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Question 1Heating, Ventilation and Air Conditioning (HVAC)

In a residential forced-air furnace, which component is responsible for transferring heat from the combustion gases to the circulating supply air without allowing the two airstreams to mix?

  1. The draft inducer blower
  2. The condensate trap
  3. The heat exchanger Correct answer.
  4. The gas valve

Explanation

The heat exchanger is a metal section that separates the flue products from the supply air, so heat passes through its walls while the two streams stay physically separate. The inducer moves flue gas, the gas valve controls fuel, and the condensate trap handles water, none of which performs the air-to-flue heat transfer.

Question 2Heating, Ventilation and Air Conditioning (HVAC)

A technician reads 72 °F dry-bulb and 60 °F wet-bulb on a sling psychrometer. What does the difference between these two readings directly indicate?

  1. The relative humidity of the air
  2. The evaporative cooling potential of the air Correct answer.
  3. The enthalpy of the air
  4. The dew point of the space

Explanation

The wet-bulb depression, the gap between dry-bulb and wet-bulb, shows how much evaporative cooling the air can still provide. A large depression means dry air with strong evaporation potential, while a small depression means humid air near saturation. Dew point, relative humidity, and enthalpy are separate properties that require additional chart work or tables.

Question 3Heating, Ventilation and Air Conditioning (HVAC)

Why must the suction line of a refrigeration circuit be insulated in a typical split air-conditioning system?

  1. To prevent refrigerant from flashing before the metering device
  2. To limit heat gain and prevent condensation on the cold line Correct answer.
  3. To reduce the pressure drop caused by refrigerant velocity
  4. To protect the copper from galvanic corrosion

Explanation

The suction line carries cold low-pressure vapor, so without insulation it absorbs heat from the surrounding air and loses capacity while sweating moisture. Insulation keeps the vapor temperature down and prevents condensation on the pipe surface. Flash gas occurs upstream at the metering device, and insulation does not meaningfully change velocity-driven pressure drop or galvanic corrosion.

Question 4Heating, Ventilation and Air Conditioning (HVAC)

A duct system is being designed for a room needing 400 cfm with a design friction rate of 0.10 in. w.c. per 100 ft. Which tool is used to select the round duct diameter that satisfies both the airflow and friction targets?

  1. A duct calculator (ductulator) Correct answer.
  2. A refrigerant charging scale
  3. A velometer
  4. A manometer

Explanation

A duct calculator relates airflow, friction rate, velocity, and duct size, so entering cfm and friction yields the required diameter. A manometer measures pressure, a velometer measures air velocity at a point, and a charging scale weighs refrigerant, so none of them selects duct size from a friction target.

Question 5Heating, Ventilation and Air Conditioning (HVAC)

During a cooling call, the compressor runs but the indoor blower does not start. Which consequence is most likely if the unit continues to operate in this condition?

  1. The metering device will open fully and flood the evaporator with warm liquid
  2. The condenser fan will overspeed and trip its overload
  3. The suction pressure will rise well above the design value
  4. The evaporator coil will freeze and may cause liquid refrigerant to reach the compressor Correct answer.

Explanation

Without airflow across the evaporator, the coil temperature drops below freezing, frost builds, and refrigerant may not fully vaporize before returning to the compressor. Liquid slugging and coil icing are the classic results. Condenser fan speed and metering-device behavior are not directly driven by indoor airflow, and suction pressure typically falls rather than rises.

Question 6Heating, Ventilation and Air Conditioning (HVAC)

A gas furnace ignites, burns for about 90 seconds, and then the main burner shuts off while the indoor blower continues to run. The flame sensor is clean. Which component should be checked first?

  1. The limit switch
  2. The gas valve regulator
  3. The flame sensor circuit Correct answer.
  4. The condensate pump

Explanation

A short burn followed by shutdown points to the flame-proving circuit: the control board is not confirming flame and drops the gas valve for safety. Even a visually clean sensor can have a poor connection, cracked ceramic, or inadequate microamp signal. The limit switch would cut the burner on high temperature but is less consistent with a fixed short burn, and the regulator and condensate pump do not prove flame.

