[MEP Center Engineering]

Starter question — a frequently asked question posted by the MEP Center team to open this forum. Share your own experience below.

On many projects the BMS ends up as a single specification clause saying “the contractor shall provide a complete BMS”, and the controls design is left to the controls subcontractor after award. This causes missing points, disputes and poor sequences.

• What drawings and schedules should the MEP designer issue for the BMS at tender stage?
• How much detail is reasonable for the designer, and what can be left to the contractor?
• Are there standards to reference?

[MEP Center Engineering]

The designer should define what is controlled and how it must behave: architecture, control schematics, point list, sequences of operation, interface schedule and field device schedules. The contractor then selects products, designs panels and wiring, and programs to that definition.

Deliverables at tender stage

1. BMS architecture diagram: head end, network levels, controllers per plant room or floor, integration interfaces, power supply (UPS) and riser routes.
2. Control schematics per system: each AHU, chiller plant, boiler plant, pump set and terminal unit type, showing every sensor, valve, damper and safety device in its real location.
3. Point list: point type, signal, alarms, trends, fail positions.
4. Sequences of operation: written step by step, including start-up, normal, alarm, failure, fire mode and restart after power loss. ASHRAE Guideline 36 is a useful model for AHU and VAV sequences.
5. Interface schedule: each third-party system, protocol, points and responsibility.
6. Field device schedules: control valves (Kvs, size, actuator, fail position), sensors (type, range, accuracy), dampers.
7. Testing and commissioning requirements: point-to-point tests, functional tests and trend review periods.

Worked example: control valve in the valve schedule

AHU heating coil, 60 kW, water 70/50 °C (20 K).

• Flow = 60 / (4.186 x 20) = 0.717 kg/s, approximately 2.58 m³/h.
• Design pressure drop across the valve chosen as 20 kPa (0.2 bar): Kv = Q / √Δp = 2.58 / √0.2 = 5.77.
• Select the next standard Kvs of 6.3. Actual valve drop = (2.58 / 6.3)² = 0.168 bar = 16.8 kPa.
• If coil and branch piping between the valve and the differential pressure control point lose 15 kPa, authority N = 16.8 / (16.8 + 15) = 0.53, above the usual target of 0.5.

Putting this on the schedule stops the common habit of selecting the valve the same size as the pipe, which usually gives an oversized valve with poor control.

What can be left to the contractor

• Controller selection and I/O allocation, within the spare capacity specified.
• Panel layouts, wiring diagrams and cable sizes, submitted for review.
• Graphics design, to the designer’s minimum content list.

Standards worth referencing

• EN ISO 16484 series for BACS design, functions and protocols.
• EN ISO 52120-1 (which replaced EN 15232) for BACS efficiency classes A-D; state the class the design targets.
• ASHRAE 135 (BACnet) and ASHRAE Guideline 36 for sequences.

Common mistakes

• Sensors drawn in the wrong place, e.g. a differential pressure sensor at the pump instead of at the index circuit.
• No fire mode sequence coordinated with the fire alarm cause and effect.
• Undefined responsibility for integration of packaged plant controls.
• Pump sets shown as twin-head pumps; specify separate duty and standby pumps with changeover logic.

Related: Fire alarm cause and effect matrix, HVAC commissioning checklist. For deeper study: HVAC systems design course.

Check the edition of the code that applies to your project.

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