LA HVAC cheat sheet

Personal fall protection is required at 6 ft or more in general industry; Louisiana-adopted OSHA construction rules also use 6 ft for fall arrest and 15 ft for controlled access zones.

A refrigerant monitor for A2L refrigerants must alarm well below the lower flammability limit; the LFL of R-32 is about 14.4% by volume in air, and R-454B is about 12.5%.

Lockout/tagout requires a personal lock and tag on the disconnect before any electrical work; verify absence of voltage with a meter rated CAT III or CAT IV for the circuit.

Nitrogen pressure testing uses dry nitrogen only; never use oxygen or compressed air on a refrigerant circuit because oil can ignite or oxidize.

Brazing requires a fire watch, a suitable extinguisher, and removal of combustible material within 35 ft; purge the system with nitrogen to prevent internal oxidation (scale).

A vacuum pump's blank-off (ultimate) vacuum should be below 100 microns; a pump that cannot pull below 500 microns needs new oil or seals.

Recovery cylinders must never be filled above 80% of capacity by weight; use a scale, and never exceed the cylinder's DOT service pressure.

Manometer and magnehelic gauges read inches of water column (in. w.c.); 1 psi equals 27.68 in. w.c., and 1 in. w.c. equals 249 pascals.

Sensible heat equation: Btu = 1.08 × cfm × ΔT for air; latent heat: Btu = 0.68 × cfm × Δgrains (or 4,500 × cfm × Δhumidity ratio).

One ton of refrigeration equals 12,000 Btu/h, 200 Btu/min, or 3.517 kW.

Superheat = actual suction line temperature minus saturated suction temperature at the same pressure; subcooling = saturated liquid temperature minus actual liquid line temperature.

In a properly charged fixed-orifice (piston) system, superheat typically runs 8–12°F; a TXV system is charged to 8–12°F subcooling with 6–10°F superheat at the evaporator.

The compressor raises pressure and temperature; the condenser rejects heat and subcools liquid; the metering device drops pressure; the evaporator absorbs heat and superheats vapor.

Pressure–temperature (PT) charts are refrigerant-specific; R-410A pressures are roughly 1.6 times those of R-22 at the same saturation temperature.

A flooded evaporator uses a float or level control; a DX (direct expansion) evaporator uses a TXV or fixed orifice and leaves superheated vapor at the outlet.

Refrigerant glide is the temperature difference between bubble and dew points of a zeotropic blend; R-407C has significant glide, R-410A has about 0.2°F, and R-134a is essentially zero.

Manual J (ACCA) residential load uses outdoor design temperature, indoor design temperature, envelope U-values, infiltration, and internal gains; Manual S selects equipment, Manual D sizes ducts.

Sensible heat gain through a wall: Btu/h = U × A × CLTD (or U × A × ΔT); total glass gain uses a solar heat gain factor and shading coefficient.

Duct sizing by equal friction typically targets 0.08–0.10 in. w.c. per 100 ft of straight duct for supply trunk lines in residential work.

Velocity limits: 600–900 fpm in main trunks, 400–700 fpm in branch runs, and under 300 fpm at return grilles to limit noise.

Static pressure budget for a residential air handler is commonly 0.5 in. w.c. total external static; a dirty filter or undersized return raises it and reduces airflow.

Airflow rule of thumb: about 400 cfm per ton of cooling for a standard system; 350 cfm/ton in humid climates improves latent removal, 450 cfm/ton raises sensible capacity.

Equivalent length converts fittings to straight-duct feet; a 90° hard elbow can equal 30–60 ft of duct, so minimize fittings on short, high-velocity runs.

Return air path must be sized nearly equal to supply; undersized returns cause negative pressure, infiltration, and high static that shortens blower life.

Complete combustion of natural gas: CH4 + 2O2 → CO2 + 2H2O; stoichiometric air is about 10 ft³ of air per ft³ of natural gas, and typical excess air is 25–50%.

Flue gas CO2 for natural gas at perfect combustion is about 11.7%; a reading well below that indicates excess air, and CO above 100 ppm in flue gas signals incomplete combustion.

Draft categories: natural draft (Category I, negative pressure, non-condensing), Category II (negative, condensing), Category III (positive, non-condensing), Category IV (positive, condensing).

A manifold gas pressure for natural gas is typically 3.5 in. w.c.; LP gas is typically 10–11 in. w.c. Verify against the data plate before adjusting.

Temperature rise = supply air temperature minus return air temperature; it must fall within the range on the rating plate, commonly 35–65°F for residential furnaces.

Gas furnace airflow formula: cfm = output Btu/h ÷ (1.08 × temperature rise).

A flame sensor reads microamps (typically 1–10 µA); below about 0.5 µA the control locks out on flame failure.

Heat exchanger cracks are detected by combustion analysis (rising CO with airflow change), visual inspection, or a smoke/candle test at the flue; any crack condemns the unit.

EPA Section 608: technicians handling refrigerant must be certified; venting refrigerant is prohibited, and recovery is required for appliances with 5 or more lb of charge.

Recovery cylinder fill limit is 80% by weight; use a scale and never exceed the DOT working pressure marked on the cylinder.