Question 7Heating, Ventilation and Air Conditioning (HVAC)

What is the primary purpose of a thermostatic expansion valve (TXV) in a direct-expansion system?

  1. To control the condenser subcooling at design conditions
  2. To maintain a constant evaporator superheat by modulating refrigerant flow Correct answer.
  3. To bypass hot gas during defrost cycles
  4. To hold a fixed suction pressure regardless of load

Explanation

A TXV senses evaporator outlet temperature and pressure and adjusts its opening to keep superheat near a set value as load changes. It does not hold a fixed suction pressure, which is the job of an evaporator pressure regulator, and it does not control condenser subcooling or serve as a defrost bypass.

Question 8Heating, Ventilation and Air Conditioning (HVAC)

A technician measures 12 °F of subcooling on a system with a fixed orifice metering device and a design target of 10 °F. What does this reading most likely suggest?

  1. The evaporator is starved of refrigerant
  2. The condenser airflow is excessive
  3. The system is overcharged with refrigerant Correct answer.
  4. The system is undercharged with refrigerant

Explanation

Subcooling rises when more liquid refrigerant backs up in the condenser, which is the signature of an overcharge. Undercharge would lower subcooling, excessive condenser airflow would also reduce it, and a starved evaporator shows high superheat rather than excess subcooling.

Question 9Heating, Ventilation and Air Conditioning (HVAC)

Which statement correctly describes the difference between a series and a parallel circuit as applied to HVAC control wiring?

  1. In a parallel circuit the total resistance increases as more branches are added
  2. In a series circuit the current has one path, so any open switch stops all loads in that loop Correct answer.
  3. In a parallel circuit an open branch interrupts current to every other branch
  4. In a series circuit each load receives the full source voltage independently

Explanation

A series loop provides a single current path, so one open device de-energizes everything in that loop, which is why safety switches are often wired in series with the control circuit. In parallel circuits, adding branches lowers total resistance, each branch sees full voltage, and one open branch does not stop the others.

Question 10Heating, Ventilation and Air Conditioning (HVAC)

A PSC condenser fan motor fails to start and hums. The capacitor is tested and reads open. What is the expected result of replacing it with a properly rated capacitor?

  1. The motor should start and run because the auxiliary winding regains its phase-shifted current Correct answer.
  2. The motor will run backward permanently because the capacitor sets rotation
  3. The motor will start but draw locked-rotor current continuously
  4. The motor will still fail because PSC motors do not use a capacitor

Explanation

A PSC motor needs the run capacitor to phase-shift current into the auxiliary winding, producing starting and running torque. Restoring a correct capacitor lets the motor start and run normally. PSC motors do require a capacitor, capacitor value does not set rotation direction, and a healthy motor drops to normal running current after startup.

Question 11Heating, Ventilation and Air Conditioning (HVAC)

In a building's variable air volume (VAV) system, a zone box with a reheat coil is at its minimum airflow setpoint while the zone is still too cold. What is the intended sequence of operation?

  1. The box damper is driven fully open to increase airflow
  2. The air-handling unit supply temperature setpoint is raised for all zones
  3. The reheat coil is energized to warm the minimum airflow delivered to the zone Correct answer.
  4. The supply fan is stopped until the zone warms naturally

Explanation

VAV boxes are limited to a minimum airflow to maintain ventilation and coil stability, so when a zone overcools at minimum flow the reheat coil adds heat to that airstream. Stopping the fan would lose ventilation, opening the damper would deliver more cold air, and raising supply temperature affects every zone rather than the one calling.

Question 12Heating, Ventilation and Air Conditioning (HVAC)

A water-cooled chiller's condenser water leaving temperature is 95 °F against a design of 85 °F, and the cooling tower fans are running at full speed. Which condition best explains this?

  1. The evaporator flow rate is above design
  2. The compressor is unloaded and rejecting less heat
  3. The chilled water supply temperature setpoint is too low
  4. The tower fill is fouled or the approach has degraded Correct answer.

Explanation

When tower fans are already at full speed and condenser water still runs hot, the tower is not achieving its design approach, which commonly results from fouled or scaled fill, poor water distribution, or airflow obstruction. Chilled water setpoint, compressor loading, and evaporator flow affect the load side rather than the tower's ability to cool condenser water.

