Welcome to Controls Traders, located in Adelaide, South Australia. We are a supplier of quality building automation controls and peripheral products for the HVAC industry. We stock a full range of controllers, sensors, valves and actuators, damper actuators and accessories to suit any application. Our aim is to provide our customers with the highest level of service, from sales to delivery and after sales support. With our extensive in-house knowledge and expertise in the industry, we can advise you on selection and application of our wide range of controls products.
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Ventilation systems sized for peak occupancy deliver far more outdoor air than necessary during periods of low or variable occupancy. A conference room designed for 40 people receiving full design airflow when only four people are present wastes substantial energy — the HVAC system overconditions outdoor air for no occupant benefit. Demand-controlled ventilation (DCV) addresses this by using CO₂ concentration as a proxy for occupancy, increasing outdoor air supply when CO₂ rises above a setpoint and reducing it when the space empties. CO₂ sensors are the measurement backbone of any DCV system. This guide covers sensor technology, placement strategies, calibration, output options, and selection criteria relevant to Australian commercial HVAC practice.
DCV is recognised in AS 1668.2 (the Australian Standard for mechanical ventilation in buildings) and is supported under the National Construction Code (NCC) as a strategy for achieving required minimum outdoor air rates while reducing energy consumption during variable occupancy. In practice, correctly commissioned DCV systems consistently deliver measurable reductions in both fan energy and conditioning load — provided the CO₂ sensors are correctly selected, placed, and maintained.
CO₂ is produced by human respiration at a rate of approximately 0.3 L/min per person at rest, rising with physical activity. Outdoor air contains approximately 420 ppm CO₂ as of the current 2024 baseline — elevated slightly from the 400 ppm figure used in older references. As occupancy increases in an enclosed space, CO₂ concentration rises above that ambient baseline. The rate of rise depends on the number of occupants, their metabolic rate, the volume of the space, and the outdoor air supply rate.
ASHRAE 62.1-2022 acknowledges CO₂ monitoring as a method for demand-controlled ventilation. A setpoint of 1,000 ppm is widely used as the upper threshold — representing the approximate equilibrium CO₂ level when ventilation is provided at the ASHRAE 62.1 person-based rate for typical office occupancy. The differential between the 420 ppm outdoor baseline and the 1,000 ppm upper setpoint provides approximately 580 ppm of working range for proportional control. An important qualification: CO₂ is an indicator of occupancy and metabolic load, not a direct measure of all indoor air quality parameters. It does not indicate elevated VOC, formaldehyde, particulate matter, or humidity. DCV based on CO₂ is appropriate for spaces where occupant-generated CO₂ is the primary driver of ventilation need — offices, conference rooms, classrooms, auditoriums, and gymnasiums. It is not appropriate as a standalone ventilation control strategy in kitchens, laboratories, car parks, or spaces with significant non-occupant-related pollutant sources.
Non-Dispersive Infrared (NDIR) is the standard CO₂ sensing technology in all HVAC-grade sensors. Alternative technologies — including electrochemical cells and metal oxide semiconductor (MOS) sensors — are occasionally found in low-cost consumer devices but are not appropriate for commercial HVAC DCV applications due to poor long-term stability, cross-sensitivity to other gases, and limited service life.
The NDIR operating principle involves four components:
NDIR has three significant advantages for HVAC applications: accuracy of ±50 ppm at calibration is readily achievable; the NDIR optical cell has no chemical reaction and does not deplete over time, giving service lives of 10–15 years; and the technology is stable across the humidity and temperature ranges encountered in occupied building environments.
Most HVAC CO₂ sensors use Automatic Baseline Calibration (ABC), sometimes labelled as self-calibration or background calibration. The ABC algorithm assumes that at some point during a regular cycle — typically 7–14 days — the monitored space will be unoccupied and the sensor will be exposed to outdoor air at near-ambient CO₂ concentration. Over each cycle, the algorithm records the minimum CO₂ reading and, after several cycles, uses a statistical average of those minima to reset the sensor's zero baseline.
ABC is effective and low-maintenance for the overwhelming majority of commercial HVAC applications — offices, schools, retail, conference facilities — where spaces are reliably unoccupied overnight and on weekends. The algorithm typically maintains calibration within ±50 ppm across the sensor's service life in these conditions.
ABC fails in continuously occupied spaces such as certain hospital wards, 24-hour call centres, some manufacturing and data centre environments, and other facilities that are never genuinely unoccupied. In these applications, commission sensors with manual calibration capability and calibrate against a known reference gas — 400 ppm reference gas in nitrogen is the standard for zero calibration. Alternatively, relocate the sensor to an adjacent space that is genuinely unoccupied overnight for its calibration cycle. Schedule annual manual verification in any application where there is uncertainty about whether the space achieves full unoccupied periods.
Two primary mounting strategies are used in DCV systems, each with distinct advantages depending on the system architecture.
Wall (zone) sensors are mounted in the occupied space at breathing height — 1.2–1.5 m above finished floor level. They directly measure the CO₂ concentration in the zone being controlled, providing the highest accuracy for individual zone DCV. Wall sensors are best practice for single VAV box control, individual room DCV, and any application where occupancy varies substantially between adjacent zones. Multiple zone sensors are required in multi-zone systems — one per independently controlled zone. Common placement errors to avoid: proximity to supply air diffusers (dilution effect will cause the sensor to underread), proximity to operable windows or doors (infiltration from corridors or outdoors will depress the reading), and proximity to return air grilles (elevated local CO₂ adjacent to the grille is not representative of the zone average).
