400 questions
What happens to the pressure in a refrigerant cylinder as its temperature rises?
- a.It changes only if the cylinder is more than half full
- b.It falls, because the vapor expands into the free space
- c.It rises, following the refrigerant's saturation curve✓
- d.It stays constant as long as liquid remains in the cylinder
A cylinder holding both liquid and vapor sits on the saturation curve, so warming it raises pressure steadily, which is why cylinders must be protected from heat. Constant pressure would require constant temperature, not merely the presence of liquid. Vapor expanding in a sealed container raises pressure rather than lowering it. And the relationship holds at any fill level, though a fuller cylinder is more dangerous because liquid expands.
A technician wants to recover refrigerant quickly from a system with a large charge. What technique speeds it up?
- a.Warming the recovery cylinder in order to raise its internal pressure
- b.Running the system's own compressor during the recovery
- c.Recovering liquid first through both service ports with short hoses✓
- d.Connecting the recovery machine only to the suction port
Pulling liquid through both ports with short, large-diameter hoses moves the bulk of the charge fast, and the remaining vapor is recovered afterward. Warming the recovery cylinder raises the pressure the machine must push against and slows it. Running the system compressor during recovery risks damaging it and is not the intended method. And a single suction connection forces slow vapor-phase recovery of the whole charge.
During a pump-down for a component replacement, what is the goal?
- a.To equalize pressures so the compressor can restart under no load
- b.To isolate the charge in the condenser and receiver so the low side can be opened✓
- c.To lower the system to a deep vacuum before opening it
- d.To transfer the charge into the recovery cylinder for disposal
Pumping down uses the system's own compressor to store refrigerant behind a closed service valve so low-side work can be done without recovering the entire charge. A deep vacuum requires a vacuum pump and comes after the system is opened and repaired. Transferring to a cylinder is recovery, a separate procedure. And equalizing pressures is a starting consideration, not the purpose of a pump-down.
What determines the evaporator's saturation temperature during normal operation?
- a.The thermostat setpoint selected by the occupant
- b.The superheat setting adjusted at the expansion valve
- c.The suction pressure the compressor and metering device establish✓
- d.The outdoor ambient temperature at the condenser
Saturation temperature is fixed by pressure, and the balance between compressor pumping and metering device feed sets the suction pressure in the coil. The thermostat decides when the system runs, not the pressure inside it. The superheat adjustment shifts how much of the coil boils but does not set the saturation point itself. And outdoor ambient controls the high side rather than the evaporator's pressure.
A condenser hums but the fan and compressor do not start, and both begin running after you spin the fan blade by hand. What component should you test first?
- a.The dual run capacitor✓
- b.The contactor coil
- c.The low-pressure switch
- d.The compressor windings
A motor that hums and then runs after a manual push has lost its phase shift, which points straight at the run capacitor. The contactor is clearly pulling in, since power is reaching the motors. A tripped low-pressure switch would leave the unit silent rather than humming. And the compressor windings are proven by the fact that it eventually runs.
A dual run capacitor is marked 45/5 microfarads. What does the smaller value serve?
- a.The defrost board's timing circuit
- b.The compressor start winding during the first second of operation
- c.The low-voltage transformer's secondary circuit
- d.The condenser fan motor✓
On a dual capacitor the large value serves the compressor and the small value serves the condenser fan motor, with a common terminal shared between them. Start assistance for the compressor comes from a separate start capacitor and potential relay when one is used. The control transformer is not capacitor driven. And defrost timing is handled electronically on the board, not by a run capacitor.
You measure a 45 microfarad capacitor at 31 microfarads. What is the correct action?
- a.Replace it, because it is well outside its tolerance✓
- b.Wait until the compressor fails to start before replacing it
- c.Add a second capacitor in series to make up the difference
- d.Leave it, because any reading above 30 is acceptable
Run capacitors carry a tolerance of a few percent, and a 31 microfarad reading on a 45 microfarad part means the motor is running with reduced torque and elevated current. There is no rule accepting anything above 30. Capacitors in series reduce total capacitance rather than increasing it. And waiting for a hard-start failure lets the weak capacitor overheat the windings in the meantime.
What must be done to a capacitor before it is tested or handled?
