NATE GS Installation cheat sheet

Sensible heat ratio (SHR) = sensible heat / total heat; a low SHR means more latent load and a lower coil ADP is needed to reach target room RH.

Airflow (cfm) = sensible load (Btu/h) / (1.08 × ΔT °F) at sea level; the 1.08 constant scales with air density (≈1.08 at 70 °F, sea level).

Total airflow (cfm) = total load (Btu/h) / (4.5 × Δh), where 4.5 = 60 min/h × 0.075 lb/ft³ and Δh is the enthalpy difference across the coil.

Latent airflow (cfm) = latent load (Btu/h) / (0.68 × Δgrains); 0.68 derives from 60 × 0.075 × 1076 Btu/lb ÷ 7000 grains/lb.

External static pressure budget is the sum of supply duct, return duct, filter, coil, and accessories; a dirty filter raises ESP and shifts the fan curve left, reducing cfm.

Duct velocity rules of thumb: ≤900 fpm in trunk ducts near occupied spaces, 600–900 fpm in branch runs, ≤300–500 fpm at supply diffusers to limit NC noise.

Equal-friction method sizes ducts at constant friction rate (typically 0.08–0.10 in. w.c. per 100 ft); velocity method caps fpm to control noise and pressure drop.

A 90° hard elbow has far higher loss than two 45° elbows; use radiused elbows or turning vanes, and keep 3 duct diameters of straight run off fan discharge.

ASHRAE 62.1 ventilation rate = Rp × people + Ra × floor area; Rp is per-person outdoor air (cfm/person) and Ra is per-unit-area outdoor air (cfm/ft²).

Filter ratings: MERV 8 captures ~20–35% of 1–3 µm particles; MERV 13 captures ≥50% of 0.3–1 µm and is typical for improved indoor air quality; MERV 16 approaches HEPA.

Pressure drop rises as a filter loads; a filter loaded to twice its clean ΔP is generally at change-out, and a high ΔP can starve airflow and freeze coils.

Duct leakage pulls unconditioned attic air (and insulation fibers) into the supply; sealing ducts and using mastic, not cloth duct tape, protects air quality.

Combustion appliance safety: a natural-draft furnace needs adequate combustion air (roughly 50 ft³ per 1,000 Btu/h input if in a confined space with no outdoor air).

CO exposure limits: OSHA PEL 50 ppm TWA; a properly tuned gas furnace should produce <100 ppm CO in the flue and near 0 ppm in supply air.

Return air must not be drawn from a garage, bathroom, kitchen, or mechanical room; those spaces require dedicated exhaust to avoid contaminant spread.

Ohm's law: V = I × R; power: P (watts) = V × I = I²R = V²/R; single-phase VA = V × A; three-phase VA = √3 × V × A.

Motor full-load amps come from the nameplate; use the NEC Table value (430.248/430.250) for conductor and overload sizing, not the nameplate FLA.

Minimum circuit ampacity (MCA) and maximum overcurrent protection (MOCP) are printed on the unit data plate; size the branch circuit and breaker to those, not to a guessed ampacity.

A run capacitor is wired in parallel with the start winding; a failed run cap causes high current, low torque, and overheating; a start capacitor is switched out by a potential relay or centrifugal switch.

Locked-rotor amps (LRA) are typically 5–7× running load amps; a compressor tripping on overload with high amps and low speed often indicates a seized or shorted winding.

Grounding: the equipment grounding conductor carries fault current and must be bonded to the cabinet; a neutral is a current-carrying conductor and is not a substitute for ground.

Voltage imbalance on a three-phase compressor should stay under 2%; imbalance causes phase current imbalance and overheating, and a blown fuse on one leg is a classic cause.

Sensible heat Btu/h = 1.08 × cfm × ΔT; latent heat Btu/h = 0.68 × cfm × Δgrains; total = 4.5 × cfm × Δh.

1 ton of refrigeration = 12,000 Btu/h = 200 Btu/min; a 3-ton system moves 36,000 Btu/h.

ΔT across a properly charged, clean evaporator is typically 16–22 °F; a low ΔT suggests low charge, low airflow, or a dirty coil.

Superheat = suction line temp − evaporating (saturated suction) temp; target 8–12 °F at the evaporator for a fixed-orifice system.

Subcooling = condensing (saturated liquid) temp − liquid line temp; target 8–12 °F for a TXV system, measured at the condenser outlet.

Standard air density 0.075 lb/ft³; specific heat of air ≈ 0.24 Btu/lb·°F; latent heat of vaporization of water ≈ 1,076 Btu/lb at 70 °F.

Latent heat of vaporization of R-410A is roughly 100 Btu/lb near 40 °F evaporating; refrigerant mass flow = capacity / (Δh across coil).

Copper line sizing: suction line is sized for low pressure drop (≤2–3 psi equivalent) to protect capacity; liquid line is sized for velocity to carry oil and avoid flash gas.

Brazing requires nitrogen purge to prevent copper oxide (scale) inside the tubing; oxide flakes can plug the metering device and compressor.

A P-trap on the condensate drain must have a depth greater than the fan's negative static pressure in inches of water column; a missing trap causes water to be pulled into the airstream.

