I85PREP
⌘ K
OK HVAC Contractor study materials
OK HVAC Contractor study resources

OK HVAC Contractor practice questions

One question at a time. Build your OK HVAC Contractor understanding.

First 50 of 200 questions. Each one lists the choices, the correct answer, and an explanation. The checker at the top of this page saves progress in this browser when it is available.

Continue in the practice tool above to load more questions, 50 at a time.

Question 1Mechanical (HVAC) trade licensing and inspection

A residential forced-air furnace is installed in a closet that also contains the air-handling unit. Which condition must the return-air path meet to prevent negative pressure in the combustion appliance zone?

  1. The closet must have a louvered door sized at one square inch per 1,000 Btu/h of input
  2. The return air must pass through a filter grille rated for the full blower airflow
  3. The closet must be sealed and provided with a barometric relief damper
  4. The return air must be ducted directly to the unit so the closet is not used as a return plenum Correct answer.

Explanation

When a furnace shares a space with the air handler, using that space as a return plenum depressurizes the combustion air zone and can back-draft the flue. Ducting return air directly to the unit keeps the closet at neutral or positive pressure relative to the combustion air source.

Question 2Mechanical (HVAC) trade licensing and inspection

What is the minimum required clearance from a single-wall metal vent connector to combustible material, as typically specified by vent manufacturer listings and mechanical code?

  1. 12 inches
  2. 2 inches
  3. 1 inch
  4. 6 inches Correct answer.

Explanation

Single-wall metal vent connectors require a 6-inch clearance to combustibles under standard listings and mechanical code provisions. Double-wall (Type B) vents reduce that clearance to 1 inch, which is why the connector type matters.

Question 3Mechanical (HVAC) trade licensing and inspection

A technician measures 0.50 in. w.c. total external static pressure on a system whose blower performance table shows 0.50 in. w.c. at the design airflow. The measured airflow is well below design. Which condition best explains this result?

  1. The refrigerant charge is low, reducing the evaporator load
  2. The static pressure reading was taken across the wrong ports or with the blower at a lower speed tap Correct answer.
  3. The system is operating exactly at design airflow because static pressure matches the table
  4. The filter is oversized and reducing the pressure drop below design

Explanation

Static pressure alone does not confirm airflow; the reading must be taken with the blower on the correct speed tap and at the correct ports (supply and return). A mismatch in tap or measurement location makes the table comparison invalid.

Question 4Mechanical (HVAC) trade licensing and inspection

Which gas piping practice is required for a newly installed natural gas furnace connector in a residential basement?

  1. Omit the drip leg because natural gas is dry and contains no moisture
  2. Use a flexible connector longer than 6 feet to allow for vibration isolation
  3. Install a manual shutoff valve within 6 feet of the appliance and upstream of the union Correct answer.
  4. Install the union upstream of the shutoff valve for easier servicing

Explanation

A manual shutoff valve must be accessible and located upstream of the union and within the specified distance of the appliance. This arrangement lets a technician isolate and disconnect the appliance safely.

Question 5Mechanical (HVAC) trade licensing and inspection

During a cooling performance check, the measured temperature split across the evaporator is 12°F, the return wet-bulb is 63°F, and the airflow is at design. What is the most appropriate conclusion?

  1. The evaporator coil is frozen and must be thawed before further testing
  2. The system is operating within a normal range for the measured entering wet-bulb Correct answer.
  3. The system is overcharged and the charge should be reduced immediately
  4. The blower is set too high and airflow should be reduced

Explanation

A 12°F temperature split at a 63°F entering wet-bulb with design airflow is consistent with normal operation. Temperature split alone is not a charge indicator; it must be compared with the manufacturer's performance data for the measured conditions.

Question 6Mechanical (HVAC) trade licensing and inspection

What is the purpose of a trap primer on a condensate drain serving a fan coil unit?

