NATE AC Installation cheat sheet

Standard ARI cooling rating conditions are 80°F dry bulb / 67°F wet bulb indoors and 95°F dry bulb outdoors.

Nominal airflow is 400 cfm per ton; the acceptable range is about 350–450 cfm per ton, and airflow below 350 cfm/ton risks coil icing.

Evaporator temperature split is 16–22°F at design conditions; a split below 14°F indicates low airflow, low charge, or a dirty coil.

Fixed-orifice superheat target is 8–12°F at 95°F outdoor and 80/67°F indoor; TXV subcooling target is 8–12°F unless the nameplate states otherwise.

Superheat equals suction line temperature minus suction saturation temperature; subcooling equals liquid saturation temperature minus liquid line temperature.

Heat pump balance point is the outdoor temperature where capacity equals building load; below it, auxiliary heat runs.

Standard residential air conditioners need a low-ambient kit below roughly 55–60°F outdoor; heat pumps typically operate to 0°F with declining capacity.

Wet-bulb temperature sets latent load; a high indoor wet bulb increases moisture removal and reduces sensible capacity.

Altitude reduces air density, so the same cfm carries less mass and capacity falls; use manufacturer altitude corrections.

A TXV senses suction line temperature at the evaporator outlet and modulates flow to hold superheat; a fixed orifice relies on charge and indoor/outdoor conditions.

Filter drier arrow points in the direction of refrigerant flow; heat pumps require a bi-flow drier.

Suction and vapor lines are insulated; liquid lines are not, except where condensation control is needed.

A condensate trap must be deeper than the blower's negative static pressure, typically 2–4 in. wc, and must be primed.

A reversing valve changes the refrigerant path to swap the indoor and outdoor coils' roles.

A crankcase heater prevents refrigerant migration into oil during the off cycle.

An accumulator on the suction line holds liquid refrigerant and protects the compressor from slugging.

Line-set sizing trades pressure drop against refrigerant velocity; excessive suction-line pressure drop lowers capacity and superheat.

Service valves must be fully back-seated or front-seated as required, and Schrader cores must be in place before evacuation.

ASHRAE 62.2 whole-house ventilation: Q = 0.03A + 7.5(N+1) cfm, with A in square feet and N bedrooms.

MERV 8 is typical residential filtration; MERV 13 captures finer particles but adds static pressure and must be within blower capacity.

Return air cannot be drawn from bathrooms, kitchens, garages, or combustion-appliance rooms.

Indoor relative humidity target is 30–50%; above 60% supports mold and dust mite growth.

CO thresholds are 9 ppm for 8 hours and 35 ppm for 1 hour; combustion appliances must be vented and tested.

A clean filter typically drops 0.1–0.2 in. wc; a dirty filter raises pressure drop and reduces airflow.

Duct leakage in unconditioned space wastes capacity and can draw humid or contaminated air into the system.

Condensate pans must drain fully and be sloped; standing water supports biological growth.

Fresh-air intakes need a filter and a means to temper or dehumidify air in humid climates.

Ohm's law: E = I × R; power: P = E × I; these apply to 24-V control and line-voltage circuits.

Run capacitors are typically good within ±6% to ±10% of rated microfarads; a bulged capacitor is replaced.

Thermostat terminals: R = 24-V power, Y = compressor, G = fan, W = heat, O/B = reversing valve, C = common.

The disconnect must be within sight of and no more than 50 ft from the equipment.

MCA is minimum circuit ampacity; MOP is maximum overcurrent protection; the breaker must not exceed the nameplate MOP.

Compressor winding resistances should balance within about 10%; a very low or open winding indicates failure.

A contactor coil is energized by the thermostat; pitted contacts cause voltage drop and compressor overheating.

Low-voltage control circuits are 24 V; a shorted or open safety switch prevents the contactor from pulling in.

Grounding and bonding are required for the equipment, conduit, and refrigerant lines per local code.

Sensible Btu/h = 1.08 × cfm × ΔT; total Btu/h = 4.5 × cfm × Δh.

TESP is supply static plus return static; residential blowers are typically rated near 0.5 in. wc.

Evacuate to below 500 microns and hold with the pump isolated; a rising reading indicates a leak or moisture.

Pressure-test with dry nitrogen at 150–300 psi for residential systems; never use oxygen or refrigerant.

Low superheat with high suction pressure suggests overcharge; high superheat suggests undercharge or a restriction.

High subcooling suggests overcharge or a liquid-line restriction; low subcooling suggests undercharge.

Amp draw above nameplate RLA indicates high load, low voltage, or a failing compressor; below RLA can indicate low charge.

A temperature split below 16°F combined with low suction pressure indicates low charge or low airflow.

Recovery must remove refrigerant before opening the system; venting is prohibited.