HI HVAC common mistakes
HI HVAC · Understand the traps and how to avoid them.
Treating sensible heat and latent heat as a single load figure, then sizing equipment on total capacity alone without checking the sensible-to-total (SHR) ratio of the selected unit against the calculated building SHR.
Candidates memorize total Btu/h and tons but under-practice the split between sensible and latent capacity; the exam often gives a design condition with high internal latent load (occupancy, infiltration) where a unit with adequate total capacity still fails to hold humidity because its SHR is too high.
What to do instead
For every sizing question, compute both sensible and latent loads, derive the required SHR, and compare it to the equipment SHR at the actual entering wet-bulb and outdoor condition. If the required SHR exceeds the unit SHR, select a unit with a lower SHR or add dehumidification, and state that reasoning explicitly.
Applying the duct sizing method's chart at the wrong friction rate or misreading the equivalent-length allowance for fittings, then accepting a duct size that meets velocity but exceeds available static pressure.
Friction-rate charts are read quickly under time pressure, and candidates forget that fitting losses (elbows, takeoffs, transitions) consume static pressure that must be subtracted from the blower's available external static pressure before sizing any run.
What to do instead
Work the problem in a fixed order: total available external static, subtract filter, coil, grille, and fitting allowances to get the usable static, then apply the friction chart at that rate. Sanity-check by confirming total pressure drop across the longest run fits within the remaining budget, and verify velocity against the noise limit for the application.
Selecting a refrigerant metering device or diagnosing a charge problem from a single gauge reading instead of correlating suction pressure, liquid-line temperature, superheat, and subcooling together.
Textbook symptoms (low suction = undercharge) are overlearned and applied without context; candidates miss that a restricted liquid line, a failed TXV, or a dirty condenser can mimic charge symptoms, so they answer with the wrong corrective action.
What to do instead
Always build a four-point picture: suction pressure, suction-line superheat, liquid-line pressure, and liquid-line subcooling, plus the indoor/outdoor conditions. Use superheat for fixed-orifice systems and subcooling for TXV systems, and eliminate non-charge causes (airflow, condenser cleanliness, restriction) before concluding charge is the fault.
Confusing combustion air requirements with dilution or ventilation air, and failing to distinguish Category I, II, III, and IV appliances when answering venting questions.
The terms overlap in field speech, and candidates memorize a single 'rule of thumb' for venting without learning that appliance category is set by flue pressure and condensate behavior, which drives material and termination requirements.
What to do instead
Anchor on the category definitions first (negative vs positive vent pressure, condensing vs non-condensing), then apply the corresponding vent material, slope, and termination rules. Separately track combustion air, dilution air, and indoor air quality ventilation as three distinct quantities with distinct code bases.
Answering control-sequence questions by naming a component (thermostat, aquastat, pressure switch) rather than tracing the actual sequence of operation through the circuit.
Candidates learn devices in isolation and never practice reading a ladder diagram or a written sequence, so they cannot predict which device opens or closes at each step or which safety should trip first.
What to do instead
Practice writing the sequence of operation in numbered steps for common systems (forced-air furnace, hydronic boiler, heat pump in heating and defrost), then map each step to the device and terminal that performs it. On exam questions, eliminate answers that skip a safety in the chain or that energize a load before its proving device.
Using the outdoor design dry-bulb as the coil entering condition, or ignoring the entering wet-bulb, when calculating or checking air-conditioning performance.
Cooling coil capacity depends on entering wet-bulb, not dry-bulb, and candidates conflate the two because both appear in the same problem statement; return-air mixing and bypass also change the entering condition.
What to do instead
For every coil or equipment selection, compute the mixed-air condition from return air and outdoor air fractions, then read capacity from the entering wet-bulb. State the entering dry-bulb and wet-bulb explicitly in your work so a wrong assumption is visible before it propagates.
Skipping the load calculation and defaulting to rule-of-thumb sizing (square footage per ton) when a question supplies envelope, orientation, and internal gain data.
Field habit and time pressure push candidates toward shortcuts, but exam items are built so that the shortcut gives a different answer than the calculated load, especially for high-performance envelopes or high-occupancy spaces.
What to do instead
When data is provided, do the calculation: conduction through assemblies using the given U-values and delta-T, solar and internal gains, infiltration or ventilation latent and sensible, then sum. Only use rules of thumb when the problem explicitly lacks data, and note that assumption in your answer.
Treating static pressure and velocity pressure as interchangeable when diagnosing airflow, or measuring static pressure at the wrong tap location.
Both are expressed in inches of water column and the instruments look similar, but static pressure drives flow through the system while velocity pressure reflects velocity; measuring across a filter versus across the blower gives different and non-comparable readings.
What to do instead
Fix the measurement plan before interpreting numbers: total external static pressure is measured between the blower outlet and the supply plenum plus the return side, not across the coil alone. Convert velocity pressure to velocity using the standard relation and confirm the reading location matches the manufacturer's rating point.
Answering refrigerant safety and code questions from general practice rather than the assigned safety group, occupancy classification, and quantity limits for the specific refrigerant.
Refrigerant classifications and charge limits are updated periodically, and candidates rely on remembered field norms instead of the current classification, which changes allowable charge per occupied volume and required detection or ventilation.
What to do instead
For each refrigerant question, first identify the safety group (toxicity and flammability class) and the occupancy classification, then apply the corresponding concentration limit and charge calculation. If a value is not certain, state the method and flag the assumption rather than guessing a number.
Ignoring airflow as the first diagnostic step and jumping to refrigerant or electrical faults when a system underperforms.
Low airflow produces symptoms that mimic charge, control, and compressor faults (high superheat, frosted coil, high head pressure, tripped limits), and candidates chase the downstream symptom because it is more familiar.
What to do instead
Adopt a fixed diagnostic order: verify airflow first (filter, blower, duct, coil cleanliness, static pressure), then electrical (supply voltage, amp draw, control circuit), then refrigerant circuit. Document each measurement so the sequence is auditable and the actual root cause is isolated rather than guessed.