NC HVAC cheat sheet

Sensible heat equation for air: Q = 1.08 × CFM × ΔT (BTU/h), where 1.08 derives from 60 min/h × 0.075 lb/ft³ × 0.24 BTU/lb·°F.

Total heat equation for air: Q = 4.5 × CFM × Δh, using enthalpy difference in BTU/lb of dry air.

Latent heat equation: Q = 4,840 × CFM × Δgrains/7,000, or approximately Q = 0.68 × CFM × Δgrains.

Gas furnace AFUE is the ratio of annual heat output to annual fuel energy input; 90%+ AFUE units are condensing and require PVC venting with condensate disposal.

Heat pump balance point is the outdoor temperature where heat pump capacity equals building load; below it, supplemental resistance or fossil heat is needed.

Electric resistance heat produces 3.412 BTU per watt-hour; COP of 1.0 equals 3.412 BTU/Wh.

Ventilation rate for residential dwellings per ASHRAE 62.2: Q = 0.01 × floor area (ft²) + 7.5 × (bedrooms + 1) CFM.

A direct-expansion evaporator coil superheat target is typically 8–12°F at design conditions for fixed-orifice systems; TXV systems target 6–10°F.

The vapor-compression cycle order is compressor → condenser → metering device → evaporator → back to compressor.

Superheat = actual suction line temperature minus evaporator saturation temperature; subcooling = condenser saturation temperature minus liquid line temperature.

Pressure-enthalpy diagram: compressor work is the enthalpy rise from suction to discharge; refrigerating effect is the enthalpy difference across the evaporator.

Common refrigerants: R-410A operates at roughly 60% higher pressure than R-22; R-134a is a medium-pressure HFC used in chillers and medium-temp equipment.

A TXV maintains constant superheat by modulating refrigerant flow; a fixed orifice meters flow based on pressure difference only.

Low superheat with high suction pressure suggests an overfeeding metering device; high superheat with low suction pressure suggests undercharge or restriction.

Non-condensables in a system raise discharge pressure and subcooling while causing high head temperatures; purge with recovery equipment, never vent refrigerant.

Recovery cylinder fill limit is 80% of capacity by weight; cylinders must be DOT-approved and hydrostatically tested.

Manual J residential load calculation accounts for envelope conduction, infiltration, internal gains, and duct losses; Manual S handles equipment selection and sensible/latent split.

Design temperature difference drives transmission load: Q = U × A × ΔT, where U is assembly conductance in BTU/h·ft²·°F.

Sensible heat ratio (SHR) = sensible load ÷ total load; equipment SHR must match or be below the calculated SHR to avoid excess latent capacity.

Infiltration estimation via the crack method: Q = A × L × ΔT or air-change method using CFM = volume × ACH ÷ 60.

Duct load in unconditioned space typically adds 10–30% to equipment sizing; Manual J applies a duct loss multiplier.

Oversizing causes short cycling, poor latent removal, higher humidity, and reduced equipment life; avoid oversizing by more than 15–20%.

Cooling equipment nominal tonnage: 1 ton = 12,000 BTU/h; verify sensible and latent capacities at design indoor and outdoor conditions.

Altitude and outdoor design conditions alter equipment capacity; use ACCA or ASHRAE design data for the specific North Carolina location.

Duct sizing methods include equal friction, static regain, and velocity reduction; Manual D is the ACCA residential duct design standard.

Friction rate for residential duct design commonly ranges 0.06–0.10 in. w.c. per 100 ft of equivalent length.

Equivalent length adds fitting losses: each 90° elbow can add 20–30 ft; each 45° elbow adds roughly 10 ft of straight duct.

Total external static pressure available equals supply static plus return static; typical residential blower ratings are 0.5 in. w.c. total.

Return air sizing should generally match supply capacity; undersized returns increase static pressure and reduce airflow.

Duct leakage in unconditioned space should be sealed with mastic or UL 181 tape; North Carolina energy code limits duct leakage in tested systems.

Flexible duct must be supported at intervals not exceeding 4 ft and cannot be compressed more than 20% of its diameter.

Air balancing uses a manometer and balancing dampers to achieve design CFM at each terminal; total system airflow should be within ±10% of design.

Ohm's law: V = I × R; power: P = V × I × PF for AC circuits; single-phase watts = volts × amps × power factor.

Motor full-load amps are read from the nameplate; conductor sizing uses 125% of FLA for continuous loads per NEC Article 440.