Question 13Heating, Ventilation and Air Conditioning (HVAC)

Which device is installed in a commercial packaged rooftop unit to bring in the correct proportion of outdoor air for ventilation as occupancy changes?

  1. A smoke detector in the return plenum
  2. An economizer with a modulating damper and enthalpy or dry-bulb control Correct answer.
  3. A barometric relief damper
  4. A condensate overflow switch

Explanation

An economizer assembly modulates outdoor and return air dampers, and with enthalpy or dry-bulb control it also provides free cooling when outdoor conditions allow. Barometric relief relieves building pressure, smoke detectors provide life safety shutdown, and overflow switches protect against condensate damage, none of which proportion ventilation air.

Question 14Heating, Ventilation and Air Conditioning (HVAC)

A technician is checking refrigerant superheat at the evaporator outlet on a fixed-orifice system. Which two measurements are required?

  1. Liquid line temperature and discharge pressure
  2. Discharge pressure and condenser entering air temperature
  3. Suction line temperature and liquid line temperature
  4. Suction line temperature and suction pressure Correct answer.

Explanation

Superheat is the difference between the actual suction line temperature and the saturation temperature corresponding to suction pressure. Suction pressure gives the saturation temperature, and the suction line thermometer gives the actual temperature. Liquid line and discharge readings relate to subcooling and high-side analysis.

Question 15Heating, Ventilation and Air Conditioning (HVAC)

A hydronic baseboard heating system has some rooms that stay cold while the boiler maintains its setpoint. The circulator runs continuously and the boiler water temperature is normal. Which cause best fits these symptoms?

  1. A failed aquastat holding the boiler below setpoint
  2. A faulty expansion tank causing high system pressure
  3. Air-bound or unbalanced branch piping limiting flow to those rooms Correct answer.
  4. A clogged burner nozzle reducing firing rate

Explanation

If the boiler reaches temperature and the circulator runs, the problem is on the distribution side, such as trapped air or imbalanced branch flows that starve some zones. A bad aquastat would show low boiler temperature, a clogged nozzle would reduce firing and recovery, and expansion tank faults affect pressure rather than zone-by-zone heat delivery.

Question 16Heating, Ventilation and Air Conditioning (HVAC)

Which refrigerant safety classification indicates lower toxicity and no flame propagation under ASHRAE Standard 34?

  1. A3
  2. B1
  3. A1 Correct answer.
  4. B2

Explanation

Under ASHRAE Standard 34 the letter denotes toxicity class and the number denotes flammability, so A1 means lower toxicity and no flame propagation. A3 and B2 indicate flammable refrigerants, and B1 indicates higher toxicity with no flame propagation.

Question 17Heating, Ventilation and Air Conditioning (HVAC)

A technician must add refrigerant to a system that uses a blended refrigerant. Which practice is correct?

  1. Charge the blend as vapor to avoid slugging the compressor
  2. Vent the cylinder to atmosphere to verify the blend ratio
  3. Charge only the higher-boiling component to correct glide
  4. Charge the blend as a liquid from the cylinder to preserve its composition Correct answer.

Explanation

Zeotropic blends fractionate when vapor is drawn from the cylinder, so the components leave in different proportions and the remaining charge drifts from the intended composition. Charging as liquid keeps the blend intact, and for systems with a small charge the liquid is throttled into the low side. Venting refrigerant is both illegal in most jurisdictions and destructive to the blend.

Question 18Heating, Ventilation and Air Conditioning (HVAC)

What is the function of a building automation system's supply air temperature reset schedule?

  1. To reset the chilled water flow setpoint based on return air humidity
  2. To lower supply air temperature whenever the outdoor air damper opens
  3. To raise supply air temperature as zone loads decrease, saving reheat and fan energy Correct answer.
  4. To hold supply air temperature constant regardless of outdoor conditions

Explanation

Supply air temperature reset raises the discharge temperature when the building's cooling load falls, allowing compressors or chillers to unload and reducing reheat energy at the zone boxes. Holding temperature constant wastes energy at part load, and the other options confuse reset with damper or water-flow control.

