NATE GS Installation common mistakes
NATE GS Installation · Understand the traps and how to avoid them.
Treating static pressure as a single number and adding return, supply, and filter pressure drops only after selecting the blower, instead of building the external static pressure budget before equipment selection.
Candidates memorize '0.5 in. w.c. per 100 ft' for duct friction but do not distinguish it from total external static pressure, so they confuse duct friction loss with the blower's available static.
What to do instead
Practice building a full ESP budget: filter, return grille, return duct, supply duct, supply registers, and any accessory. Know that 0.5 in. w.c./100 ft is a duct friction design guideline, not an equipment limit, and that the blower must cover the sum of all external drops at design airflow.
Sizing return air paths the same as supply paths, or ignoring return static entirely when checking airflow complaints.
Return ductwork is often hidden in ceiling plenums and overlooked; candidates focus on supply registers because those are visible in the conditioned space.
What to do instead
Always include the return path (grille free area, filter, duct, and fittings) in the pressure budget. Use the rule that return grilles are typically sized for lower face velocity than supply grilles and check filter loading as a common cause of high return static.
Confusing ventilation rate requirements with indoor air quality pollutant control, and applying ASHRAE 62.2 whole-building ventilation rates to a single room or to commercial spaces.
Candidates memorize a cfm number without knowing which standard applies to residential versus commercial, or which rate is per person versus per floor area.
What to do instead
Separate the concepts: residential whole-building ventilation (ASHRAE 62.2) versus commercial ventilation (ASHRAE 62.1, which uses per-person and per-area rates). Learn that source control and filtration are separate from dilution ventilation, and be able to explain when each applies.
Assuming a higher MERV rating always improves system performance, without checking blower capacity and pressure drop.
Candidates equate filtration quality with air quality and forget that a restrictive filter raises external static pressure and can reduce airflow below design.
What to do instead
State the tradeoff explicitly: higher MERV means higher pressure drop, which reduces airflow unless the blower has reserve capacity. Check the filter's rated pressure drop at design cfm and verify the blower can maintain design airflow before recommending an upgrade.
Reading a wiring diagram by following wire colors instead of terminal designations and the ladder logic of the schematic.
Field experience teaches color conventions, but manufacturers vary colors and the exam tests schematic reading, where the same color can serve different functions.
What to do instead
Trace circuits by terminal labels and line/load relationships (L1, L2, R, C, Y, G, W), not by color. Practice identifying the control circuit versus the power circuit and locating the safety switches in series with the contactor coil.
Confusing the function of a capacitor, contactor, and relay, or failing to recognize a start capacitor versus a run capacitor in a diagram.
These components look similar and candidates memorize 'capacitor starts the motor' without understanding the phase-shift role of the run capacitor versus the momentary boost of a start capacitor.
What to do instead
Learn each component's job: run capacitor provides continuous phase shift for the auxiliary winding; start capacitor is switched in briefly by a potential relay or start thermistor; contactor switches line voltage; relay switches control voltage. Match the symbol to the function in a schematic.
Mixing up sensible heat, latent heat, and total heat when calculating load or interpreting a psychrometric chart.
Candidates memorize formulas but do not connect them to the chart, so they add latent load as if it were sensible or ignore the difference between dry-bulb and wet-bulb when reading enthalpy.
What to do instead
Practice reading the psychrometric chart to separate dry-bulb, wet-bulb, dew point, and enthalpy. Remember sensible heat changes dry-bulb; latent heat changes moisture content; total heat is the sum and is read from enthalpy difference. Work sample problems converting cfm and temperature difference into Btu/h for each component.
Confusing superheat and subcooling targets, or measuring them at the wrong location on the system.
Candidates learn 'superheat is for fixed orifice, subcooling is for TXV' as a slogan but do not know where to place gauges or how to interpret a reading that is too high or too low.
What to do instead
Know the definitions: superheat is measured at the evaporator outlet (suction line temperature minus suction saturation temperature); subcooling is measured at the condenser outlet (condensing saturation temperature minus liquid line temperature). Memorize that high superheat with low subcooling suggests undercharge or a restriction, and practice interpreting both together.
Selecting duct size from a rule of thumb (such as 400 cfm per ton) without converting to actual velocity and friction for the specific duct shape and fitting count.
Rules of thumb are memorable and widely repeated, so candidates apply them as if they were design calculations, ignoring equivalent length of elbows, transitions, and takeoffs.
What to do instead
Use the duct sizing method (equal friction or velocity reduction) and account for equivalent length of fittings. Convert cfm and duct area to velocity (fpm = cfm / area in sq ft) and check against the allowable velocity for the application (supply, return, or branch).
Treating refrigerant handling and electrical lockout as separate topics, and skipping the code basis for each (EPA Section 608, OSHA lockout/tagout, NEC requirements).
Safety questions feel like common sense, so candidates answer from intuition rather than citing the specific regulation, and they miss questions that ask which code or standard governs a practice.
What to do instead
Map each safety practice to its source: refrigerant recovery and certification to EPA Section 608; energy isolation to OSHA 1910.147 lockout/tagout; grounding and bonding to NEC Article 250; ventilation of occupied spaces to ASHRAE 62. Study the code citation alongside the practice so you can answer 'which standard requires this' questions.