NATE HVAC cheat sheet

Ohm's law: E = I × R; single-phase power P = E × I × power factor; 1 horsepower = 746 watts.

One ton of refrigeration = 12,000 Btu/h = 200 Btu/min.

Superheat = measured suction-line temperature minus the saturated suction temperature at the same measured pressure.

Subcooling = saturated condensing temperature minus the measured liquid-line temperature.

At a fixed pressure a pure refrigerant has exactly one saturation temperature; temperature glide appears only with zeotropic blends.

Pressure conversions: 1 psi = 2.31 ft of water = 27.7 in. wc; 1 in. wc = 0.036 psi.

Brazing requires a slow dry-nitrogen purge through the tubing; never pressure-test with oxygen and never use compressed air on a refrigerant circuit.

Discharge and verify capacitors with a resistor before touching terminals; lock out and tag out every power source before opening a panel.

Reference constants: 1 Btu raises 1 lb of water 1°F; standard air weighs about 0.075 lb/ft³.

Nominal sensible airflow is 400 cfm per ton; 350 cfm per ton favors latent removal in humid climates.

Sensible Btu/h = 1.08 × cfm × ΔT dry bulb.

Latent Btu/h = 0.68 × cfm × Δgrains; total Btu/h = 4.5 × cfm × Δenthalpy.

Fixed-orifice charging: match measured superheat to the manufacturer target chart, commonly 10–15°F.

TXV charging: set by subcooling, typically 8–12°F measured at the condensing unit.

Normal evaporator air temperature drop is 16–22°F; a drop below 14°F points to low airflow or an overcharge.

R-410A operates at roughly 50–60% higher pressures than R-22 and uses POE oil, which absorbs moisture rapidly.

Weighing in the nameplate charge is the only accurate method on fixed-orifice systems with short line sets.

Standard air density is 0.075 lb/ft³; velocity V (fpm) = 4005 × √VP with VP in inches of water column.

Residential duct design commonly uses a friction rate of about 0.1 in. wc per 100 ft of equivalent length.

Typical velocities: supply trunk 600–900 fpm, branch 400–700 fpm, return 600–900 fpm.

Total external static pressure = supply static + return static, read at the plenum taps with a manometer.

A typical residential air handler budget is 0.5 in. wc total external static including filter and coil resistance.

Flex duct bends need a centerline radius of at least one duct diameter; limit sag to about ½ in. per foot of run.

Seal joints with mastic or UL 181 tape; duct leakage is measured in cfm25 with a duct blaster.

Diffuser selection uses throw, drop, and noise criteria; a ceiling diffuser mounted 8–10 ft high throws roughly 10–20 ft.

Heating values: natural gas ≈ 1,030 Btu/ft³; propane vapor ≈ 2,500 Btu/ft³.

Clocking the meter: input Btu/h = (3600 ÷ seconds per ft³) × Btu per ft³.

Manifold pressure: natural gas 3.5 in. wc; LP 10–11 in. wc. Inlet supply: natural gas 5–7 in. wc; LP 11–13 in. wc.

Combustion air: 50 ft³ per 1,000 Btu input in unconfined space; 1 in² free area per 2,000 Btu with two grilles.

Temperature rise °F = output Btu/h ÷ (1.08 × cfm); it must land inside the nameplate range, often 35–65°F.

Category I appliances vent under negative pressure through metal pipe; Category IV condensing units vent under positive pressure through PVC.

Combustion targets: CO air-free below 100 ppm, ambient CO below 9 ppm, O2 roughly 5–9% on natural draft.

Hot-surface igniters read about 40–90 ohms cold; a cracked igniter reads open circuit.

A standing-pilot thermocouple produces roughly 25–30 mV open circuit and must hold the safety valve open at about 10 mV.

Balance point is the outdoor temperature where heat pump capacity equals building heat loss; below it, auxiliary heat carries the remainder.

Auxiliary heat is staged by outdoor thermostat or control logic; emergency heat locks the compressor out entirely.

Defrost is initiated by time/temperature or on demand, and terminates on coil temperature near 50–70°F or on a time limit.

On most residential units the reversing valve solenoid is energized in heating and de-energized in cooling.

Charge a heat pump in cooling mode when outdoor ambient is above 60°F; use subcooling on TXV systems, superheat on fixed orifice.

COP = heat output (Btu) ÷ electrical input (Btu); HSPF2 is annual heating Btu divided by annual watt-hours.

Suction-line accumulators and crankcase heaters protect the compressor from liquid floodback and off-cycle migration.

A heat pump with equal suction and discharge pressures and no temperature split often has a stuck reversing valve or failed compressor valves.

