Commissioning Records: A Worked Example of Air Balancing Figures

A commissioning sheet is a set of measurements an owner can check. This example reads a real looking air balance report and shows what the numbers reveal.

Commissioning is where a system is adjusted to deliver what was designed and the measurements are recorded. For air conditioning, the central document is the air balance report: design air volume against measured air volume at every grille, with the deviation. It is one of the few handover documents an owner can genuinely audit without technical training, because it is arithmetic. This example walks through one.

The situation

An illustrative villa floor is served by one air handling unit feeding five spaces. The contractor submits an air balance report at handover. The specification allows each terminal to be within plus or minus 10 per cent of its design air volume, and the total at the unit to be within plus or minus 5 per cent of design. The owner wants to know whether the report shows a balanced system.

Assumptions (illustrative)

• Design air volumes: majlis 200, living 150, bedroom one 120, bedroom two 100, study 80 litres per second.
• Total design air volume at the unit: 650 litres per second.
• First measured volumes: majlis 150, living 130, bedroom one 110, bedroom two 95, study 75 litres per second.
• Acceptance tolerance per terminal: plus or minus 10 per cent of design.
• Acceptance tolerance at the unit: plus or minus 5 per cent of design.
• Second set of measurements after balancing: majlis 195, living 155, bedroom one 125, bedroom two 98, study 82 litres per second.

Step 1: Add the design volumes and confirm the total

200 plus 150 plus 120 plus 100 plus 80 gives 650 litres per second, which matches the stated design total at the unit. If those two figures had not agreed, the report would be internally inconsistent before any measurement was taken.

The first check on any commissioning sheet is whether it adds up. A report whose terminal figures do not sum to its system figure has either a missing terminal or a transcription error, and both matter.

Step 2: Total the first measured set and compare with design

150 plus 130 plus 110 plus 95 plus 75 gives 560 litres per second against a design of 650. The deviation is 560 minus 650, which is negative 90, or 90 divided by 650, about 13.8 per cent below design.

The system as installed is moving substantially less air than designed, which is outside the 5 per cent tolerance at the unit. This is a system problem, not a balancing problem, and no amount of adjusting dampers will fix it.

Step 3: Calculate the deviation at each terminal

Majlis: 150 against 200, a shortfall of 50, which is 25 per cent low. Living: 130 against 150, 13.3 per cent low. Bedroom one: 110 against 120, 8.3 per cent low. Bedroom two: 95 against 100, 5 per cent low. Study: 75 against 80, 6.3 per cent low.

Only bedroom two and the study are inside the 10 per cent tolerance. The majlis, the largest terminal and the furthest on the branch, is dramatically short, which is the classic signature of a system starved of total air with the loss concentrated at the index run.

Step 4: Diagnose the shortfall before adjusting anything

With every terminal low and the total 13.8 per cent down, the fault is upstream. The candidates are a fan not running at the required speed, a filter already loaded, a damper left partly closed at the unit, a duct leakage rate higher than assumed, or ductwork installed with more resistance than the design allowed through sharp bends and flexible connections.

Balancing distributes the air that exists; it cannot create air. Reading the total first prevents a balancer from spending a day adjusting dampers and reporting a system that is uniformly short.

Step 5: Check the second set after the system fault is corrected

195 plus 155 plus 125 plus 98 plus 82 gives 655 litres per second, which is 5 above the design of 650, or 0.8 per cent high and comfortably inside the 5 per cent tolerance. Per terminal: majlis 2.5 per cent low, living 3.3 per cent high, bedroom one 4.2 per cent high, bedroom two 2 per cent low, study 2.5 per cent high. Every terminal is inside 10 per cent.

This second set is what an acceptable report looks like: a total close to design and every terminal within tolerance. Note that some terminals are now above design, which is normal in a balanced system where air has been redistributed.

Step 6: Check what the report should carry besides the numbers

A usable report names the instrument and its calibration date, the measurement method at each terminal, the date and the person who took the readings, the fan speed or drive setting at the time, the filter condition, and the position of each regulating damper after balancing. Without the damper positions, nobody can restore the balance after a filter change or a repair.

The numbers prove the system was balanced once. The settings and instrument details are what let it be kept balanced, which is why a sheet of figures alone is only half a commissioning record.

What changes the answer

• Instrument calibration: an uncalibrated instrument makes the whole report unverifiable.
• Filter condition at the time of testing: a clean filter at commissioning and a loaded one in service give different flows.
• Fan speed setting: a system balanced at one drive setting is not balanced at another.
• Duct leakage: high leakage shows as a total shortfall with the loss greatest at the furthest terminal.
• Measurement method: readings taken at a grille face and in the duct are not directly comparable.

What the numbers tell you

• Check that the design figures sum to the system total before reading any measurement.
• Read the total deviation first, because a uniformly short system is an upstream fault, not a balancing one.
• Per terminal deviation identifies the index run, which is where the system resistance is concentrated.
• A balanced report has terminals both above and below design within tolerance, not all below it.
• Record damper positions, instrument calibration and fan settings, or the balance cannot be restored later.

Limits of this example

Every figure above is an illustrative assumption used to show the method, not a quote, valuation or market rate. Costs, rates, fees and regulations change and must be confirmed with the relevant authority, lender or supplier before you commit.

Domira builds and supervises projects across Saudi Arabia, and this is the level of detail its site teams work at. Send the drawings or the site, and Domira will tell you what the specification has to say before anyone prices it.