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HVAC Excellence Oil Heat study materials
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HVAC Excellence Oil Heat Certification Exam practice questions

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Question 1Oil Heat Systems

A technician is called to an oil furnace that has locked out three times in one day. The reset button on the primary control restarts the burner, but it fails again within minutes. Which diagnostic step should be taken first?

  1. Replace the primary control to rule out a control fault
  2. Increase the nozzle size to raise the firing rate
  3. Open the bleed port on the pump and run it continuously
  4. Check the cad cell resistance during the trial-for-ignition period Correct answer.

Explanation

The cad cell reading during trial for ignition tells you whether the control is seeing flame. A reading that stays high (typically above about 1600 ohms) shows the control is locking out because it does not detect light, which points to ignition, fuel, or cell problems rather than the control itself. Measuring first avoids replacing parts on a guess.

Question 2Oil Heat Systems

Which component on a conventional oil burner transforms the low-voltage control circuit into the high-voltage spark at the electrodes?

  1. The stack relay
  2. The ignition transformer Correct answer.
  3. The nozzle assembly
  4. The cad cell

Explanation

The ignition transformer steps up line voltage to the high voltage needed to jump the electrode gap and light the oil spray. The stack relay and cad cell are flame-sensing devices, and the nozzle meters and atomizes oil.

Question 3Oil Heat Systems

A technician measures 0.85 inches of vacuum at the pump inlet on a two-line oil system with a 100-foot run. What does this reading most likely indicate?

  1. Excessive vacuum from a restricted or undersized suction line Correct answer.
  2. Normal vacuum for that length of run
  3. Air leaking into the suction line
  4. A partially clogged nozzle

Explanation

Two-line systems should show very little inlet vacuum because the return line keeps the pump supplied. A reading near 0.85 inches Hg on a 100-foot run points to a restriction such as a clogged filter, kinked line, or undersized tubing. Air leaks usually show as erratic readings and foaming, not steady high vacuum.

Question 4Oil Heat Systems

What is the purpose of the delay in the primary control's pre-purge and post-purge timing?

  1. To reduce the nozzle firing rate during start-up
  2. To clear residual oil vapors and combustion gases from the chamber before and after firing Correct answer.
  3. To raise the pump pressure gradually
  4. To test the cad cell for light sensitivity

Explanation

Pre-purge and post-purge use the blower to sweep the combustion chamber so unburned oil vapor and gases do not accumulate and cause puffs or explosions. They do not change firing rate, pump pressure, or test the cad cell.

Question 5Oil Heat Systems

An oil furnace produces a smoky, orange flame with soot on the heat exchanger. Which adjustment is most likely needed?

  1. Advance the cad cell position
  2. Increase the electrode gap
  3. Reduce the pump pressure
  4. Increase combustion air to the burner Correct answer.

Explanation

A smoky orange flame with soot indicates incomplete combustion from insufficient air. Increasing combustion air settings on the burner restores the air-fuel balance and yields a clean flame. Electrode gap, pump pressure, and cad cell position do not correct an air-starved flame.

Question 6Oil Heat Systems

Before performing combustion analysis on an oil furnace, the technician should verify which condition to obtain meaningful readings?

  1. The barometric damper is fully closed
  2. The cad cell is bypassed
  3. The furnace has reached steady-state operating temperature Correct answer.
  4. The nozzle has been removed

Explanation

Combustion readings are only representative once the appliance has stabilized and reached steady-state operation. Bypassing the cad cell, removing the nozzle, or closing the barometric damper would alter operation or create unsafe conditions and invalidate the test.

Question 7Oil Heat Systems

Which oil burner component is responsible for filtering out particles before oil reaches the nozzle?

  1. The stack relay
  2. The barometric damper
  3. The cad cell
  4. The oil filter and nozzle strainer Correct answer.

Explanation

Oil filters and the small strainer inside the nozzle body trap particles that would otherwise clog the tiny nozzle orifice. The cad cell, stack relay, and barometric damper serve flame sensing, flame proving, and draft control respectively.