Number 2 heating oil ≈ 140,000 Btu/gal; a 1.00 gph nozzle at 100 psi fires about 140,000 Btu/h.

Nozzle flow scales with the square root of pressure: gph₂ = gph₁ × √(psi₂ ÷ psi₁).

Standard pump pressure is 100 psi; some retention-head burners specify 140 psi.

Typical electrode settings are a ⅛ in. gap, 1/16 in. ahead of and above the nozzle centerline - always follow the burner spec sheet.

Cad cell reads 400–1,000 ohms with flame; the primary control locks out above about 1,600 ohms after the trial for flame.

Targets: smoke 0 to a trace, overfire draft −0.02 to −0.04 in. wc, CO2 11–13% on retention-head burners.

Net stack temperature = gross stack temperature minus room temperature; a high net stack indicates a fouled heat exchanger.

Bleed the pump fully and replace the nozzle, oil filter, and pump strainer at every annual service.

One-pipe systems require a bypass or two-pipe conversion when tank lift exceeds the pump's rated vacuum capability.

1 gpm carries 500 Btu/h per °F of temperature drop (8.33 lb/gal × 60 min × 1 Btu/lb·°F).

gpm = Btu/h ÷ (500 × ΔT); a 100,000 Btu/h load at 20°F ΔT needs 10 gpm.

Residential boiler aquastat: 180°F operating with a 200°F high limit; relief valve typically set at 30 psi.

Cold fill pressure = 12 psi for one to two stories, plus 0.433 psi per foot of height above the boiler.

Diaphragm expansion tank air precharge equals the cold fill pressure and is checked with the system at zero pressure.

Finned-tube baseboard delivers roughly 550–600 Btu/h per linear foot at 180°F average water and 65°F room air.

Primary/secondary piping uses closely spaced tees no more than four pipe diameters apart to decouple the loops.

Air removal needs a scoop or separator on the boiler supply plus an automatic vent at the system high point.

Steam systems run below 1 psi; 1 psi of pressure lifts water about 28 in. up a riser.

Three-phase power: watts = volts × amps × 1.732 × power factor.

Reverse any two line leads to correct a three-phase compressor or blower spinning backward.

Economizers change over at 55–65°F dry bulb or on an enthalpy setpoint; verify minimum position and damper linkage.

Crankcase heaters must be energized 8–24 hours before compressor start to drive liquid out of the oil.

Belt drives: sheave alignment within 1/32 in. and about ½ in. belt deflection at mid-span for rated tension.

Cooling below roughly 55°F outdoor ambient requires fan cycling, head pressure control, or a low-ambient kit.

Condensate trap depth must equal 1 in. per inch of negative cabinet static plus 1 in. of reserve.

Rooftop units need a curb gasket, accessible filters, and a return-duct smoke detector where code requires it.

TEV superheat at the evaporator outlet is typically 4–6°F; compressor superheat runs 20–40°F.

Evaporator TD (box temperature minus coil temperature) is 10–12°F for coolers and about 10°F for freezers.

Suction risers need 700–1,500 fpm to return oil; use double risers when load falls below minimum velocity.

Pump-down closes the liquid-line solenoid and pulls the low side to the low-pressure control setting before the compressor stops.

Head pressure controls - fan cycling, flooding valves, VFDs - maintain minimum condensing pressure in low ambient.

Safety groups: A1 low toxicity and no flame propagation; A2L low toxicity and mildly flammable; B1 higher toxicity.

With zeotropic blends, evaluate superheat at the dew point and subcooling at the bubble point because of glide.

Ammonia (R-717) is B2L: highly efficient but toxic, requiring detection and relief venting to atmosphere.

Liquid-line subcooling at the TEV inlet should be 5–15°F; flash gas causes low capacity and valve hunting.

Reach-in cooler product space 35–38°F; freezers −10 to 0°F, with box temperature set by cold-control differential.

Capillary-tube systems are critically charged: charge by weight only and never add refrigerant to fix a partial frost pattern.

Freezer defrost terminates near 45–55°F coil temperature, then a fan delay holds fans off until the coil re-chills.

Anti-sweat and frame heaters add sensible load; verify their operation before condemning the refrigeration circuit.

On a receiver system the sight glass should be clear; bubbles indicate a shortage or flash gas, not always low charge.

Test door gaskets with a dollar-bill or tear test; a torn gasket causes frame frost and ice buildup.

Evaporator-outlet superheat on a reach-in is typically 6–10°F.

Ice machine low production is most often a dirty condenser or a restricted water filter, not a refrigerant fault.