  1. To filter debris from the condensate before it enters the drain
  2. To increase the drain slope for better drainage
  3. To prevent freezing of the condensate line in cold weather
  4. To maintain the water seal in the condensate trap during dry periods Correct answer.

Explanation

A trap primer adds a small amount of water to keep the condensate trap seal intact when the unit is not producing condensate. Without the seal, sewer gas or air can be drawn into the airstream.

Question 7Mechanical (HVAC) trade licensing and inspection

A heat pump in heating mode has a suction line that is cold and sweating at the outdoor unit, and the indoor coil is not warming the space. Which component is most likely failed?

  1. The reversing valve is stuck in the cooling position Correct answer.
  2. The outdoor fan motor has failed
  3. The indoor blower is running at high speed
  4. The defrost control board is in defrost mode

Explanation

A cold, sweating suction line at the outdoor unit during heating suggests the outdoor coil is acting as the evaporator, which occurs when the reversing valve remains in cooling position. The system then removes heat from the space instead of adding it.

Question 8Mechanical (HVAC) trade licensing and inspection

Which duct sealing method is acceptable for a supply duct joint in a conditioned space where the duct will be enclosed before inspection?

  1. Seal the joint with cloth-backed duct tape only
  2. Apply mastic or listed tape to the joint and allow it to cure before concealment Correct answer.
  3. Seal only the joints that are visible from the access panel
  4. Rely on the slip-and-drive connection alone because it is mechanically fastened

Explanation

Duct joints must be sealed with an approved method such as mastic or listed tape, and the seal must be completed and inspected before the duct is concealed. Mechanical fastening alone does not provide an airtight seal.

Question 9Mechanical (HVAC) trade licensing and inspection

A contractor is sizing a return grille for a system moving 1,200 cfm with a desired face velocity of 500 fpm. What free area is required?

  1. 0.6 square feet
  2. 2.4 square feet Correct answer.
  3. 1.2 square feet
  4. 4.8 square feet

Explanation

Free area equals airflow divided by face velocity: 1,200 cfm divided by 500 fpm equals 2.4 square feet. The grille must be selected so its published free area meets or exceeds this value.

Question 10Mechanical (HVAC) trade licensing and inspection

What is the primary function of a barometric draft diverter on a natural draft gas furnace?

  1. To prevent flue gases from entering the room during normal operation
  2. To provide combustion air to the burner assembly
  3. To increase the flue gas temperature for better draft
  4. To stabilize draft at the flue collar by relieving excess draft to the room Correct answer.

Explanation

A barometric draft diverter opens to spill excess draft and maintains a relatively constant negative pressure at the appliance flue collar. This protects the burner from variable chimney draft conditions.

Question 11Mechanical (HVAC) trade licensing and inspection

An inspector finds a flexible duct in a crawlspace with a 90-degree bend and no support within 4 feet of the turn. Which correction is required?

  1. Increase the bend radius by adding an elbow fitting
  2. Remove the bend and run the duct straight through the joist space
  3. Wrap the bend with additional insulation to prevent kinking
  4. Support the duct at intervals not exceeding the listed spacing and within 4 feet of the bend Correct answer.

Explanation

Flexible duct must be supported at the manufacturer's listed intervals and within a specified distance of fittings and bends to prevent sagging and kinking. Correct support maintains the duct's cross-section and airflow.

Question 12Mechanical (HVAC) trade licensing and inspection

Which refrigerant handling practice is required when a technician opens a sealed system to replace a compressor?

  1. Vent the refrigerant slowly to the atmosphere to avoid oil slugging
  2. Recover the refrigerant into a certified recovery cylinder before opening the system Correct answer.
  3. Release the charge into the return air stream for dilution
  4. Store the refrigerant in an unmarked container for later reuse

Explanation

Refrigerant must be recovered using certified equipment and stored in a properly marked recovery cylinder before a sealed system is opened. Venting refrigerant to the atmosphere is prohibited by federal regulation.