Question 8Oil Heat Systems

A technician finds the oil pump pressure at 100 psi on a burner rated for a 1.00 gph nozzle at 100 psi. The flame appears long and lazy. What is the most appropriate action?

  1. Verify the nozzle is the correct size and not worn or oversized Correct answer.
  2. Increase pump pressure to 140 psi
  3. Close the barometric damper
  4. Reduce the electrode gap to 1/16 inch

Explanation

A long lazy flame with correct pump pressure suggests the nozzle is passing more oil than rated, often from wear or an incorrect size. Verifying and replacing the nozzle with the specified size restores the intended firing rate. Raising pressure or closing the damper would mask rather than correct the problem.

Question 9Oil Heat Systems

What is the function of the barometric draft regulator on an oil-fired appliance?

  1. To atomize oil at the nozzle
  2. To sense flame at the burner
  3. To maintain a steady negative draft in the flue Correct answer.
  4. To regulate oil pump pressure

Explanation

The barometric draft regulator opens and closes a damper to hold a constant negative draft in the flue, compensating for changes in chimney draft. It does not atomize oil, sense flame, or control pump pressure.

Question 10Oil Heat Systems

A service technician smells oil near the burner and finds a small puddle under the pump. Which condition should be investigated first?

  1. A leaking pump seal or loose fitting on the suction or pressure line Correct answer.
  2. A clogged flue passage
  3. A misadjusted electrode gap
  4. A failed cad cell

Explanation

Oil pooling under the pump points to a leaking seal, gasket, or loose fitting on the oil lines. Fuel leaks are a safety concern and must be corrected before other service. Cad cell, electrode gap, and flue blockage do not produce oil puddles.

Question 11Oil Heat Systems

Which statement correctly describes the resistance behavior of a cad cell exposed to burner flame?

  1. Resistance rises sharply when light strikes the cell
  2. Resistance remains constant regardless of light
  3. Resistance drops only when the cell is cold
  4. Resistance drops when light strikes the cell Correct answer.

Explanation

A cadmium sulfide cell is a photoresistor: its resistance falls when light reaches it, so the primary control sees low resistance when flame is present. High resistance in the presence of flame indicates the cell is dirty, misaimed, or failed.

Question 12Oil Heat Systems

A technician is setting up a new oil burner and needs to set the electrodes. Which dimension is most critical for reliable ignition?

  1. The height of the oil tank above the burner
  2. The diameter of the flue pipe
  3. The length of the cad cell leads
  4. The distance from the nozzle center to the electrode tips and the gap between electrodes Correct answer.

Explanation

Electrode position relative to the nozzle and the gap between the tips determine whether the spark lands in the oil spray. Manufacturer specifications for that nozzle and burner must be followed. Cad cell lead length, flue diameter, and tank height do not set the spark geometry.

Question 13Oil Heat Systems

On a single-line oil system, why must the oil tank outlet be positioned above the burner pump inlet?

  1. To allow gravity to supply oil to the pump and avoid suction lift problems Correct answer.
  2. To increase the nozzle firing rate
  3. To prevent the cad cell from overheating
  4. To keep the barometric damper closed

Explanation

A single-line system relies on gravity or minimal lift to deliver oil reliably; a tank outlet below the pump can cause air ingestion and loss of prime. Cad cell temperature, damper position, and firing rate are unrelated to tank elevation.

Question 14Oil Heat Systems

A technician observes a puffback on start-up of an oil furnace. Which condition is the most likely cause?

  1. A pump pressure set too low
  2. A cad cell with low resistance
  3. Excessive combustion air
  4. Delayed ignition allowing oil to accumulate in the chamber Correct answer.