Question 19Heating, Ventilation and Air Conditioning (HVAC)

A technician measures 0.5 in. w.c. total external static pressure on an air handler rated for 0.5 in. w.c. at design airflow, but measured airflow is well below design. Which explanation is most consistent?

  1. The static pressure reading was taken in the wrong units
  2. The filter and coil are both clean and unrestricted
  3. The blower is turning faster than its rated speed
  4. The duct system has high resistance, so the fan operates far out on its curve at reduced flow Correct answer.

Explanation

A fan's airflow depends on the system resistance it works against; a restrictive duct system moves the operating point up the pressure curve and reduces delivered cfm even when static pressure equals the rating. Higher blower speed would increase flow, and clean components would not explain a large airflow deficit.

Question 20Heating, Ventilation and Air Conditioning (HVAC)

Why is a trap required on the drain line of a condensing furnace or an air-conditioning evaporator coil?

  1. To increase the condensate flow rate to the floor drain
  2. To create a water seal that prevents air from being drawn or blown through the drain Correct answer.
  3. To provide a pressure relief point for the drain pan
  4. To filter particulates from the condensate before disposal

Explanation

A trap forms a water seal that blocks the cabinet pressure from pushing or pulling air through the drain opening, which would otherwise cause water to back up or escape. It does not accelerate drainage, filter water, or act as a pressure relief device.

Question 21Heating, Ventilation and Air Conditioning (HVAC)

A technician is commissioning a constant-volume system and must verify that the outdoor air quantity meets the design ventilation rate. Which measurement approach is appropriate?

  1. Record the fan motor amperage and convert it to cfm
  2. Read the filter pressure drop and divide by the design airflow
  3. Measure the return air temperature and assume the outdoor air fraction
  4. Measure the pressure drop across the outdoor air louver and apply its manufacturer's flow curve Correct answer.

Explanation

Airflow through a louver or an outdoor air intake can be determined from a measured pressure differential and the manufacturer's published flow characteristics. Temperature and filter readings do not quantify outdoor cfm, and motor amperage reflects power draw rather than airflow.

Question 22Heating, Ventilation and Air Conditioning (HVAC)

A heat pump in heating mode is operating with a reversing valve energized and the outdoor coil frosted. The defrost control initiates a defrost cycle. What happens to the reversing valve and indoor fan during a typical demand defrost?

  1. The reversing valve stays in heating mode and the indoor fan runs at high speed
  2. The compressor stops and the auxiliary heat alone serves the space
  3. The reversing valve shifts to cooling mode and the indoor fan is often disabled to avoid cold blow Correct answer.
  4. The outdoor fan runs at high speed while the indoor fan is disabled

Explanation

During defrost the system temporarily reverses to cooling mode so hot discharge gas melts frost from the outdoor coil, and the indoor fan is commonly stopped or the auxiliary heat is staged to prevent cold air delivery. The compressor keeps running, and the outdoor fan is usually stopped to speed frost melt.

Question 23Heating, Ventilation and Air Conditioning (HVAC)

A technician is evaluating a duct run and finds that a 90-degree hard elbow is used where a long-radius elbow or turning vanes would fit. What effect does the hard elbow most likely have?

  1. Reduced pressure loss because the air changes direction quickly
  2. Higher airflow at the branch takeoff with unchanged total pressure
  3. Increased pressure loss and turbulence compared with a gradual turn Correct answer.
  4. No measurable effect on system static pressure

Explanation

Abrupt direction changes create turbulence and separation, which raise the fitting's pressure loss coefficient and increase system static pressure. A long-radius elbow or turning vanes guide the air more smoothly and reduce that loss. The other options contradict the basic behavior of duct fittings.

Question 24Heating, Ventilation and Air Conditioning (HVAC)

A boiler room inspection reveals a spill switch installed at the draft hood of a naturally drafted appliance. What is the purpose of this device?

  1. To provide make-up air to the combustion chamber
  2. To measure the combustion efficiency of the appliance
  3. To regulate the manifold gas pressure at the burner
  4. To shut off the burner if flue products spill into the room Correct answer.

