KY HVAC cheat sheet

Superheat = actual suction-line temperature minus saturated suction temperature at the same pressure; it is measured at the evaporator outlet.

Subcooling = saturated condensing temperature minus liquid-line temperature; it is measured at the condenser outlet.

Saturation temperature is the temperature at which refrigerant changes phase at a given pressure; evaporator and condenser operation is near saturation.

Compression ratio = absolute discharge pressure divided by absolute suction pressure; ratios above roughly 10:1 raise discharge temperature and reduce volumetric efficiency.

A TXV modulates refrigerant flow to maintain relatively constant evaporator superheat as load changes; a fixed orifice varies superheat with load.

R-410A is a near-azeotropic blend and must be charged as a liquid; it operates at roughly 50 to 70 percent higher pressures than R-22.

A standing vacuum decay test verifies tightness and dehydration; reaching and holding below 500 microns is a common acceptance target.

Bubbles in a liquid-line sight glass indicate flash gas from restriction, undercharge, or loss of subcooling, not necessarily a complete lack of charge.

Complete combustion of methane: CH4 + 2 O2 produces CO2 + 2 H2O; stoichiometric air is about 9.5 cubic feet of air per cubic foot of natural gas.

Carbon monoxide forms from incomplete combustion caused by insufficient air, poor fuel-air mixing, flame impingement, or a blocked flue.

Typical natural-gas manifold pressure is about 3.5 in. w.c.; typical LP manifold pressure is about 10 to 11 in. w.c.

A limit switch opens the burner circuit on excessive plenum or heat-exchanger temperature; it is a safety control, not an operating control.

A flame rollout switch is a manual-reset device that trips when flame exits the heat-exchanger inlet, often from a blocked flue or overfiring.

A thermocouple generates millivolts from pilot-flame heat to hold a standing pilot valve open; a thermopile generates higher voltage for control boards.

Temperature rise = supply air temperature minus return air temperature; it must fall within the furnace nameplate rise range at design airflow.

A combustion analyzer measures O2, CO2, CO, and flue temperature; air-free CO is used to compare readings independent of dilution.

Sensible heat in Btu/h = 1.08 times standard-air CFM times dry-bulb temperature difference.

Total heat in Btu/h = 4.5 times standard-air CFM times enthalpy difference.

The equal-friction method sizes ducts so each run has approximately the same friction loss per unit length.

External static pressure is the sum of supply and return static pressures the blower must overcome; it is compared with the fan performance table.

MERV rates filter capture efficiency; higher MERV generally increases pressure drop and may reduce airflow if the fan cannot compensate.

ASHRAE 62.2 whole-house ventilation rate is approximately 3 CFM per 100 square feet of conditioned floor area plus 7.5 times the bedroom count plus one.

Duct leakage testing measures leakage to outside; sealing with mastic at joints and seams reduces loss better than tape alone.

A room becomes positively pressured when supply airflow exceeds return and exhaust airflow; it becomes negatively pressured when exhaust exceeds supply.

Flex duct pressure loss rises sharply with sag, compression, and tight bends; support and straighten runs to preserve capacity.

Ohm's law: E = I times R; for AC single-phase loads, power in watts = E times I times power factor.

Locked rotor amps is current at motor standstill; running load amps is normal operating current.

The branch-circuit breaker protects conductors; the motor overload protects the motor winding from sustained overcurrent.

A run capacitor remains in the circuit during operation to improve phase angle and motor torque.

A potential relay removes the start capacitor from the circuit when motor back-EMF voltage rises after startup.

A 24-volt control transformer must have VA capacity at least equal to the connected control load.

A contactor coil is controlled by the thermostat; its contacts switch line-voltage power to the compressor or fan.

Typical furnace ignition sequence: call for heat, inducer starts, pressure switch proves, igniter heats, gas valve opens, flame sensor proves.

Manual J calculates room-by-room heating and cooling loads from envelope, infiltration, internal gains, and solar effects.

Manual S selects equipment using Manual J loads and manufacturer capacity tables at design conditions.

Manual D sizes ducts from room airflows, available static pressure, and friction rate.

Q = U times A times temperature difference calculates conduction heat transfer; U = 1 divided by R-value.

Sensible heat ratio = sensible load divided by total load; a low ratio means more latent moisture removal is needed.

Cooling design typically uses 1 percent outdoor dry-bulb and indoor 75 degrees Fahrenheit at 50 percent relative humidity; heating design uses 99 percent outdoor dry-bulb and indoor 70 degrees Fahrenheit.

Oversizing shortens run time, reduces latent removal, and increases cycling; equipment capacity is matched to calculated load rather than a rule of thumb.

Duct loads and ventilation loads must be included in total equipment capacity; they are not free capacity.

HVAC permits are issued by local code authorities before installation or alteration; work is inspected against the adopted code.

EPA Section 608 certification is required to service or dispose of appliances containing regulated refrigerants; intentional venting is prohibited.

Kentucky HVAC contractor licensing is administered by the Department of Housing, Buildings and Construction.

The Kentucky Building Code adopts model codes with state amendments; local jurisdictions may add permit and inspection requirements.

A mechanic's lien is a legal claim against property for unpaid labor or materials; notice and filing deadlines vary by state.

A written change order documents scope, price, and schedule changes before extra work proceeds.

Contract documents typically include scope, exclusions, payment terms, warranty, and insurance; they define the contractor's obligations.