Explanation

A puffback occurs when oil accumulates in the combustion chamber and then ignites suddenly. Delayed ignition from a weak spark, poor electrode setting, or a nozzle that dribbles is the usual cause. Excess air, a healthy cad cell, and low pressure do not create accumulated fuel.

Question 15Oil Heat Systems

Which safety device on an oil furnace is designed to shut off the burner if the flue stops drafting properly?

  1. The high-limit aquastat
  2. The stack relay or flue gas sensor Correct answer.
  3. The primary control with cad cell
  4. The nozzle strainer

Explanation

A stack relay or flue gas sensor proves that combustion gases are leaving the appliance and can shut down the burner if the flue temperature or draft indicates a problem. The cad cell proves flame at the burner, the aquastat limits water or air temperature, and the nozzle strainer filters oil.

Question 16Oil Heat Systems

What is the correct procedure when replacing a nozzle on an oil burner?

  1. Install the nozzle finger-tight only
  2. Reuse the old nozzle gasket to save time
  3. Match the nozzle flow rate, spray angle, and pattern to the manufacturer's specification Correct answer.
  4. Use any nozzle with the same flow rate regardless of spray pattern

Explanation

Nozzles are specified by flow rate, spray angle, and pattern because these determine flame shape and combustion quality. Substituting a different pattern or angle can cause impingement, soot, or poor efficiency. The nozzle should be tightened to specification with a new gasket.

Question 17Oil Heat Systems

A technician measures smoke at a #2 spot on the Bacharach scale after tuning an oil burner. What does this indicate?

  1. The nozzle is undersized
  2. The combustion air is excessive
  3. The burner is operating at peak efficiency
  4. The burner is producing acceptable but not clean combustion and should be adjusted toward zero smoke Correct answer.

Explanation

Smoke spot readings should be as close to zero as practical; a #2 spot shows visible smoke and indicates incomplete combustion. The technician should adjust air and draft to reduce smoke while monitoring CO2 and stack temperature. Excess air and nozzle size are not indicated by a smoke spot alone.

Question 18Oil Heat Systems

Which type of oil is most commonly specified for residential oil-fired heating appliances in the United States?

  1. No. 6 heavy oil
  2. Biodiesel B100
  3. No. 4 residual oil
  4. No. 2 heating oil Correct answer.

Explanation

No. 2 heating oil is the standard fuel for residential furnaces and boilers because it flows well and burns cleanly in small burners. No. 4 and No. 6 are heavier residual fuels used in larger commercial equipment, and B100 is not a drop-in residential fuel without equipment changes.

Question 19Oil Heat Systems

An oil boiler short-cycles on the high-limit aquastat. Which condition is the most likely cause?

  1. The nozzle is clogged
  2. The circulator is running continuously
  3. The boiler water temperature rises faster than the load can remove it Correct answer.
  4. The cad cell is misaimed

Explanation

Short cycling on high limit occurs when the burner adds heat faster than the system removes it, often from low water flow, a failed circulator, or a limit set too low. A misaimed cad cell or clogged nozzle would cause lockout or flame problems, not high-limit tripping.

Question 20Oil Heat Systems

What is the primary purpose of the oil burner's retention head?

  1. To filter oil before the nozzle
  2. To regulate pump pressure
  3. To sense flame light
  4. To hold the flame at the burner and stabilize combustion Correct answer.

Explanation

The retention head shapes airflow and holds the flame near the burner for stable ignition and combustion. Filtration, flame sensing, and pressure regulation are handled by other components.

Question 21Oil Heat Systems

A technician is bleeding air from a single-line oil system after running the tank dry. What is the correct approach?

  1. Leave the bleed valve open during normal operation
  2. Open the bleed valve on the pump and run the burner until oil flows free of air, then close it Correct answer.
  3. Increase the nozzle size to help prime
  4. Remove the cad cell during bleeding

Explanation

Air must be purged from the pump and lines before the burner will fire reliably. The bleed valve is opened briefly while the pump runs until bubble-free oil appears, then closed. Leaving it open wastes oil and prevents pressure buildup, and removing the cad cell or changing the nozzle does not aid priming.