Duct return sensors are mounted in the return air duct serving the air handling unit (AHU), or in the AHU return air section itself. They measure a blended CO₂ concentration representing all zones returning air to that AHU. A single duct sensor per AHU replaces the need for individual zone sensors and can be used to control the AHU's outdoor air damper directly. The trade-off is accuracy: the mixed-air reading averages occupancy across all zones, so a high-occupancy conference room returning air to the same AHU as an empty office will have its occupancy signal diluted. Duct return sensors are most appropriate for large open-plan areas with relatively uniform occupancy distribution — not for buildings with a mix of high and low occupancy rooms on the same AHU.
| Placement Option | Best Use Case | Measurement Accuracy | Installation Complexity |
|---|---|---|---|
| Wall (zone) sensor | Single zone or individual room control | Highest — direct zone measurement | Higher (one sensor per controlled zone) |
| Duct return sensor | Central AHU serving uniform occupancy | Lower — averaged mixed-air reading | Lower (one sensor per AHU) |
| Zone + duct combination | Large buildings with mixed space types | Comprehensive | Highest cost and complexity |
A proportional DCV control strategy using CO₂ as the process variable is straightforward to implement in any BMS capable of reading an analogue input. The following setpoints are typical for Australian commercial office applications under AS 1668.2 design criteria:
The CO₂ sensor output — typically 4-20 mA or 0-10 V scaled to 0–2,000 ppm — is wired to a BMS analogue input. The BMS modulates the outdoor air damper actuator using a PID control loop with CO₂ concentration as the process variable and outdoor air damper position as the control output. Wind-up protection and minimum position overrides are important configuration considerations: the outdoor air damper must never close below the hygienic minimum regardless of CO₂ level, and the control loop must be tuned to the relatively slow time constant of zone CO₂ response (typically 5–20 minutes for a well-mixed office zone).
The energy benefit of DCV is most pronounced in spaces with highly variable occupancy. Conference rooms, auditoriums, lecture theatres, and gymnasium spaces are frequently designed for peak occupancy loads that are achieved only a fraction of the time. DCV in these spaces can reduce HVAC energy consumption by 15–30% compared to constant-volume outdoor air systems, with the majority of savings occurring in the conditioning energy required to treat outdoor air (cooling, dehumidification, or heating depending on climate and season).
CO₂ sensor output type must match the BMS or controller input available at the installation. Three output formats are in common use in Australian commercial BAS practice:
Specifying CO₂ sensor accuracy correctly requires understanding both the initial calibration accuracy and the long-term drift behaviour. Typical HVAC-grade NDIR CO₂ sensor specifications are:
| Parameter | Typical Specification |
|---|---|
| Measurement range | 0–2,000 ppm (HVAC DCV) or 0–5,000 ppm (industrial / safety monitoring) |
| Accuracy at calibration | ±50 ppm ± 3% of reading (e.g., ±80 ppm at 1,000 ppm reading) |
| Long-term drift (with ABC) | <20 ppm per year, corrected by ABC algorithm |
| Temperature compensation range | 0–50°C (automatic); some models to 60°C for duct applications |
| Operating humidity | 0–95% RH, non-condensing |
| Rated service life | 10–15 years (NDIR optical cell) |
Temperature compensation is particularly important for duct-mount sensors, where the air temperature may differ substantially from the ambient room temperature, and where rapid airflow can affect the thermal equilibrium of the sensor cell. Verify that the sensor's stated temperature compensation range covers the expected duct temperature at the installation point — in mixed-air duct sections in Australian climates, temperatures between 10°C and 40°C are common depending on season and system configuration.
BAPI (Building Automation Products Inc.) offers a comprehensive range of wall-mount and duct-mount CO₂ sensors for HVAC DCV applications, available in Australia through Controls Traders. The BAPI CO₂ sensor range includes standalone CO₂ units for straightforward DCV applications as well as combination sensors integrating CO₂ with relative humidity and temperature measurement in a single housing — an arrangement that reduces wiring and installation time significantly where all three parameters are required in a zone.
BAPI CO₂ sensors support multiple output options including 4-20 mA, 0-10 V, BACnet MS/TP, and Modbus RTU, making them compatible with all major BMS platforms used in Australian commercial and industrial buildings. Combination sensors with digital outputs are particularly efficient in multi-zone systems, where a single RS-485 bus can carry CO₂, humidity, and temperature data from multiple sensors to the BMS without the wiring overhead of individual analogue inputs per parameter. For sensor selection, product availability, and technical assistance, contact the Controls Traders team.
1,000 ppm is the widely used upper setpoint, based on ASHRAE 62.1 guidance and representing approximately the equilibrium CO₂ level when ventilation is provided at the standard person-based rate for office occupancy. A proportional control band of 700–1,000 ppm is common, where the outdoor air damper modulates from minimum to maximum outdoor air as CO₂ rises through this range. Some projects specify tighter setpoints — 800–850 ppm upper limit — where higher indoor air quality targets are required. Confirm the setpoint strategy with the project mechanical specification and the relevant authority in your jurisdiction, as NCC DtS provisions and Green Star or NABERS credits may impose specific requirements.
Sensors with Automatic Baseline Calibration (ABC) self-calibrate by periodically resetting their zero to the minimum reading over a 7–14 day cycle, assuming the space is unoccupied during that period. In genuinely variable-occupancy spaces, ABC typically maintains calibration within ±50 ppm over multiple years without manual intervention. For spaces that are never fully unoccupied, schedule annual manual calibration using 400 ppm reference gas. Sensors that are consistently reading more than 100 ppm above a calibrated reference despite ABC correction, or that have exceeded the manufacturer's stated service life of 10–15 years, should be replaced rather than recalibrated.
