Installation of HVAC and Mechanical Systems
What this domain tests
This section tests field judgement: pressure class, support spacing, clearances, damper access, and connections to equipment. The exam wants installation to the SMACNA manual and the specification, not to habit.
Duct system basics
- Pressure classes: 1/2 in. wg, 1, 2, 3, 4, 6, and 10 in. wg. The pressure class sets gauge, reinforcement, joint type, and seal class together; you cannot upgrade one and ignore the rest.
- Fittings matter. Use radius elbows with turning vanes, or a centreline radius of at least 1.5 times duct width, where space allows; avoid tight square elbows on high-velocity runs.
- Provide access doors at dampers, fire dampers, coils, and controls, and test ports for balancing.
Supports, clearances, and fire stopping
- Hanger spacing follows the SMACNA table by duct size and pressure class, commonly 8 ft for rectangular and 10 ft for spiral. Supports must not crush insulation or distort the duct.
- Fire dampers go in rated assemblies, in a sleeve, with an access panel and listed firestop. A damper nobody can reach fails inspection.
- Maintain clearance to combustibles and structure, and isolate fan vibration with neoprene or spring isolators.
Equipment connections
- Flexible duct: fully extended, minimal sag, supported at intervals, never used as an elbow.
- Condensate drains need a trap sized to the fan's negative static; a shallow trap will not drain and the pan overflows.
- Kitchen exhaust follows NFPA 96: liquid-tight welded duct, cleanouts, clearance to combustibles, and a fan interlocked with the cooking equipment.
Worked example: hanger spacing and load
A 48 ft run of 24 x 12 in. duct at 1 in. wg. Perimeter = 2(2 + 1) = 6 ft. At 1.16 lb/ft2 for 24 ga galvanized, weight = 6 x 1.16 = 6.96 lb per foot. Supports at 8 ft on centre give 7 hanger points on 48 ft, each carrying 8 x 6.96 = about 56 lb plus insulation and hardware. That load is fine for 3/8 in. rod; the field failure is usually the fastener into the structure, not the rod.
Traps that cost marks
- Mixing pressure classes and seal classes.
- Hanging duct from sprinkler pipe, conduit, or ceiling grid.
- Sagging flex duct, which cuts airflow and ices coils.
- Omitting fire damper access.
- Blocking a condensate trap with mastic or debris.
Testing, Balancing, and Commissioning
What this domain tests
This domain tests whether the system you installed actually delivers design airflow, and whether you can prove it. Items cover duct leakage testing, air measurement with a pitot tube, balancing method, and commissioning documentation.
Duct leakage testing
- SMACNA leakage classes rate ductwork by allowable leakage in cfm per 100 ft2 of duct surface at a stated test pressure.
- The relationship is F = CL x P to the 0.65, where F is leakage in cfm/100 ft2, CL is the leakage class, and P is test pressure in in. wg.
- The test section is temporarily sealed, pressurized with a calibrated fan, and the leakage read at the specified pressure. Test the specified section, not the whole system by default.
Air measurement
- Velocity from velocity pressure: V (fpm) = 4005 x square root of VP (in. wg), for standard air at 0.075 lb/ft3, 70 F, 29.92 in. Hg. Correct for altitude and temperature away from standard conditions.
- Traverse on a grid of equal areas with at least 16 to 20 points, about 7.5 duct diameters downstream and 3 diameters upstream of a disturbance.
- Average the velocities, not the velocity pressures; averaging VP understates airflow in a non-uniform profile.
- Balance proportionally: set the index run first, then work back, so one damper does not fight another.
Commissioning
- Pre-functional checklists confirm installation and safety before start-up; functional performance tests prove sequences, interlocks, and capacity.
- Deliverables: TAB report, O&M manuals, as-built drawings, training records, and a deficiency log with closeout.
- Fire and smoke dampers, smoke-control systems, and refrigerant charge are verified and documented.
Worked example: airflow and allowable leakage
A 24 x 12 in. duct reads average VP = 0.36 in. wg. V = 4005 x square root of 0.36 = 4005 x 0.6 = 2403 fpm. Area = 2.0 x 1.0 = 2.0 ft2, so airflow = 2403 x 2.0 = 4806 cfm. If the section has 240 ft2 of surface, is Class 3, and is tested at 3 in. wg: F = 3 x (3 to the 0.65) = 3 x 2.04 = 6.1 cfm/100 ft2, so allowable leakage = 6.1 x 2.4 = 14.7 cfm.
Traps that cost marks
- Averaging velocity pressures instead of velocities.
- A single centre-line reading in a disturbed airstream.
- Using 4005 without correcting for temperature or altitude, such as on a hot roof.
- Balancing with doors open or filters dirty.
- Testing leakage before sealant has cured.