What is a BMS Points List (IO Schedule)?
The points list is the single most important document in BMS design — the master schedule of every input and output the system monitors and controls, listed equipment by equipment. It drives everything downstream: DDC controller selection, panel sizing, cable and containment schedules, graphics count, software licensing and — most visibly — the system price. BMS vendors quote per point, so an inflated or missing points list flows straight into commercial disputes at tender stage.
DI, DO, AI, AO — the Four Point Types
- DI (Digital Input) — reads on/off states: fan run status, trip alarm, auto/manual switch, flow switch, high/low level.
- DO (Digital Output) — sends on/off commands: start/stop, enable/disable, open/close.
- AI (Analog Input) — reads variable values: temperature, pressure, humidity, filter ΔP (4–20 mA / 0–10 V / RTD).
- AO (Analog Output) — sends variable commands: valve position, damper position, VFD speed reference.
How the Calculation Works
Points per type = Σ (Equipment Qty × Points per Unit)
Hard Total = DI + DO + AI + AO
With Spare = Hard Total × (1 + Spare %) [ASHRAE G13: 10–25%]
DDC Count ≈ With Spare ÷ IO per Controller (budget estimate)
The per-unit defaults in this tool reflect common hardwired control philosophies — a VFD AHU at ~17 points, a DOL pump at 4, an FCU at 4. Every value is editable because the sequence of operations decides the points: add a return-air humidity sensor and the AHU gains an AI; move FCUs to networked thermostats and their hard points drop to zero (they become soft points instead).
Worked Example: Mid-Size Commercial Building
4 AHUs, 60 FCUs, 2 chillers (hardwired safeties), 4 VFD pumps, 2 cooling towers, 10 fans, 2 tanks — with 20% spare and 64-point DDCs:
- DI = 142 · DO = 90 · AI = 94 · AO = 78 → 404 hard points
- +20% spare = 485 points → ~8 × 64-point DDC controllers (budget)
- Chiller BACnet interfaces add ~100 soft points → system total ≈ 585 for licensing
Design Tips
- Distribute, don't just divide. The DDC count here is a budget figure — real layouts assign controllers per plant room, per AHU and per floor, so field wiring stays short. A project can need more controllers than the pure math suggests.
- Spare per type, not just total. Specifications usually require spare capacity in each point type per controller — 20% spare AO is not covered by unused DIs.
- Soft points still cost money. Integrated chillers, energy meters and VFDs don't consume IO terminals, but they count toward front-end licensing tiers and engineering hours.
- Match the spec's control philosophy. Hardwired safeties on chillers/boilers are often mandatory even when the equipment is BACnet-integrated — check the specification before zeroing those rows.
- Coordinate with electrical. Every DO to a starter and every VFD needs a corresponding interface in the MCC — share the points list with the electrical designer early.
Frequently Asked Questions
What is a BMS points list?
The master IO schedule of every DI/DO/AI/AO the BMS handles, per equipment — it drives controller selection, panel design, cabling and pricing.
What's the difference between DI, DO, AI and AO?
DI reads on/off (status, trip); DO commands on/off (start/stop); AI reads variables (temp, pressure); AO commands variables (valve, VFD speed).
How much spare capacity should I include?
10–25% per ASHRAE Guideline 13 practice — 20% is the most common specification value, ideally applied per point type per controller.
How many points does an AHU need?
A typical VFD AHU runs 15–20 hard points (status/trip DIs, start DO, temps + filter ΔP AIs, valve/damper/speed AOs) — the exact count follows the sequence of operations.
What are soft points?
Points read over BACnet/Modbus integration (chillers, meters, VFDs) instead of hardwired IO — no terminals used, but they count for licensing and engineering.
Disclaimer: Point counts are indicative for budgeting and tender-stage design. The final points list must follow the project's approved sequence of operations and specifications.