Refrigerant lines should be insulated on the suction side; liquid lines in hot attics may also need insulation to prevent flash gas and capacity loss.

Duct sizing: round duct area (ft²) = π × (diameter/2)²; cfm = velocity (fpm) × area (ft²); a 10-in. round duct at 900 fpm moves ≈ 490 cfm.

Flex duct must be fully extended and supported; a sag or sharp bend can cut effective capacity by 30–50% and raise static pressure.

Pressure test and evacuate to ≤500 microns with a micron gauge before charging; a decay rate indicates a leak or moisture in the system.

Four components: compressor raises pressure/temperature; condenser rejects heat and subcools liquid; metering device drops pressure; evaporator absorbs heat and superheats vapor.

Saturated temperatures come from pressure via a PT chart; R-410A at 118 psig is ≈ 40 °F evaporating; at 340 psig is ≈ 100 °F condensing.

Superheat = actual suction temp − saturated suction temp; high superheat with low suction pressure points to undercharge or a restricted metering device.

Subcooling = saturated condensing temp − liquid line temp; low subcooling with high superheat indicates undercharge; high subcooling with high head pressure indicates overcharge or a restricted condenser.

A restriction (e.g., plugged filter drier) creates a temperature drop across it; frost or condensation at the drier means restriction.

Compressor amps track mass flow; high head pressure raises amps and can trip the overload; low charge reduces mass flow and amps while raising superheat.

Heat pump reversing valve swaps evaporator and condenser roles; in heating, the outdoor coil is the evaporator and needs defrost to clear frost.

Refrigerant safety: R-410A is A1 (lower toxicity, no flame propagation); never use air or oxygen to pressure test-use dry nitrogen.

Refrigerant cylinders must be upright, secured, and never filled above 80% liquid by volume; a full cylinder heated can hydrostatically rupture.

Recover refrigerant before opening a system; venting is prohibited under Section 608 of the Clean Air Act; EPA certification is required to purchase or handle refrigerant.

Lockout/tagout: de-energize and verify with a meter before servicing; capacitors can retain a charge-discharge with a resistor, not a screwdriver.

Gas furnace: check for spillage at the draft hood, verify combustion air, and confirm the flue is clear; CO monitors and a combustion analyzer are standard service tools.

Electrical code: size conductors and overcurrent protection to the unit MCA/MOCP; install a disconnect within sight of the equipment and bond the cabinet.

Personal protective equipment for brazing: safety glasses, gloves, and adequate ventilation; flux fumes and refrigerant decomposition products are irritants.

Sensible heat ratio (SHR) = sensible heat / total heat; a low SHR means more latent load and a lower coil ADP is needed to reach target room RH.

Airflow (cfm) = sensible load (Btu/h) / (1.08 × ΔT °F) at sea level; the 1.08 constant scales with air density (≈1.08 at 70 °F, sea level).

Total airflow (cfm) = total load (Btu/h) / (4.5 × Δh), where 4.5 = 60 min/h × 0.075 lb/ft³ and Δh is the enthalpy difference across the coil.

Latent airflow (cfm) = latent load (Btu/h) / (0.68 × Δgrains); 0.68 derives from 60 × 0.075 × 1076 Btu/lb ÷ 7000 grains/lb.

External static pressure budget is the sum of supply duct, return duct, filter, coil, and accessories; a dirty filter raises ESP and shifts the fan curve left, reducing cfm.

Duct velocity rules of thumb: ≤900 fpm in trunk ducts near occupied spaces, 600–900 fpm in branch runs, ≤300–500 fpm at supply diffusers to limit NC noise.

Equal-friction method sizes ducts at constant friction rate (typically 0.08–0.10 in. w.c. per 100 ft); velocity method caps fpm to control noise and pressure drop.

A 90° hard elbow has far higher loss than two 45° elbows; use radiused elbows or turning vanes, and keep 3 duct diameters of straight run off fan discharge.

ASHRAE 62.1 ventilation rate = Rp × people + Ra × floor area; Rp is per-person outdoor air (cfm/person) and Ra is per-unit-area outdoor air (cfm/ft²).

Filter ratings: MERV 8 captures ~20–35% of 1–3 µm particles; MERV 13 captures ≥50% of 0.3–1 µm and is typical for improved indoor air quality; MERV 16 approaches HEPA.

Pressure drop rises as a filter loads; a filter loaded to twice its clean ΔP is generally at change-out, and a high ΔP can starve airflow and freeze coils.

Duct leakage pulls unconditioned attic air (and insulation fibers) into the supply; sealing ducts and using mastic, not cloth duct tape, protects air quality.

Combustion appliance safety: a natural-draft furnace needs adequate combustion air (roughly 50 ft³ per 1,000 Btu/h input if in a confined space with no outdoor air).

CO exposure limits: OSHA PEL 50 ppm TWA; a properly tuned gas furnace should produce <100 ppm CO in the flue and near 0 ppm in supply air.

Return air must not be drawn from a garage, bathroom, kitchen, or mechanical room; those spaces require dedicated exhaust to avoid contaminant spread.

