Free HVAC Excellence Exam Flashcards

Describes the relationship between voltage, current, and resistance: E = I x R, so I = E / R and R = E / I. Example: 120 V applied across a 15-ohm load draws 8 A. Technicians use it to find any one value in an HVAC circuit when the other two are known.

A run capacitor stays in the circuit continuously to phase-shift the start winding of a single-phase motor, improving running torque and efficiency. A start capacitor has a much higher microfarad rating, adds starting torque for a second or two, and must be switched out by a potential relay or similar starting device because it is not built for continuous duty.

All three use an electromagnetic coil (commonly 24 V, 120 V, or 240 V) to open and close contacts. A relay is a light-duty switch for control circuits and small loads, a contactor is a heavy-duty version for compressors and other large loads, and a motor starter is a contactor with built-in overload protection.

Converts line voltage, commonly 120 V or 240 V, down to the 24 V control voltage used by thermostats, relays, and gas valves. Control transformers are rated in VA (volts x amps): a 40 VA, 24 V secondary can carry about 1.67 A, so adding accessories beyond that rating overloads it.

The designed order in which a system's components energize and de-energize in response to a control signal, such as a call for heat starting the inducer, then the pressure switch proving, then ignition, the gas valve, and finally the blower. Finding the first step that fails to happen is the standard way to isolate an electrical fault on a wiring diagram.

A voltmeter is connected across (in parallel with) a load or switch on a live circuit to read potential difference. An ohmmeter is used only with power off and the component isolated, because it supplies its own battery voltage. A clamp-on ammeter closes around one conductor of a live circuit and reads current without opening the circuit.

Every mechanical refrigeration circuit has a compressor, condenser, metering device, and evaporator. The compressor and metering device are the two points that divide the circuit into its high-pressure and low-pressure sides.

The temperature rise of refrigerant vapor above its saturation (boiling) temperature at the measured pressure. Example: R-410A at about 118 psig boils at 40 degrees F, so a 50 degree F suction line means 10 degrees F of superheat. Superheat is the charge check for fixed-orifice and capillary-tube systems, and it confirms that no liquid is returning to the compressor.

The temperature drop of liquid refrigerant below its saturation (condensing) temperature at the measured pressure. Example: R-410A at about 365 psig condenses at 110 degrees F, so a 100 degree F liquid line means 10 degrees F of subcooling. Subcooling is the standard charge check on TXV systems, because the TXV holds superheat roughly constant regardless of charge.

ASHRAE Standard 34 rates refrigerants by toxicity (A = lower toxicity, B = higher toxicity) and flammability (1 = no flame propagation, 2L = lower flammability, 2 = flammable, 3 = higher flammability). R-410A is classified A1; R-454B and R-32, the common R-410A replacements, are A2L; propane (R-290) is A3.

Primary air is drawn into the burner and mixes with the fuel gas before the mixture reaches the burner ports; secondary air is supplied around the flame to complete combustion. Too little primary air gives a lazy yellow flame that can soot and raise carbon monoxide, while too much can make the flame lift off the ports.

Complete combustion of natural gas or propane with sufficient air produces carbon dioxide, water vapor, and heat. Incomplete combustion, caused by insufficient air, flame impingement, or a dirty burner, produces carbon monoxide (CO), a colorless, odorless, toxic gas that can only be confirmed with a test instrument.

An electronic combustion analyzer typically measures oxygen, carbon monoxide, and stack (flue gas) temperature directly, then calculates carbon dioxide, excess air, and combustion efficiency from those readings. Technicians compare the results against the equipment manufacturer's specifications to confirm safe, efficient combustion.

The pressure difference, measured in inches of water column, that moves combustion products out of an appliance and up the vent. Natural draft relies on the buoyancy of hot flue gas, induced draft uses a fan at the heat exchanger outlet, and forced draft uses a fan at the inlet. Exhaust fans or dryers that depressurize the room can overpower natural draft and cause spillage.

A time-delay relay used on electric furnaces to bring multiple heating elements online one stage at a time instead of all at once. Staging avoids a sudden large current demand and voltage dip on the electrical service, and it also stages the elements off in steps when the call for heat ends.

The limit switch is an automatic-reset safety that opens the circuit to a heating element if the air around it overheats, usually from low airflow. The fusible link (thermal fuse) is a one-time backup set to open at a higher temperature if the limit fails; once open it must be replaced, and the airflow cause must be found.

Electric furnace heat kits are rated in kilowatts (kW); multiplying kW by 3,412 converts that output to Btu/h, so a 10 kW kit delivers about 34,120 Btu/h. Dividing watts by volts gives the current: 10,000 W at 240 V is about 41.7 A, which drives the required circuit ampacity and overcurrent protection.

