GA HVAC common mistakes
GA HVAC · Understand the traps and how to avoid them.
Sizing equipment from rule-of-thumb square-footage factors (e.g., 400–500 sq ft per ton) instead of performing a room-by-room Manual J style load calculation, then selecting the unit on nominal capacity alone without checking sensible/latent split at design conditions.
Candidates memorize shortcut multipliers from field experience and skip the psychrometric and envelope inputs; the exam then presents a design condition where the shortcut gives a wrong answer and the candidate cannot recognize the mismatch.
What to do instead
Practice full load-calc workflows on paper: gather orientation, U-factors, infiltration, internal gains, then compute sensible and latent separately and compare with manufacturer expanded performance data at the actual entering wet-bulb and outdoor design temperature. Always verify total and sensible capacity at design conditions rather than nominal tonnage.
Mixing up superheat and subcooling targets and applying a fixed charging chart value without converting for the metering device type (fixed orifice vs. TXV) or for low-ambient/low-load conditions.
Candidates conflate the two measurements and treat published target values as universal constants rather than device- and condition-specific, a habit formed from rote memorization of charging charts.
What to do instead
Build a decision rule: TXV systems are charged to subcooling, fixed-orifice/piston systems to superheat, and always at stabilized conditions per the manufacturer's chart. Drill problems that give a metering device, indoor wet-bulb, and outdoor temperature and require you to state which measurement governs and why.
Diagnosing a heat pump or furnace problem by swapping parts (board, capacitor, gas valve) before completing a systematic sequence-of-operation check of safeties, pressure switches, and control signals.
Time pressure and familiarity with common failures lead candidates to jump to the most frequent culprit; exam scenarios are written so the obvious part is not the failed component, and the candidate picks it anyway.
What to do instead
Use a written sequence-of-operation checklist for each equipment type and answer troubleshooting items by locating where in the sequence the fault occurs first. Practice tracing the circuit from call for heat/cool through each safety and proving device before naming a failed component.
Confusing duct static pressure, velocity pressure, and total pressure and adding them incorrectly when computing available static or fan performance, or selecting a fan from the wrong curve point.
Candidates treat static and total pressure as interchangeable and read fan curves at the intersection with the wrong system resistance line, an error reinforced by casual field use of 'static' for everything.
What to do instead
Relearn the relationships: total = static + velocity pressure, velocity pressure from V = 4005√VP, and available static = external static minus pressure drops of filters, coils, and grilles. Work problems that require plotting a system resistance curve against a fan curve and identifying the actual operating point after a filter or duct change.
Ignoring code-mandated combustion air, venting category, and gas piping sizing (length plus fittings plus appliance BTU input) and answering from generic pipe-sizing tables without applying the longest-length/equivalent-length method.
Candidates size gas lines by nominal pipe diameter intuition and overlook that developed length, fitting allowances, and specific gravity/altitude corrections change the answer; venting category questions are answered from memory of 'B-vent is fine' rather than the appliance's listed category.
What to do instead
Practice the full NFPA 54 method: compute appliance load, add fitting equivalent lengths, apply the longest-run table with the correct specific gravity and pressure drop column, and confirm the appliance vent category and manufacturer's listing before choosing vent material. Memorize the combustion-air opening rules by appliance input thresholds.
Selecting or troubleshooting motors and controls without accounting for actual operating conditions - not checking nameplate voltage/phase, service factor, ampacity at the applied voltage, and whether the control transformer or contactor is rated for the load.
Candidates read full-load amps at the nominal voltage and ignore derating, dual-voltage wiring, and the difference between running and locked-rotor current, so breaker, wire, and overload answers come out wrong.
What to do instead
For every electrical item, first write down nameplate data, then apply the correct column of the code ampacity table, then verify conductor and overcurrent sizing, and check control-circuit VA against transformer rating. Drill dual-voltage motor wiring and overload-heater selection problems explicitly.
Treating TAB as a paperwork step - accepting design airflow values without measuring at the correct traverse location, not converting velocity pressure to CFM, and not documenting fan RPM, static, and amp draw after adjustment.
Candidates underestimate the measurement discipline of balancing and answer with 'set the damper to design' rather than the actual procedure for a proper Pitot-tube traverse and proportional balancing.
What to do instead
Study the traverse method (grid of readings across the duct, average velocity pressure, CFM = area × 4005√VP), the proportional balancing sequence, and the required final report fields. Practice explaining how to verify total airflow, then branch airflow, then room airflow with the correct instruments.
Skipping commissioning prerequisites - energizing equipment before verifying rotation, phasing, refrigerant charge, and control safeties, or failing to record as-found versus as-left readings.
Candidates memorize startup checklists as ordered lists to recite, not as a logic sequence, so they mis-order steps or omit the documentation that proves the system was commissioned.
What to do instead
Reorganize the startup checklist by system: pre-start (electrical, mechanical, controls), first-start (rotation, amp draw, pressures, temperatures), and handover (documented readings, training). Answer commissioning questions by asking 'what must be true before the next step is safe?' and always include as-found/as-left comparison.
Applying permit, code-edition, and licensing requirements generically instead of identifying which code (International Mechanical Code, International Fuel Gas Code, local amendments) and which edition governs the described jurisdiction.
Candidates learn one code family's numbers and reuse them everywhere, missing that the question hinges on scope, adopted edition, or the AHJ's authority.
What to do instead
For each code-domain question, first identify the scope (mechanical, fuel gas, electrical, plumbing) and the adopted edition, then recall the specific requirement. Study the permit/inspection triggers and the AHJ's role, and practice scenario questions where the local amendment changes the answer.
Calculating heating or refrigeration performance without converting units and matching conditions - mixing sensible and total heat, using 1.08/4.5 factors at the wrong conditions, or neglecting altitude and wet-bulb corrections.
Candidates plug the standard air constants into every problem even when the problem states non-standard conditions or asks for total vs. sensible capacity, and they skip the latent heat term for moisture.
What to do instead
Before any calculation, write the given state points and the quantity requested (sensible, latent, or total), then choose the correct constant and correct for density/altitude if stated. Practice the three side-by-side equations (sensible, latent, total) with the same number set so the differences become automatic.