Wall-mount (zone) sensors in the occupied space provide the most accurate reading for individual zone DCV — they measure actual occupant CO₂ without the averaging effect of mixing with air from adjacent zones. Duct-return sensors are suitable for central AHUs serving large, relatively uniform occupancy spaces, where a mixed-air reading is an acceptable proxy for zone-level CO₂ concentration. For conference rooms, classrooms, and other variable-occupancy rooms, wall-mount sensors in each room give significantly better control performance and are recommended where budget permits. A duct return sensor can still be used as a secondary check or system-level override in combination with zone sensors.
NDIR CO₂ sensors are designed specifically to measure CO₂ concentration by targeting the 4.26 μm absorption wavelength; they do not measure VOCs, formaldehyde, particulate matter, carbon monoxide, or other indoor air quality parameters. For spaces where multiple pollutant sources exist — commercial kitchens, laboratories, print rooms, car parks — DCV based on CO₂ alone is insufficient to ensure adequate ventilation for all contaminants. In those applications, CO₂ monitoring may still be included as part of a broader IAQ strategy, but ventilation rates must not be reduced below the level required to dilute non-occupant-generated pollutants to safe concentrations.
NDIR CO₂ sensors typically carry a rated service life of 10–15 years. Unlike electrochemical sensors, which rely on a chemical reaction that gradually depletes a reagent, the NDIR optical cell operates without a consumable element — degradation occurs slowly through contamination of the optical surfaces and gradual ageing of the infrared source. Sensors that are consistently reading more than 100 ppm above a calibrated reference after ABC correction, or that have exceeded the manufacturer's stated service life, should be scheduled for replacement as part of building maintenance planning. Record sensor installation dates in the BMS or asset register to facilitate planned replacement programmes.
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Siemens actuators are among the most commonly specified in Australian commercial HVAC and building automation systems. The SSA, SAS, SAY, and SKB series cover the dominant damper and valve control applications — from compact VAV box dampers through to large AHU mixing boxes and modulating chilled water coil valves. Each series has a defined application domain; selecting the correct series avoids field rework and ensures the actuator meets the mechanical and control requirements of the application. Specifying a damper actuator with insufficient torque, or a linear valve actuator where a rotary actuator is needed, will result in either field failure or a costly replacement during commissioning.
This guide covers the technical characteristics of each series, the model number decoding logic for the HVAC actuator range, actuator sizing methodology, and the key selection criteria for Australian commercial projects. Fail-safe behaviour — which series and variants offer spring return, and in which direction — is addressed specifically, as this is a common source of specification errors and non-compliance with damper actuator requirements under AS 1668 and the National Construction Code.
| Series | Application | Motion | Spring Return | Typical Torque / Force | Key Applications |
|---|---|---|---|---|---|
| SSA | Dampers | Rotary | Yes | 3–45 Nm | Outdoor air dampers, exhaust dampers, AHU mixing boxes, economiser dampers, relief dampers |
| SAS | Dampers | Rotary | Yes | 2.5–10 Nm | VAV terminal unit dampers, small HVAC zone dampers |
| SAY | Valves (globe) | Linear (push-pull) | Yes | 300–800 N | 2-way and 3-way globe valves on AHU cooling and heating coils |
| SKB | Valves (ball/butterfly) | Rotary (quarter-turn) | Optional | 10–150 Nm | Ball valves, butterfly valves, isolation and modulating applications on larger pipe sizes |
All series are available through Controls Traders as an authorised Siemens distributor for South Australia and nationally.
The SSA series is Siemens' primary line of spring-return rotary actuators for air-side damper applications. Spring return is the defining attribute of the series: the electric motor drives the actuator toward its operating position while simultaneously compressing a mechanical spring. On loss of power or control signal, the spring drives the actuator to a defined fail-safe position without requiring any external power or control input. This behaviour is a code requirement for outdoor air dampers under AS 1668.2 (to close and isolate the air stream in cold climates, preventing freeze damage to heating coils) and for any damper serving a life safety function under AS 1668.1.
The SSA range spans the torque requirements of most commercial HVAC damper installations. Key torque classes within the range include:
Control signal variants are identified in the model number by a two-digit decimal suffix:
Fail-safe position must be specified separately from the torque class and control variant. SSA actuators are available as normally-closed (NC — spring drives the actuator to the closed position on power loss, standard for outdoor air dampers and mixed-air dampers) or normally-open (NO — spring drives the actuator to the fully open position, used for relief dampers and exhaust dampers where opening on power loss prevents over-pressurisation). Confirm the required fail-safe direction with the mechanical engineer or BAS contractor before ordering.
Many SSA models support integral or field-fitted auxiliary switches — end-of-travel feedback contacts that close when the damper reaches the fully open or fully closed position. These dry contact outputs provide a binary position proof signal to the BMS, which is essential in fire/smoke control sequences where the BMS logic must confirm damper position before permitting a fan to start. The presence and configuration of auxiliary switches is identified in the suffix codes of the full model number.
