Two 0–10 V signals are enough to make a VAV box work. They are not enough to operate the system it belongs to.
“How many of your VAV dampers are wide open right now?” In most buildings the answer is: nobody knows. You have the airflow, not the position.
What the 0–10 V pair carries
The classic wiring of a variable air volume box comes down to two analogue signals: an output from the controller to the box, carrying the airflow setpoint, and an input bringing back the measured airflow. Add the 24 V supply and, often, a separate wire for the room sensor.
It is simple, proven, and it works: the box holds its airflow. That is not the problem. The problem is everything those two wires do not carry.
The box controller knows its damper position at every moment. It computes it, it uses it, and it tells no one. It is a measurement already paid for, already available, that analogue wiring leaves behind.
Why that position is the useful one
A box's airflow tells you about the room. Its damper position tells you about the duct network.
A damper close to fully open means the box is struggling to reach its setpoint: available pressure is marginal. A damper almost closed means the opposite: the fan is delivering far more than needed, and the box spends its time throttling.
Yet in the vast majority of installations the duct static pressure setpoint is a fixed value. It was computed once, at design stage, for the worst case — that is, for a few dozen hours a year. For the rest of the year the fan pushes against dampers that resist. The energy is spent twice: once to produce it, once to destroy it in the dampers. And it comes back as noise at the diffuser.
Resetting pressure on the most unfavourable damper
As soon as the automation station reads every damper position, one function becomes possible, and it costs nothing in hardware.
The automation station continuously looks for the most open damper — the most unfavourable one, the hardest to serve — and lowers the duct pressure setpoint until that damper sits at around 90 % open. It is still served, since it still has margin; and every other box, less in demand, throttles far less.
The usual implementation is incremental, not instantaneous: every few minutes the setpoint is trimmed down by one step while too few dampers are near full opening, and raised by one step as soon as too many are. The range is bounded by a minimum and a maximum, and the system is given time to react between steps.
Published measurements on real installations put the gain at around 25 % of fan energy, with a spread of 19 to 60 % depending on network size, number of boxes and the gap between the original fixed setpoint and actual need. These are operating orders of magnitude, not guaranteed values: the more generous the original setpoint, the more spectacular the saving.
A word on efficiency classes, since the question always comes up. SIA 386.111 (SN EN ISO 52120-1) classifies automation functions by level, and room-level air flow control is one of them: on/off operation, a schedule, occupancy detection and demand-based control — on air quality, for instance — are not worth the same. At air handling unit level, the top of the ranking assumes a setpoint driven by actual room demand, not a frozen pressure. Without damper positions, that step is out of reach.
Two diagnostics that come for free
Damper position does not only save electricity. It exposes, with no measurement campaign at all, two very common faults.
- A box whose damper sits at 100 % all year is not holding its airflow. The duct run is undersized, a balancing damper was left shut, or the air handling unit filter is clogged. With 0–10 V this only shows if the airflow feedback was wired — and only if someone manually compares measurement and setpoint.
- A box whose damper never exceeds 15 % is oversized. It regulates poorly at low opening, and it whistles. This is behind a good share of meeting-room noise complaints.
Both are invisible on an analogue installation, and obvious on a damper position trend.
What else the bus brings
Damper position is the main argument. It is not the only one.
| Data | 0–10 V | Communicating box |
|---|---|---|
| Airflow setpoint | yes | yes |
| Measured airflow | yes, if feedback is wired | yes |
| Damper position in % | no | yes |
| Room temperature | separate wire to the controller | yes, if the sensor is wired to the box |
| Local setpoint, occupancy | no | yes, depending on the box model |
| CO2 / air quality | no | yes, if the sensor is wired to the box |
| Minimum and maximum airflow | set at the box | read and changed remotely |
| Actuator fault, damper travel | no | yes |
| Running hours | no | yes |
| Wiring | 4 to 6 wires + 24 V | daisy-chained bus + 24 V |
The most underrated line in that table is the second to last. Minimum and maximum airflows are project parameters: they depend on how the room is occupied. After a floor is refitted — an open space split into offices, a meeting room turned into storage — they should be revisited. On an analogue installation that means a ladder, an access hatch and a laptop under every suspended ceiling; which is to say it never happens. On a communicating installation it is a change made from the supervision.
Finally, the wiring. Four to six conductors per box, home-run to the panel, against a two-wire bus daisy-chained from box to box: over sixty boxes the difference shows in the cable schedule, in the terminal count and in the controller's I/O count.
A bus brings its own discipline: addressing, segment length, device count, termination. That is not an installation detail — it is sized and it is written down. But it is design work, done once, against a missing data point for twenty years.
What to write in the specification
- Communicating boxes. On an open protocol — BACnet MS/TP or Modbus — with no proprietary gateway in between.
- A points list per box. At minimum: airflow setpoint, measured airflow, damper position in %, status and fault — and room temperature where the sensor is wired to the box.
- Minimum and maximum airflows accessible for reading and writing from the supervision.
- Duct pressure continuously reset from damper positions, bounded by a minimum and a maximum.
- The function demonstrated at handover: damper positions and duct pressure recorded before and after activation, minuted in the acceptance report.
- Standardised tagging and an EDE export at handover — without which damper position will exist in the controller and never reach the supervision.
A VAV damper that never reports its position is a sensor you have already paid for and chosen not to read. The extra cost of communication is measured in metres of cable; its absence is paid in kilowatt-hours, in noise and in call-outs above the ceiling.
wall-i is an independent MCR and BMS engineering practice based in Valais, Switzerland. We write points lists and specifications in which damper position is a data point in its own right. A topic you would like us to cover: hello@wall-i.ch