Question 22Oil Heat Systems

Which combustion reading indicates the greatest efficiency concern on an oil furnace?

  1. Steady draft with stable flame
  2. Low smoke with normal draft
  3. Moderate CO2 with normal stack temperature
  4. Low CO2 with high stack temperature Correct answer.

Explanation

Low CO2 combined with high stack temperature signals excess air and heat loss up the flue, both of which reduce efficiency. Normal CO2, stack temperature, and draft readings indicate acceptable operation.

Question 23Oil Heat Systems

What does the term "trial for ignition" mean in an oil burner primary control sequence?

  1. The interval between oil deliveries
  2. The warm-up period for the ignition transformer
  3. The time required to heat the boiler water
  4. The period when the control allows the burner to run while checking for flame Correct answer.

Explanation

Trial for ignition is the timed interval during which the primary control runs the burner and looks for a flame signal. If no flame is proven before the trial period ends, the control locks out. It is not related to boiler heating time, deliveries, or transformer warm-up.

Question 24Oil Heat Systems

A technician notices the oil burner runs but the boiler never reaches setpoint and the stack temperature stays low. Which condition should be suspected?

  1. A cad cell with too much light exposure
  2. An oversized pump
  3. A nozzle that is partially clogged or undersized Correct answer.
  4. A barometric damper stuck fully open

Explanation

Low firing rate from a clogged or undersized nozzle delivers less heat than the load requires, so the boiler never reaches setpoint and stack temperature stays low. A damper stuck open would raise heat loss but the burner would still deliver its rated input; cad cell light and pump size do not reduce heat output this way.

Question 25Oil Heat Systems

What is the recommended action when a technician finds a cracked electrode insulator on an oil burner?

  1. Bend the electrode to compensate
  2. Increase the transformer output
  3. Replace the electrode assembly Correct answer.
  4. Coat the crack with high-temperature cement

Explanation

A cracked insulator allows the spark to arc to ground instead of across the electrode gap, causing unreliable ignition. The electrode assembly should be replaced. Coating or bending may temporarily mask the symptom but will not restore reliable sparking.

Question 26Oil Heat Systems

A technician measures the inside diameter of a residential oil tank fill pipe and finds it smaller than the minimum allowed by code. What is the minimum nominal pipe size for an oil tank fill pipe?

  1. 1 1/4 inches Correct answer.
  2. 2 inches
  3. 1/2 inch
  4. 3/4 inch

Explanation

Standard oil installation codes require the fill pipe to be at least 1 1/4 inch nominal pipe size so the tank can be filled at a reasonable rate without pressure buildup or spillage. Smaller sizes are permitted only in specific cases such as certain inside tanks, but the general rule is 1 1/4 inch.

Question 27Oil Heat Systems

Which grade of fuel oil is typically dyed red and sold for use in residential heating oil burners without road tax?

  1. No. 6 bunker oil
  2. No. 1 kerosene
  3. No. 2 heating oil Correct answer.
  4. No. 4 heavy oil

Explanation

No. 2 heating oil is the standard residential fuel and is dyed red to mark it as untaxed heating fuel, distinguishing it from on-road diesel. No. 1 kerosene is a lighter, clearer blend used for special cold-weather or vaporizing applications.

Question 28Oil Heat Systems

What is the purpose of the oil burner nozzle's swirl (turbulator) element?

  1. It closes off flow when the burner is off.
  2. It filters grit from the oil before atomization.
  3. It regulates the oil pump discharge pressure.
  4. It imparts a spinning motion to the oil so it breaks into a fine cone of droplets. Correct answer.

Explanation

The swirl element inside the nozzle forces the pressurized oil into a rotational path, and when it exits the orifice it forms a hollow cone of finely atomized droplets. Good atomization is essential for clean, efficient combustion.