The SAS series serves small dampers and VAV terminal unit applications where the physical dimensions of the SSA actuator exceed the available mounting space in the unit. The torque range of 2.5–10 Nm covers single-blade and small multi-blade dampers typically found inside VAV boxes and small duct-mounted zone dampers. The mounting arrangement — shaft clamp geometry, bracket profile, and overall actuator footprint — is designed to fit the smaller damper shafts and mounting configurations found in manufactured VAV terminal units from major HVAC manufacturers.
Control variants in the SAS series parallel those in the SSA: on/off, three-point floating, and 0–10 V modulating variants are available, with spring return as standard. The selection between SSA and SAS for a given application is primarily determined by physical fit — damper shaft diameter, available mounting depth, and the space envelope inside the terminal unit or duct section — rather than by control or torque characteristics alone.
The SAY series drives valve stems in a linear (push-pull) motion, making it the correct actuator type for globe valves with a rising stem. Globe valves are the predominant valve type on AHU chilled water and hot water coils in Australian commercial HVAC systems, as their flow characteristic (typically equal-percentage) is well-matched to coil heat transfer behaviour. Spring return in the SAY provides fail-safe valve positioning — fail-closed is standard for heating and cooling coil valves, ensuring that the coil valve closes on power loss and prevents uncontrolled heating or cooling of the conditioned space.
Key technical specifications for the SAY series:
Close-off pressure verification is a critical step in SAY selection. The actuator closing force must exceed the hydraulic force acting on the valve disc at maximum system differential pressure. The required close-off force (N) can be estimated as: Kvs (m³/h) × system ΔP (kPa) × a valve-type factor. For most secondary HVAC circuits at standard ΔP, the SAY31 at 300 N provides adequate close-off force for DN15–32 globe valves. Verify against the valve manufacturer's published close-off force data for the specific valve and system pressure combination before finalising the selection.
The SKB series drives quarter-turn applications: ball valves and butterfly valves, which require a 90° rotation to move between fully open and fully closed positions. Unlike the SSA damper actuator — which is designed for continuous modulating positioning across a 95° rotation — the SKB's mechanical design is optimised for the torque profile of ball and butterfly valves, which require significantly higher breakaway torque at the start of the opening stroke than running torque during normal modulation.
Spring-return and non-spring-return variants are both available in the SKB range. Spring-return SKB variants are selected for isolation applications where the valve must reach a defined fail-safe position — typically fail-closed for shutoff applications on condenser water or chilled water pipework. Non-spring-return (hold last position) variants are appropriate for modulating applications where the BMS can reliably reposition the valve after a power interruption, and where the fail-safe requirement is to remain in the last commanded position rather than drive to an end-of-travel position.
The torque range of 10–150 Nm in the SKB series covers ball valve and butterfly valve sizes from DN20 through to large-diameter butterfly valves on major plant. Torque selection requires matching the actuator's rated torque to the valve's required breakaway torque at maximum system differential pressure, with a minimum safety margin of 25% above the valve manufacturer's published breakaway torque figure. Breakaway torque for ball valves is typically 2–3 times the running torque; for butterfly valves it varies with disc geometry and seat type.
Siemens uses a structured part number system for the HVAC actuator range. Reading the model number correctly is essential for confirming a specification and for identifying replacement parts in the field. The general structure is:
Two practical examples illustrate the decoding:
For engineers and technicians familiar with Belimo actuator model numbers, the naming philosophy differs — Belimo encodes product family, torque, and options differently — but the underlying approach of reading torque class, signal type, and spring-return status from the part number is similar. The Controls Traders article on Belimo model numbers explained covers the Belimo decoding methodology for comparison.
Correct actuator sizing prevents both field failure (undersized actuator cannot overcome damper or valve resistance) and unnecessary cost (oversized actuator is larger and more expensive than required). A systematic approach reduces the likelihood of specification errors.
Damper actuators (SSA and SAS series):
Required torque (Nm) = Damper area (m²) × Specific torque (Nm/m²)
Typical specific torque values by damper type:
Apply a minimum 25% safety margin to the calculated torque to account for manufacturing tolerances, seal ageing, and blade linkage wear over the actuator's service life. Where the damper manufacturer publishes a required actuator torque specification, use that value — with the 25% margin applied — in preference to the area-based calculation. Select the next standard SSA or SAS torque class above the calculated requirement.
Linear valve actuators (SAY series):
Required force (N) must equal or exceed the valve's close-off force at maximum system differential pressure. This is published in the valve datasheet as a "required actuator force at rated ΔP" figure for Siemens VVI/VVF/VXF valves. The SAY31 at 300 N covers the majority of AHU coil valve applications on secondary HVAC circuits at standard design differential pressures. Where the calculation indicates a required force approaching the SAY31 rating, specify the SAY61 (800 N) to maintain an adequate margin at maximum system pressure transient conditions.
Rotary valve actuators (SKB series):
Required torque (Nm) ≥ valve breakaway torque at maximum system ΔP × 1.25 safety margin. Ball valve breakaway torque is typically 2–3 times running torque; the exact figure is published in the valve manufacturer's data. Butterfly valve breakaway torque varies with disc geometry, seat material, and the direction of flow relative to disc position — consult the valve datasheet.
For complex selections — particularly where system ΔP varies significantly across operating conditions, or where the damper or valve manufacturer's data is not available — contact Controls Traders for selection support. The Siemens HVAC product catalogue (Catalogue S) provides the full torque and force data for each model in the SSA, SAS, SAY, and SKB series.
Controls Traders stocks the Siemens HVAC actuator range across the SSA, SAS, SAY, and SKB series for Australian commercial projects. For selection assistance, lead time confirmation, or to discuss actuator requirements for a specific project, contact the Controls Traders team.
