Testing, adjusting and balancing and the people who do it

An air system that has been fabricated well, hung straight and sealed tight can still fail the people who use it. A classroom can be stuffy while the corridor outside is drafty, an isolation room can leak air the wrong way, and a kitchen hood can spill smoke into the dining room. In every one of those cases the duct may be perfect and the airflow wrong. Testing, adjusting and balancing, usually shortened to TAB, is the work of measuring what a finished system actually does, setting its dampers, fans and controls so that each space gets the air the designer intended, and recording the result so the owner can keep it that way.

This course follows Kai Ferreira, now a third-year apprentice at Keelson Sheet Metal in Halifax, as he moves from Sean Doucette's installation crew to the small testing and balancing group Halima Noor started three years ago. The group is run by Marisol Quintero, a certified TAB supervisor who came to Keelson from an independent balancing firm. Kai already knows the Ashgrove Elementary library system intimately: he helped fabricate it in SMW-220, hung it and leak-tested it in SMW-310, and now he will help balance it. Along the way the group also works at the Eastgate Hospital addition, where pressure relationships between rooms protect patients and staff, and at the Brine & Ember restaurant, whose kitchen exhaust and make-up air have to be matched. The kitchen hood itself, and the code rules around it, are covered in SMW-410; here the focus is the measuring and the balancing.

What TAB is

Testing means measuring: airflows at fans, in ducts and at every outlet; static pressures across fans, coils and filters; fan and motor speeds; motor current and voltage; and, on hydronic systems, water flows and pressure drops. Adjusting means changing the system to move those readings toward the design: setting volume dampers, changing fan speed with a sheave or a drive, setting the minimum and maximum flows of variable-air-volume boxes and checking that controls respond. Balancing is the method that ties the two together, so that the right proportion of air reaches each branch and outlet with the least wasted fan energy.

TAB is done after the system is complete, clean and working, and before the building is handed over. It sits between installation and commissioning. Installation builds the system to the drawings; TAB makes it deliver the design flows; commissioning, which is often run by a separate commissioning authority, verifies that the whole building meets the owner's requirements, including the controls sequences, and documents it. A TAB report is one of the main documents the commissioning authority reviews.

Why balancing is needed at all

A duct system is a set of parallel paths of different lengths and resistances. Air, like water, takes the easiest path. The outlet nearest the fan, on a short branch with few fittings, gets more than its share, and the outlet at the end of a long branch gets less. Designers size ducts to reduce the difference, but fittings, field changes, ceiling offsets and the real performance of the fan never exactly match the calculation. Without adjustment, a system typically delivers too much air to some rooms and too little to others, even when the total from the fan is close to design.

Balancing corrects that by adding resistance where there is too much flow, using volume dampers in branches and at outlets, and by setting the fan to deliver the total the system needs. Done well, balancing also saves energy. A system with every damper partly closed and the fan running fast to make up for it wastes power in the dampers; a system balanced with at least one path wide open lets the fan run at the lowest speed that still satisfies every room. Lesson 7 shows how the proportional method finds that condition.

Who does TAB

TAB is a specialty. On large projects it is usually done by an independent firm hired by the owner or the general contractor, so that the people measuring the system are not the people who installed it. On smaller projects, and in many union markets, a mechanical or sheet metal contractor's own certified TAB group does the work. Either way, specifications for public buildings, hospitals and schools almost always require the work to be done under a recognized certification program and to follow a published procedural standard.

In Canada, the main certification bodies that operate here are the Associated Air Balance Council (AABC), whose national standards cover total system balance, and the National Environmental Balancing Bureau (NEBB), whose procedural standards cover testing, adjusting and balancing and other building systems work. The Testing, Adjusting and Balancing Bureau (TABB), associated with the sheet metal workers' union in the US, also certifies technicians and supervisors working in Canada through its international program. Specifications for provincial and federal buildings typically name one or more of these programs. The work itself is usually done by sheet metal workers and refrigeration mechanics under a certified TAB supervisor, and the report is reviewed by the engineer of record. Keelson's group works under one of these programs, and Kai's hours in the group count toward his apprenticeship.

Each program publishes its own standard. AABC publishes the National Standards for Total System Balance, updated in recent years with a chapter on health care facilities. NEBB publishes Procedural Standards for Testing, Adjusting and Balancing of Environmental Systems. TABB-certified work follows the procedures in the program's own manuals and the industry references they cite, including the SMACNA HVAC Systems Testing, Adjusting and Balancing manual. All of them rely on ASHRAE Standard 111, which describes methods for measuring and balancing building HVAC systems. The specification for a project says which standard applies, and the report must state it.