- a.Note the polarity marking on the case
- b.Verify the microfarad rating with the manufacturer
- c.Warm it to room temperature so that the reading is accurate
- d.Discharge it safely across a resistor with power removed✓
A capacitor holds a charge after power is off, so it must be discharged through a resistor before it is touched or tested. Temperature has little effect on a field capacitance reading. Motor run capacitors are non-polarized, so there is no polarity marking to observe. And confirming the rating with the manufacturer is a parts question, not a safety step.
A contactor's contacts are visibly pitted and the compressor chatters at startup. What is the risk of leaving it in service?
- a.Voltage drop and arcing across the contacts can damage the compressor✓
- b.The condenser fan will run backwards
- c.The low-voltage transformer will be overloaded
- d.The capacitor will discharge through the contacts
Pitted contacts add resistance and cause voltage drop and repeated arcing, which subject the compressor to low-voltage starting and heat. The transformer supplies only the small coil current and is not overloaded by contact condition. Capacitors discharge through motor windings, not through line contacts. And single-phase fan rotation is fixed by the motor's internal wiring.
A 24-volt control transformer is replaced twice and burns out both times. What should you investigate?
- a.A shorted control wire or a failed contactor coil drawing excess current✓
- b.A refrigerant overcharge raising compressor amperage
- c.A thermostat mounted on an exterior wall
- d.An oversized filter causing high static pressure
Repeat transformer failures come from an overloaded secondary, which usually means a pinched or shorted thermostat wire or a coil that has shorted turns. Airflow restriction and refrigerant charge act on the line-voltage side and do not load the control transformer. And a poorly located thermostat causes comfort complaints and short cycling, not transformer burnout.
The correct sequence of operation for a modern gas furnace begins with which event after the thermostat calls?
- a.The main blower starts at heating speed
- b.The inducer starts and proves the pressure switch✓
- c.The gas valve opens to purge the manifold
- d.The hot surface igniter warms while the blower runs
The control energizes the inducer first and waits for the pressure switch to close, proving that the venting path is open before ignition can proceed. Opening the gas valve before proving draft would release unburned gas. Heating the igniter comes after the pressure switch proves. And the main blower starts on a timed delay after flame is established.
A furnace inducer runs but the pressure switch never closes. Which cause should be checked first?
- a.A cracked hot surface igniter
- b.A dirty flame sensor coated with oxide
- c.A weak thermostat battery reducing the call voltage below spec
- d.A blocked vent, plugged drain, or disconnected sensing hose✓
The pressure switch proves the inducer is actually moving flue gas, so blockages in the vent, the condensate trap, or the sensing hose are the usual causes. A cracked igniter fails later in the sequence, after the switch closes. A weak thermostat battery would prevent the call from being made at all. And a dirty flame sensor causes a lockout after ignition, not a failure to prove draft.
How does a flame sensor prove the presence of flame on a modern furnace?
- a.It measures the temperature rise at the burner face
- b.It passes a small DC current through the flame by rectification✓
- c.It detects the pressure pulse ignition creates
- d.It measures the infrared light the flame emits
A flame rectification circuit sends alternating current through the flame, which conducts in only one direction, producing a microamp DC signal the board reads. Optical sensing is used on oil burners and some commercial equipment, not on this circuit. Thermocouples measure heat and are used on standing pilot systems. And no pressure pulse is used to prove flame.
A furnace lights, runs for about six seconds, then shuts down and retries. What is the most probable cause?
- a.A dirty or misaligned flame sensor✓
- b.A failed inducer motor bearing assembly
- c.An undersized gas supply line
- d.A cracked heat exchanger
Ignition followed by a short run and a retry is the classic signature of a flame sensing failure, since the board proves flame for a few seconds and then loses the signal. A failed inducer would stop the sequence before ignition. A cracked heat exchanger produces flame disturbance and rollout symptoms rather than a clean lockout pattern. And an undersized gas line usually shows as a weak or unstable flame at high fire, not a consistent short cycle.
A rollout switch on a gas furnace has tripped. What must the technician do?
