TX HVAC Contractor cheat sheet

Nominal capacity: 1 ton = 12,000 Btu/h; nominal airflow is about 400 cfm per ton (design range roughly 350–450 cfm/ton).

Total capacity = sensible + latent; sensible heat ratio (SHR) = sensible capacity ÷ total capacity.

Air-side total capacity (Btu/h) ≈ 4.5 × cfm × Δh, where Δh is the enthalpy difference across the coil in Btu/lb dry air.

Sensible capacity (Btu/h) ≈ 1.08 × cfm × ΔT at standard air conditions.

Evaporator superheat is measured at the suction line outlet; typical target for fixed-orifice systems is 8–12°F, while TXV systems are charged by subcooling.

Typical evaporator air temperature drop is 16–22°F; low ΔT suggests low load, high airflow, or dirty coil; high ΔT suggests low airflow.

R-410A is a near-azeotropic HFC blend with small glide and requires POE oil; operating pressures are roughly 1.5–2 times R-22 at the same saturation temperature.

Dehumidification requires the coil surface temperature to be below the entering air dew point; apparatus dew point is the coil surface condition at the air leaving condition.

Compressor short cycling can result from oversizing, low refrigerant charge, faulty thermostat, or a tripped safety control.

Electric resistance heat: 1 kW = 3,413 Btu/h; capacity (Btu/h) = kW × 3,413.

Furnace temperature rise (°F) = output Btu/h ÷ (1.08 × cfm); typical natural-gas furnace rise is 35–65°F.

Natural gas heating value is about 1,000 Btu/ft³; LP gas about 2,500 Btu/ft³.

Standard combustion air rule: 50 ft³ of space per 1,000 Btu/h input for unconfined spaces; grilles sized for free area.

Natural gas manifold pressure is typically 3.5 in. w.c.; LP manifold pressure is typically 10–11 in. w.c.

AFUE measures annual fuel utilization efficiency; condensing furnaces (90%+) recover latent heat and produce acidic condensate.

A cracked heat exchanger can introduce carbon monoxide; inspect with combustion analysis and visual/leak testing.

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

Limit switches and rollout switches are safety devices that open the gas valve on excess temperature or flame rollout.

Saturation temperature is the temperature at which a refrigerant changes phase at a given pressure; superheat is vapor temperature above saturation.

Subcooling is liquid temperature below saturation; TXV charging targets subcooling, while fixed orifice targets superheat.

Compression ratio = absolute discharge pressure ÷ absolute suction pressure; high ratios increase discharge temperature and reduce capacity.

A TXV modulates refrigerant flow to maintain a constant evaporator superheat as load changes.

Refrigerant recovery cylinders must not be filled above 80% of capacity by weight/volume.

Deep vacuum for dehydration is typically 500 microns; triple evacuation with nitrogen sweeps removes moisture and noncondensables.

HFC refrigerants such as R-410A and R-134a require POE lubricant; mineral oil is used with CFC/HCFC refrigerants.

Noncondensables raise discharge pressure and reduce capacity; they are removed by proper evacuation, not by adding refrigerant.

Refrigerant leaks are located with electronic detectors, soap bubbles, or UV dye; EPA Section 608 certification is required to handle refrigerant.

Ohm’s law: E = I × R; power in a resistive circuit: P = E × I = I²R.

Single-phase power (W) = volts × amps × power factor; three-phase power = √3 × volts × amps × power factor.

A 24V control circuit is Class 2 low voltage; R is 24V hot, C is common, Y calls cooling, W calls heat, G calls fan.

Start capacitors increase starting torque and are removed by a potential relay or centrifugal switch after startup.

Run capacitors are rated in microfarads (µF); a bulged or failed run capacitor causes high current and poor motor performance.

Contactors switch line voltage; overloads protect motors from sustained overcurrent, not short circuits.

NEC requires a disconnecting means within sight of and within 50 ft of the equipment; it must be lockable.

Continuous loads are sized at 125% of the load for conductor and overcurrent sizing.

Motor service factor above 1.0 allows limited overload; nameplate RLA/LRA values are used for branch-circuit and breaker sizing.

ACCA Manual J calculates room-by-room heating and cooling loads; Manual S selects equipment; Manual D designs ducts; Manual T selects outlets.

Heat loss through an assembly: Q = U × A × ΔT, where U is the overall heat-transfer coefficient.

Sensible infiltration/ventilation load: Q ≈ 1.08 × cfm × ΔT; latent load uses the enthalpy or moisture difference.

Typical residential duct design friction rate is about 0.1 in. w.c. per 100 ft of equivalent length.

Duct cross-sectional area (ft²) = cfm ÷ velocity (ft/min); round duct area = π × radius².

Equivalent length adds straight duct plus fitting losses; elbows and transitions can dominate total pressure drop.

A typical fan static-pressure budget is about 0.5 in. w.c. total, split between supply and return; flex duct adds more resistance if compressed.

Flex duct must be fully extended, supported, and kept with gradual bends; sharp bends and sagging reduce airflow.

Condensate drains require a trap sized to the fan static pressure and a secondary drain or float switch for overflow protection.

Texas Occupations Code Chapter 1302 governs air conditioning and refrigeration contracting; the Texas Department of Licensing and Regulation (TDLR) administers the license.

16 Texas Administrative Code Chapter 75 contains the ACR licensing, technician registration, and enforcement rules.

A licensed A/C contractor is responsible for the work performed under the license; technicians work under contractor supervision as required by rule.

Local permits and inspections are separate from the state license; state licensure does not replace municipal code requirements.

EPA Section 608 certification is required for technicians who service, repair, or dispose of refrigerant-containing equipment.

OSHA requires fall protection for work at 6 ft or more in construction; ladders and roof work require documented safety procedures.

Lockout/tagout isolates electrical and gas energy before service; verify with a meter and bleed stored pressure.

Carbon monoxide is produced by incomplete combustion; install CO alarms and perform combustion analysis on fuel-burning equipment.

Texas Deceptive Trade Practices Act and licensing rules prohibit false advertising and require accurate representations of services and credentials.