ID HVAC common mistakes
ID HVAC · Understand the traps and how to avoid them.
Adding up the sensible and latent loads and then selecting equipment on total capacity alone, without checking the Sensible Heat Ratio (SHR) against the coil's rated SHR at the actual entering wet-bulb and airflow.
Candidates memorize Q = 1.08 x CFM x ΔT and Q = 4.5 x CFM x Δh but treat the two as independent checks. In practice the coil must satisfy both simultaneously, and a unit whose total capacity matches but whose SHR is too high will leave the space over-humidified (or too dry in dry climates).
What to do instead
For every load calculation, compute SHR = sensible/total and compare it with the manufacturer's coil SHR at the design entering air wet-bulb and CFM. If they diverge, adjust airflow (lower CFM lowers SHR) or select a coil with a different face velocity or row count. Practice problems that give total and sensible capacity separately and ask whether the selection is acceptable.
Treating the psychrometric chart as a lookup for one property at a time instead of plotting a process line and reading Δh, Δw, and apparatus dew point together.
Exam items often give dry-bulb and relative humidity and ask for enthalpy, humidity ratio, dew point, and the resulting condensate rate. Candidates who grab a single value from memory (e.g., assuming 400 CFM/ton always yields a 20°F ΔT) miss that the answer depends on the entering wet-bulb.
What to do instead
Drill full chart traversals: locate state point from DB/RH, read WB, DP, h, and w; then draw the process to the leaving state and read the slope. Rehearse the rule that a 20°F split is only nominal at ~80°F DB/67°F WB entering air, and recompute Q = 4.5 x CFM x Δh for off-design conditions.
Sizing ductwork on velocity alone without converting to equivalent length and checking available static pressure against the fan curve.
Candidates remember the friction chart (e.g., 0.1 in. w.c. per 100 ft) but forget to add fitting equivalent lengths, filter and coil pressure drops, and terminal-unit drops. The result is a duct sized for the wrong total external static pressure, so the selected fan operates off its design point.
What to do instead
Build a pressure-budget worksheet every time: straight-run friction plus fitting equivalent lengths, plus coil, filter, heat exchanger, and diffuser drops, compared with the fan's rated external static pressure at design CFM. Then verify the operating point by intersecting the system curve with the fan curve rather than assuming the fan delivers nameplate CFM.
Applying the fan laws to a variable-air-volume system as if airflow, static pressure, and power all scale independently, and ignoring that system resistance changes when dampers or VAV boxes modulate.
The cube-law shortcut (power varies with the cube of speed) is memorable and gets over-applied. In a real VAV system the system curve shifts as boxes close, so static pressure control, minimum airflow settings, and fan curves matter; candidates also forget that motor and drive efficiency change at low speed.
What to do instead
State the assumptions before using fan laws: same fan, same system curve, same air density. For VAV questions, work through the part-load sequence (static pressure sensor setpoint, inlet guide vanes versus VFD, minimum box airflow) and use the affinity laws only to compare two points on the same curve. Practice a part-load energy comparison where the system curve is explicitly given.
Confusing combustion efficiency, steady-state efficiency, and annual fuel utilization efficiency (AFUE) when answering furnace questions, and mixing up gross versus net calorific value.
The terms sound interchangeable and exam stems often quote one while asking about another. Candidates also forget that condensing furnaces recover latent heat, so efficiencies above 90% require accounting for flue-gas water vapor, and that derating for altitude changes the input rate.
What to do instead
Write one-line definitions before answering: combustion efficiency (heat released versus fuel input, from flue CO2/O2 and stack temperature), steady-state efficiency (useful heat output at steady operation), AFUE (seasonal, includes cycling and jacket losses). For condensing equipment, always note whether the efficiency is based on HHV or LHV and whether latent recovery is included.
Selecting a boiler or chiller without checking the part-load performance and the actual load profile, relying only on full-load efficiency or COP.
Full-load ratings are the easiest number to find, so candidates anchor on them. Real buildings spend most hours at 30-60% load, where cycling losses, unloading steps, and condenser-water temperature reset dominate; a unit with a great full-load COP can be a poor seasonal performer.
What to do instead
For equipment-selection items, compute a simple load-duration profile and compare integrated part-load value (IPLV) or seasonal COP, not just full-load ratings. Note staging and unloading characteristics (e.g., screw versus centrifugal versus scroll), and include condenser-water reset and minimum-load constraints in the answer.
Mixing up the roles of outdoor-air economizer, demand-controlled ventilation, and energy recovery, and applying them in the wrong climate or code context.
All three reduce energy but for different reasons: economizer uses cool outdoor air for free cooling, DCV reduces outdoor air below design when occupancy is low, and energy recovery transfers heat/moisture between exhaust and intake. Candidates often propose DCV where code requires minimum ventilation regardless of occupancy, or economizers in humid climates without considering latent load.
What to do instead
For each strategy, state the condition under which it saves energy and the condition under which it hurts. Rehearse ASHRAE 62.1 ventilation-rate procedure (Rp per person plus Ra per area, then system ventilation efficiency), and check economizer high-limit and lockout settings against climate zone. Practice items that ask which strategy applies to a given climate and occupancy pattern.
Calculating static pressure or head in mixed units (feet of head versus psi versus inches of water) and forgetting the density correction for hot water or steam.
Pump and hydronic problems mix units freely, and candidates plug feet of head into formulas expecting psi, or use cold-water density for a 180°F loop. Steam systems add another layer because pressure and temperature are linked through saturation.
What to do instead
Convert everything to consistent units before calculating: 1 psi = 2.31 ft of water, 1 ft of water = 0.433 psi, and correct head for specific gravity at the operating temperature. For steam, use the saturation table to relate pressure and temperature and size traps and piping accordingly. Do a unit-dimension check on every answer.
Answering refrigerant and refrigeration-cycle questions from memory of R-22 behavior, ignoring glide, oil miscibility, and the pressure-enthalpy diagram for the actual refrigerant named.
Many candidates trained on R-22 and apply its saturation temperatures and charging rules to R-410A or R-32 blends. Zeotropic blends have temperature glide, so dew point and bubble point differ, and charging by subcooling or superheat must follow the manufacturer's method.
What to do instead
For any refrigerant item, identify whether it is a pure fluid or zeotropic blend, then read the P-h diagram for the named refrigerant. State the charging method (weigh-in, subcooling for TXV, superheat for fixed orifice) and note glide effects on evaporator and condenser approach temperatures. Memorize only the properties the exam actually tests, and always name the refrigerant in the answer.
Skipping the controls sequence and safety interlock logic in troubleshooting scenarios, jumping straight to a component failure.
Exam scenarios present a symptom (e.g., compressor short-cycling, no heat call) and candidates name the most familiar failed part. But most field failures are sequence, sensor, or interlock issues: a dirty filter tripping a high-limit, a bad thermostat anticipator, or a pressure switch preventing ignition.
What to do instead
Use a fixed diagnostic order for every scenario: verify the call and thermostat, check safety interlocks and limits, confirm sequence of operation step by step, then test components. Write out the expected sequence (e.g., call for heat, inducer proves, pressure switch closes, ignition, flame proves, blower on) and locate where it breaks before naming a part.