Question 13Mechanical (HVAC) trade licensing and inspection

A gas furnace has a steady-state temperature rise of 55°F, but the manufacturer's rating plate specifies a range of 35°F to 65°F. What does this indicate?

  1. The heat exchanger is cracked and must be replaced
  2. The blower is moving too much air and the speed should be reduced
  3. The furnace is overfired and the gas valve must be derated
  4. The temperature rise is within the listed range and airflow is acceptable Correct answer.

Explanation

The measured rise of 55°F falls inside the manufacturer's 35°F to 65°F range, which indicates acceptable airflow and firing rate. Temperature rise outside the range would point to airflow or gas input problems.

Question 14Mechanical (HVAC) trade licensing and inspection

Which condition requires a dedicated combustion air opening when a Category I furnace is installed in a tightly constructed utility room?

  1. The room has a louvered door to the hallway
  2. The furnace input is less than 50,000 Btu/h
  3. The furnace has a draft diverter installed
  4. The room volume is insufficient to provide the required combustion air per the code method used Correct answer.

Explanation

When the confined space volume cannot supply the required combustion and dilution air, dedicated openings or ducts must be provided. The code's volume calculation determines whether the room qualifies as confined.

Question 15Mechanical (HVAC) trade licensing and inspection

What is the correct orientation for a condensate drain trap on a draw-through evaporator coil?

  1. Install the trap on the leaving-air side of the coil with no depth requirement
  2. Install the trap only when the coil is above the drain connection
  3. Install the trap level with the drain pan outlet to avoid air binding
  4. Install the trap with a depth greater than the fan static pressure and slope the drain to a suitable disposal point Correct answer.

Explanation

A draw-through coil operates under negative pressure, so the trap depth must exceed the fan static pressure to hold the water seal. The drain must also slope continuously to disposal.

Question 16Mechanical (HVAC) trade licensing and inspection

A technician is verifying a gas furnace ignition sequence. After the thermostat calls for heat, what is the correct order of events?

  1. Pressure switch proves, gas valve opens, inducer starts, igniter energizes
  2. Inducer starts, pressure switch proves, igniter energizes, gas valve opens, flame sensed Correct answer.
  3. Gas valve opens, igniter energizes, inducer starts, flame sensed
  4. Igniter energizes, gas valve opens, inducer starts, pressure switch proves

Explanation

In a induced-draft furnace, the inducer must establish draft and prove the pressure switch before ignition and gas valve opening. The flame sensor then confirms ignition and keeps the valve open.

Question 17Mechanical (HVAC) trade licensing and inspection

Which factor most directly determines the required size of a return-air duct for a given airflow?

  1. The number of supply registers served
  2. The furnace flue diameter
  3. The allowable pressure drop and resulting velocity through the duct Correct answer.
  4. The color of the duct insulation jacket

Explanation

Return duct sizing is governed by the allowable friction loss and velocity for the design airflow, which sets the duct cross-section. Insulation color and register count do not determine duct size.

Question 18Mechanical (HVAC) trade licensing and inspection

A heat pump auxiliary heat strip is energized during a defrost cycle. What is the purpose of this operation?

  1. To increase refrigerant subcooling at the outdoor unit
  2. To raise the outdoor coil temperature to speed defrost
  3. To prevent the reversing valve from shifting
  4. To temper the cool air delivered indoors while the system is in defrost Correct answer.

Explanation

During defrost the heat pump temporarily runs in cooling mode, so auxiliary heat temperates the supply air to avoid a cold draft. It does not affect the outdoor coil defrost rate or refrigerant subcooling.

Question 19Mechanical (HVAC) trade licensing and inspection

What is the minimum size of a working platform required at a permanently installed appliance located in an attic with limited access?

  1. A platform at least 18 inches deep and 18 inches wide
  2. No platform is required if the appliance is reachable from a ladder
  3. A platform at least 12 inches deep and 24 inches wide
  4. A platform at least 30 inches deep and 30 inches wide in front of the appliance Correct answer.