Both are spring-return rotary damper actuators for air-side HVAC applications. The SSA covers a wider torque range (3–45 Nm) and offers more control signal variants and auxiliary switch configurations, making it the standard selection for medium and large commercial dampers from approximately 0.3 m² upward. The SAS is more compact in its physical form factor, intended specifically for smaller dampers and VAV terminal unit applications where the SSA's body dimensions do not fit within the available mounting envelope inside the unit or duct section. The selection is primarily determined by physical fit rather than control requirements.
Use spring-return whenever the damper or valve must reach a defined fail-safe position — open or closed — on loss of power or control signal. This is mandatory for outdoor air dampers under AS 1668.2 (fail-closed to isolate the air stream and protect heating coils in cold climates), for fire and smoke dampers under AS 1668.1, and for any isolation valve where fail-safe closure is a safety or process requirement. Use non-spring-return (hold last position) where the fail-safe requirement is to maintain the last commanded position, or where the BMS can reliably reposition the actuator within an acceptable time after power restoration — typically modulating valves on secondary circuits in systems with reliable power supply.
Yes. Siemens and third-party suppliers provide adapter kits to mount the SAY series to compatible globe valve stems. Before specifying an adapter, verify two critical parameters: first, that the valve stem travel is compatible with the SAY's 5.5 mm stroke — some globe valves use a different stroke and require a stem nut adjustment or a different actuator entirely; second, that the SAY's force rating (300 N for SAY31, 800 N for SAY61) meets the valve's published close-off force requirement at the maximum system differential pressure. Where neither of these can be confirmed from the valve manufacturer's data, contact Controls Traders for guidance.
The first two letters identify the series: SS for spring-return rotary damper actuators, SA for linear or non-spring rotary actuators, SK for rotary ball or butterfly valve actuators. The following letter identifies the application variant: A for rotary damper, Y for linear valve, B for rotary ball valve. The digits (31, 61, 81, 161) identify the torque or force class — higher numbers indicate higher rated values. The decimal suffix identifies the control input: .03 is on/off two-position, .53 is three-point floating, .73 is 0–10 V modulating. Trailing letters indicate options: S for integral auxiliary switch, U for universal voltage (24–240 V AC/DC). The full decoding table is published in Siemens Catalogue S — HVAC Products.
Using a specific torque of 12 Nm/m² for a parallel-blade damper with standard seals: 1.2 m² × 12 Nm/m² = 14.4 Nm. Applying a 25% safety margin: 14.4 × 1.25 = 18 Nm. The next standard SSA class above 18 Nm is the SSA81 at 25 Nm, which is the correct selection for this damper. For a damper fitted with tight low-leakage blade seals (Class 1 or Class 2 leakage rating), increase the specific torque assumption to 16–20 Nm/m² and recalculate — in this case the SSA81 at 25 Nm remains the appropriate selection but with a reduced safety margin, and the SSA161 at 45 Nm may be warranted if the damper manufacturer's published torque specification exceeds 25 Nm.
Differential pressure (DP) measurement is fundamental to HVAC system operation. Whenever air or fluid passes through a restriction — a filter bank, a coil, a duct fan — there is a pressure difference across it. Differential pressure sensors measure that difference and convert it to a signal a BMS or controller can act on. The sensor class covers an enormous range of applications: from detecting a 5 Pa pressure differential across a door gap in a hospital isolation room, to monitoring the 1,000+ Pa pressure rise across a supply air fan. Selecting the wrong sensor — one with a range that is too wide, or an output that does not match the BMS analogue input — results in poor measurement resolution, unnecessary field rework, or incorrect alarm set points.
This guide covers the operating principles behind DP sensors, the pressure ranges and output signal types available, and a step-by-step selection process applicable to the most common HVAC applications. It references products available through Controls Traders, including the BAPI differential pressure sensor range, which covers room pressurisation through to high-pressure industrial air handling. For a broader overview of HVAC sensors available from Controls Traders, including temperature, humidity, and CO₂ types, visit the sensors category page.
A differential pressure sensor has two pressure ports — a high-pressure port and a low-pressure port — each connected to the measurement point via tubing or direct-mount fittings. The sensor measures the pressure at each port and outputs a signal proportional to the difference between them. When both ports are at equal pressure (the reference condition), the output represents zero differential pressure. When the high-pressure port is at greater pressure than the low-pressure port, the output increases proportionally.
Three primary sensing element technologies are used in HVAC-grade DP sensors:
Signal conditioning electronics convert the raw sensor element output to a standard electrical signal — 4–20 mA, 0–10 V, or a digital protocol such as BACnet MS/TP or Modbus RTU. Accuracy of ±2% of full scale is typical for standard HVAC-grade instruments; high-accuracy process transmitters achieve ±0.25% of full scale but are rarely required in building HVAC applications.
