NATE GS Installation flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Sensible heat formula

Btu/h = 1.08 × cfm × ΔT in °F at sea level. Use it to find airflow from a known sensible load or to check delivered capacity against the equipment rating.

Total heat formula

Btu/h = 4.5 × cfm × Δh, where 4.5 combines 60 minutes per hour and 0.075 lb/ft³ air density. Δh is the enthalpy difference across the coil from a psychrometric chart.

External static pressure

The resistance the blower must overcome, equal to the sum of supply duct, return duct, filter, coil, and accessory pressure drops. High ESP shifts the fan curve left and reduces delivered cfm.

Duct velocity limits

Trunk ducts near occupied space are typically kept at or below 900 fpm, branch runs at 600–900 fpm, and supply diffusers at 300–500 fpm to control noise and pressure drop.

Equal-friction duct sizing

Sizes each duct section at a constant friction loss, commonly 0.08–0.10 in. w.c. per 100 ft of equivalent length. It balances material cost against fan energy and noise.

MERV 13 filter

A filter rating that captures at least 50% of particles in the 0.3–1 µm range. It is a common upgrade for improved indoor air quality without the pressure drop of HEPA.

ASHRAE 62.1 ventilation rate

Outdoor air cfm = Rp × number of people + Ra × floor area. Rp is per-person outdoor air and Ra is per-unit-area outdoor air from the standard's table for the space type.

Combustion air requirement

A natural-draft appliance in a confined space needs roughly 50 ft³ of combustion air per 1,000 Btu/h input when no outdoor air is provided. Inadequate air causes spillage and carbon monoxide.

Carbon monoxide exposure limit

OSHA sets the permissible exposure limit at 50 ppm as an eight-hour time-weighted average. A properly tuned gas furnace should produce near zero CO in the supply air.

Return air restrictions

Return air must never be drawn from a garage, bathroom, kitchen, or mechanical room. Those spaces need dedicated exhaust so contaminants and odors are not distributed through the building.

Ohm's law and power

V = I × R and P = V × I = I²R = V²/R. For three-phase loads, apparent power in VA equals √3 × line voltage × line current.

MCA and MOCP

Minimum circuit ampacity and maximum overcurrent protective device are printed on the unit data plate. The branch circuit conductors and breaker are sized to these values, not to a guess.

Run capacitor function

A run capacitor is wired in parallel with the start winding to shift phase and improve running torque. A failed run capacitor causes high current, low torque, and motor overheating.

Locked rotor amps

LRA is the current a motor draws at standstill, typically five to seven times running load amps. A compressor tripping on overload with high amps often has a seized or shorted winding.

Three-phase voltage imbalance

Keep voltage imbalance under 2%. Imbalance causes unequal phase currents and overheating, and a blown fuse on one leg is a common cause of a compressor failure.

Ton of refrigeration

One ton equals 12,000 Btu/h, or 200 Btu/min, the heat absorbed by melting one ton of ice in 24 hours. A three-ton system moves 36,000 Btu/h.

Evaporator delta T

A properly charged, clean evaporator with correct airflow shows 16–22 °F between return and supply air. A low delta T suggests low charge, low airflow, or a dirty coil.

Superheat definition

Superheat is actual suction line temperature minus saturated suction temperature from a PT chart. For a fixed-orifice system, target about 8–12 °F at the evaporator outlet.

Subcooling definition

Subcooling is saturated condensing temperature minus liquid line temperature. For a TXV system, target about 8–12 °F measured at the condenser outlet to confirm a solid liquid column.

Sensible heat ratio

SHR equals sensible heat divided by total heat. A low SHR means a large latent load, requiring lower coil apparatus dew point or reduced airflow to control humidity.

Nitrogen purge while brazing

Flowing dry nitrogen through the tubing during brazing displaces oxygen and prevents copper oxide scale. Oxide flakes can plug the metering device and damage the compressor.

Condensate trap depth

The trap depth must exceed the fan's negative static pressure in inches of water column. Without an adequate trap, condensate is pulled into the airstream and overflows the drain pan.

Round duct airflow

Cfm = velocity in fpm × area in ft², and area = π × (diameter/2)². A 10-inch round duct at 900 fpm carries about 490 cfm.

Flex duct installation

Flex duct must be fully extended, supported, and free of sharp bends. A sag or kink can cut effective capacity by 30–50% and raise static pressure on the blower.

Evacuation to 500 microns

Pull a vacuum to 500 microns or lower and hold it with a micron gauge. A rising reading indicates a leak or moisture still boiling off inside the system.

Refrigeration cycle components

Compressor raises pressure and temperature, condenser rejects heat and subcools liquid, metering device drops pressure, and evaporator absorbs heat and superheats vapor before return.

R-410A pressure relationship

Saturated temperatures come from a PT chart. R-410A at about 118 psig corresponds to 40 °F evaporating, and about 340 psig corresponds to 100 °F condensing.

High superheat diagnosis

High superheat with low suction pressure points to undercharge or a restricted metering device. Low superheat with high suction pressure suggests overcharge or too much airflow.

Filter drier restriction

A plugged filter drier creates a temperature drop across it, often shown by frost or condensation. It starves the evaporator, raising superheat and lowering capacity.

Heat pump defrost cycle

In heating, the outdoor coil is the evaporator and frosts. Defrost temporarily reverses the cycle to melt frost, and the auxiliary heat stages to keep supply air warm.

Refrigerant cylinder safety

Store cylinders upright and secured, and never fill above 80% liquid by volume. A liquid-full cylinder heated by the sun can rupture hydrostatically because liquid does not compress.

Section 608 recovery rule

The Clean Air Act prohibits venting refrigerant to the atmosphere. Technicians must recover refrigerant before opening a system and hold EPA Section 608 certification to handle it.