Explanation

A spill switch is a temperature-sensing safety device placed at the draft hood that opens the control circuit when hot flue gases escape into the space instead of venting. It does not regulate gas pressure, measure efficiency, or supply combustion air.

Question 25Heating, Ventilation and Air Conditioning (HVAC)

A system uses a fixed-orifice metering device, and the technician measures low suction pressure, high superheat, and normal subcooling. Which conclusion is most reasonable?

  1. The compressor valves are worn and bypassing
  2. The system is overcharged and flooding the evaporator
  3. The evaporator is starved, possibly from a restriction or low load-side airflow Correct answer.
  4. The condenser is rejecting too much heat

Explanation

Low suction pressure with high superheat means the coil is not receiving enough refrigerant to wet its surface, which can result from a restriction in the liquid line, a clogged orifice, or low airflow reducing load. Normal subcooling argues against an overcharge, and worn compressor valves usually show reduced head pressure with elevated suction pressure.

Question 26Heating, Ventilation and Air Conditioning (HVAC)

A technician is checking a heat pump in heating mode and finds the outdoor coil frosted while the defrost control is in its normal operating sequence. Which component is responsible for reversing the refrigerant flow during defrost?

  1. The defrost thermostat
  2. The outdoor fan motor
  3. The reversing valve solenoid Correct answer.
  4. The compressor contactor

Explanation

The reversing valve is the component that changes the direction of refrigerant flow, allowing the system to switch between heating and cooling modes. During defrost, the reversing valve is energized (or de-energized, depending on design) to put the unit back into cooling mode so hot discharge gas can melt frost on the outdoor coil. The defrost thermostat only senses temperature and initiates or terminates defrost; it does not reverse flow.

Question 27Heating, Ventilation and Air Conditioning (HVAC)

A technician is measuring airflow in a duct and obtains a velocity pressure of 0.25 in. w.c. using a Pitot tube. What is the approximate air velocity in feet per minute (fpm)?

  1. 2000 fpm Correct answer.
  2. 500 fpm
  3. 1000 fpm
  4. 4000 fpm

Explanation

The relationship between velocity pressure (VP) and velocity (V) in standard air is V = 4005 × √VP, where V is in feet per minute and VP is in inches of water column. For VP = 0.25 in. w.c., √0.25 = 0.5, so V = 4005 × 0.5 ≈ 2000 fpm. This formula is widely used in duct traverse calculations.

Question 28Heating, Ventilation and Air Conditioning (HVAC)

Which type of fan is best suited for a residential furnace application where the system must overcome moderate static pressure and operate quietly?

  1. Tubeaxial fan
  2. Axial propeller fan
  3. Forward-curved centrifugal fan Correct answer.
  4. Backward-inclined centrifugal fan

Explanation

Forward-curved centrifugal fans are commonly used in residential furnaces and air handlers because they can move large volumes of air at relatively low noise levels and handle moderate static pressures. Backward-inclined fans are more efficient for higher static pressures and larger commercial systems, while axial and tubeaxial fans are typically used for low-pressure, high-volume applications such as exhaust or condenser airflow.

Question 29Heating, Ventilation and Air Conditioning (HVAC)

A technician is troubleshooting a hot surface ignition (HSI) furnace that sometimes fails to light. The igniter glows but the main burner does not ignite, and the technician observes that the gas valve opens briefly. Which component is most likely causing the intermittent failure?

  1. The flame sensor Correct answer.
  2. The control board
  3. The igniter
  4. The gas valve

Explanation

If the igniter glows and the gas valve opens but the burner does not stay lit, the flame sensor may not be proving flame, causing the control to shut the gas valve. A dirty or failing flame sensor is a common cause of intermittent ignition failures. The igniter is already glowing, so it is not the likely cause; the gas valve is opening, so it is also less likely; and the control board is opening the valve, indicating it is functioning at least partially.

Question 30Heating, Ventilation and Air Conditioning (HVAC)

Which refrigerant is classified as A1 by ASHRAE Standard 34 and is commonly used in new residential air-conditioning systems?

  1. R-22
  2. R-134a
  3. R-410A Correct answer.
  4. R-123

Explanation

R-410A is an A1 refrigerant, meaning it has low toxicity and no flame propagation, and it has been the standard replacement for R-22 in residential air conditioning. R-22 is also A1 but is being phased out due to ozone depletion; R-134a is A1 but used more in refrigeration and automotive; R-123 is B1, indicating higher toxicity.