The pressure range of a DP sensor determines both its measurement ceiling and its resolution. A sensor with a 0–6,250 Pa range used on a 25 Pa room pressurisation application will produce only 0.4% of its full output at the target set point — effectively unusable for control. Matching the sensor range to the application is critical.
| Range Category | Pressure Range (Pa) | Typical HVAC Application |
|---|---|---|
| Ultra-low | <25 Pa | Hospital isolation room pressurisation, clean room pressure monitoring |
| Low | 25–125 Pa | Filter monitoring in low-velocity systems, pharmaceutical clean room pressure |
| Medium | 125–1,250 Pa | VAV box airflow measurement, supply duct static pressure, AHU filter banks, fan monitoring |
| High | 1,250–6,250 Pa | High-velocity ducted systems, industrial air handling units |
| Bidirectional | ±12.5 to ±250 Pa | Room pressurisation where differential direction may reverse |
The bidirectional range category deserves specific attention for room pressurisation applications. A unidirectional sensor (0 to +X Pa) can only report pressure in one direction. If the controlled space momentarily goes negative relative to the reference — which can occur during door opening, a supply fan trip, or poor initial commissioning — a unidirectional sensor will either read zero or saturate at its lower limit. It cannot communicate to the building automation system that the pressure has reversed. A bidirectional sensor (±X Pa) produces an output above mid-scale for positive pressure and below mid-scale for negative pressure, allowing the BMS to detect and respond to pressure reversals.
Filter monitoring is the most common DP sensor application in commercial HVAC. A sensor is installed across the filter bank — high-pressure port upstream, low-pressure port downstream — and the output is monitored by the BMS. As filters accumulate dust and particulate, the pressure drop across the bank increases from the clean filter resistance toward the final resistance at which the filter should be changed.
ASHRAE recommends DP-based filter monitoring rather than time-based scheduling because identical filters in different systems — one handling clean office air, another handling dusty warehouse return air — will reach their final resistance at very different rates. Time-based replacement wastes filters that still have useful life, or leaves loaded filters in service beyond their design change point.
For MERV 8–13 filters at typical AHU face velocities of 2–3 m/s, initial resistance ranges from 40–80 Pa and final resistance (the filter change alarm point) is typically 150–250 Pa. Select a sensor with a range approximately 20% above the expected final resistance. A 0–250 Pa sensor is appropriate for many commercial HVAC filter banks; a 0–500 Pa sensor provides additional headroom for higher face velocity applications.
VAV box airflow measurement uses a DP sensor paired with a Pitot tube, flow cross, or factory-fitted flow station inside the VAV terminal unit. The flow element creates a measurable velocity pressure differential by separating total pressure (measured at the upstream stagnation point) from static pressure. The VAV controller derives volumetric airflow using the Bernoulli relationship: V = √(2ΔP/ρ), where V is velocity (m/s), ΔP is velocity pressure (Pa), and ρ is air density (approximately 1.2 kg/m³ at standard conditions). Volumetric flow is then calculated by multiplying velocity by the known duct cross-section area.
Typical velocity pressure at an airflow of 5 m/s in a 300 mm round duct is approximately 15 Pa. At minimum design airflow — often 30–40% of maximum — velocity pressure drops to approximately 1.5–2.5 Pa, placing demanding resolution requirements on the sensor. Select a sensor with a range of 0–125 Pa to cover the full VAV operating range, and confirm that the sensor's resolution is adequate at the minimum flow condition.
Building pressurisation control is a critical function in hospitals, clean rooms, pharmaceutical laboratories, and isolation rooms. Controlled pressure differentials prevent cross-contamination between spaces: positive pressure rooms (+12.5 Pa relative to adjacent corridor) push air outward, preventing particulates from entering; negative pressure rooms (−12.5 Pa) draw air inward, preventing airborne contaminants from escaping. These requirements are governed by AS 1668 and the National Construction Code (NCC/BCA) for healthcare facilities.
Sensors for this application must be bidirectional with adequate resolution at the set point. A ±25 Pa bidirectional sensor is appropriate for a ±12.5 Pa target. Resolution of 0.1 Pa or better is required to support tight pressure control within ±1–2 Pa of set point. Mount the sensor in a location with stable temperature — avoid direct exposure to supply air diffusers or locations subject to temperature swings that cause zero drift. Reference the low-pressure port to a location representative of corridor or adjacent space pressure, not immediately adjacent to a supply diffuser or door gap.
AHU performance monitoring uses DP measurement across the complete air handling unit — from inlet plenum to supply fan outlet — or across individual components such as cooling coils and heating coils. The DP baseline is established at commissioning and stored in the BMS. Deviation from the baseline indicates component fouling: a rising DP across a cooling coil indicates biofilm or scale accumulation; a rising DP across the complete AHU may indicate a combination of filter loading and coil fouling. DP-based AHU monitoring allows maintenance to be targeted to actual condition rather than calendar schedules.
The terms DP sensor and DP transmitter are used interchangeably throughout HVAC specifications and product catalogues. Technically, a sensor refers to the detecting element — the silicon diaphragm, capacitive cell, or mechanical element that responds to pressure. A transmitter includes signal conditioning circuitry and a standard electrical output that can be connected directly to a BMS analogue input. In HVAC practice, when a project specification calls for a "DP sensor, 0–250 Pa, 4–20 mA output," it is invariably referring to a complete transmitter. The technical distinction matters only when sourcing replacement detecting elements for an existing transmitter, or when specifying a field-mount transmitter to be connected to a remote sensing element.
Three output signal types are common in HVAC DP sensors:
BAPI (Building Automation Products Inc.) manufactures a comprehensive range of differential pressure sensors calibrated specifically for HVAC applications, available in Australia through Controls Traders' BAPI product range. BAPI sensors are designed for stable performance in the variable temperature and humidity conditions typical of mechanical rooms and duct interiors, where ambient conditions can fluctuate significantly over daily and seasonal cycles.
