KY HVAC common mistakes
KY HVAC · Understand the traps and how to avoid them.
Checking charge with a superheat target on a system with a TXV, or a subcooling target on a system with a fixed-bore orifice, because the candidate applies the same charging method to every unit.
Charging procedures are memorized as a single script instead of being selected from the metering device type. Under exam pressure the candidate sees a gauge set and reaches for the familiar number.
What to do instead
Before touching gauges, state the metering device out loud from the nameplate or component list, then commit to the matching method: TXV = charge to subcooling (and verify superheat as a health check), fixed orifice/piston = charge to superheat at the evaporator outlet, corrected for indoor wet-bulb and outdoor dry-bulb.
Reading a saturation temperature straight off the gauge face without confirming the refrigerant in the system or using the correct pressure-temperature chart, so R-410A pressures get interpreted on an R-22 scale.
Many manifold gauge sets are only color-coded, and the candidate treats the pressure reading as the temperature reading. Blends, glide, and the different pressure ranges then throw every diagnostic conclusion off.
What to do instead
Read the nameplate refrigerant first, use that refrigerant's PT table, and where a blend has glide use the dew point for evaporator superheat and the bubble point for condenser subcooling. Practice converting five pressures by hand for each refrigerant the exam covers.
Declaring a gas furnace safe because the flame is blue and steady, without measuring flue CO, draft, or performing a spillage test at the draft hood.
Visual flame checks feel like a complete combustion analysis, and CO/draft instruments are the part of the trade candidates practice least. Blue flame does not rule out CO from a cracked heat exchanger or blocked flue.
What to do instead
Build a fixed combustion sequence in memory and rehearse it in order every time: draft/pressure verification, flue gas CO and O2/CO2 sample, steady-state temperature rise against the nameplate rating, and a spillage test at the draft hood with the door closed and a fan running. Never sign off without all four.
Mixing up the sign and units of vent pressure, calling an induced-draft furnace 'positive pressure' or reading inches of water column as inches of mercury.
Draft readings are small numbers near zero, and candidates reason from the blower instead of the flue. A reversed sign makes a properly drafting appliance look failed and a failed one look fine.
What to do instead
Anchor the physics: the vent on a natural-draft or induced-draft appliance is negative relative to the room, the burner compartment of a condensing furnace is negative at the collector box, and all combustion-air pressures are in inches of water column with 1 in. w.c. about 249 Pa. Write the sign convention on scratch paper before answering.
Applying fan laws linearly, assuming a 20 percent increase in blower RPM gives a 20 percent increase in airflow and a 20 percent increase in motor power.
The relationships are square and cube functions, which is easy to memorize wrong and easy to forget under time pressure. The candidate defaults to the simplest proportionality.
What to do instead
Memorize the three exponents as a set: airflow varies with RPM, static pressure varies with the square of RPM, and brake horsepower varies with the cube of RPM. Rehearse with one worked problem (RPM doubled = CFM doubled, static pressure quadrupled, horsepower eight times) before the exam.
Measuring total external static pressure at a single point, or with the filter removed, or across only one side of the blower, then comparing that number to the air handler's rated external static.
The candidate has one manometer and taps the easiest hole. Total external static is defined across the whole appliance boundary, so a single-port reading is meaningless against the rating.
What to do instead
Take one port downstream of the filter and upstream of the first component (return side, after filter) and one port after the last component on the supply side (before the coil if the coil is field-installed, per the manufacturer's label), subtract, and compare to the nameplate maximum. Note on scratch paper which boundary the rating applies to.
Testing a capacitor with only an ohmmeter continuity beep, or replacing a run capacitor with a start capacitor of different microfarads because the physical size matched.
Capacitors are the most commonly failed electrical part in service, so candidates get comfortable with a single crude test. The candidate also treats capacitors as interchangeable because they look alike.
What to do instead
Use a capacitance meter and compare to the nameplate tolerance (commonly plus or minus 6 percent for run capacitors). Discharge before testing, verify microfarads and voltage rating, and keep start versus run functions distinct: start capacitors are high-microfarad, short-duty, often with a bleed resistor, and never substitute for a run capacitor.
Tracing a low-voltage control circuit from memory of the color code without following the actual sequence, so a call for cooling is diagnosed as a bad thermostat when the open safety is a float switch or a high-pressure lockout.
Control wiring is learned as 'red is 24V, yellow is compressor' rather than as a series circuit with safeties. The candidate jumps to the most familiar component instead of finding the open leg.
What to do instead
Meter the 24V transformer output, then walk the circuit terminal by terminal to find where voltage stops. Draw the ladder diagram on scratch paper before testing. Confirm that all series safeties (float switch, high-limit, pressure controls, freeze stat) are closed before blaming the thermostat or control board.
Sizing equipment from a square-footage rule of thumb (for example, a ton per 500 square feet) instead of performing a room-by-room load calculation, and conflating sensible load with total load.
Rules of thumb are widely repeated in the field. The candidate also confuses the sensible capacity shown at design conditions with the total capacity, particularly at high latent loads.
What to do instead
Always start from a full load calculation: envelope, orientation, window area and U-factor, infiltration, internal gains, and duct loss. Select capacity against the design sensible and latent conditions, not the nominal tonnage, and confirm airflow and temperature rise against those conditions. If a number in a question seems to come from a rule of thumb, look for the design conditions the question supplies.
Skipping the duct loss and infiltration allowances in a sizing problem, or treating the duct system as outside the load calculation because the equipment is selected first.
The equipment is mentally chosen before the distribution is considered. Duct leakage and gains are invisible, and the candidate assumes the load equals the room load.
What to do instead
Apply supply and return duct loss factors to the equipment selection, and reduce delivered capacity accordingly. Verify that the selected blower delivers design CFM at the static pressure the duct system will actually produce. If the exam gives a duct loss percentage, apply it explicitly and show the arithmetic.
Answering permit, inspection, and licensing questions from general trade habit, for example describing when a permit is 'usually' required or who may pull it, instead of from the code and licensing framework the exam tests.
Field practice varies by jurisdiction and the candidate generalizes from one employer's custom. Kentucky has statewide HVAC licensing and code adoption, but local inspection authorities add requirements, so habit is an unreliable guide.
What to do instead
Answer from the governing documents: the adopted mechanical code for what work requires a permit and what inspection sequence follows, and the state licensing statute and regulations for scope of a journeyman versus a master, who may contract, and continuing-education requirements. Where the exam cites a code section or regulation, read the question as asking about that text, and study the state-specific licensing chapter rather than relying on field custom.