Question 29Oil Heat Systems

A technician needs to check the oil pump's cut-off pressure to confirm the valve is sealing. Which instrument is appropriate?

  1. A manometer on the flue
  2. A vacuum gauge on the inlet line
  3. A voltmeter across the primary control
  4. A pressure gauge on the pump discharge line Correct answer.

Explanation

Cut-off pressure is measured with a pressure gauge teed into the pump discharge so the technician can watch how quickly pressure drops after the burner stops. A slow drop indicates a leaking cut-off valve that can cause after-drip and smoke.

Question 30Oil Heat Systems

What does the term 'off-cycle loss' refer to on an oil-fired boiler?

  1. Heat that escapes up the chimney while the burner is not firing Correct answer.
  2. Pressure lost through the boiler relief valve
  3. Electrical energy used by the primary control in standby
  4. Oil that drips from the nozzle after shutdown

Explanation

Off-cycle loss is the heat that continues to travel up the flue when the burner is off because the boiler stays warm. It is reduced by a barometric damper, flue damper, or boiler insulation.

Question 31Oil Heat Systems

Which condition indicates a blocked oil tank vent on a residential oil system?

  1. The cad cell reads low resistance during operation.
  2. The burner runs hotter than normal.
  3. The stack temperature rises steadily.
  4. The tank collapses slightly inward or the burner starves after delivery. Correct answer.

Explanation

A blocked vent prevents air from entering the tank as oil is drawn out, creating a vacuum that can collapse the tank or starve the burner, especially right after a fill. The vent must be kept clear and sized to the tank.

Question 32Oil Heat Systems

Which component on an oil burner ignites the atomized oil spray?

  1. The cad cell
  2. The ignition transformer and electrodes Correct answer.
  3. The primary control
  4. The retention head

Explanation

The ignition transformer steps voltage up to around 10,000 volts, and the electrodes create a spark across the gap that ignites the atomized oil. The cad cell only senses flame, and the retention head shapes the air pattern.

Question 33Oil Heat Systems

A technician is checking the oil burner's cad cell circuit and needs to confirm the cell is seeing flame. Which meter reading indicates a properly detected flame?

  1. A few hundred to roughly 1,000 ohms or less Correct answer.
  2. About 500 ohms or higher
  3. Infinite resistance
  4. About 1,600 ohms or higher

Explanation

When the cad cell sees flame, its resistance drops to a low value, typically a few hundred to about 1,000 ohms depending on the control. High or infinite resistance indicates no flame.

Question 34Oil Heat Systems

A nozzle rated 0.85 gph at 100 psi is operated at 125 psi. What is the approximate firing rate?

  1. About 1.20 gph
  2. About 0.85 gph
  3. About 0.95 gph Correct answer.
  4. About 0.75 gph

Explanation

Nozzle flow varies with the square root of the pressure ratio. Multiplying 0.85 by the square root of 1.25 gives about 0.95 gph, so raising pressure above the rated 100 psi increases the firing rate.

Question 35Oil Heat Systems

A two-line oil system is being installed with the tank below the burner. What is the function of the bypass plug in the pump?

  1. It returns oil from the pressure regulator to the suction side for two-pipe operation. Correct answer.
  2. It vents air from the nozzle line.
  3. It limits maximum discharge pressure.
  4. It converts the pump from two-pipe to one-pipe operation.

Explanation

In two-pipe operation the bypass plug must be installed so surplus oil from the regulator returns to the tank rather than deadheading against the pump seal. Removing the plug converts the pump to single-pipe operation.

Question 36Oil Heat Systems

During a combustion test the technician reads 9.5 percent CO2 and a stack temperature of 500 F with a net stack temperature of 450 F. What does the high net stack temperature most likely indicate?