The BAPI DP sensor range covers ±12.5 Pa bidirectional sensors for hospital room pressurisation, through standard ranges of 0–125 Pa, 0–250 Pa, 0–500 Pa, 0–1,250 Pa, and 0–6,250 Pa for filter monitoring, VAV airflow, duct static pressure, and high-pressure industrial applications. Output options include 4–20 mA, 0–5 V, 0–10 V, and BACnet MS/TP digital output. The digital output models provide simultaneous DP and temperature measurements on the same device, reducing the analogue input count on BMS panels. BAPI sensors can be specified with field-adjustable ranges and zero suppression, allowing a single stocked model to serve multiple site pressure ranges.
For selection assistance — particularly for room pressurisation applications where sensor resolution and mounting location require careful consideration — contact Controls Traders. The Controls Traders technical team can confirm sensor selection against project specifications and advise on site-specific installation requirements.
For MERV 8–13 filters at typical AHU face velocities of 2–3 m/s, initial resistance is 40–80 Pa and final resistance (filter change point) is 150–250 Pa. Select a sensor rated 0–250 Pa or 0–500 Pa so the output does not saturate before the filter reaches its change point. Add a 20% range margin above the highest expected pressure to provide headroom for variations in face velocity and filter loading rate across different operating conditions.
Yes, when paired with a Pitot tube, flow cross, or flow station that creates a measurable velocity pressure differential. The DP sensor measures the difference between total pressure (stagnation) and static pressure, which is equal to velocity pressure at that point. The VAV controller or BMS then applies V = √(2ΔP/ρ) and the duct cross-section area to calculate volumetric flow in m³/s or L/s. Sensor resolution at minimum flow conditions is the critical selection parameter for this application.
In HVAC practice, the terms are used interchangeably. Technically, a sensor refers to the detecting element — the silicon diaphragm or capacitive cell — while a transmitter includes signal conditioning circuitry and a standard electrical output (4–20 mA, 0–10 V, or digital). When a specification calls for a "DP sensor, 4–20 mA output," it means a complete transmitter. The distinction is relevant only when sourcing replacement detecting elements for an existing transmitter housing, or when specifying a separate sensing element and signal conditioner for a specialised installation.
Use 4–20 mA for any cable run longer than 30 metres, or where the signal cable shares a conduit with power wiring. A current signal is not affected by cable resistance or induced voltage noise, and the 4 mA live-zero means a broken wire can be detected — the BMS input reads below 4 mA rather than zero, allowing a wiring fault to be distinguished from a genuine zero-pressure reading. Use 0–10 V where cable runs are short and wiring simplicity is the priority, noting that a broken wire will be indistinguishable from a zero-pressure reading.
Select a bidirectional sensor covering the expected range with margin — for a ±12.5 Pa target, a ±25 Pa bidirectional sensor is appropriate. Confirm the sensor resolution is 0.1 Pa or better, since tight pressure control within ±1–2 Pa of set point requires the sensor to detect small deviations reliably. Mount the sensor body in a stable environment away from supply air diffusers and direct airflow, and reference the low-pressure port to a location representative of the adjacent corridor or space pressure, not immediately adjacent to a door gap or supply diffuser.
If you've ever stared at a Belimo part number like NMB24-MFT-T or R2025-B2 and wondered what each segment means, you're not alone. Belimo model numbers follow a precise alphanumeric logic — once you understand the system, you can read any Belimo code off a datasheet and know immediately what you're looking at: torque, fail-safe type, voltage, control signal, valve size.
This guide decodes the system end to end, covering damper actuators, control valves, and the most commonly specified variants used in Australian commercial HVAC and building automation projects.
Controls Traders stocks the full Belimo product range as an authorised Australian distributor. If you need to cross-reference a model or check availability, the Belimo brand page is the place to start.
Belimo model numbers are not random. They encode, from left to right:
Some positions are letters, some are numbers, and the suffixes add further layers. Work through each position in sequence and the code becomes readable in under a minute.
The first letter of any Belimo damper actuator indicates how much torque it produces. This is the most critical spec — undersize the actuator and the damper won't close properly; oversize and you've wasted money and potentially damaged the damper linkage.
| First Letter | Torque | Typical Application |
|---|---|---|
| T | 2 Nm | Small zone dampers, VAV boxes |
| L | 5 Nm | Small to medium dampers |
| N | 10 Nm | Medium commercial dampers |
| A | 20 Nm | Large AHU dampers |
| G | 40 Nm | Very large dampers, multi-stack assemblies |
| E | Variable | Large or specialty applications |
As a rule of thumb, calculate 1 Nm per 1.5–2 m² of damper area, then select the next torque class up for safety margin. Tight-sealing dampers, cold climates, and ageing linkages all increase the effective torque requirement.
The second letter defines how the actuator behaves on power loss — critical for life safety applications and sequence-of-operations design.
This is why LF and LM are different products despite looking similar at first glance. An LF will spring-return on power loss; an LM will hold position.
The digits following the configuration letter indicate supply voltage:
A full actuator code up to this point looks like: NMB24 — a 10 Nm, non-spring return, basic actuator running on 24 V.
The suffix after the dash defines how the actuator receives its control signal:
NMB24-MFT-T
If you're specifying damper actuators for AHUs or VAV systems, the MFT suffix gives you maximum flexibility for future reprogramming without changing hardware — a smart investment on any project that will run a modern BAS.
Fire and smoke actuators follow the same base logic but are prefixed with FS to denote compliance with fire-rated damper requirements:
These actuators are independently tested and certified for operation at elevated temperatures (up to 177°C / 350°F). Never substitute a standard damper actuator on a fire/smoke damper regardless of torque match — the independent certification is what matters, and the installation will not meet NCC or AS 1668 requirements without it.
