There's a version of this that plays out regularly on handover day. The BMS is installed, the contractor is ready to sign off, and the client gets a demonstration of a system that appears to be working. The graphics are there, the plant seems to be responding, the alarms are set up. Six months later, the building manager is fielding comfort complaints from every floor, the energy bills are higher than the design projections, and the maintenance team can't work out why the system is doing what it's doing. When a controls engineer eventually gets in to look, they find sequences that were never properly tested, sensors reading plausible but incorrect values, and setpoints that were left at commissioning defaults that nobody ever revisited.
Poor BMS commissioning is one of the most expensive problems in commercial building services because it's invisible until it isn't. The system appears to function — plant runs, temperatures are roughly maintained — but the gap between what was specified and what's actually happening can cost a building tens of thousands of pounds a year in wasted energy and unnecessary maintenance. This guide explains what proper commissioning actually involves and what to demand from any contractor carrying it out on your site.
Commissioning is the systematic process of verifying that a BMS — and the plant it controls — performs in accordance with the design intent. CIBSE Commissioning Code M — the management code for building services commissioning — defines commissioning as a quality-oriented process that verifies all components are installed, adjusted, and operating in accordance with the design intent; it establishes that commissioning is not a single event at project completion but a continuous activity from design through to occupation. The key word is verifying. Commissioning is not the same as installation, and it is not the same as a handover demonstration. Installation puts the hardware in place and makes the wiring connections. Commissioning proves that every control sequence works correctly under real operating conditions, that every sensor is reading accurately, that every actuator is responding as specified, and that the system behaves correctly across all its operating modes — including night setback, frost protection, fire mode, and demand-controlled ventilation.
The relevant guidance in the UK includes BSRIA BG8 (the commissioning specification for building services), CIBSE TM31 (building log book toolkit), and for new public-sector and healthcare buildings, the BSRIA commissioning process guides that form part of the building's technical file. When a specification calls for commissioning "in accordance with BSRIA BG8," that's not a vague aspiration — it defines a documented process with specific outputs including a commissioning method statement, a test record for every point on the system, and a signed-off performance report before the building is handed over.
Commissioning is typically the last thing that happens before handover, which makes it the first thing that gets compressed when a project runs over programme. The consequences are predictable. Sensors that are reading slightly high or low — close enough to pass a visual check — cause persistent comfort complaints because the control loops are chasing the wrong value. PID parameters that were never properly tuned result in plant that hunts continuously, wearing out actuators and valves far faster than they should. Time schedules that were left at design defaults run plant outside occupied hours, adding thousands of pounds to the energy bill. Heat recovery wheels that were bypassed to speed up fan testing are never re-enabled. CO₂-based demand-controlled ventilation sequences that look correct in the BMS graphics but were never tested with real occupancy levels.
Each of these faults individually is minor. Collectively, they represent a building operating at perhaps 70% of its design efficiency from day one — and the gap widens over time as the equipment drifts further from its optimum settings without a commissioning record to refer back to.
Proper BMS commissioning follows a defined sequence. Pre-commissioning work — before any functional testing begins — involves verifying every point on the I/O schedule against the physical installation, checking power supplies and network connectivity, confirming wiring continuity on every sensor and actuator circuit, and validating that all controllers are online and communicating correctly. This work should be completed and signed off before any sequence testing starts. Testing control logic against a system where half the field devices are not yet confirmed as wired correctly wastes time and produces misleading results.
Functional testing then proves each control sequence under simulated conditions — overriding sensors to drive the system through its operating modes and confirming that the plant responds as the design specifies. Every AHU sequence, every FCU group, every heating zone, every alarm condition, and every interlock is tested individually and the result recorded. Sensor calibration is verified against a reference instrument, not just checked against a plausible reading. Actuator travel is confirmed across its full range, including end stops and fail positions. This stage typically takes between one and three days per major plant item for a thorough job.
Integration testing follows — verifying that systems which are specified to interact actually do so correctly. That the BMS correctly executes a morning warm-up sequence based on outside air temperature. That CO₂ levels in the return air duct cause the AHU to increase fresh air supply. That a fire signal from the fire alarm panel correctly overrides BMS sequences to the specified fail-safe positions. That the energy metering integration is pulling accurate data through to the BEMS dashboard.
The final phase of commissioning — performance monitoring after the building is occupied — is the one most commonly omitted. A system that passes all its functional tests in an empty building may still fail to maintain comfort when 400 people arrive on a Monday morning and the solar gain on a south-facing open-plan floor is nothing like the design assumption. BSRIA's Soft Landings framework (BG 11/2010) extends commissioning responsibility beyond practical completion, requiring the contractor to remain engaged with building performance during the initial occupation period — for BMS commissioning specifically, this means verifying that the control sequences work as intended under real occupancy conditions, not just under the controlled conditions of a formal witnessing exercise. CIBSE TM31 recommends a structured post-occupation review at three and twelve months after handover, using trended data from the BMS to compare actual energy consumption and comfort performance against design targets. Where the gap is significant, a recommissioning exercise identifies whether the cause is control sequences, sensor drift, plant degradation, or a mismatch between design assumptions and actual building use.
In practice, this monitoring period is rarely carried out unless it's explicitly specified in the contract. Building owners who don't know to ask for it don't get it. The result is that design-versus-actual performance gaps persist indefinitely, and the cost of poor commissioning is quietly absorbed into the energy bill rather than being identified and corrected.
When specifying or procuring BMS commissioning, the key questions are: Does the contractor produce a point-by-point test record for every device on the I/O schedule? Do they use calibrated reference instruments for sensor verification? Do they provide a commissioning report that includes as-built performance data — actual sensor readings, actual control responses, actual energy metrics — not just a declaration that the system has been commissioned? Are they independent from the installation contractor, or are they commissioning their own work under programme pressure?
Alpha Controls carries out BMS commissioning across London and the South East, including recommissioning of existing systems where performance has degraded since original handover. If you're planning a new BMS installation or have concerns about how an existing system is performing, get in touch to discuss your site requirements.
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