Elements need airflow any time they are energized, so the blower is interlocked with the heat. To verify airflow, use CFM = (kW x 3,412) / (1.08 x temperature rise). Example: 10 kW with a 30 degree F rise is 34,120 / 32.4, or about 1,050 CFM. A higher-than-expected rise means low airflow and explains nuisance limit trips.

A standing pilot burns continuously and heats a thermocouple, whose output of roughly 25 to 30 millivolts holds the safety valve open to prove the pilot. Hot surface ignition instead heats a silicon carbide or silicon nitride igniter electrically on each call for heat, saving the gas a standing pilot would otherwise consume.

A method of proving burner flame using the flame itself as a rectifier: the control applies AC voltage to a flame sensor rod, and the flame conducts it to the grounded burner as a weak DC (microamp) signal. A dirty or oxidized sensor rod or a poor burner ground weakens that signal, so the burner lights and then drops out within seconds.

The gas pressure measured at the burner manifold, downstream of the gas valve's regulator. Typical values are about 3.5 in. w.c. for natural gas and about 10 in. w.c. for propane, but the appliance nameplate governs. Incorrect manifold pressure under-fires or over-fires the appliance relative to its rated Btu input.

Gas appliances are grouped into four venting categories by vent pressure and condensing potential. Category I is negative-pressure, non-condensing (conventional furnaces); Category II is negative-pressure, condensing; Category III is positive-pressure, non-condensing; Category IV is positive-pressure, condensing, requiring sealed vent material rated for wet, corrosive flue gas.

A furnace, typically 90% AFUE or higher, with a secondary heat exchanger that extracts additional heat from flue gas, causing water vapor in the exhaust to condense into acidic condensate. This raises efficiency but requires a condensate drain and trap plus the vent material the manufacturer specifies, commonly PVC, CPVC, or polypropylene.

The thermal balance point is the outdoor temperature at which a heat pump's heating capacity equals the building's heat loss; below it, supplemental heat is needed to hold setpoint. The economic balance point is different: it is the outdoor temperature below which a dual-fuel system's furnace becomes cheaper to run than the heat pump.

On a heat pump the larger line is called the vapor line, because it carries cool suction vapor in cooling mode and hot discharge gas in heating mode; the liquid line carries liquid in both modes but the flow direction reverses. Installers size both lines from manufacturer charts for capacity, run length, and vertical lift, not by reusing whatever size the old system had.

Weighing in the nameplate charge plus the manufacturer's line-length adjustment is the most reliable method, and often the only practical one in cold weather. In cooling mode, check subcooling on TXV units or superheat on fixed-orifice units; in heating mode, use the manufacturer's heating-mode charging chart rather than a generic target number.

The thermostat's O terminal energizes the reversing valve in cooling mode, which is the convention most manufacturers use; the B terminal energizes it in heating mode, used notably by Rheem and Ruud. Setting the thermostat for the wrong type makes the system heat on a call for cooling and cool on a call for heat.

A pilot-operated four-way valve, shifted by a solenoid, that swaps the coil roles: in cooling mode the outdoor coil is the condenser and the indoor coil is the evaporator, and in heating mode the outdoor coil is the evaporator and the indoor coil is the condenser. The slide needs a pressure difference across the system to shift, so it will not change position with the compressor off.

A heater installed on or in a compressor's crankcase to keep the oil warmer than the rest of the system, so refrigerant does not migrate to the crankcase and condense in the oil during the off cycle. Starting a compressor with refrigerant-diluted oil causes foaming, loss of lubrication, and possible liquid slugging.

In defrost, a heat pump shifts to cooling mode for a few minutes, stops the outdoor fan, and energizes auxiliary heat to temper the supply air while hot gas melts frost off the outdoor coil. Time-temperature controls start defrost from a fixed timer when a coil thermostat is closed; demand defrost starts only when sensors indicate actual frost, such as the difference between coil and outdoor air temperature.

Auxiliary heat is supplemental heat staged on automatically alongside the compressor when heat pump capacity alone cannot meet demand, such as during defrost or below the balance point. Emergency heat is a manual thermostat setting that runs only the backup heat source and locks out the compressor entirely.

A self-contained HVAC unit, typically mounted on a roof curb, that houses the compressor, condenser, evaporator coil, blower, and often a gas or electric heat section in one cabinet, with ductwork connecting through the curb or the side. RTUs are common in light commercial buildings because they keep mechanical equipment and service work off the occupied floor.

A damper assembly on a rooftop unit that brings in outdoor air for 'free cooling' when outdoor conditions are suitable, reducing compressor run time. A dry-bulb or enthalpy sensor tells the economizer controller when outdoor air is cool and dry enough to use instead of mechanical cooling.