Explanation

Mechanical code requires a level working platform of at least 30 inches by 30 inches in front of appliances that require service access. This allows safe servicing without standing on joists or a ladder.

Question 20Mechanical (HVAC) trade licensing and inspection

A ductless mini-split indoor unit is mounted on an exterior wall in a bedroom. Which installation detail most affects condensate disposal reliability?

  1. Height of the indoor unit above the floor
  2. Pitch of the condensate drain line continuously downward toward the exterior Correct answer.
  3. Length of the refrigerant line set
  4. Refrigerant charge adjustment by weight

Explanation

Condensate must drain by gravity, so the drain line needs continuous downward pitch to the exterior and must not trap water. Line set length, mounting height, and charge method do not control condensate flow.

Question 21Mechanical (HVAC) trade licensing and inspection

A technician finds a gas furnace flue with a blocked chimney and elevated carbon monoxide in the supply air. Which immediate action is required?

  1. Adjust the gas manifold pressure to reduce carbon monoxide production
  2. Increase the blower speed to dilute the carbon monoxide
  3. Open a window near the furnace and leave the unit running
  4. Shut off the appliance, tag it out, and correct the venting before returning it to service Correct answer.

Explanation

A blocked flue with elevated carbon monoxide is an immediate safety hazard, so the appliance must be shut down and tagged out until venting is repaired. Dilution or gas pressure adjustment does not correct the underlying venting failure.

Question 22Mechanical (HVAC) trade licensing and inspection

Which method correctly verifies that a condensate pump discharge does not create a cross-connection with a sanitary drain?

  1. Discharge through an air gap or approved air-gap fitting above the receptor Correct answer.
  2. Discharge directly into the drain with a hose clamped to the pipe
  3. Discharge into a floor drain below the flood-level rim
  4. Discharge into the soil stack through a wye fitting

Explanation

Condensate must be discharged through an air gap or approved air-gap fitting to prevent backflow and cross-connection with the sanitary system. Direct connections to drains or stacks are not permitted.

Question 23Mechanical (HVAC) trade licensing and inspection

What is the required clearance between a Type B gas vent and combustible material?

  1. 12 inches
  2. 2 inches
  3. 6 inches
  4. 1 inch Correct answer.

Explanation

Listed Type B double-wall gas vents require a 1-inch clearance to combustibles. Single-wall connectors require 6 inches, so the vent type determines the clearance.

Question 24Mechanical (HVAC) trade licensing and inspection

A contractor is installing a furnace in a garage. Which requirement applies to the ignition source and burner location?

  1. The furnace must be installed on a platform at least 6 inches above the floor
  2. The furnace may be installed at any height if a carbon monoxide alarm is present
  3. The furnace may be installed at floor level if the garage door is sealed
  4. Burners and ignition sources must be at least 18 inches above the floor unless the unit is listed for garage installation Correct answer.

Explanation

In garages, ignition sources must be elevated at least 18 inches above the floor to reduce the risk of igniting heavier-than-air vapors, unless the appliance is specifically listed for the location. A CO alarm does not substitute for this protection.

Question 25Mechanical (HVAC) trade licensing and inspection

Which test confirms the integrity of a newly installed gas piping system before the appliance is connected?

  1. A combustion analysis at the flue after startup
  2. A leak detection test with the burner operating
  3. A pressure test with the gas valve isolated, held for the code-specified duration Correct answer.
  4. A soap bubble test on the union only

Explanation

New gas piping must be pressure-tested with the appliance isolated for the duration and pressure specified by code. Soap testing or operating the burner does not verify the whole piping system.

Question 26Mechanical (HVAC) trade licensing and inspection

A technician is brazing copper refrigerant lines and purging with dry nitrogen. What is the primary reason for the nitrogen flow during brazing?

