A building automation system or BAS is a network of controlling and monitoring devices mainly responsible for the heating, ventilation, and air conditioning (HVAC) control system in large facilities and commercial buildings. Building automation systems are most commonly implemented in large facilities such as healthcare facilities, schools, universities, hotels, residential buildings, and data centers. A BAS network allows building operators to supervise and control the HVAC systems from a centralized location known as the Building Management System or BMS. VAV system, the air handling unit (AHU) and the VAV boxes are physically separated from each other, and each one has its own controller, also distanced from one another. However, for a VAV system to operate effectively, the VAV Boxes and the AHU have to talk to each other; in other words, they need to share information to properly adjust their setpoints based on zone heating or cooling demand.
AHU Sensors: what they do, where to put them, and how to use them
1️⃣ Outside‑air T/RH 🌤️
• Purpose: economizer enable, coil reset, frost logic.
• Location: north side or shaded, away from exhausts and sun; use a shield.
• Spec: accuracy ±0.3–0.5 °C and ±2–3 %RH.
2️⃣ CO₂ sensor 🫁
• Purpose: demand‑controlled ventilation.
• Location: return duct or a representative zone.
• Setpoint: 800–1000 ppm typical; minimum OA per ASHRAE 62.1 still enforced.
3️⃣ Mixed‑air averaging sensor 🔀
• Purpose: detect low mixed‑air temperature, protect coils.
• Location: serpentine across the full plenum cross‑section upstream of the coil.
• Alarm: ~7–10 °C low limit; drive OA closed and enable preheat.
4️⃣ Freeze stat 🧊
• Purpose: coil freeze protection.
• Location: capillary laced on the leaving‑air face of the coil.
• Trip: 3–5 °C; action = stop supply fan, close OA, open heating/preheat valve.
5️⃣ Duct smoke detector 🚨
• Purpose: life safety interlock.
• Location: per NFPA 90A/72 and OEM spacing; avoid turbulent elbows; sample straight runs.
• Action: stop fans and close smoke/fire dampers.
6️⃣ Filter differential pressure 🧱
• Purpose: filter loading and alarms.
• Location: high side upstream, low side downstream; use proper pitot rings or static tips.
• Numbers: note clean DP (e.g., 90 Pa). Alarm at ≈2× clean DP or manufacturer limit.
7️⃣ Supply duct static pressure 📈
• Purpose: VAV static reset and fan control.
• Location: 2/3 of the main supply trunk; avoid takeoffs and elbows.
• Setpoint: 250–500 Pa typical VAV; reset to keep the “most‑open” VAV at ~90%.
• Fan law (plain text): ΔP ∝ Q^2, Power ∝ ΔP × Q.
8️⃣ Airflow/pressure switch (AFS) 💨
• Purpose: proof of airflow for safety sequences.
• Location: straight duct runs with correct pickup orientation.
• Setpoint: verify against design velocity; avoid nuisance trips at low flow.
9️⃣ Motor current switch ⚡
• Purpose: fan run status feedback to BMS.
• Setting: trip just below normal running current; avoids false “off” during soft‑start.
🔟 Duct temperature sensors 🌡️
• Purpose: supply, return, and discharge control and trending.
• Location: ≥5 duct diameters downstream of fittings; average or multi‑point in large ducts.
• Spec: ±0.3–0.5 °C; immersion wells for coils, strap‑on for casings.
1️⃣1️⃣ Space/Return RH sensor 💧
• Purpose: dehumidification, reheat, and IAQ alarms.
• Setpoints: comfort 40–60 %RH; critical spaces per ASHRAE 55 and project specs.
1️⃣2️⃣ Pressure transmitters (coils/fans) 🧪
• Use across coil to trend fouling; across fans to validate curves.
• Equation (plain text): Q = k × √(ΔP/ρ) for calibrated flow rings; document k.