Ohm's law: V = I × R; power: P (watts) = V × I = I²R = V²/R; single-phase VA = V × A; three-phase VA = √3 × V × A.

Motor full-load amps come from the nameplate; use the NEC Table value (430.248/430.250) for conductor and overload sizing, not the nameplate FLA.

Minimum circuit ampacity (MCA) and maximum overcurrent protection (MOCP) are printed on the unit data plate; size the branch circuit and breaker to those, not to a guessed ampacity.

A run capacitor is wired in parallel with the start winding; a failed run cap causes high current, low torque, and overheating; a start capacitor is switched out by a potential relay or centrifugal switch.

Locked-rotor amps (LRA) are typically 5–7× running load amps; a compressor tripping on overload with high amps and low speed often indicates a seized or shorted winding.

Grounding: the equipment grounding conductor carries fault current and must be bonded to the cabinet; a neutral is a current-carrying conductor and is not a substitute for ground.

Voltage imbalance on a three-phase compressor should stay under 2%; imbalance causes phase current imbalance and overheating, and a blown fuse on one leg is a classic cause.

Sensible heat Btu/h = 1.08 × cfm × ΔT; latent heat Btu/h = 0.68 × cfm × Δgrains; total = 4.5 × cfm × Δh.

1 ton of refrigeration = 12,000 Btu/h = 200 Btu/min; a 3-ton system moves 36,000 Btu/h.

ΔT across a properly charged, clean evaporator is typically 16–22 °F; a low ΔT suggests low charge, low airflow, or a dirty coil.

Superheat = suction line temp − evaporating (saturated suction) temp; target 8–12 °F at the evaporator for a fixed-orifice system.

Subcooling = condensing (saturated liquid) temp − liquid line temp; target 8–12 °F for a TXV system, measured at the condenser outlet.

Standard air density 0.075 lb/ft³; specific heat of air ≈ 0.24 Btu/lb·°F; latent heat of vaporization of water ≈ 1,076 Btu/lb at 70 °F.

Latent heat of vaporization of R-410A is roughly 100 Btu/lb near 40 °F evaporating; refrigerant mass flow = capacity / (Δh across coil).

Copper line sizing: suction line is sized for low pressure drop (≤2–3 psi equivalent) to protect capacity; liquid line is sized for velocity to carry oil and avoid flash gas.

Brazing requires nitrogen purge to prevent copper oxide (scale) inside the tubing; oxide flakes can plug the metering device and compressor.

A P-trap on the condensate drain must have a depth greater than the fan's negative static pressure in inches of water column; a missing trap causes water to be pulled into the airstream.

Refrigerant lines should be insulated on the suction side; liquid lines in hot attics may also need insulation to prevent flash gas and capacity loss.

Duct sizing: round duct area (ft²) = π × (diameter/2)²; cfm = velocity (fpm) × area (ft²); a 10-in. round duct at 900 fpm moves ≈ 490 cfm.

Flex duct must be fully extended and supported; a sag or sharp bend can cut effective capacity by 30–50% and raise static pressure.

Pressure test and evacuate to ≤500 microns with a micron gauge before charging; a decay rate indicates a leak or moisture in the system.

Four components: compressor raises pressure/temperature; condenser rejects heat and subcools liquid; metering device drops pressure; evaporator absorbs heat and superheats vapor.

Saturated temperatures come from pressure via a PT chart; R-410A at 118 psig is ≈ 40 °F evaporating; at 340 psig is ≈ 100 °F condensing.

Superheat = actual suction temp − saturated suction temp; high superheat with low suction pressure points to undercharge or a restricted metering device.

Subcooling = saturated condensing temp − liquid line temp; low subcooling with high superheat indicates undercharge; high subcooling with high head pressure indicates overcharge or a restricted condenser.

A restriction (e.g., plugged filter drier) creates a temperature drop across it; frost or condensation at the drier means restriction.

Compressor amps track mass flow; high head pressure raises amps and can trip the overload; low charge reduces mass flow and amps while raising superheat.

Heat pump reversing valve swaps evaporator and condenser roles; in heating, the outdoor coil is the evaporator and needs defrost to clear frost.

Refrigerant safety: R-410A is A1 (lower toxicity, no flame propagation); never use air or oxygen to pressure test-use dry nitrogen.

Refrigerant cylinders must be upright, secured, and never filled above 80% liquid by volume; a full cylinder heated can hydrostatically rupture.

Recover refrigerant before opening a system; venting is prohibited under Section 608 of the Clean Air Act; EPA certification is required to purchase or handle refrigerant.

Lockout/tagout: de-energize and verify with a meter before servicing; capacitors can retain a charge-discharge with a resistor, not a screwdriver.

Gas furnace: check for spillage at the draft hood, verify combustion air, and confirm the flue is clear; CO monitors and a combustion analyzer are standard service tools.

Electrical code: size conductors and overcurrent protection to the unit MCA/MOCP; install a disconnect within sight of the equipment and bond the cabinet.

Personal protective equipment for brazing: safety glasses, gloves, and adequate ventilation; flux fumes and refrigerant decomposition products are irritants.