  1. To prevent oxidation and scale formation inside the tubing Correct answer.
  2. To pressurize the line for a leak check
  3. To cool the joint so the filler metal does not flow
  4. To carry refrigerant oil back to the compressor

Explanation

Heating copper in the presence of air forms copper oxide scale on the inside of the tubing, and that scale later migrates through the system and can plug metering devices and damage the compressor. A slow nitrogen purge displaces oxygen so the interior stays clean during brazing.

Question 27Mechanical (HVAC) trade licensing and inspection

A condensing furnace is installed with a PVC flue that slopes back toward the furnace. Why is this slope required?

  1. To drain condensate back to the furnace's condensate collection point Correct answer.
  2. To keep the inducer motor from overheating
  3. To reduce the visible plume at the termination
  4. To allow combustion air to enter the flue during standby

Explanation

Condensing appliances produce acidic flue condensate continuously, and the flue must slope so that liquid drains toward a collection point rather than pooling or running out the termination. Pooling in low spots can block the flue and damage the pipe or appliance.

Question 28Mechanical (HVAC) trade licensing and inspection

A residential split system has a liquid line filter-drier installed with the arrow pointing toward the outdoor unit on a system where the outdoor unit is the condenser. What is the result?

  1. The system charge becomes permanently overcharged
  2. Refrigerant flows through the drier in the reverse of its intended direction Correct answer.
  3. The compressor suction pressure rises above design
  4. The drier acts as an additional metering device

Explanation

Filter-driers are directional; the arrow must point in the direction of normal refrigerant flow. Installing it backward means the internal screen and desiccant are loaded from the wrong side, which can release desiccant fines downstream and reduce filtration effectiveness.

Question 29Mechanical (HVAC) trade licensing and inspection

A technician is checking a PSC blower motor and finds the run capacitor bulged and reading far below its rated microfarads. What is the most likely operating symptom before replacement?

  1. The motor hums, runs slowly, and draws high current Correct answer.
  2. The motor runs at full speed but draws less current
  3. The motor reverses rotation direction
  4. The motor runs normally but the thermostat loses power

Explanation

The run capacitor provides phase shift and improves torque on a PSC motor. A failed capacitor leaves the motor with little starting and running torque, so it hums, turns slowly, and draws elevated current that can trip overloads or damage the winding.

Question 30Mechanical (HVAC) trade licensing and inspection

An inspector observes a gas-fired appliance connected to a Type B vent that terminates 2 feet above a flat roof within 10 feet of a parapet wall. Which concern applies?

  1. The vent must be increased one pipe size
  2. The vent must be converted to a power vent
  3. The vent is too tall for the appliance category
  4. The termination is below the required height relative to the parapet Correct answer.

Explanation

Type B vent terminations must clear the roof and any obstruction within the specified horizontal distance, including parapet walls, so flue gas does not recirculate or enter the building. A termination that is lower than the parapet within that distance fails the clearance rule.

Question 31Mechanical (HVAC) trade licensing and inspection

A technician measures 12°F superheat at the suction service valve of a fixed-orifice residential system operating at design conditions with a matching indoor coil. What does this reading most directly indicate?

  1. The condenser is rejecting too much heat
  2. The evaporator is flooding and liquid may reach the compressor
  3. The compressor valves are leaking
  4. The system is undercharged or the evaporator is starved Correct answer.

Explanation

Superheat is the temperature of vapor above saturation at the measuring point, and it reflects how much of the evaporator is being used. Excessive superheat means the refrigerant is fully boiled off early, indicating low charge or a restricted metering device.

Question 32Mechanical (HVAC) trade licensing and inspection

A furnace is installed in a closet that also contains a clothes washer and dryer. Which requirement most directly governs the installation?

  1. The furnace must be converted to propane
  2. The closet must have a floor drain
  3. The dryer vent must terminate above the furnace flue
  4. The furnace must be listed for installation in a combined appliance space with those appliances Correct answer.

Explanation

Appliances installed in the same space must be evaluated together for combustion air, clearances, and listing restrictions. A furnace listed for closet installation may still be prohibited where other fuel-burning or moisture-producing appliances share the space unless the listing and code allow it.