  1. A clogged nozzle
  2. Heat being wasted up the flue Correct answer.
  3. A clean, efficient heat exchanger
  4. Excess combustion air

Explanation

A high net stack temperature means a large amount of heat is leaving through the flue rather than being transferred to the space. Causes include a dirty heat exchanger, overfiring, or excessive draft.

Question 37Oil Heat Systems

An oil furnace pressure switch used to prove draft is failing to close. Which condition should the technician check first?

  1. The cad cell resistance
  2. The nozzle size
  3. The transformer output
  4. The sensing tube and flue draft Correct answer.

Explanation

A draft-proving pressure switch closes only when it senses adequate negative pressure, so a blocked sensing tube, disconnected hose, or poor draft will keep it open. The technician should verify the tube and actual draft before condemning the switch.

Question 38Oil Heat Systems

A technician finds the oil burner short-cycling on the cad cell safety lockout with a strong flame present. Which condition is the most likely cause?

  1. A low oil tank level
  2. A worn nozzle
  3. A closed oil shutoff valve
  4. A dirty or misaimed cad cell Correct answer.

Explanation

If the flame is strong but the control still locks out, the cad cell is not reliably seeing the flame, often due to dirt, misalignment, or a failing cell. Cleaning or repositioning the cell usually resolves the problem.

Question 39Oil Heat Systems

Which practice is correct when setting the air shutter and combustion head on an oil burner?

  1. Set the air shutter fully open to guarantee low smoke.
  2. Adjust air only after the nozzle is removed.
  3. Set the head as far forward as possible to reduce noise.
  4. Adjust the head and air to achieve a clean flame with smoke near zero and the highest practical CO2. Correct answer.

Explanation

Proper setup balances air and head position to produce a stable flame with minimal smoke and the highest practical CO2 without CO rise. Excess air lowers efficiency, while too little air causes smoke.

Question 40Oil Heat Systems

A technician measures a vacuum of 12 inches Hg on the inlet of an oil pump with the tank located at normal height. What does this reading suggest?

  1. The pump is overpressurized.
  2. The cut-off valve is leaking.
  3. A restricted suction line or partially plugged filter Correct answer.
  4. The nozzle is oversized.

Explanation

High inlet vacuum indicates resistance on the suction side, such as a clogged filter, kinked line, or blocked tank outlet. Normal single-line vacuum is typically much lower, so 12 inches Hg warrants inspection.

Question 41Oil Heat Systems

What is the correct sequence when starting up an oil burner after a nozzle replacement?

  1. Set the electrodes, run the burner, then install the nozzle.
  2. Run combustion tests, then install the nozzle.
  3. Bleed the pump, install the nozzle, then set the electrode gap.
  4. Install the nozzle, bleed the pump, set the electrodes and head, then run combustion tests. Correct answer.

Explanation

The nozzle must be installed first, followed by bleeding air from the pump so oil reaches the nozzle, then electrode and head settings, and finally combustion testing to confirm proper operation. Skipping bleeding causes air lock and no ignition.

Question 42Oil Heat Systems

An oil-fired boiler's relief valve is weeping and the technician finds the system pressure near 30 psi with a cold boiler. Which condition should be investigated?

  1. A clogged nozzle
  2. A weak ignition transformer
  3. A dirty cad cell
  4. A failed expansion tank or a stuck fill valve Correct answer.

Explanation

High cold-system pressure usually means the expansion tank has lost its air charge or the fill valve is passing city water into the loop. The relief valve opens near its rating to protect the boiler.

Question 43Oil Heat Systems

Which statement best describes the purpose of the primary control's pre-purge feature on an oil burner?

  1. It heats the nozzle before ignition.
  2. It raises pump pressure before firing.
  3. It removes unburned oil vapors from the combustion chamber before the spark is energized. Correct answer.
  4. It vents combustion gases from the boiler before ignition.

Explanation

Pre-purge runs the blower for a set period before ignition to clear any accumulated oil vapors from the chamber, reducing the risk of puffback or explosion on light-off. The purge time is set by the primary control.

