IA HVAC common mistakes
IA HVAC · Understand the traps and how to avoid them.
Using the dry-bulb temperature alone to select a cooling coil or to estimate supply-air conditions, instead of locating the state point on a psychrometric chart using both dry-bulb and wet-bulb (or relative humidity) and then reading humidity ratio and enthalpy.
Candidates memorize the chart layout but practice by reading single properties, so under time pressure they substitute the familiar dry-bulb value and skip the second coordinate needed to fix the state point.
What to do instead
Drill with a two-property rule: every state point must be fixed by two independent properties before any other value is read. Practice ten problems that give dry-bulb plus wet-bulb and require humidity ratio, dew point, enthalpy and specific volume, and ten that give dry-bulb plus relative humidity. Check each answer against the saturation curve position to catch misplots.
Confusing sensible heat ratio with relative humidity, or computing the apparatus dew point without first establishing the room sensible heat ratio and the entering coil conditions.
Sensible heat ratio, relative humidity and apparatus dew point all describe moisture behaviour, so candidates collapse them into one concept and skip the graphical construction that links room load line to coil performance.
What to do instead
Work the construction explicitly: plot the room condition, draw the room sensible heat ratio line, intersect it with the saturation curve to find the apparatus dew point, then apply the coil bypass factor to locate the leaving-air condition. Do five full constructions and state in words what each of the three terms means before calculating.
Performing a load calculation from square footage rules of thumb, or omitting internal, infiltration and ventilation latent loads, when the question supplies construction details and occupancy.
Field habit and exam time pressure push candidates toward a single multiplier, and latent loads are invisible in the dry-bulb arithmetic so they are forgotten entirely.
What to do instead
Build a one-page checklist before every load problem: envelope conduction (walls, roof, windows, doors), solar through glass, people sensible and latent, lighting, equipment, infiltration, and outdoor-air load. Total each category separately, then sum. When a question gives U-factors, areas and design conditions, use them; treat rules of thumb as a sanity check on the final number only.
Selecting equipment on nominal tonnage or nominal capacity rather than on the capacity at the actual entering wet-bulb, entering dry-bulb and outdoor design condition.
Catalog model numbers and nominal ratings are easier to recall than performance tables, and candidates do not realize net capacity falls as entering wet-bulb rises or as outdoor temperature rises.
What to do instead
Always identify the entering air wet-bulb and the outdoor condition in the question, then apply the manufacturer performance table or the sensible and latent capacity correction. Practice reading a capacity table in both directions and confirm that total capacity equals sensible plus latent at the selected point.
Mixing up AFUE, combustion efficiency and steady-state efficiency, or treating a condensing appliance as able to vent in the same category as a non-condensing one.
The efficiency terms sound interchangeable and the venting categories are learned as isolated facts, so candidates apply a high-efficiency number to a standard appliance or ignore return-water temperature limits.
What to do instead
Learn the definitions as a set: combustion efficiency from flue gas analysis, steady-state efficiency at steady operation, AFUE as the annual fuel utilization efficiency including cycling and jacket losses. Pair each with its appliance type and vent category, and note that condensing operation requires flue gas below the dew point and return water low enough to allow it.
Analyzing the refrigeration cycle with the wrong pressure levels, or forgetting that the expansion device is isenthalpic and that superheat and subcooling are measured at specific locations.
Candidates sketch the cycle from memory and place the state points by shape rather than by the component sequence, so superheat is read at the wrong line or the expansion process is drawn as isentropic.
What to do instead
Draw the cycle with all four components labeled and number the states: compressor suction (superheated vapor), discharge, condenser outlet (subcooled liquid), expansion outlet (low-quality mixture), evaporator outlet. State the assumption for each process before calculating, and practice locating superheat as evaporator outlet minus evaporator saturation temperature and subcooling as condenser saturation minus liquid-line temperature.
Adding static pressure losses in parallel instead of in series when totaling external static pressure for a fan selection, or ignoring the effect of a filter, coil and balancing damper on the fan curve.
Duct layouts drawn as branches invite treating each branch as an additive loss, and candidates forget that the fan must overcome the critical path plus the terminal and coil resistances.
What to do instead
Trace the critical path from return inlet through filter, coil, fan, supply trunk and the longest branch to the farthest terminal, summing losses along that single path. Compare the result with the fan curve at the design airflow. Practice identifying which branch is critical when branch losses differ, and remember that the fan operates where its curve intersects the system curve.
Sizing a duct by velocity alone, or applying the equal-friction method without checking that the resulting velocity and pressure drop stay within acceptable limits for the application.
Velocity is easy to compute and remember, while friction charts and equivalent-length tables feel laborious, so candidates pick a velocity and stop.
What to do instead
Use a two-step check: size for the friction rate, then verify velocity against the limit for the duct location and application (main, branch, return, near occupied space for noise). Practice reading a friction chart for round duct and converting to rectangular with the equivalent-diameter relation, and always record both the friction rate and the resulting velocity.
Describing a control sequence in vague terms such as 'modulates to maintain temperature' instead of specifying the controlled variable, sensor location, setpoint, action, and fail-safe position.
Candidates know the intent of a control loop but not the vocabulary of sequences of operation, and exam prompts reward specificity that casual reading does not supply.
What to do instead
Write every sequence as a five-part sentence: sensor and location, controlled variable, setpoint, controller action (direct or reverse), and output device with its normal or fail-safe position. Practice converting a paragraph of intent into that structure, and include economizer, staging and safeties explicitly.
Ignoring lockout and tagout, stored energy and refrigerant safety when answering electrical or troubleshooting questions, or diagnosing a component without first verifying power, control signal and safeties.
Technical diagnosis feels like the real task, and safety steps are treated as boilerplate, so candidates jump to replacing parts or reading a gauge before confirming isolation and the presence of voltage.
What to do instead
Adopt a fixed diagnostic order and state it in answers: verify the call for operation, confirm line voltage and control voltage, check safeties and limits, then evaluate the component. For any service task, state lockout and tagout, capacitor discharge, stored pressure release, and refrigerant handling rules before touching the equipment.
Treating commissioning as a startup visit, or omitting functional performance testing, trend logging and documentation of design intent from the answer.
Commissioning is often learned as a checklist performed at the end of a project, so candidates describe balancing and a punch list without verification against design intent.
What to do instead
Structure answers around the commissioning phases: pre-design and design review, submittal and installation verification, startup, functional performance testing with acceptance criteria, seasonal and deferred testing, and documentation with issues log and training. For any equipment question, name the specific functional test and the pass criterion.
Citing a code or standard from memory without naming the edition or the specific section, or mixing ventilation-rate requirements with indoor air quality guidance.
Codes are learned as general principles and the edition matters less in day-to-day work, so candidates give a plausible requirement with no traceable basis.
What to do instead
Build a short reference sheet pairing each topic with its governing document and the parameter it controls, and practice stating answers as 'per the adopted edition of the applicable standard, the requirement is X'. Flag any item where the adopted edition is unknown and note the jurisdiction's amendment process.