- a.Find and correct the cause of flame escaping the burner box✓
- b.Reset it and monitor the furnace on the next several cycles
- c.Jumper it temporarily so the customer has heat overnight
- d.Replace it with a switch of a higher temperature rating
A rollout switch trips because flame left the combustion chamber, usually from a blocked flue, a plugged heat exchanger, or a cracked passage, and that cause must be corrected. Simply resetting it leaves a combustion hazard in service. Raising the trip temperature defeats the protection the switch provides. And jumpering a safety device on a fuel-burning appliance is never acceptable.
A furnace repeatedly trips its high-limit switch during heating. Which cause is most likely?
- a.A thermostat set too low for the occupant's comfort
- b.An oversized return duct delivering excess air
- c.Restricted airflow from a dirty filter, closed registers, or a failing blower✓
- d.A refrigerant undercharge in the cooling circuit
The high limit opens when heat exchanger temperature climbs, and the usual reason is that not enough air is carrying that heat away. The cooling circuit's charge has no role in a heating cycle. A low setpoint would shorten the run rather than overheat the exchanger. And too much return air would lower temperature rise, moving away from the limit rather than toward it.
A combustion analysis on a natural gas furnace shows 400 ppm of carbon monoxide air-free in the flue. How should the technician respond?
- a.Increase the manifold pressure to burn the gas more completely
- b.Record it as acceptable and complete the service call
- c.Open a nearby window and retest to dilute the reading
- d.Investigate the cause and correct the combustion problem✓
An air-free carbon monoxide reading in the hundreds of parts per million signals incomplete combustion from a dirty burner, restricted venting, or a cracked exchanger, and it must be diagnosed. Recording it as acceptable ignores a life-safety indicator. Raising manifold pressure typically overfires the burner and makes carbon monoxide worse. And ventilating the room changes the ambient reading without changing what the appliance produces.
What does a yellow, lazy flame on a natural gas burner most often indicate?
- a.A cracked hot surface igniter near the flame
- b.Insufficient primary air or a dirty burner✓
- c.Excess combustion air reaching the burner
- d.Gas pressure below the manifold rating
Yellow tipping and a lazy flame show incomplete mixing of gas and primary air, usually from lint, rust, or a blocked burner port. Excess air produces a lifting, noisy blue flame rather than a yellow one. Low gas pressure gives a small, weak flame that is still blue. And a cracked igniter fails to light the burner instead of changing flame color.
A hot surface igniter glows but the burner never lights. What should be checked next?
- a.The condensate float switch
- b.The evaporator coil for ice
- c.The blower motor capacitor
- d.Gas supply and the gas valve's operation✓
If the igniter reaches temperature and no flame appears, the missing element is fuel, so the gas supply, shutoff position, and valve operation are the next checks. Blower components affect air movement after ignition. A tripped float switch would interrupt the call before the igniter energized. And evaporator ice belongs to the cooling side and has no role in the ignition sequence.
A technician measures 0.95 in. w.c. of total external static on a system whose blower table is built around 0.50 in. w.c. What is the most productive next measurement?
- a.Line voltage at the disconnect
- b.Compressor amperage under load
- c.Superheat at the evaporator outlet
- d.Pressure drops across the filter, coil, and each duct section✓
Once total external static is known to be high, breaking it into component drops locates the restriction so the right thing gets fixed. Compressor amperage reflects refrigerant load and reveals nothing about which duct component is restrictive. Line voltage is an electrical supply check. And superheat is a refrigerant-side reading that will be distorted by the airflow problem rather than explaining it.
Which instrument gives the most direct measurement of the air a supply register is actually delivering?
- a.An infrared thermometer aimed at the grille face
- b.A digital manometer at the supply plenum
- c.An anemometer held in the center of the duct
- d.A powered flow hood placed over the register✓
A flow hood captures the entire discharge and reports CFM at that outlet directly. A single anemometer reading in the middle of a duct gives a velocity at one point, which must be traversed and converted. A manometer measures pressure, not volume. And an infrared thermometer reads surface temperature, which says nothing about airflow.
A blower wheel is caked with dust. What happens to airflow?
- a.It falls, because the dirty blades no longer grip and move air✓
- b.It stays the same because the motor compensates automatically
- c.It increases because the wheel is heavier and carries more momentum
- d.It falls only on ECM motors, not on PSC motors
Dirt fills the space between the blades so each pocket carries less air, and delivered CFM drops sharply even though the wheel still spins. Added mass does not push more air. A PSC motor holds speed but cannot recover the lost airflow, and an ECM raises torque and may partially compensate while drawing more watts. So the loss occurs with either motor type, not just one.
