LA HVAC common mistakes
LA HVAC · Understand the traps and how to avoid them.
On refrigerant charging items, the candidate adds charge until suction pressure reaches a target value and stops there, without converting to superheat or subcooling or checking the manufacturer's charging chart.
Suction pressure is the most visible gauge reading and feels like a direct measure of 'how much refrigerant is in there.' Candidates who learned on small appliances carry over the habit and never internalize that pressure alone is ambiguous because it moves with indoor and outdoor conditions.
What to do instead
Before the exam, drill the fixed decision rule: for a fixed-orifice or piston system, set charge by superheat at the evaporator outlet using the manufacturer's chart with indoor wet-bulb and outdoor dry-bulb; for a TXV system, set charge by subcooling at the condenser outlet. Practice reading a chart cold, and on every charging question write down which metering device is named before choosing a method.
The candidate treats superheat and subcooling as interchangeable numbers, or reports superheat measured at the compressor instead of at the evaporator outlet.
Both are temperature differences computed from a saturation temperature, and under time pressure the candidate grabs whichever formula they remember. The compressor location is easier to picture, so the measurement point gets misplaced without the candidate noticing.
What to do instead
Memorize the definitions as location-specific: superheat = actual suction line temperature minus the saturation temperature corresponding to suction pressure, taken at the evaporator outlet (or as the manufacturer specifies); subcooling = saturation temperature corresponding to liquid-line pressure minus actual liquid-line temperature, taken at the condenser outlet. Practice with a pressure-temperature chart until converting gauge pressure to saturation temperature is automatic in both directions.
On combustion items, the candidate analyzes the flue gas or adjusts the burner before confirming that the heat exchanger is intact and that the appliance has adequate combustion air and venting.
Flue-gas numbers look like the 'real' combustion science, so candidates jump to them. Safety sequencing is a habit learned on the job, and candidates who have not done service work underestimate how often exam items test order of operations rather than calculation.
What to do instead
Adopt a fixed sequence for every combustion question: verify venting and combustion air, inspect the heat exchanger, measure gas supply pressure and manifold pressure against the data plate, then evaluate draft and flue products. If a question asks what to do first, the answer is almost always the step that protects occupants, not the step that improves efficiency.
The candidate sizes ductwork by matching the equipment's nominal tonnage to a rule-of-thumb CFM and stops, skipping friction rate, equivalent length, and available static pressure.
Rules of thumb circulate widely in the trade and are fast. Duct design items often present a simple layout that invites a shortcut, and candidates who have not worked through a friction chart recently avoid the multi-step method.
What to do instead
Work at least a dozen practice layouts end to end: total equivalent length including fittings, available static pressure from the blower performance table, friction rate in inches of water per 100 feet, then duct sizing from the friction chart. Be able to state why equivalent length, not measured length, governs fitting losses.
On load calculation items, the candidate uses square footage times a factor, or uses the existing equipment's capacity as the load, instead of performing a room-by-room calculation with orientation, glazing, infiltration, and internal gains.
Manual J-style calculations are tedious and the exam clock pressures candidates toward a single multiplication. Existing equipment feels like a validated answer because it 'worked' in the building.
What to do instead
Practice a full room-by-room calculation at least three times, including a room with west-facing glass and one over an unconditioned space. Internalize that oversizing degrades humidity control and cycling, so 'the old unit was 3 tons' is a distractor, not evidence.
The candidate answers electrical troubleshooting items by replacing parts in sequence rather than by measuring voltage drop, amp draw, and continuity to isolate the fault.
Part-swapping is common in the field under time pressure, and candidates reproduce field habits on the exam. Reading a wiring diagram and following a sequence of measurements feels slower than guessing at the likely component.
What to do instead
Practice the measurement-first method: confirm supply voltage, then control voltage, then the specific switch or load, using the diagram to predict what each reading should be before taking it. For any 'unit will not start' item, write out the sequence of operation and identify where the sequence breaks.
The candidate conflates the Louisiana licensing framework, attributing statewide scope, exam, or enforcement authority to the wrong body or assuming a parish license covers the state.
Licensing rules are administrative and rarely taught in technical training. Candidates learn the trade from experienced technicians who repeat folklore about which license is required, and municipal and parish rules vary.
What to do instead
Build a one-page reference from the current statute and board rules covering which work requires a state mechanical license, which classifications exist, what insurance and continuing education are required, and where local permits and parish requirements add on top. Verify each line against the primary source rather than a forum post, and re-check before the exam because rules change.
The candidate answers permit and code items from general recollection instead of from the adopted code edition and the local authority's amendment.
Codes differ by edition and jurisdiction, and candidates often work in one parish while the exam reflects statewide adopted editions. Confident memory of 'how we do it here' produces wrong answers when the exam tests the adopted text.
What to do instead
Identify which code family and edition Louisiana has adopted for mechanical, fuel gas, and refrigeration work, and study clearance, venting, combustion air, and refrigerant concentration-limit requirements from that edition. For any question about inspection or permit sequencing, reason from the stated authority having jurisdiction rather than from habit.
On contract and estimating items, the candidate treats a change order, a verbal authorization, or a signed proposal as interchangeable, and answers payment and lien questions without reference to the contract documents and notice deadlines.
These items read like general business common sense, so candidates answer quickly without applying the specific legal mechanics. The distinctions between an offer, a change order, and a directive are not part of most technical training.
What to do instead
Study the contract-formation chain and the change-order process as a sequence: written scope, priced change, signed authorization, revised schedule and payment terms. Learn Louisiana's notice and lien deadlines for private and public work and the difference in remedies. On estimating items, separate direct cost, overhead, and markup explicitly rather than blending them into one multiplier.
The candidate answers recovery, evacuation, and refrigerant-handling items from memory of field practice, skipping the required vacuum level, leak-check sequence, or cylinder and transport rules.
Field practice is often faster and looser than regulation and manufacturer procedure, and the candidate has seen evacuation 'verified' by time rather than by micron gauge. Regulatory details about cylinder filling and transport are rarely part of daily work.
What to do instead
Memorize the sequence: recover to the required level, leak-check, evacuate to the manufacturer's micron level with a micron gauge and hold, then charge by weight or by the specified superheat or subcooling method. Separately review refrigerant classification, cylinder filling limits, and transport and disposal requirements so handling questions are answered from rules rather than habit.