The TAB process from start to finish

Marisol starts every job at a desk, not on a ladder. The first step is the design review: she reads the mechanical drawings and specifications, the equipment schedules and the fan and diffuser submittals, and marks up a set of reduced drawings with every outlet numbered and its design flow written beside it. She checks that the outlet flows on each branch add up to the branch flow and that the branches add up to the fan, and she lists the places where readings will have to be taken. If a branch has no volume damper, or a traverse location has no straight duct, she raises it with the engineer before the ceiling closes, because adding a damper or a test port to a finished ceiling costs far more than adding it during installation.

The second step is a pre-balance inspection. Before any readings, the technician confirms that the system is complete and ready: filters are clean and installed, coils are clean, all dampers are present and open, fire dampers are open and their access doors close, ceilings are in, doors and windows are installed, the fan rotates the right way, belts are tight, and the controls contractor has the system running in a mode suitable for balancing. Balancing a system that is not ready wastes time and gives misleading numbers.

Then come the readings themselves: total airflow at the fan, usually by a duct traverse; static pressure profile across the unit; motor amperage and speed; branch flows; and every outlet. After that come the adjustments, done in a logical order so that each adjustment does not undo the one before. Finally the technician takes final readings, compares them with design, writes up deficiencies, marks the final damper positions so they can be restored, and the supervisor prepares and certifies the report.

Tolerances

Measured airflow never exactly equals design, and the specification says how close is close enough. A common tolerance for outlets is plus or minus 10 percent of design, and for fans and main ducts plus or minus 5 to 10 percent, but specifications vary and health care spaces with pressure relationships may be held tighter or judged by pressure rather than flow. The TAB standards also require that instruments be calibrated and that readings be repeatable. A reading inside the tolerance but outside the instrument's own accuracy is still a reading to check twice.

# Check measured readings against design and a tolerance band
design = [200, 200, 115, 115]        # design flows, L/s
measured = [231, 196, 98, 112]      # measured flows, L/s
tol = 0.10          # specification tolerance (plus or minus)
for k, (d, m) in enumerate(zip(design, measured), 1):
    pct = 100 * m / d
    ok = abs(m - d) <= tol * d
    print(f"outlet {k}: design {d} L/s, measured {m} L/s, {pct:.0f}% of design ->", "within" if ok else "adjust")

outlet 1: design 200 L/s, measured 231 L/s, 116% of design -> adjust
outlet 2: design 200 L/s, measured 196 L/s, 98% of design -> within
outlet 3: design 115 L/s, measured 98 L/s, 85% of design -> adjust
outlet 4: design 115 L/s, measured 112 L/s, 97% of design -> within

In this example the first outlet, the one closest to the fan, is running at 116 percent of design and the third, at the end of a long branch, at 85 percent. Both are outside the band and both need adjustment, but not in the obvious way: closing the first damper will push a little more air to the others, including the third. Lesson 7 shows how to plan the adjustments so that the system converges instead of going round in circles.

Safety on TAB work

TAB technicians spend their days on ladders and lifts, in ceiling spaces, on roofs and inside mechanical rooms with running equipment. The same rules apply as for installation: fall protection where required, mobile elevating work platforms used by trained operators, and ladders set up properly. Two hazards are particular to the work. The first is rotating equipment: belt guards come off to read fan speed or change a sheave, and that is done only with the unit locked out under the site's lockout procedure, never with a guard off and the fan running. The second is electrical: motor current and voltage readings are taken at live starters and disconnects, which requires the training, protective equipment and approach limits set by the employer's electrical safety program. Kai takes amperage readings only with Marisol present and after his electrical safety training, using a clamp meter rated for the circuit.

TAB also involves opening ducts: test holes are drilled for pitot traverses and sealed afterward with plugs, and access doors are opened at dampers. Duct lining, sharp edges and the occasional surprise in an old system, including dust and debris in a renovated building, mean gloves and eye protection are routine. In occupied buildings like Ashgrove, readings are scheduled when rooms are empty or with the school's permission, and in hospitals like Eastgate the infection prevention rules described in SMW-310 still apply whenever a ceiling tile is lifted.