Why this measurement is difficult to get right
Commissioning work is done against a programme, with limited access to the plant and with instruments that have to be portable and battery powered. A reading has to be obtained in minutes, frequently through an access hole cut on the day, at a location chosen by what is reachable rather than by what the standard would prefer. The instrument that measures best in a laboratory is often not the instrument that gets used.
The measurements themselves are routine - air velocity, water flow, differential pressure, temperature and humidity - but they are frequently taken at conditions well away from design. A system is rarely at full load when the commissioning engineer is on site, and the plant may not be complete. Recording the operating condition alongside each reading therefore matters as much as the reading, because without it the figure cannot be judged against the design value it is meant to demonstrate.
This is also where differential pressure earns its place. It is the fastest indicator of whether a filter is loaded, a coil is fouled or a fan is delivering, and it needs two pressure connections rather than a full flow measurement. For troubleshooting, that speed is worth more than the absolute accuracy of a flow survey.
What has to be measured
- Air velocity and volumetric flow at terminals, in ducts and at fan inlets.
- Water flow in heating and chilled water circuits.
- Differential pressure across coils, filters, valves and fans.
- Supply, return and surface temperature, plus relative humidity.
- The operating condition each reading was taken at, since a figure without it cannot be compared against design.
Which instrument measures what
| Measurement | What the reading has to establish | Instruments commonly specified |
| Duct and terminal airflow | That each terminal is delivering its share of the design flow, which is often a balance between outlets rather than an absolute figure | Thermal mass and vane anemometers, with pitot tubes for duct traverses |
| Water flow in heating and chilled circuits | That the design flow reaches the coil or the terminal unit, sometimes without a valve being disturbed | Ultrasonic and electromagnetic flow meters |
| Differential pressure and static pressure | The condition of filters, coils and fans, which is faster to establish than a full flow measurement | Differential pressure instruments |
| Temperature and humidity | Supply and return conditions, and whether the space is being held where the design intends | Temperature measurement and environmental monitoring |
| Recording over an operating cycle | What the system does over a day rather than at the moment the engineer happened to visit | Data acquisition and monitoring |
Portability, probe length and battery life weigh more heavily here than long-term stability does.
Narrowing the choice for a commissioning visit
- Establish what has to be demonstrated - a design flow, a balance between terminals, or a control sequence - because that decides which quantity is recorded.
- Check the access available for each measurement, including the duct size and the hole that can be cut, since probe length follows from it.
- Decide whether readings are compared against a design value or only tracked against each other, which sets the accuracy requirement.
- Confirm the operating condition at the time of measurement and record it, because the system is rarely at design load.
- Where results go to a building management system, confirm the protocol, addressing and scaling before ordering.
- Weigh battery life and portability against resolution, because a commissioning instrument is carried all day.
- Plan for the measurements that need a second person or two instruments at once, such as differential readings across a coil.
- Decide whether the readings have to be exported into a commissioning record, which makes logging part of the requirement.
Where this application ends and the next begins
| Adjacent application | What separates it from HVAC commissioning |
| Water flow measurement | Flow is a permanent, often commercial duty there, while here it is a temporary check on a closed circuit taken during a short visit. |
| Energy efficiency testing | Commissioning verifies that a system delivers its design duty, whereas efficiency testing measures what it costs to run across a cycle. |
| Tank & silo level monitoring | Level is a stored quantity in a vessel; nothing in a commissioning duty is stored, and the fluid is moving through a closed circuit. |
| Industrial test bench & calibration | A bench produces a known value under controlled conditions, while commissioning measures whatever the installed system happens to be doing. |
| Product quality inspection | That is a manufacturing judgement, whereas this work verifies an installation against its design intent on site. |
What to send with the enquiry
- What has to be demonstrated, and the design values it will be compared against.
- The quantities to be measured at each position.
- Duct sizes and the access available for probes.
- Line sizes of the water circuits and whether they can be isolated.
- The operating condition expected during the visit.
- Whether readings must be recorded or exported, and into what format.
- Whether any result has to go to a building management system.
- How many measurements are needed, and over how many visits.
- Quantity required and the date it has to be in use.
- Whether calibration documentation is expected with the instruments.