What advantage does a constant-airflow ECM blower have when duct static pressure is high?
- a.It shuts down and signals a fault on the control board
- b.It maintains a constant wattage regardless of resistance
- c.It reduces its speed to protect the motor from overload
- d.It increases speed and torque to hold the commanded CFM✓
A constant-airflow ECM senses torque and speed and ramps up to keep delivering the target CFM, though it consumes more watts doing it. It does not back off in response to static, which is the PSC behavior. Faulting out on high static is not how these motors respond within their range. And wattage rises rather than staying constant when the motor works harder.
Why can an ECM blower mask a duct problem from the homeowner while still costing them money?
- a.It holds airflow up by drawing more power, hiding the restriction✓
- b.It shifts the load to the compressor instead of the blower
- c.It quietly lowers airflow while still reporting normal operation to the board
- d.It defers the problem to the defrost board's fault log
Because the motor compensates for restriction, comfort stays acceptable while the electrical consumption climbs, so the defect shows up on the utility bill rather than as a complaint. It does not quietly reduce airflow; holding airflow is precisely what it does. Blower work is not transferred to the compressor. And a defrost board logs heat pump defrost events, not duct static problems.
A compressor draws locked rotor amps for a few seconds and then trips on overload repeatedly. What should be checked?
- a.The thermostat's heat anticipator setting
- b.The evaporator coil for a dirty filter media
- c.The condensate drain trap for standing water
- d.The start components, supply voltage, and whether pressures are equalized✓
A compressor stuck at locked rotor current is failing to start, so start capacitor and relay condition, supply voltage under load, and whether the system pressures have equalized are the diagnostic path. A condensate trap has nothing to do with starting torque. Anticipator settings affect cycle length in older thermostats. And a dirty filter reduces airflow without preventing the compressor from starting.
What does a hard start kit do for a struggling single-phase compressor?
- a.It lowers the voltage the compressor sees during startup
- b.It bypasses the run capacitor entirely while the motor is accelerating
- c.It delays the compressor start until head pressure builds
- d.It adds start winding current briefly to increase starting torque✓
A start capacitor and relay put extra current through the start winding for the first fraction of a second, boosting torque, and then drop out. It does not reduce supply voltage, which would make starting worse. The run capacitor stays in the circuit throughout. And a hard start kit does not create a timed delay; that is a separate anti-short-cycle control.
A compressor's common-to-ground resistance reads near zero ohms. What does that mean?
- a.The start winding has a higher resistance than normal
- b.The compressor is normal, since windings connect to the shell
- c.The motor windings are shorted to the shell and the compressor is failed✓
- d.The run winding is open and must be jumpered
Windings must be electrically isolated from the shell, so continuity to ground means the insulation has failed and the compressor is scrap. An open run winding would read infinite resistance between terminals, not to ground. Start winding resistance is measured terminal to terminal, not to the shell. And a healthy compressor reads very high resistance from any terminal to ground.
A heat pump runs constantly on a 35 F morning with the auxiliary heat energized the whole time. What should the technician verify first?
- a.Whether the low-pressure switch has been bypassed
- b.Whether the outdoor coil is frosted and the defrost control is working✓
- c.Whether the condensate float switch has tripped
- d.Whether the supply registers are set to the summer position
At that temperature a frosted outdoor coil that is not being defrosted destroys capacity and forces auxiliary heat to carry the load. A tripped float switch would stop the equipment rather than leave it running. Register position affects distribution but not the outdoor coil's condition. And a bypassed low-pressure switch is a wiring defect that would not by itself cause continuous auxiliary operation.
A customer reports the heat pump blows lukewarm air compared with their old furnace. What is the correct explanation?
- a.The auxiliary heat strips have failed open at the sequencer
- b.The system is undercharged and needs refrigerant added
- c.Heat pumps normally deliver a lower supply temperature over longer run times✓
- d.The reversing valve is leaking internally and must be replaced
A heat pump typically supplies air in the 90 to 105 degree range, which feels cool against skin but heats the house through longer run times. Adding refrigerant to a correctly charged system creates a different problem. An internally leaking reversing valve would show as poor capacity with an abnormally warm suction line, not as normal operation. And failed strips would matter only when supplemental heat was actually required.