Question 33Mechanical (HVAC) trade licensing and inspection

During a startup check, a technician finds the supply-air temperature rise across a gas furnace is 30°F while the rating plate specifies 40°F to 70°F. Which condition is most consistent with this reading?

  1. A restricted return-air path
  2. Excessive airflow through the heat exchanger Correct answer.
  3. A manifold pressure set too high
  4. A cracked heat exchanger

Explanation

Temperature rise equals heat input divided by airflow, so a low rise with normal firing rate points to too much air moving across the heat exchanger. Common causes include an oversized blower speed or a duct system with less resistance than expected.

Question 34Mechanical (HVAC) trade licensing and inspection

A technician is evacuating a residential split system and the micron gauge stalls at about 1,500 microns and rises slowly after the pump is isolated. What is the best interpretation?

  1. Moisture or a small leak is preventing the vacuum from holding Correct answer.
  2. The micron gauge is reading in the wrong units
  3. The vacuum pump oil is too clean
  4. The system has reached a deep vacuum and is ready to charge

Explanation

A decay test isolates the pump and watches the vacuum; a slow rise indicates either a leak or outgassing from moisture in the system. Reaching a deep vacuum and holding it is the pass criterion, and a stall with a rise fails that test.

Question 35Mechanical (HVAC) trade licensing and inspection

A technician is charging a TXV system in cooling mode. Which measurement pair is used to verify charge?

  1. Suction superheat and liquid-line subcooling Correct answer.
  2. Return-air wet-bulb and supply-air dry-bulb only
  3. Suction pressure and outdoor ambient only
  4. Liquid-line pressure and compressor current only

Explanation

TXV systems are charged by subcooling because the valve modulates to hold superheat, so subcooling reflects the liquid seal in the condenser. Superheat is still checked to confirm the valve is controlling properly.

Question 36Mechanical (HVAC) trade licensing and inspection

An inspector finds a flexible duct connector used to join two sections of rigid rectangular supply trunk in a conditioned ceiling plenum. Which evaluation is correct?

  1. It is acceptable if the connector is listed for 1-hour fire resistance
  2. It is acceptable if wrapped with mastic
  3. It is acceptable because the plenum is conditioned
  4. It is not acceptable because flexible connectors are limited to connections at the appliance or terminal Correct answer.

Explanation

Flexible duct connectors are listed for joining duct to equipment or terminals, not as a substitute for rigid trunk sections. Using one as a trunk splice can sag, restrict airflow, and violate the listing and duct construction rules.

Question 37Mechanical (HVAC) trade licensing and inspection

A technician is diagnosing a heat pump that runs continuously in heating mode and never satisfies the thermostat, with outdoor temperature near 35°F. Auxiliary heat is energized and supply air is only slightly warm. Which check is most appropriate first?

  1. Increase the refrigerant charge to the nameplate value
  2. Verify the reversing valve is energized in heating mode Correct answer.
  3. Disable the auxiliary heat to raise supply temperature
  4. Check the outdoor fan blade pitch

Explanation

On most heat pumps the reversing valve solenoid is energized in cooling and de-energized in heating, so a valve stuck in cooling or a miswired solenoid will produce low heating capacity. Confirming valve position and coil temperature difference is a logical first diagnostic step.

Question 38Mechanical (HVAC) trade licensing and inspection

A technician is installing a condensate drain from a fan coil in a ceiling plenum above an occupied space. Which detail best protects the building from water damage if the drain plugs?

  1. Insulating the primary drain line
  2. A trap primer on the primary drain
  3. A secondary drain pan with a float switch wired to shut down the unit Correct answer.
  4. A larger primary drain pipe

Explanation

A secondary pan with a float switch provides a backup that stops the unit before overflow reaches the ceiling. Primary drain sizing, trapping, and insulation address flow and sweating but do not protect against a blocked drain.