Locked-rotor amps (LRA) exceed running amps by roughly 5–6 times; hard-start kits add a capacitor to improve starting torque.

A run capacitor improves motor efficiency by shifting phase; a failed run capacitor causes high current draw and overheating.

Thermostat wiring: R (24 VAC power), C (common), W (heat), Y (cooling compressor), G (fan), O/B (reversing valve).

A heat pump reversing valve is energized in cooling on most brands (O terminal); B terminal configurations energize in heating.

Defrost controls initiate defrost based on coil temperature and time; demand defrost uses sensors to reduce unnecessary cycles.

Control transformers step 240/208 V or 120 V down to 24 VAC; a shorted control circuit often blows the low-voltage fuse.

Complete combustion of natural gas requires approximately 10 cubic feet of air per cubic foot of gas; 15 ft³ is used for design safety margin.

Combustion products include CO₂, water vapor, and CO; carbon monoxide is produced by incomplete combustion and is lethal.

A properly adjusted natural gas flame is blue with a slight yellow tip; a yellow, lazy flame indicates insufficient primary air.

Gas piping sizing uses the longest-run method and the NFPA 54 / International Fuel Gas Code tables based on BTU/h input and specific gravity.

Sediment trap (drip leg) must be installed on the gas line upstream of the appliance controls to protect the gas valve.

Combustion air: a confined space requires two permanent openings sized per code; a 50 ft³ per 1,000 BTU/h standard applies to unconfined spaces.

Flue venting for Category I appliances requires a minimum vertical rise and proper termination; Category IV condensing appliances use PVC and positive pressure venting.

Gas leak testing uses a manometer or bubble solution; never use a flame to test for leaks, and purge lines before ignition.

North Carolina adopts the International Residential Code and International Mechanical Code with state amendments; the NC Building Code is enforced by local code officials.

The North Carolina State Board of Examiners of Plumbing, Heating, and Fire Sprinkler Contractors licenses heating contractors; a license is required to contract for HVAC work.

HVAC license classifications include H-1 (unlimited), H-2, and H-3, with H-3 limited in scope; licensing requires examination and continuing education.

North Carolina requires a permit before installation or replacement of HVAC equipment; inspections occur at rough-in and final stages.

The NC Energy Conservation Code references the IECC with state amendments, including duct leakage and equipment efficiency requirements.

Refrigerant handling requires EPA Section 608 certification; North Carolina enforces federal refrigerant rules through licensed technicians.

Changeout permits must match equipment sizing to the load calculation; equipment replacement cannot increase capacity without justification.

License renewal requires continuing education hours per the Board's current rules; verify current cycle requirements directly with the Board.

Gross profit margin = (price − cost) ÷ price; markup = (price − cost) ÷ cost; a 30% margin equals a 42.9% markup.

Break-even volume = fixed costs ÷ contribution margin per job; contribution margin = price minus variable cost.

A written contract should specify scope, price, payment schedule, change-order procedure, warranty terms, and dispute resolution.

North Carolina mechanics' lien rights must be perfected by filing within the statutory period after last furnishing labor or materials.

Change orders must be documented in writing and signed before performing out-of-scope work to preserve payment rights.

Job costing tracks labor, material, equipment, and overhead per job; comparing estimated to actual cost identifies profit leakage.

Retainage is commonly 5–10% withheld until substantial completion; track it as a receivable, not as collected revenue.

Warranty obligations should be defined separately from manufacturer equipment warranties, which typically cover parts only.

Personal protective equipment for HVAC work includes safety glasses, gloves, hearing protection, and steel-toe boots; lockout/tagout is required before servicing energized equipment.

Refrigerant exposure causes frostbite and asphyxiation; use a refrigerant monitor in confined spaces and ventilate before entering.

Never vent refrigerant to the atmosphere; recover, recycle, or reclaim per EPA Section 608 requirements.

Carbon monoxide alarms should be installed outside sleeping areas and on each level of a dwelling per code.

Confined-space entry requires atmospheric testing for oxygen, CO, and combustible gas, plus an attendant and permit.

Ladder safety: maintain a 4:1 ratio of working height to base distance and maintain three points of contact.

Electrical safety: verify de-energization with a meter, treat all circuits as live until tested, and use GFCI protection on temporary power.

Refrigerant cylinders must be stored upright, secured, and away from heat; never heat a cylinder with an open flame to speed transfer.