Question 31Heating, Ventilation and Air Conditioning (HVAC)

A technician is brazing a refrigerant line and notices that the copper tubing has turned black inside the joint. What is the most likely cause and the proper preventive measure?

  1. Oxygen was not purged; purge with nitrogen during brazing. Correct answer.
  2. The tubing was contaminated with oil; clean with solvent.
  3. Flux was not applied; apply flux to the joint.
  4. The tubing was overheated; use a smaller torch tip.

Explanation

When copper is heated in the presence of oxygen during brazing, copper oxide forms as a black scale on the inside of the tubing. This scale can flake off and damage compressors or metering devices. The standard practice is to purge the system with dry nitrogen while brazing to displace oxygen and prevent oxidation.

Question 32Heating, Ventilation and Air Conditioning (HVAC)

A technician is testing a compressor and finds that the resistance between the common terminal and the start terminal is 2 ohms, and between common and run is 1 ohm. What should the resistance between start and run be?

  1. 2 ohms
  2. 4 ohms
  3. 3 ohms Correct answer.
  4. 1 ohm

Explanation

In a single-phase compressor motor, the start winding and run winding are connected at the common terminal. The resistance measured between start and run is the sum of the start winding resistance and the run winding resistance: 2 + 1 = 3 ohms. This is a standard check for winding integrity.

Question 33Heating, Ventilation and Air Conditioning (HVAC)

A technician is adjusting the airflow in a variable air volume (VAV) system and needs to set the minimum airflow for a zone. Which factor primarily determines the minimum airflow setpoint?

  1. The heating load and ventilation requirements Correct answer.
  2. The cooling load at design conditions
  3. The outdoor air temperature
  4. The static pressure at the supply fan

Explanation

In VAV systems, the minimum airflow setpoint is typically determined by the need to maintain adequate ventilation (outdoor air) and to satisfy heating loads, especially when reheat is used. Cooling load determines maximum airflow, while static pressure and outdoor temperature are not direct determinants of the minimum setpoint.

Question 34Heating, Ventilation and Air Conditioning (HVAC)

Which type of boiler is required to have a low-water cutoff installed according to typical mechanical codes?

  1. Condensing boiler
  2. Hot water boiler
  3. Electric boiler
  4. Steam boiler Correct answer.

Explanation

Steam boilers require a low-water cutoff because a loss of water level can lead to overheating and a dangerous condition. Hot water boilers may also require low-water cutoffs in some applications, but it is a mandatory safety requirement for steam boilers. Condensing and electric boilers may have other safety controls but the low-water cutoff is specifically critical for steam boilers.

Question 35Heating, Ventilation and Air Conditioning (HVAC)

A technician is commissioning a hydronic system and observes that the pump is cavitating. Which of the following is the most likely cause?

  1. Excessive system pressure
  2. Pump running at too low a speed
  3. Insufficient net positive suction head available (NPSHA) Correct answer.
  4. Air in the expansion tank

Explanation

Cavitation occurs when the pressure at the pump suction drops below the vapor pressure of the fluid, causing vapor bubbles to form and collapse. This is primarily caused by insufficient net positive suction head available (NPSHA). Excessive system pressure, air in the expansion tank, and low pump speed do not directly cause cavitation.

Question 36Heating, Ventilation and Air Conditioning (HVAC)

What is the primary function of an economizer in a commercial HVAC system?

  1. To reduce the load on the chiller by precooling supply air
  2. To control the humidity of the supply air
  3. To recover heat from exhaust air
  4. To use outdoor air for cooling when conditions are favorable Correct answer.

Explanation

An economizer uses outdoor air to provide free cooling when the outdoor air temperature and humidity are suitable, reducing the need for mechanical cooling. Heat recovery and humidity control are separate functions, and precooling supply air is not the primary purpose of an economizer.

Question 37Heating, Ventilation and Air Conditioning (HVAC)

A technician is measuring the temperature difference across an evaporator coil and finds it to be 8 °F lower than the design ΔT. Which of the following is the most likely cause?