A condensate float switch keeps interrupting the cooling call on an attic air handler. What is the correct repair?
- a.Wire the switch to the alarm circuit instead of the control circuit
- b.Clear the blockage in the drain and verify the trap and slope✓
- c.Remove the switch since a secondary pan is already installed
- d.Replace the switch with one rated for a higher water level
The switch is doing its job, so the drain restriction that is letting water rise must be found and cleared. Changing the trip level lets water climb closer to the ceiling before anything happens. Moving it to an alarm circuit removes the shutoff protection entirely. And a secondary pan is not a substitute for the shutoff device where one is required.
A cooling system shows low suction pressure, high superheat, and a frosted liquid line just past the filter drier. What is the diagnosis?
- a.A restriction at the drier causing a pressure drop and local cooling✓
- b.A blower running at too high a speed
- c.A reversing valve leaking internally
- d.An overcharge of refrigerant in the condenser
Frost appearing right after a component on the liquid line means refrigerant is flashing there, which is the signature of a restriction starving the evaporator. An overcharge raises suction pressure and subcooling instead. Excess blower speed raises suction pressure and lowers superheat. And a leaking reversing valve produces a warm suction line and poor capacity without a localized liquid line frost line.
How can a technician distinguish low airflow from low refrigerant charge when both produce low suction pressure?
- a.Low airflow shows a large temperature split; low charge shows high superheat with low subcooling✓
- b.Low airflow shows high head pressure; low charge shows high head pressure as well
- c.Low airflow trips the low-pressure switch; low charge never does
- d.Low airflow shows a wet suction line; low charge shows a dry suction line
Restricted airflow leaves the coil overcooling the small amount of air passing through it, producing a wide split, while a shortage of refrigerant shows itself as high superheat with little or no subcooling. Head pressure falls in both cases rather than rising in one. Suction line sweating depends on humidity and superheat and does not separate the two. And a severe airflow restriction can also drop suction low enough to trip the switch.
A system's head pressure is high and subcooling is high on a 90 F day with a clean condenser. What is the most likely cause?
- a.An overcharge of refrigerant✓
- b.A weak compressor with worn valves
- c.Low indoor airflow across the evaporator
- d.A restricted liquid line filter drier
Excess refrigerant stacks liquid in the condenser, which raises both subcooling and head pressure even when the coil and air are clean. A restricted drier would raise subcooling but starve the low side and lower head pressure. Low indoor airflow drops suction and head pressure together. And a weak compressor lowers head pressure while raising suction.
A customer's system cools well at night but struggles every afternoon. Which check addresses the pattern directly?
- a.Verify the thermostat's programmed schedule
- b.Check the condensate drain for a partial blockage
- c.Measure winding resistance on the compressor
- d.Measure condensing temperature against outdoor ambient at peak load✓
A complaint that tracks outdoor temperature points to heat rejection, so the condenser's approach temperature under peak conditions is the measurement that confirms or clears it. A thermostat schedule would produce the same behavior every day regardless of weather. Winding resistance is an electrical test unrelated to load. And drain condition affects water removal, not afternoon capacity.
What does SEER2 measure that EER does not?
- a.The electrical demand of the system at startup
- b.The efficiency of the ductwork attached to the system
- c.Seasonal performance across a range of conditions rather than one rating point✓
- d.The efficiency of the system in heating mode
SEER2 aggregates performance over a simulated cooling season, while EER reports efficiency at a single set of rating conditions. Heating efficiency is covered by HSPF2 and COP. Startup demand is a locked rotor characteristic, not an efficiency metric. And duct performance is measured separately by leakage testing, not by either rating.
Why were SEER2 and HSPF2 introduced to replace the earlier SEER and HSPF ratings?
- a.The ratings now include the cost of electricity in the region
- b.The ratings now cover only variable-speed equipment
- c.Testing moved from cooling season to heating season conditions
- d.Testing now uses a higher external static pressure closer to real installations✓
The revised test procedure raises the external static pressure used during testing so published numbers better reflect ducted systems in the field. The cooling and heating metrics still address their own seasons. Energy price is a local economic factor and is not part of any efficiency rating. And the ratings apply to single-stage equipment as well as variable-speed.