Question 44Oil Heat Systems

A technician observes a delayed ignition with a loud rumble on an oil furnace. Which cause is most consistent with this symptom?

  1. A cad cell that is too clean
  2. A transformer with excessive secondary voltage
  3. An oil tank that is overfilled
  4. A nozzle with a restricted strainer delivering a weak spray Correct answer.

Explanation

A restricted nozzle strainer or partially blocked nozzle can delay atomization and allow oil to pool before ignition, producing a delayed light-off and rumble. The nozzle and strainer should be replaced and combustion retested.

Question 45Oil Heat Systems

Which oil-fired appliance component is designed to prevent backflow of flue gases into the space when the chimney is cold?

  1. The combustion air blower
  2. The aquastat
  3. The barometric draft regulator Correct answer.
  4. The stack (flue) damper

Explanation

The barometric draft regulator opens to admit room air into the flue, stabilizing draft and helping prevent spillage when the chimney is cold or draft is excessive. It maintains a steady negative pressure at the appliance outlet.

Question 46Oil Heat Systems

A technician is diagnosing an oil burner that trips the primary control's lockout after about 30 seconds of flame. Which test best distinguishes a flame-sensing problem from a fuel problem?

  1. Measure the cad cell resistance with the burner running. Correct answer.
  2. Check the flue draft.
  3. Measure the oil pump vacuum.
  4. Check the nozzle size.

Explanation

If the burner lights and runs but still locks out, measuring cad cell resistance during flame confirms whether the control is receiving a valid flame signal. Low resistance with lockout points to the control or wiring, while high resistance points to the cell or sighting.

Question 47Oil Heat Systems

An oil burner is firing at 1.25 gph with a measured CO2 of 8 percent and zero smoke. What does the low CO2 most likely indicate?

  1. Too little combustion air
  2. A clogged flue
  3. Excess combustion air Correct answer.
  4. A nozzle that is too small

Explanation

CO2 below the typical 10 to 12 percent range for oil usually means excess air is diluting the flue gases, which lowers efficiency by carrying heat up the stack. Reducing excess air while keeping smoke at zero improves efficiency.

Question 48Oil Heat Systems

A technician must determine why an oil boiler's stack temperature is high while CO2 is normal and smoke is zero. Which cause is most consistent with these readings?

  1. A leaking pump seal
  2. An undersized nozzle
  3. A low water level in the boiler
  4. A fouled heat exchanger surface Correct answer.

Explanation

Normal CO2 and zero smoke indicate clean combustion, so a high stack temperature points to poor heat transfer, often from soot or scale on the heat exchanger. Cleaning the exchanger restores efficiency.

Question 49Oil Heat Systems

On a two-line oil system, the technician notices the pump inlet vacuum rises steadily while the burner runs and the tank is nearly empty. Which action is most appropriate?

  1. Increase pump pressure to compensate.
  2. Install a larger nozzle.
  3. Reduce the nozzle size.
  4. Check the tank outlet and suction line for restriction, and refill the tank. Correct answer.

Explanation

Rising inlet vacuum with a low tank indicates the pump is struggling to draw oil, often from a nearly empty tank or a restricted outlet. The correct action is to restore fuel supply and clear the restriction rather than alter pump or nozzle settings.

Question 50Oil Heat Systems

A technician measures 300 ppm CO in the flue gas of an oil furnace with a visible flame and no smoke. Which interpretation is most accurate?

  1. CO indicates excess air and should be corrected by closing the air shutter.
  2. CO readings are irrelevant on oil-fired equipment.
  3. CO at this level is normal for oil and needs no action.
  4. Elevated CO with no smoke suggests incomplete combustion and requires immediate investigation. Correct answer.

Explanation

Any significant CO in flue gas indicates incomplete combustion and a potential safety hazard, even without visible smoke. The technician should check air adjustment, flame impingement, and the heat exchanger before returning the appliance to service.