  1. Low refrigerant charge Correct answer.
  2. Restricted airflow
  3. High outdoor ambient temperature
  4. Excessive refrigerant charge

Explanation

A lower-than-design temperature difference across the evaporator coil often indicates a lack of cooling capacity, which can be caused by a low refrigerant charge. Restricted airflow would typically increase the ΔT because the air is in contact with the coil longer, and high outdoor ambient would reduce capacity but not necessarily lower ΔT directly. Excessive charge would increase ΔT.

Question 38Heating, Ventilation and Air Conditioning (HVAC)

Which type of thermostat is most suitable for a line-voltage electric baseboard heater?

  1. A line-voltage thermostat Correct answer.
  2. A low-voltage thermostat
  3. A wireless thermostat
  4. A programmable communicating thermostat

Explanation

Electric baseboard heaters operate at line voltage (120/240 V) and require a line-voltage thermostat that can directly switch the higher current. Low-voltage and communicating thermostats are designed for control circuits and would require a relay or contactor to handle the load.

Question 39Heating, Ventilation and Air Conditioning (HVAC)

A technician is calculating the sensible heat gain of a room and knows the airflow is 400 cfm and the temperature difference is 20 °F. What is the sensible heat gain in Btu/h?

  1. 10,800 Btu/h Correct answer.
  2. 9,600 Btu/h
  3. 12,000 Btu/h
  4. 8,640 Btu/h

Explanation

The sensible heat equation is Qs = 1.08 × cfm × ΔT. Substituting the values: Qs = 1.08 × 400 × 20 = 8,640 Btu/h. This formula is standard for estimating sensible heat in air.

Question 40Heating, Ventilation and Air Conditioning (HVAC)

A technician is troubleshooting a ductless mini-split system that is not cooling adequately. The indoor unit is running, but the outdoor unit fan is not turning. Which component is most likely faulty?

  1. The thermistor
  2. The reversing valve
  3. The indoor blower motor
  4. The outdoor fan capacitor Correct answer.

Explanation

If the outdoor fan is not turning while the compressor is running, a common cause is a failed fan capacitor. The indoor blower is running, so it is not the issue. The reversing valve affects heating/cooling mode but would not stop the fan, and a thermistor would affect temperature sensing, not fan operation directly.

Question 41Heating, Ventilation and Air Conditioning (HVAC)

What is the purpose of a barometric damper in a natural-draft gas furnace venting system?

  1. To increase the flue gas velocity
  2. To provide combustion air to the burner
  3. To prevent backdraft of flue gases into the space
  4. To regulate the draft and maintain a stable negative pressure in the vent Correct answer.

Explanation

A barometric damper is a draft control device that opens or closes to regulate the draft in the vent connector, maintaining a consistent negative pressure and preventing excessive draft that could affect combustion. It does not prevent backdraft (that is the role of a draft hood or spill switch), increase flue velocity, or supply combustion air.

Question 42Heating, Ventilation and Air Conditioning (HVAC)

A technician is checking a refrigerant circuit and finds frost on the liquid line after the metering device. Which of the following is the most likely cause?

  1. Low refrigerant charge
  2. A faulty compressor
  3. High superheat at the evaporator
  4. A restriction in the liquid line or metering device Correct answer.

Explanation

Frost on the liquid line after the metering device indicates a pressure drop due to a restriction, which causes the refrigerant to flash and cool rapidly. Low charge would cause frost at the evaporator inlet, not the liquid line. High superheat and compressor issues would not cause liquid line frost.

Question 43Heating, Ventilation and Air Conditioning (HVAC)

Which of the following is a common symptom of a failed run capacitor in a permanent split capacitor (PSC) condenser fan motor?

  1. The motor runs at full speed
  2. The motor runs backwards
  3. The motor hums and does not start Correct answer.
  4. The motor overheats and trips

Explanation

A failed run capacitor in a PSC motor prevents the motor from developing starting torque, causing it to hum without rotating. The motor may eventually overheat and trip on overload, but the immediate symptom is humming and failure to start. Running at full speed or backwards would not be typical of a failed capacitor.

