An aspirating smoke detector uses a fan to draw air continuously from the protected space through a network of small pipes with sampling holes, and passes it through a very sensitive detection chamber. Because it actively collects air instead of waiting for smoke to drift to a ceiling detector, it can sense smoke at very low levels and raise staged warnings well before a conventional detector would respond.
VESDA is a trademark of its owner, Xtralis, now part of Honeywell, and is so widely used that people often say "VESDA" for any aspirating system. Several manufacturers make aspirating smoke detectors. This guide describes the technology in general terms; details vary by product and listing.
The parts of the system
An aspirating system has two halves: a pipe network in the protected space and a detector unit, usually mounted on a wall where it can be reached for maintenance.
- Sampling pipes run through the space, typically under the ceiling, in a ceiling void, under a raised floor or across the return air openings of cooling units. Small holes drilled at calculated positions are the sampling points.
- The aspirator is a fan in the detector that draws air through the pipes at a steady rate.
- A flow sensor watches airflow through each pipe. A drop in flow suggests a blocked pipe or holes; a rise suggests a broken pipe or a missing end cap. Either produces a trouble signal.
- A filter removes dust and larger particles that would cause nuisance alarms and contaminate the chamber.
- The detection chamber shines a light source, commonly a laser, through the air sample. Smoke particles scatter the light, and a sensor measures how much. More scattered light means more smoke.
- The exhaust returns sampled air to the space, or in some designs back to the same pressure zone the pipes sample from.
Why it detects smoke earlier
A ceiling spot detector waits for smoke to reach it. In a space with strong airflow, such as a server room, smoke can be diluted, cooled and swept past the ceiling before it builds up enough to trigger a spot detector. In a tall space, smoke may stratify below the ceiling and never get there at all.
An aspirating system works differently in three ways. It samples air actively from many points, including where the airflow goes, such as the return air of cooling units. Its detection chamber can be set far more sensitive than a typical spot detector. And because it combines the air from many holes, it sees the total smoke across an area rather than waiting for one point to reach a threshold.
Staged alarm levels
Most aspirating detectors have several thresholds, so a slowly developing problem produces a sequence of warnings rather than one alarm. VESDA products name them Alert, Action, Fire 1 and Fire 2; other products use similar stages. The settings are chosen during design and commissioning to suit the space and its normal background level.
The value of the early levels depends entirely on what happens when they trigger. If Alert and Action signals go nowhere, or go to an inbox nobody reads at night, the system is just an expensive smoke detector. The design should state who is notified at each level, how, and what they are expected to do.
Where the sampling holes go
Pipe design is where most of the engineering happens. Each sampling hole is treated much like a spot detector location for coverage purposes, following CAN/ULC-S524 and the manufacturer's listing. The holes are sized so that each draws a similar share of air, which matters because holes nearest the detector naturally draw more. Manufacturers provide pipe modelling software that calculates hole sizes, flow balance and transport time, the time it takes air to travel from the farthest hole to the detector. The applicable standard and listing set a maximum transport time, and the design has to meet it.
Other sampling methods suit other spaces. Capillary tubes drop from a hidden pipe to small, discreet sampling points in finished ceilings, which suits heritage interiors. In-cabinet sampling runs small pipes into individual equipment cabinets. In cold storage, pipes run in the freezer while the detector sits in a heated area outside.
Common design mistakes
| Mistake | What happens | How to avoid it |
|---|---|---|
| Pipes too long or too many holes for one detector | Transport time too long; distant holes barely sampled | Model the network and split it across more pipes or detectors |
| Holes placed only at the ceiling in a high-airflow room | Smoke is carried to the cooling units before it reaches the holes | Add sampling at return air openings and in the airflow path |
| Thresholds copied from another site | Constant nuisance alerts, or alerts too late to help | Set levels from the measured background in this space |
| Detector exhaust in a different pressure zone | Airflow and readings drift as doors and fans change pressure | Return exhaust to the sampled zone where the manufacturer requires it |
| No plan for early warnings | Alert and Action signals ignored | Write the response for each level and test it |
A good design record includes the pipe model outputs, hole locations and sizes on the drawings, commissioned airflow and transport time for each pipe, the threshold settings and the reasons for them. That record is what the service technician compares against at every inspection.
Testing and maintenance
Like every part of the fire alarm system, aspirating detectors are inspected and tested under CAN/ULC-S536 as your fire code requires, by qualified technicians, and the system is verified to CAN/ULC-S537 when it is installed or changed. Routine work typically includes:
- Smoke testing at the end of each pipe and measuring the time for the detector to respond, compared with the commissioning record.
- Checking airflow readings against the commissioned baseline and investigating any drift.
- Replacing filters on the manufacturer's schedule, sooner in dusty spaces.
- Inspecting pipes for damage, disconnected joints, painted-over holes and missing end caps.
- Confirming each alarm level reaches the right people and the fire alarm control unit.
- Reviewing the event log for patterns of alerts that point to a recurring source.
Renovations break pipe networks. Ceiling work, new partitions and painting commonly block holes, crack joints or move pipes. Tell your fire alarm service provider before work starts in a protected space, and have the system tested afterwards.
How ehZee can help
Our VESDA and OSID smoke detection service covers pipe network design, threshold and response planning, integration with your fire alarm system and review of existing systems that give nuisance alarms. For the wider picture, see where very early smoke detection makes sense and how a fire alarm system works.
Common questions
Does aspirating detection replace sprinklers or suppression?
No. It detects; it does not put out fires. It is often paired with sprinklers or a clean agent suppression system, and its later alarm levels may be part of how suppression is triggered.
Why does our system keep going into Alert?
Common causes include thresholds set too close to the normal background, dust from construction, diesel exhaust drawn in from outside, or a real overheating component. The event log and a walk through the space usually narrow it down.
Can one detector cover several rooms?
Some detectors can identify which pipe the smoke came from, and some can identify individual sampling points. Whether one detector can serve several rooms depends on the product, its listing and how the fire alarm zones must be arranged.