GA HVAC cheat sheet

ACCA Manual J calculates room-by-room heating and cooling loads; Manual S selects equipment; Manual D sizes ducts; Manual T addresses air distribution.

Sensible Btu/h = 1.08 × cfm × ΔT; latent Btu/h = 0.68 × cfm × Δgrains; total Btu/h = 4.5 × cfm × Δh for standard air near sea level.

1 ton of cooling = 12,000 Btu/h; 1 kW = 3,412 Btu/h.

Sensible heat ratio (SHR) = sensible load ÷ total load; selected equipment should meet the design sensible and latent loads, not merely nominal tonnage.

Typical design conditions use 1% cooling design dry-bulb and 99% heating design temperature; indoor design is commonly 75°F cooling and 70°F heating unless specified.

Equipment capacity must be checked at design entering wet-bulb, outdoor temperature, and airflow using manufacturer performance data.

Oversized cooling equipment short-cycles, dehumidifies poorly, and can reduce equipment life.

Available static pressure for duct design = fan rated external static pressure minus pressure drops of filters, coils, humidifiers, and accessories.

Superheat = suction-line temperature minus saturation temperature at suction pressure; target value depends on metering type and manufacturer data.

Subcooling = saturation temperature at liquid-line pressure minus liquid-line temperature; TXV systems are commonly charged by subcooling.

The four basic components are compressor, condenser, metering device, and evaporator; the cycle moves heat from a low-temperature source to a higher-temperature sink.

A TXV modulates refrigerant flow to maintain evaporator outlet superheat; a fixed orifice relies on charge and operating conditions.

Low superheat indicates overfeed or floodback risk; high superheat indicates a starved evaporator, undercharge, or restriction.

Zeotropic refrigerant blends have temperature glide; they must be charged as liquid to avoid fractionation.

EPA Section 608 prohibits venting regulated refrigerants and requires certified technicians for recovery.

R-410A operates at roughly 50–70% higher pressure than R-22 and uses POE lubricant, which readily absorbs moisture.

A2L refrigerants such as R-32 and R-454B are mildly flammable and subject to charge, ventilation, and ignition-source rules.

AFUE = annual heat output ÷ annual fuel energy input × 100; condensing appliances typically exceed 90% AFUE.

Furnace temperature rise in °F = output Btu/h ÷ (1.08 × measured cfm); measured rise must fall within the nameplate range.

Natural gas manifold pressure is commonly about 3.5 in. w.c.; LP manifold pressure is commonly 10–11 in. w.c.; the appliance nameplate governs.

A condensing furnace recovers latent heat, produces acidic condensate, and requires a trapped drain and approved vent material.

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

Defrost cycles remove outdoor-coil frost by reversing refrigerant flow or using hot gas; auxiliary heat may temper supply air during defrost.

A boiler relief valve setting must not exceed the boiler maximum allowable working pressure; the expansion tank absorbs thermal expansion.

Heat pump COP = heating output ÷ electrical input; COP varies with outdoor temperature and refrigerant lift.

Category I natural-draft appliances require proper flue draft and cannot share venting unless listed and approved for common venting.

Friction rate in in. w.c. per 100 ft = available static pressure ÷ total effective length.

Total effective length = straight duct length plus equivalent lengths of fittings, transitions, and components.

Airflow: cfm = 4005 × √VP for standard air, where VP is velocity pressure in in. w.c.

ASHRAE 62.2 governs residential whole-building and local exhaust ventilation; ASHRAE 62.1 governs most commercial buildings.

Return air must have a path from each room; undersized returns raise static pressure and reduce supply airflow.

Flex duct should be fully extended, properly supported, and kept with gradual bends; sharp bends add pressure loss.

Balancing dampers belong in branch ducts and at terminals; volume dampers should not be used to compensate for undersized ducts.

Duct leakage testing pressurizes the duct system and measures leakage airflow; sealed ducts reduce delivered-air losses.

Gas pipe sizing uses the longest run, total connected load, allowable pressure drop, gas specific gravity, and pipe material tables.

A sediment trap or drip leg is installed at the appliance connection before controls; it is capped and accessible.