Question 44Heating, Ventilation and Air Conditioning (HVAC)

A technician is evaluating a building's ventilation system and needs to determine the required outdoor air intake. Which standard provides the minimum ventilation rates for commercial buildings?

  1. ASHRAE Standard 90.1
  2. ASHRAE Standard 15
  3. ASHRAE Standard 55
  4. ASHRAE Standard 62.1 Correct answer.

Explanation

ASHRAE Standard 62.1 specifies minimum ventilation rates and indoor air quality requirements for commercial buildings. Standard 90.1 covers energy efficiency, Standard 55 addresses thermal comfort, and Standard 15 covers refrigerant safety.

Question 45Heating, Ventilation and Air Conditioning (HVAC)

A technician is working on a high-efficiency condensing furnace and notices that the condensate trap is dry. What is the most likely consequence?

  1. The furnace will operate more efficiently
  2. Flue gases can escape into the conditioned space Correct answer.
  3. The furnace will short-cycle
  4. The burner will not light

Explanation

The condensate trap creates a water seal that prevents flue gases from escaping through the condensate drain into the conditioned space. If the trap is dry, flue gases can bypass the heat exchanger and enter the space, posing a safety hazard. Efficiency, short-cycling, and ignition are not directly affected by a dry trap.

Question 46Heating, Ventilation and Air Conditioning (HVAC)

A technician is measuring the static pressure drop across a pleated air filter and finds it to be 0.35 in. w.c. The filter is rated for 0.20 in. w.c. at 400 fpm. What is the most appropriate action?

  1. Increase the fan speed
  2. Clean the filter
  3. Replace the filter Correct answer.
  4. Reduce the airflow

Explanation

A static pressure drop significantly higher than the filter's rated value indicates that the filter is loaded with dirt and should be replaced. Increasing fan speed or reducing airflow does not address the root cause, and cleaning is not typically effective for pleated filters.

Question 47Heating, Ventilation and Air Conditioning (HVAC)

Which of the following best describes the function of a desuperheater in a refrigeration system?

  1. To reduce the pressure of the refrigerant
  2. To increase the subcooling of the liquid refrigerant
  3. To control the evaporator superheat
  4. To remove superheat from hot discharge gas for useful heating Correct answer.

Explanation

A desuperheater is a heat exchanger that recovers heat from the superheated discharge gas, typically to produce domestic hot water or for space heating. It does not reduce pressure, increase subcooling, or control evaporator superheat.

Question 48Heating, Ventilation and Air Conditioning (HVAC)

A technician is troubleshooting a hydronic heating system where the boiler is operating but some radiators remain cold. The circulator is running and the boiler aquastat is satisfied. Which of the following is the most likely cause?

  1. A faulty aquastat
  2. A failed circulator
  3. Low boiler water pressure
  4. Air trapped in the radiators Correct answer.

Explanation

Air trapped in radiators prevents hot water from filling them, causing cold sections. The circulator is running and the aquastat is satisfied, so those are not the issue. Low boiler pressure could cause other problems but would typically affect the entire system, not individual radiators.

Question 49Heating, Ventilation and Air Conditioning (HVAC)

What is the primary purpose of a reversing valve in a heat pump?

  1. To regulate the compressor capacity
  2. To control the refrigerant charge
  3. To defrost the outdoor coil
  4. To change the direction of refrigerant flow for heating or cooling Correct answer.

Explanation

The reversing valve switches the refrigerant flow direction so that the indoor coil can act as either an evaporator (cooling) or a condenser (heating). It does not control charge, defrost, or compressor capacity.

Question 50Heating, Ventilation and Air Conditioning (HVAC)

A technician is calculating the total external static pressure of an air handler and obtains a supply static of 0.30 in. w.c. and a return static of -0.20 in. w.c. What is the total external static pressure?

  1. 0.30 in. w.c.
  2. 0.50 in. w.c. Correct answer.
  3. 0.20 in. w.c.
  4. 0.10 in. w.c.

Explanation

Total external static pressure is the sum of the absolute values of the supply and return static pressures: 0.30 + 0.20 = 0.50 in. w.c. The return static is negative because it is below atmospheric pressure, but the magnitude is used in the sum.