MD HVAC common mistakes
MD HVAC · Understand the traps and how to avoid them.
Calculates superheat using the suction-line temperature at the compressor service valve paired with the low-side pressure read at the same port, and calculates subcooling using a liquid-line temperature taken at the condenser outlet paired with a discharge pressure read at the compressor.
Superheat and subcooling are defined at a single point (evaporator outlet and condenser outlet respectively). Suction and discharge lines have pressure drop and heat gain/loss between the coil and the service ports, so mixing locations shifts the saturation temperature and produces values that look like a charge or metering problem when the system is fine.
What to do instead
Always pair temperature and pressure at the same location: superheat = suction-line temp at the evaporator outlet minus saturation temp from suction pressure at that same outlet; subcooling = saturation temp from liquid-line pressure at the condenser outlet minus liquid-line temp at that same point. Practice reading a PT chart for a stated point, not a port.
Enters a gauge reading in psig directly into a pressure-temperature table printed in psia, or uses an R-22 table for an R-410A system because the pressures 'look close enough' after a conversion.
Most field gauges read psig while many reference tables, especially in textbook problems, are absolute. Candidates also default to the refrigerant they learned first. The resulting saturation temperature is off by roughly 15 psi of offset or by the full refrigerant difference, so every downstream superheat/subcooling answer is wrong even though the arithmetic is right.
What to do instead
Before touching a chart, write down the refrigerant and whether the table is gauge or absolute; convert with psia = psig + 14.7 at sea level. When a stem gives a pressure and a refrigerant, state the saturation temperature first, then do the subtraction, and sanity-check it against a known value (for example R-410A suction near 118 psig corresponds to about 40 F).
Confuses gas supply (line) pressure with manifold (burner) pressure when diagnosing a furnace, and clocks the meter using the wrong dial value or the wrong heating value of the fuel.
Both are measured in inches of water column with the same manometer, so the numbers get swapped under time pressure. Meter clocking compounds the error because the test dial size (often 2 cubic feet per revolution) and the local heating value (about 1,000 Btu per cubic foot for natural gas) must both be applied.
What to do instead
Memorize the order of the test: supply pressure upstream of the gas valve with the unit running, then manifold pressure downstream at the burner orifice. For clocking, Btu/hr = (3,600 x cubic feet per dial revolution) / seconds per revolution x heating value, and state each assumption out loud before substituting numbers.
Reports the combustion analyzer's raw CO reading as the CO safety number, or mixes up O2 and CO2 when checking dilution air.
Analyzers display both as-measured CO (undiluted by excess air effects) and air-free CO, and the two differ a lot when the stack has excess air. Candidates also treat the O2 cell reading as if it were CO2, so a normal 6-9 percent O2 is read as a dangerously low CO2.
What to do instead
Fix the formula in memory: CO air-free = measured CO x [20.9 / (20.9 - measured O2)]. On every combustion question, write down which value is being asked for, convert if needed, and compare against the air-free limit, not the raw number.
Computes total external static pressure by subtracting return static from supply static instead of adding their absolute values, then plots that number on the fan table.
Return-side static is measured as a negative (vacuum) value. Candidates instinctively subtract the two numbers because the gauge shows a minus sign, which understates the true external static and makes an undersized duct system look like it is moving the rated airflow.
What to do instead
Take absolute values first: TESP = |supply static| + |return static|. Probe locations matter too - total external static is measured outside the cabinet, so filter and coil pressure drops inside the cabinet are handled by the manufacturer's table, not added by hand unless the table says to.
Applies commercial ventilation rate tables (cfm per person plus cfm per square foot) to a single-family house, or applies the residential dwelling-unit rate to an office build-out.
The two standards look interchangeable and both are cited as 'the ventilation code,' but residential ventilation is driven by bedroom count and floor area for the whole dwelling, while commercial ventilation is a per-person plus per-area calculation on a zone-by-zone basis. Using the wrong one changes the answer by an order of magnitude.
What to do instead
Before computing, classify the occupancy in one sentence: dwelling unit versus commercial space. Then select the residential dwelling-unit method or the commercial breathing-zone method, and show the rate basis (per person, per square foot, per bedroom) in the work so a wrong basis is visible.
Puts the multimeter in resistance mode across a component while the control circuit is still energized, or leaves the meter in amps and probes across a line-voltage terminal.
Continuity checks feel like the fastest way to test a switch or a winding, and the disconnect or thermostat is easy to overlook when the unit is quiet. The result is a meaningless reading, a blown meter fuse, or an arc-flash hazard, and the candidate then reasons from bad data.
What to do instead
Use a fixed sequence: de-energize, lock out, verify zero volts with the meter in voltage mode, then switch to ohms. Never change function while the probes are on a live circuit. In questions, treat 'verify no voltage before ohming' as a required step, not an optional one.
Reads a ladder diagram symbol as the switch's energized state - assuming a normally closed contact shows no continuity, or tracing a circuit without allowing for the fact that a NC limit or safety opens only on a fault.
Candidates learn the symbols as pictures rather than as de-energized states, so a normally closed high-limit drawn closed is treated as a closed path only when hot. Troubleshooting sequences are then inverted, and the candidate blames the wrong component in the chain.
What to do instead
Redraw or trace the circuit in its de-energized, normal condition first, listing every device that is closed and every device that is open. Then re-trace with the call for heat or cool present. Answer sequence questions by stating 'at this step, this device is closed/open because...' before naming the failed part.
Answers code questions from memory of the national model code (IRC or IMC) without accounting for the edition Maryland has adopted and its state amendments, or cites a permit or licensing rule that is actually a local jurisdiction overlay.
The exam tests the adopted code with amendments, and many candidates studied a model code from a different year or from another state. Maryland also layers state licensing and permit requirements on top of local jurisdiction rules, so the 'generic' answer is often a distractor.
What to do instead
Build a one-page adoption sheet: which mechanical, fuel gas, and electrical code editions Maryland currently enforces, the key state amendments, and where permitting and licensing authority sits. For each practice question, ask 'is this a model-code question or an adoption/amendment question?' before answering.
Calculates selling price by adding a percentage markup to cost and then reports that percentage as gross margin, or mixes up contract, lien, and insurance requirements when answering business and licensing items.
Markup and margin are both expressed as percentages, so the two get treated as the same number. On the licensing side, candidates study the trade material and skim the administrative portion, so notice, lien, and written-contract thresholds are answered from intuition.
What to do instead
Drill the conversion until automatic: gross margin = markup / (1 + markup). Work two or three price problems with both figures shown side by side. For the administrative items, keep a short list of the dollar thresholds and written-contract, notice, and lien deadlines you must know, and review it in the last week before the exam.