Belimo valve model numbers encode valve type, pipe size, and — for pre-assembled units — the actuator pairing. The logic is separate from the actuator naming but equally systematic.
The R2.. and R3.. series are characterised control valves for modulating control of chilled water, hot water, and condenser water circuits.
The digits following indicate DN (nominal bore) size:
| Model Code | DN Size | Common Application |
|---|---|---|
| R2015 | DN 15 (½") | Fan coil units, small terminal units |
| R2020 | DN 20 (¾") | Larger FCUs, small AHUs |
| R2025 | DN 25 (1") | AHU coils, medium capacity systems |
| R2032 | DN 32 (1¼") | Medium-large coils |
| R2040 | DN 40 (1½") | Large AHU coils |
| R2050 | DN 50 (2") | Large capacity coils, chiller connections |
Important: Valve sizing is based on Kv/Cv flow coefficient and system flow rate — not on pipe connection diameter alone. Oversizing a valve (selecting a larger Kv than required) causes the valve to run at very low percentage open, destroying control authority and making the system difficult to commission. Use Belimo's selection tool or the Kv calculation method at the design stage.
For fan coil unit and terminal unit control applications, the R2015 and R2020 characterised valves paired with a rotary actuator are the most common specification in Australian hotel and commercial projects.
The B2.. and B3.. series are factory-assembled valve and actuator combinations:
These arrive from the factory with a matched Belimo actuator already installed, which reduces installation time and the risk of mismatched torque selection. The actuator designation follows the valve body code and determines control type, voltage, and fail-safe behaviour.
If you're comparing control valve options for chilled or hot water systems, the B-series simplifies procurement and installation but offers less field-configuration flexibility than a separately mounted R-series valve with a chosen actuator.
Belimo's pressure-independent valve range uses distinct model designations:
These products are specified at design stage and selected by maximum design flow (L/s or GPM), not by Kv. No manual commissioning of flow balancing is required after installation.
1. Confusing -SR with spring return. The -SR suffix on a Belimo actuator means 2–10 V DC proportional signal, not spring return. For spring return, look at the second character — 'F' for spring return. An NM24-SR is a non-spring-return actuator with proportional control; an NF24-SR is spring return with proportional control.
2. Incorrect voltage specification. Ordering a 24 V actuator into a 230 V panel causes equipment damage and a field rework. Australian commercial installations commonly use 230 V panels — verify the panel supply voltage before specifying.
3. Torque undersizing. Default to the next torque class up from your calculated requirement. Tight-sealing dampers, cold climates, and ageing linkage hardware all increase effective torque demand beyond the theoretical calculation.
4. Sizing a valve by connection diameter rather than Kv. The pipe size printed on a valve does not determine whether it's right for your application. An oversized valve running at 5% open has very poor control authority. Calculate Kv from design flow and acceptable pressure drop.
5. Forgetting auxiliary switch requirements. If your sequence of operations requires a damper-open proof signal to enable a fan start, the base actuator will not provide it. Order the -S variant at design stage — retrofitting auxiliary switches in the field is difficult and usually requires a unit swap.
The 'F' in the second position indicates spring return (fail-safe) operation. When power is removed, an internal spring drives the actuator to its fail position — either fully open or fully closed depending on how the actuator is mounted and wired. This behaviour is distinct from 'M' (non-spring return — holds last position on power loss) and 'K' (electronic fail-safe — capacitor-driven return to fail position).
The -3 suffix indicates floating (three-point) control: separate open and close pulse signals drive the actuator motor in each direction. The -SR suffix indicates proportional control via a 2–10 V DC input that positions the actuator continuously across its full stroke. Proportional control provides finer modulation and is preferred for chilled water and heating coil valve applications. Floating control is simpler, lower cost, and entirely adequate for on/off and two-position damper applications.
MFT stands for Multi-Function Technology. MFT actuators are field-programmable via Belimo's PC-Tool software — control direction, signal range, fail position, and operating time can all be configured on site without hardware changes. When used with a Belimo ZipLink module or bus interface, MFT actuators support BACnet MS/TP and Modbus RTU communication, enabling full integration into a building automation system.
Size the valve by Kv (flow coefficient) value, not by pipe connection diameter. Calculate the required Kv from your coil design flow rate (L/s) and the acceptable pressure drop across the valve (typically 25–50% of the coil pressure drop for good control authority). Use Belimo's online selector tool or the formula Kv = Q / √ΔP to arrive at the correct model number. Always aim for a valve authority of 0.5 or above at design conditions.
Yes, for separately specified R-series valve and actuator combinations. Belimo actuators use a direct-coupled shaft interface that allows the actuator to be removed and replaced without disturbing the valve body or pipework. The replacement actuator must have equal or greater torque than the original and a compatible shaft interface. For B-series pre-assembled ball valve units, the valve and actuator are typically replaced as a complete unit since the assembly is factory-set.
Belimo model numbers are systematic, not arbitrary. Reading from left to right: torque class, fail-safe mode, configuration type, voltage, then control signal and options as suffixes. For valves, the series prefix tells you valve type and configuration, and the following digits give DN size or flow class.
Understanding the code means specifying correctly the first time, identifying cross-references quickly, and verifying that what arrives on site matches the design intent — a small investment that pays off every time a delivery note lands on the workbench.
Controls Traders is an authorised Belimo distributor in Australia. Browse the full Belimo product range or contact the team for selection support on your next project.