Eg:
🏢 VAV AHU 40,000 m³/h, clean filter DP 90 Pa, design supply static 350 Pa.
• Static reset band: 250→350 Pa; trend fan kW before/after reset.
• Freeze stat trip: 4 °C; mixed‑air low limit: 8 °C.
• CO₂ DCV: setpoint 900 ppm; minimum OA = code value even at low occupancy.
• Filter alarm: 180 Pa; change‑out target 200 Pa.
• Purpose: economizer enable, coil reset, frost logic.
• Location: north side or shaded, away from exhausts and sun; use a shield.
• Spec: accuracy ±0.3–0.5 °C and ±2–3 %RH.
2️⃣ CO₂ sensor 🫁
• Purpose: demand‑controlled ventilation.
• Location: return duct or a representative zone.
• Setpoint: 800–1000 ppm typical; minimum OA per ASHRAE 62.1 still enforced.
3️⃣ Mixed‑air averaging sensor 🔀
• Purpose: detect low mixed‑air temperature, protect coils.
• Location: serpentine across the full plenum cross‑section upstream of the coil.
• Alarm: ~7–10 °C low limit; drive OA closed and enable preheat.
4️⃣ Freeze stat 🧊
• Purpose: coil freeze protection.
• Location: capillary laced on the leaving‑air face of the coil.
• Trip: 3–5 °C; action = stop supply fan, close OA, open heating/preheat valve.
5️⃣ Duct smoke detector 🚨
• Purpose: life safety interlock.
• Location: per NFPA 90A/72 and OEM spacing; avoid turbulent elbows; sample straight runs.
• Action: stop fans and close smoke/fire dampers.
6️⃣ Filter differential pressure 🧱
• Purpose: filter loading and alarms.
• Location: high side upstream, low side downstream; use proper pitot rings or static tips.
• Numbers: note clean DP (e.g., 90 Pa). Alarm at ≈2× clean DP or manufacturer limit.
7️⃣ Supply duct static pressure 📈
• Purpose: VAV static reset and fan control.
• Location: 2/3 of the main supply trunk; avoid takeoffs and elbows.
• Setpoint: 250–500 Pa typical VAV; reset to keep the “most‑open” VAV at ~90%.
• Fan law (plain text): ΔP ∝ Q^2, Power ∝ ΔP × Q.
8️⃣ Airflow/pressure switch (AFS) 💨
• Purpose: proof of airflow for safety sequences.
• Location: straight duct runs with correct pickup orientation.
• Setpoint: verify against design velocity; avoid nuisance trips at low flow.
9️⃣ Motor current switch ⚡
• Purpose: fan run status feedback to BMS.
• Setting: trip just below normal running current; avoids false “off” during soft‑start.
🔟 Duct temperature sensors 🌡️
• Purpose: supply, return, and discharge control and trending.
• Location: ≥5 duct diameters downstream of fittings; average or multi‑point in large ducts.
• Spec: ±0.3–0.5 °C; immersion wells for coils, strap‑on for casings.
1️⃣1️⃣ Space/Return RH sensor 💧
• Purpose: dehumidification, reheat, and IAQ alarms.
• Setpoints: comfort 40–60 %RH; critical spaces per ASHRAE 55 and project specs.
1️⃣2️⃣ Pressure transmitters (coils/fans) 🧪
• Use across coil to trend fouling; across fans to validate curves.
• Equation (plain text): Q = k × √(ΔP/ρ) for calibrated flow rings; document k.
Eg:
🏢 VAV AHU 40,000 m³/h, clean filter DP 90 Pa, design supply static 350 Pa.
• Static reset band: 250→350 Pa; trend fan kW before/after reset.
• Freeze stat trip: 4 °C; mixed‑air low limit: 8 °C.
• CO₂ DCV: setpoint 900 ppm; minimum OA = code value even at low occupancy.
• Filter alarm: 180 Pa; change‑out target 200 Pa.
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