A homeowner asks whether a higher AFUE furnace will lower their bill in a mild coastal climate. What is the honest technical answer?
- a.AFUE only matters when the furnace is oversized for the load
- b.The savings depend on how many heating hours the house actually runs✓
- c.AFUE applies to the blower's electrical use rather than the fuel
- d.AFUE gains apply equally in every climate because efficiency is fixed
AFUE is a percentage of fuel converted to useful heat, so the dollar savings scale with how much fuel is burned across the season. The percentage is fixed, but the size of the savings is not, because it depends on run hours. Oversizing hurts efficiency in practice but does not determine whether AFUE matters. And AFUE addresses fuel conversion, while blower electricity is accounted separately.
A two-stage furnace never leaves low fire even on the coldest days. What should be checked?
- a.The staging control wiring and the second-stage thermostat call✓
- b.The condensate neutralizer for saturation
- c.The vent termination clearance from the wall
- d.The flame sensor microamp reading
Second-stage operation depends on a thermostat that can call for it and control wiring that carries the call, so those are the first things to verify. A saturated neutralizer affects drainage, not staging. The flame sensor proves flame at either stage and does not select one. And vent termination clearance is an installation dimension unrelated to staging logic.
An air conditioner short cycles, running four minutes and stopping for three all afternoon. Which cause fits best?
- a.The condensate trap has lost its water seal
- b.The blower motor is failing intermittently on its start winding
- c.The equipment is significantly oversized for the load✓
- d.The supply registers are open too far
Rapid on-off cycling under a steady load is the classic behavior of equipment with far more capacity than the house needs. A failing blower would produce airflow and overload symptoms rather than clean short cycles. A dry trap causes drainage and air-leakage problems, not cycling. And registers that are open too far would extend run time rather than shorten it.
A thermostat's cooling setpoint is satisfied but the indoor blower keeps running continuously. What is the first thing to check?
- a.Whether the compressor contactor has welded itself closed
- b.Whether the evaporator coil is frozen solid
- c.Whether the fan switch is set to ON rather than AUTO✓
- d.Whether the outdoor unit's capacitor has failed
A fan switch left in the ON position runs the blower continuously by design, and it is the simplest explanation to rule out first. A welded contactor would keep the compressor running, not just the blower. A frozen coil affects capacity and can extend run time but does not command continuous fan operation. And an outdoor capacitor failure affects the condenser, not the indoor blower.
You measure 197 volts at a condenser rated 208/230 volts while the compressor is trying to start. What is the concern?
- a.Low voltage will make the compressor run cooler and last longer
- b.Low voltage has no effect because the compressor is single phase
- c.Low voltage raises current draw and can overheat the windings✓
- d.Low voltage only matters on three-phase equipment
A motor delivering the same mechanical work at reduced voltage draws more current, and that extra current heats the windings. Low voltage does not make a motor run cooler. Single-phase compressors are just as vulnerable as three-phase, arguably more so at starting. And the effect is a property of induction motors generally, not of phase count.
A technician measures 240 volts across an open contactor's line terminals with the unit off and 0 volts across the contacts when it is pulled in. What does that indicate?
- a.The line voltage supply is unstable
- b.The contactor coil is open
- c.The contacts are burned and adding resistance
- d.The contactor is operating normally✓
Full voltage across an open contactor and no measurable drop across the closed contacts is exactly what a healthy contactor does. An open coil would prevent it from pulling in at all. Burned contacts would show a measurable voltage drop while closed. And these readings show a stable supply rather than an unstable one.
A technician performs annual maintenance on a residential split system. Which task most directly protects capacity on hot days?
- a.Replacing the thermostat batteries
- b.Tightening the electrical lugs at the disconnect
- c.Cleaning the outdoor coil and clearing the surrounding area✓
- d.Lubricating the blower motor's sealed bearings
A clean condenser with unobstructed airflow keeps condensing temperature and head pressure down, which is exactly what protects capacity when it is hottest. Tight electrical connections prevent failures but do not improve heat rejection. Sealed bearings are not field lubricated. And fresh thermostat batteries prevent nuisance outages but add no capacity.
During maintenance you find the outdoor coil packed with cottonwood. What is the correct cleaning approach?