Superheat charging (fixed orifice): measure suction pressure, convert to saturated temperature, subtract from suction line temperature; target 8–12°F at design indoor conditions.

Subcooling charging (TXV): measure liquid line pressure converted to saturated temperature, subtract from liquid line temperature; target 8–12°F.

A2L refrigerants (R-32, R-454B) are mildly flammable; they require leak detection, no ignition sources, and charge limits per ASHRAE 15 and the equipment nameplate.

Vacuum decay test: pull to 500 microns, isolate the pump, and confirm the vacuum holds within 500 microns for 10–15 minutes; a rise indicates moisture or a leak.

R-410A operates at roughly 118 psig suction and 340–400 psig head at 95°F outdoor ambient; R-22 runs about 68 psig suction and 250 psig head under the same conditions.

A system that is overcharged shows high subcooling, high head pressure, and elevated discharge temperature; an undercharge shows low subcooling, low suction pressure, and high superheat.

Ohm's law: E = I × R; power: P = E × I; for single-phase, watts = volts × amps × power factor.

Three-phase power: watts = volts × amps × 1.732 × power factor; a balanced motor draws equal current on all three legs within about 10%.

Motor winding resistance: a single-phase compressor run winding has lower resistance than the start winding; the common terminal is the one with the highest resistance to both others.

Capacitor test: a good run capacitor reads within 6% of its marked µF value on a capacitance meter; a bulged or leaking capacitor is replaced, not reused.

A PSC blower motor's speed is set by the number of energized taps; a failed run capacitor causes the motor to hum and overheat without starting.

Thermostat wiring: R (24V power), C (common), G (fan), W (heat), Y (cooling contactor), O/B (reversing valve); a blown low-voltage fuse usually means a shorted control wire.

Contactors and relays are tested by coil resistance and by voltage across the coil; a contactor that chatters usually has low control voltage or a loose connection.

A compressor that trips on internal overload after running suggests high head pressure, low voltage, or a failing start component; check amp draw against the nameplate RLA.

Copper line sizing follows the manufacturer's table; a common residential rule is 3/8 in. liquid and 3/4 in. suction for 1.5–3 tons, with 7/8 in. suction for 3.5–5 tons.

Suction line velocity should stay between 700 and 1,200 fpm to return oil; below 500 fpm oil pools in traps, above 1,500 fpm noise and pressure drop rise.

A P-trap at the evaporator coil drains condensate; the trap depth must exceed the fan's static pressure, typically 2–3 in., and the drain must slope at least 1/8 in. per ft.

Brazing copper requires a nitrogen purge; the joint is heated to a dull cherry red and the filler flows by capillary action.

Sheet metal gauges: 26 ga is common for residential trunk, 24 ga for larger ducts; galvanized steel duct weight is about 1.6 lb/ft² for 26 ga.

Duct sealant must be applied to all joints and seams; the SMACNA seal classes range from A (most leak-tight) to C, with residential work commonly requiring Class C or better.

Flexible duct must be supported every 4–5 ft, not compressed, and limited to short runs; sharp bends reduce airflow and increase static pressure.

A line-set length beyond the manufacturer's maximum requires an oil trap every 20 ft of vertical rise and may require additional charge per foot of liquid line.

Louisiana mechanical work is regulated by the Louisiana State Licensing Board for Contractors (LSLBC); a mechanical (air conditioning and heating) license is required for contracts over the residential threshold.

Louisiana requires a state mechanical license for commercial HVAC work; local parishes and municipalities may require a separate permit and inspection.

Permits are required before installing or replacing HVAC equipment in most Louisiana parishes; work without a permit can result in stop-work orders and fines.

The Louisiana Mechanical Code is based on the International Mechanical Code (IMC); local amendments apply in parishes such as Orleans and East Baton Rouge.

Refrigerant handling is governed by EPA Section 608 in addition to state rules; Louisiana does not issue a separate refrigerant certification, so the federal card is the credential.

Contractors must carry workers' compensation and general liability insurance; LSLBC requires proof of insurance for license renewal.

Bonding requirements: Louisiana contractors must post a surety bond; the amount depends on license classification and must be maintained for the license to stay active.

Continuing education is required for license renewal; the LSLBC sets the hours and approved course categories for each license type.

Gross profit margin = (price − cost) ÷ price; markup = (price − cost) ÷ cost. A 30% margin equals a 42.9% markup, so never confuse the two.

Break-even volume = fixed costs ÷ (unit price − unit variable cost); the result is the number of jobs or units needed to cover overhead.

A labor rate must cover wages, payroll taxes, insurance, truck, tools, and overhead; a common HVAC service labor burden is 35–60% of the base wage.

Change orders must be written and signed before the work is performed; verbal changes are unenforceable in most disputes.

A contract should state scope, price, payment schedule, exclusions, warranty terms, and a dispute resolution clause; Louisiana's Private Works Act governs mechanics' liens on private projects.

Progress billing typically uses a schedule of values; retainage of 5–10% is common on commercial work and is released at substantial completion.

Estimate takeoff: count fixtures and equipment, measure duct by the linear foot by size, and price refrigerant and materials at current supplier cost plus waste factor.

Job cost tracking compares estimated hours to actual hours per task; a variance over 10% on a task signals a pricing or productivity problem to correct in the next bid.