like a VAV fume hood. Where flows change frequently and unpredictably, variable valves are needed.

Shut-off air valves

Purpose

A shut-off valve can close to a low-leakage position, isolating a room or duct branch from the rest of the system. Common reasons include:

• Gaseous decontamination: rooms treated with vaporized hydrogen peroxide or formaldehyde must be sealed so the agent stays in the room for the required contact time and does not leak into the duct system.
• Containment labs: BSL-3 and similar suites may need to isolate supply and exhaust during decontamination or in failure scenarios.
• Vivarium room isolation: allowing a room to be taken offline for cleaning or quarantine while the rest of the facility operates.
• Maintenance: isolating a branch to service equipment without disturbing adjacent rooms.

How tight is tight

Shut-off performance is expressed as leakage at a stated pressure. Manufacturers publish leakage rates for their shut-off products, and bubble-tight isolation dampers are a separate category of device for the most demanding applications. A venturi shut-off valve with a sealing gasket can achieve low leakage suitable for many decontamination tasks, but it is not the same as a bubble-tight damper.

Requirements for containment labs are set by the Canadian Biosafety Standard, which lists physical containment and operational requirements by containment level. Where the standard or the owner's biosafety officer requires gastight isolation, designers often specify bubble-tight dampers in addition to, or instead of, shut-off air valves. See containment labs: BSL-2 and BSL-3 HVAC.

Controls and sequencing

Switching flows or closing valves changes room pressure. The order of operations matters.

Safe sequencing principles

1. Move toward the safer pressure first. For a negative room entering a higher flow mode, increase exhaust before or with supply. When reducing flow, reduce supply first.
2. Close supply before exhaust when isolating a negative room, so it does not briefly go positive.
3. Use interlocks and timers to prevent simultaneous moves that cause transients.
4. Confirm position with end switches or feedback before proceeding to the next step.
5. Alarm on failure if a valve does not reach its commanded position within a set time.

Fail-safe positions

Each valve should have a defined position on loss of power or control signal. For an exhaust valve serving a lab, failing to maximum flow often keeps the room negative. For a supply valve, failing to minimum or closed can be the safer choice. The right answer depends on the room and the fan system, and should be decided in the design with the owner's safety team. Spring-return electric actuators or pneumatic fail positions provide this behaviour.

Integration with the BAS

Two-position and shut-off valves are typically commanded from a room controller or the building automation system. Useful points to expose:

• Commanded state and confirmed position.
• Calculated flow at each state.
• Alarm for failure to reach position.
• Mode status for the room, displayed at the entrance where relevant.
• History of mode changes and who initiated them.

Access to change modes, especially isolation or decontamination modes, should be restricted. Air valve controls integration with BACnet covers the networking side.

Testing and verification

At commissioning and periodically after:

1. Measure airflow at each valve in each state and compare with design.
2. Verify room pressure direction in each mode.
3. Test sequencing during mode changes, logging room pressure to check for reversal.
4. Verify fail-safe positions by removing power or control signal.
5. For shut-off valves, perform a leakage test at the specified pressure, or a room decay or tightness test when decontamination is planned.
6. Confirm alarms and BAS reporting.

These tests belong in the building commissioning plan, with acceptance criteria agreed in advance.

Frequently asked questions

Can a two-position valve be converted to variable later?

Often the valve body is the same and only the actuator and controller change. Check with the manufacturer, since this is model specific.

Is a shut-off valve a fire damper?

No. Shut-off air valves are not listed as fire dampers and do not replace fire or smoke dampers required by code.

Do decontamination rooms always need bubble-tight dampers?

Not always. The required tightness depends on the agent, the room design and the biosafety requirements. The owner's biosafety officer and the decontamination provider should define it.

Key takeaways

• Two-position valves provide accurate, pressure-independent control for rooms with distinct operating modes at lower cost and complexity.
• Shut-off valves isolate rooms or branches for decontamination, quarantine or maintenance.
• Bubble-tight dampers remain the choice where gastight isolation is required by biosafety standards.
• Sequence valve moves so rooms move toward the safer pressure first, and define fail-safe positions.
• Test flows, sequencing, fail-safe action and leakage at commissioning and periodically.