- a.Brush the fins vigorously across their face with a wire brush
- b.Rinse from the inside out with low-pressure water after disconnecting power✓
- c.Spray a caustic cleaner and leave it on the coil to dry
- d.Blast the outside face of the coil with a pressure washer set at full pressure
Rinsing from the inside outward pushes debris back the way it came, and low pressure with the power off protects the fins and the technician. High-pressure washing folds fins flat and drives debris deeper. A wire brush across the face bends and tears the aluminum. And leaving a caustic cleaner to dry on the coil corrodes fins and tubing.
A customer replaced a 1-inch pleated filter with a high-MERV version and now complains the house is not cooling. What is the likely mechanism?
- a.Higher MERV filters remove humidity from the return air
- b.The added pressure drop cut airflow, reducing capacity✓
- c.The filter media has changed the refrigerant charge requirement
- d.The denser filter causes the compressor to short cycle on high pressure
A dense filter in a slot sized for a low-resistance one raises static pressure and drops airflow, which lowers capacity and can freeze the coil. Filter selection has no bearing on how much refrigerant the system holds. Filters capture particles, not water vapor. And the resulting problem shows up as low suction and possible icing rather than a high-pressure trip.
A homeowner asks why closing registers in unused rooms did not lower their bill. What is the correct explanation?
- a.Closed registers cause the compressor to run at reduced capacity
- b.Closing registers has no measurable effect on any other part of the duct system
- c.Closing registers raises system static pressure and can reduce total airflow✓
- d.Closed registers redirect air to the return and shorten run times
Closing outlets raises static pressure, which reduces total airflow and can push a system toward coil icing or high limit trips rather than saving energy. Compressor capacity is set by refrigerant conditions, not by damper position. Air blocked at a register does not usefully return to the equipment. And the effect is measurable, which is exactly why the practice causes problems.
A technician suspects duct leakage is hurting performance in an existing home. What test provides the evidence?
- a.An amp draw reading on the blower motor
- b.A combustion analysis at the furnace flue
- c.A temperature split reading at the coil
- d.A pressurized duct leakage test with a calibrated fan✓
A duct pressurization test measures leakage in CFM at a reference pressure, which is the direct evidence of how much conditioned air is escaping. Temperature split reflects the coil's performance, not the duct's tightness. Combustion analysis addresses the burner. And blower amperage responds to total system resistance without isolating leakage.
An older thermostat has a heat anticipator setting. What does it do?
- a.Adds a small heat source that shuts the burner off slightly early to limit overshoot✓
- b.Runs the blower before the burner to warm the ducts
- c.Prevents the compressor from starting within five minutes of shutdown
- d.Delays the burner start until the room has dropped a full degree
The anticipator warms the bimetal slightly so the contacts open before the room reaches setpoint, letting residual heat finish the job and reducing temperature swing. It does not add a starting delay based on room temperature. Pre-purging air through the ducts is a fan control function. And short-cycle protection for the compressor is a separate timing control.
A smart thermostat is installed on a system with no common wire, and it resets intermittently. What is the standard fix?
- a.Replace the thermostat with one rated for line voltage
- b.Run a common conductor or install a listed adapter at the equipment✓
- c.Increase the transformer's secondary voltage to 30 volts
- d.Move the thermostat to an exterior wall to improve its wireless signal
These thermostats need continuous 24-volt power, and either a dedicated common conductor or a manufacturer's adapter supplies it without power stealing. Transformers are not field adjustable and raising voltage would damage controls. Wall location affects temperature accuracy, not power supply. And residential systems use low-voltage controls, so a line-voltage thermostat is not an option.
A technician needs to confirm actual airflow on an installed system without a flow hood. Which method is most practical?
- a.Measure temperature rise on the furnace and calculate CFM from the output✓
- b.Estimate from the register face velocity multiplied by the grille free area
- c.Compare the amp draw to the blower motor's rated amps
- d.Read the nameplate CFM for the equipment model
With a known firing rate and efficiency, measuring the temperature rise and applying the sensible heat formula gives a reliable airflow figure with tools every technician carries. Register velocity estimates are crude and ignore the effective free area of the grille. Nameplate CFM is a rated value, not what the installed system delivers. And motor amperage varies with static in ways that do not translate directly to CFM.