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Starter question — a frequently asked question posted by the MEP Center team to open this forum. Share your own experience below.
I have to issue a DDC point list with the mechanical design of an office building (AHUs, chillers, boilers, pumps and fan coils). Previous lists I received from others were just counts of AI, AO, DI and DO per panel, and the controls contractor priced them very differently.
- What columns and information should a good point list contain?
- How do I count points for a typical AHU?
- How much spare capacity should be specified?
A point list should define every point well enough for a contractor to price, wire and program it: tag, description, point type, signal and range, alarm and trend requirements, and whether it is hardwired or read over a network. Point counts alone are not enough.
Recommended columns
- System and equipment tag (e.g. AHU-01), point tag and plain-language description.
- Point type: AI, AO, DI, DO, or network (integrated) point.
- Signal: 4-20 mA, 0-10 V, PT1000/NTC, volt-free contact, pulse, with range and engineering units.
- Alarm limits and priority, trend interval, display on graphics, schedule, and safe or fail position (e.g. heating valve opens on failure for frost protection).
- Controller or panel, and field device reference (to the valve or sensor schedule).
- Hardwired or integration (BACnet object, Modbus register map reference).
Worked example: AHU with supply and return fans on VFDs
- DI (8): supply fan status, return fan status, supply VFD fault, return VFD fault, pre-filter differential pressure, bag filter differential pressure, frost thermostat, fire alarm shutdown.
- DO (2): supply fan start, return fan start.
- AI (6): supply air temperature, return air temperature, outdoor air temperature, supply duct static pressure, return air humidity, return air CO2.
- AO (5): supply fan speed, return fan speed, heating valve, cooling valve, mixing damper.
Total 21 hardwired points. With 20 % spare, rounded up per type: DI 8 x 1.2 = 9.6, so 10; DO 2.4, so 3; AI 7.2, so 8; AO 6.0, so 6. The controller must therefore provide at least 27 I/O. Many VFD values (power, current, running hours, fault codes) are better read over the network than wired, and should be listed as network points.
Pumps
Pumps are specified as separate duty and standby units, not twin-head pumps. Each pump gets its own start command, run status and fault input, plus speed command and feedback if on a VFD. The list should state duty rotation (e.g. weekly or by running hours) and automatic changeover to the standby pump on fault or loss of differential pressure.
Spare capacity and general items
- Specify spare I/O per controller (commonly 10-20 %) and spare space in panels.
- Include virtual points that need trends or alarms, such as setpoints, valve positions and calculated energy.
- Add meters (heat, electricity, water) with their interface type.
Common mistakes
- Giving counts without signal types, so the contractor cannot select controllers.
- Missing safety points: frost thermostat, fire alarm shutdown, smoke damper feedback.
- Wiring points that could be integrated, or integrating safety functions that must stay hardwired.
- No fail positions for valves and dampers.
Related: HVAC commissioning checklist, AHU sizing quick rules. For deeper study: HVAC systems design course.
Check the edition of the code that applies to your project.
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