Question 39Mechanical (HVAC) trade licensing and inspection

A gas furnace is installed with a Category I vent connector that rises 1/4 inch per foot toward the chimney. The connector passes through a framed wall opening. Which requirement applies to that penetration?

  1. The connector must be converted to a direct-vent termination
  2. The connector must be enclosed in a listed thimble or ventilated sleeve when passing through a combustible wall Correct answer.
  3. The connector may pass through the opening with no additional protection if clearance to combustibles is maintained
  4. The opening must be sealed airtight around the connector

Explanation

Vent connectors that pass through combustible walls require listed thimbles or ventilated metal sleeves that maintain clearance and allow heat dissipation. Simply keeping clearance in the open portion of the run does not satisfy the penetration requirement.

Question 40Mechanical (HVAC) trade licensing and inspection

A technician is checking a natural gas manifold pressure on a furnace and reads 3.5 inches water column with the burners firing. The rating plate specifies 3.5 inches water column. What does this confirm?

  1. The orifice size is incorrect
  2. The manifold pressure matches the rating plate at firing conditions Correct answer.
  3. The supply pressure is too low
  4. The gas valve is defective

Explanation

Manifold pressure is checked with the appliance firing because that is the condition the rating plate specifies. A reading that matches the plate confirms the gas valve regulator is set correctly for that input.

Question 41Mechanical (HVAC) trade licensing and inspection

An inspector observes a return-air grille installed in a closet that contains a gas furnace with a standing pilot. Which concern is most significant?

  1. Return air drawn from the closet can depressurize the space and interfere with combustion air Correct answer.
  2. The grille will increase the temperature rise
  3. The return grille will reduce furnace efficiency
  4. The grille will cause the pilot to burn too rich

Explanation

Return air taken from an appliance space can create negative pressure that pulls flue gases into the space or starves combustion air. Return air must come from conditioned space, not from a closet containing a fuel-burning appliance.

Question 42Mechanical (HVAC) trade licensing and inspection

A contractor is installing a ducted system and must select a supply register for a room with a 10-foot ceiling and a heating load. Which placement best promotes proper mixing?

  1. In the return plenum directed toward the door
  2. High on the wall with throw directed along the ceiling Correct answer.
  3. Low on the wall with throw directed at the floor
  4. In the center of the floor directed upward

Explanation

For heating, warm supply air should be delivered high and thrown along the ceiling so it mixes before dropping into the occupied zone. This reduces stratification and avoids cold floors caused by short-circuiting.

Question 43Mechanical (HVAC) trade licensing and inspection

A technician is measuring static pressure across a furnace and finds 0.9 inches water column total external static with a rated maximum of 0.5 inches. What is the most likely consequence?

  1. Lower current draw on the blower motor
  2. Improved dehumidification with no other effect
  3. Reduced airflow, higher temperature rise, and possible limit trips Correct answer.
  4. Increased airflow and lower temperature rise

Explanation

High external static pressure means the duct system is restrictive, so the blower delivers less air. Reduced airflow raises the temperature rise, which can trip the high-limit switch and shorten heat exchanger life.

Question 44Mechanical (HVAC) trade licensing and inspection

A technician is brazing a new line set and notices the nitrogen regulator is set so high that flow cannot be felt at the open end. What is the correct practice?

  1. Purge with refrigerant vapor instead of nitrogen
  2. Increase flow until the tubing is visibly pressurized
  3. Cap both ends and braze without purge
  4. Use a low, steady purge that displaces air without pressurizing the line Correct answer.

Explanation

The purge only needs to displace air inside the tubing; a low, steady flow is sufficient and avoids wasting nitrogen or pressurizing the joint. Pressurizing the line while brazing can blow the molten filler and create a poor joint.

Question 45Mechanical (HVAC) trade licensing and inspection

A heat pump is installed with the outdoor unit below the indoor coil and the line set rises 30 feet. Which installation feature is most important for compressor protection?