Many light commercial units use two or more smaller compressors instead of one large compressor so capacity can be staged to match building load and provide some redundancy if one compressor fails. Staging is typically controlled by the thermostat or a rooftop control board based on how far indoor temperature is from setpoint.

Air handlers installed above a ceiling need overflow protection, such as an auxiliary drain pan or a float switch that shuts the unit down if the primary drain blocks. On draw-through units the drain pan is under negative pressure, so the trap must be deep enough to overcome blower suction or condensate is held in the pan and overflows.

Commercial mechanical codes, generally based on ASHRAE Standard 62.1, require a minimum volume of outdoor air whenever the building is occupied, unlike a typical residential system that simply recirculates air. The outdoor air damper therefore has a minimum position that is set and measured during commissioning, balancing ventilation against energy use.

A walk-in cooler holds box temperature above freezing, so off-cycle (air) defrost works: the compressor stops and the fans melt the frost with box air. A freezer's box air is below freezing, so it needs electric or hot gas defrost started by a time clock, ended by coil temperature with a time fail-safe, followed by a fan delay so water is not blown into the box.

An evaporator pressure regulator (EPR) sits at the evaporator outlet and keeps evaporator pressure, and therefore coil temperature, from falling below its setting; it lets one compressor serve evaporators at different temperatures. A crankcase pressure regulator (CPR) sits at the compressor inlet and limits how high suction pressure can rise, protecting the motor from overload after defrost or on a hot pull-down.

In low ambient conditions, a commercial refrigeration condenser can produce head pressure too low for the metering device to feed refrigerant properly. Head pressure controls, such as a condenser fan cycling switch or a condenser flooding valve, hold head pressure up by reducing condenser airflow or effective condenser surface.

Sweating, or surface condensation, on a refrigerated case or door frame points to high store humidity, a bad gasket, or failed anti-sweat heaters rather than a refrigerant issue. Heavy frost on the coil itself, beyond a normal light pattern, more often points to low airflow, low charge, or a defrost failure.

A control sequence in which the thermostat closes a liquid-line solenoid valve, the compressor pumps the low side down, and a low-pressure control then stops the compressor. This stores the refrigerant in the condenser and receiver and keeps liquid out of the crankcase during the off cycle; it is common on commercial refrigeration and rare on residential AC.

A thermostatic expansion valve (TXV) uses a sensing bulb at the evaporator outlet to modulate refrigerant flow and hold a target superheat across varying loads. A fixed orifice (piston) or capillary tube has no moving parts, so its flow depends only on the pressure difference across it and it relies on an exact refrigerant charge for the conditions.

Seasonal Energy Efficiency Ratio 2, the U.S. Department of Energy cooling efficiency metric for central AC and heat pumps manufactured since January 1, 2023. Its M1 test raised external static pressure from 0.1 to 0.5 in. w.c. to better reflect real ductwork. Minimums for split AC under 45,000 Btu/h are 13.4 SEER2 in the North and 14.3 SEER2 in the Southeast and Southwest.

Sensible cooling lowers air temperature, while latent cooling removes moisture from the air as it condenses on the evaporator coil. An oversized air conditioner that short-cycles removes plenty of sensible heat but not enough latent heat, leaving a space cold but humid.

Technicians flow dry nitrogen at very low pressure through copper refrigerant lines while brazing to displace oxygen, which would otherwise form copper oxide scale inside the pipe. That scale later breaks loose and clogs metering devices, screens, and filter driers.

The pressure the blower must overcome to move air through the duct system, filter, and coil, measured with a manometer in inches of water column. Many residential furnaces and air handlers are rated for about 0.5 in. w.c. of total external static pressure; exceeding the nameplate rating cuts airflow, which lowers capacity and can freeze the evaporator coil.

A temperature-sensing control on a boiler that turns the burner, or heat source, on and off to maintain water temperature within a set range, and acts as a high limit. It performs a similar role for hydronic heat that a thermostat performs for forced-air heat, but senses water instead of air.

An electric centrifugal pump that moves heated water from the boiler through the piping loop to terminal units such as baseboard, radiators, or radiant tubing, then back to the boiler. It only has to overcome the friction of the closed loop, not lift the water, because the system is already filled and pressurized.

A tank, usually with an air-charged diaphragm or bladder, that absorbs the volume increase of heated water in a closed hydronic system. Its air precharge is set to match the cold fill pressure, commonly about 12 psi. A waterlogged or failed tank lets pressure climb until the relief valve, commonly set at 30 psi on residential boilers, discharges.

A zone valve is a motorized valve that opens one heating zone while a shared circulator serves the whole system; its end switch signals the boiler and circulator to start once the valve is open. Circulator zoning gives each zone its own pump and needs a flow-check valve per zone to stop unwanted gravity flow into zones that are not calling.