Natural gas service pressure is typically about 7 in. w.c.; appliance manifold pressure is set according to the nameplate.

CSST must be bonded according to manufacturer instructions and code; bonding reduces lightning-related arc damage risk.

Dielectric unions separate copper and steel to limit galvanic corrosion at piping connections.

Condensate drains require a trap to prevent flue gas or air bypass; trap depth must exceed the pressure difference.

Refrigerant line sizing must provide oil return at minimum capacity and acceptable pressure drop at design capacity.

Flue connectors require upward slope and clearance from combustibles according to listing and code.

Ohm's law: E = I × R; power: P = E × I for resistive loads; single-phase P = V × A × power factor.

MCA is the minimum circuit ampacity listed on the nameplate; MOP is the maximum overcurrent protective device size.

A capacitor stores charge; discharge it before testing microfarads, and replace it if the reading is outside nameplate tolerance.

Low-voltage thermostat terminals: R = 24V power, W = heat, Y = cooling, G = fan, C = common.

PSC motors use a run capacitor; ECM motors are electronically commutated and often operate in constant-torque or constant-airflow modes.

VFDs change motor speed and frequency; they require proper grounding and shielded cable to limit electrical noise.

A disconnect must be within sight of and readily accessible from the equipment it serves.

Motor current above nameplate FLA can indicate overload, bad bearings, low voltage, or restricted airflow.

Braze with dry nitrogen flowing through the piping to prevent internal oxidation scale.

Evacuate to 500 microns or lower and perform a standing vacuum decay test before charging.

Weigh in refrigerant when the system is empty or when manufacturer instructions require total charge.

Verify supply voltage and phase balance against nameplate; incorrect phase rotation reverses three-phase compressors.

Record airflow, temperature rise or drop, superheat, subcooling, static pressure, and amp draw at start-up.

Perform combustion analysis on fuel-burning equipment by measuring O2, CO2, CO, flue temperature, and draft.

Confirm condensate drainage, trap operation, and absence of water leaks before completing commissioning.

Functional testing verifies economizer sequences, safeties, and lockout functions.

Pitot traverse: average velocity pressure across a duct; cfm = 4005 × √VP × duct area in ft².

Fan laws: cfm varies with rpm; static pressure with rpm squared; brake horsepower with rpm cubed.

Proportional balancing adjusts branch dampers so each terminal receives its design percentage before final readout.

Static pressure profile readings locate restrictions and excessive losses across filters, coils, and ducts.

Temperature difference across coils and equipment verifies airflow and capacity within expected ranges.

Instruments must be calibrated; record model, serial number, and calibration date in the TAB report.

The final TAB report includes design versus actual airflow, pressures, temperatures, and adjustments made.

An air temperature difference outside the expected range can indicate low airflow, incorrect charge, or a dirty coil.

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

A failed run capacitor causes motor humming, low torque, and overheating; test microfarads against the rating.

A dirty filter reduces airflow, lowers evaporator temperature, and can freeze the coil.

Frozen evaporator causes include low airflow, low charge, restricted metering device, and low return-air temperature.

Electronic leak detectors and UV dye locate refrigerant leaks; after repair, evacuate and recharge.

Compressor winding checks use ohms among terminals and to ground; open or grounded windings indicate failure.

Heat pump defrost problems can involve the defrost board, sensors, reversing valve, or outdoor fan.

EPA Section 608 certification is required to service refrigerant-containing equipment; venting refrigerant is prohibited.

ASHRAE 34 classifies refrigerants by toxicity and flammability; ASHRAE 15 addresses refrigeration system safety.

OSHA construction fall protection is generally required at 6 feet; general industry fall protection at 4 feet.

Lockout/tagout isolates electrical, gas, and other hazardous energy before maintenance.

NEC rules govern disconnects, grounding, bonding, GFCI protection, and conductor sizing for HVAC equipment.

NFPA 54/IFGC governs fuel gas piping, combustion air, venting, and appliance connections.

The International Mechanical Code governs HVAC installation, ventilation, and appliance clearances where adopted.

Permits and inspections are required for regulated installations; final inspection confirms code compliance.