  1. An accumulator at the indoor coil
  2. An oil return trap or proper line sizing and velocity for oil return Correct answer.
  3. A larger liquid-line filter-drier
  4. A crankcase heater sized for the indoor coil

Explanation

When the outdoor unit is below the indoor coil, oil must be carried upward against gravity, so line sizing and refrigerant velocity must be adequate and traps may be needed. Poor oil return can starve the compressor and cause premature failure.

Question 46Mechanical (HVAC) trade licensing and inspection

An inspector is evaluating a new furnace installation where the vent connector passes through an unheated attic before reaching the chimney. Which condition most likely requires correction?

  1. The connector is supported every 4 feet
  2. The connector has a single-wall section in the unheated attic Correct answer.
  3. The connector is sloped upward toward the chimney
  4. The connector has a listed thimble at the wall penetration

Explanation

Single-wall vent connectors in unconditioned spaces are prone to cooling, condensation, and corrosion, and many codes require listed double-wall or insulated vent where the connector passes through cold spaces. The other listed details are normal good practice.

Question 47Mechanical (HVAC) trade licensing and inspection

A technician is troubleshooting a system with a piston metering device and finds frost forming at the outdoor unit on the liquid line after the piston. Which condition best explains this?

  1. There is a restriction at the piston or in the liquid line Correct answer.
  2. The compressor is running backward
  3. The system is overcharged
  4. The piston is oversized for the system

Explanation

Frost on the liquid line downstream of a restriction indicates a pressure drop that chills the refrigerant below freezing at that point. A plugged piston screen or a kinked liquid line is the typical cause.

Question 48Mechanical (HVAC) trade licensing and inspection

A contractor is installing a gas furnace in a basement with a Category I vent and a clothes dryer nearby. Which arrangement best satisfies combustion air requirements?

  1. Provide two permanent openings sized per the combustion air method used, or a listed combustion air duct Correct answer.
  2. Rely on infiltration through the basement windows
  3. Install a powered exhaust fan to pull air from the basement
  4. Seal the basement tighter to reduce drafts

Explanation

Fuel-burning appliances need a reliable, calculated supply of combustion air, and the code provides specific methods such as two permanent openings or a listed duct. Infiltration and exhaust fans are not reliable or code-compliant substitutes.

Question 49Mechanical (HVAC) trade licensing and inspection

A technician is evaluating a compressor that trips the internal overload after running for several minutes. Suction pressure is low, head pressure is low, and current is below rated. Which diagnosis fits best?

  1. A failing compressor or low refrigerant charge Correct answer.
  2. A stuck reversing valve in cooling
  3. A restricted liquid line
  4. A failed condenser fan motor

Explanation

Low suction, low head, and low current together indicate the compressor is moving little mass, which can come from low charge or internal wear. A restricted liquid line would show high head and high subcooling, and a failed condenser fan would show high head pressure.

Question 50Mechanical (HVAC) trade licensing and inspection

A contractor is installing a new gas-fired furnace and must size the combustion air openings for a confined space using the two-permanent-opening method. The space has two vertical ducts, each running from the attic down to within 12 inches of the floor and terminating within 12 inches of the ceiling. Which sizing basis applies to each opening?

  1. Each opening must have a free area of at least 1 square inch per 1,000 Btu/h of total input rating of all appliances in the space.
  2. Each opening must have a free area of at least 1 square inch per 5,000 Btu/h of total input rating of all appliances in the space.
  3. Each opening must have a free area of at least 1 square inch per 4,000 Btu/h of total input rating of all appliances in the space. Correct answer.
  4. Each opening must have a free area of at least 1 square inch per 2,000 Btu/h of total input rating of all appliances in the space.

Explanation

When combustion air is supplied through two permanent openings connected to vertical ducts, each opening is sized at 1 square inch of free area per 4,000 Btu/h of the combined input rating of all appliances in the confined space. The vertical-duct allowance is more generous than the horizontal-duct basis because the ducts provide a taller stack effect, but it is still less generous than a direct opening to outdoors.