I85PREP
⌘ K
MD HVAC study materials
MD HVAC study resources

MD HVAC practice questions

One question at a time. Build your MD HVAC 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 1Refrigeration Principles and System Components

For a pure refrigerant, the saturation temperature is the temperature at which:

  1. the refrigerant is neither liquid nor vapor
  2. only liquid refrigerant is present in the vessel
  3. only vapor refrigerant is present in the vessel
  4. liquid and vapor are in equilibrium Correct answer.

Explanation

A pure refrigerant at saturation has liquid and vapor coexisting in equilibrium, and each saturation pressure corresponds to one saturation temperature. Heat added or removed at that point changes state rather than temperature. This pressure-temperature relationship is the basis for using gauge readings to determine evaporating and condensing temperatures.

Question 2Refrigeration Principles and System Components

The heat a refrigerant absorbs while changing from liquid to vapor at a constant temperature is called:

  1. latent heat Correct answer.
  2. sensible heat
  3. superheat
  4. specific heat

Explanation

Latent heat is energy absorbed or released during a change of state with no accompanying temperature change. In the evaporator, most of the cooling comes from latent heat as liquid refrigerant boils. Sensible heat, by contrast, changes the temperature of a substance without changing its state.

Question 3Refrigeration Principles and System Components

Superheat is the number of degrees the refrigerant vapor is:

  1. above its saturation temperature Correct answer.
  2. below its evaporating temperature
  3. above its condensing temperature
  4. below its condensing temperature

Explanation

Superheat measures how far vapor has traveled past its saturation temperature at the existing pressure. It is calculated by subtracting the saturation temperature of the suction pressure from the measured suction line temperature. Superheat protects the compressor by confirming that refrigerant arrives as vapor rather than liquid.

Question 4Refrigeration Principles and System Components

The main job of the condenser in a vapor-compression cycle is to:

  1. reject heat so the refrigerant vapor condenses Correct answer.
  2. raise the refrigerant from suction to discharge pressure
  3. remove moisture and acid from the refrigerant
  4. control the flow of refrigerant into the evaporator

Explanation

In the condenser, high-pressure discharge vapor gives up heat to the surrounding air or water and condenses into liquid. Removing heat at essentially constant pressure is what allows the refrigerant to change state and return to the liquid line. Pressure rise is produced by the compressor, not the condenser.

Question 5Refrigeration Principles and System Components

Which function does the metering device perform in the refrigeration cycle?

  1. it drops the pressure and feeds the evaporator Correct answer.
  2. it separates refrigerant oil from the vapor
  3. it raises the refrigerant pressure from suction to discharge
  4. it removes heat from the refrigerant vapor

Explanation

The metering device throttles high-pressure liquid down to evaporator pressure and regulates how much refrigerant enters the coil. The pressure drop causes part of the refrigerant to flash into vapor and cools the remaining liquid to the evaporating temperature. Flow regulation lets the device match coil feed to the load.

Question 6Refrigeration Principles and System Components

If the pressure exerted on a liquid refrigerant is reduced, its boiling point will:

  1. remain unchanged
  2. equal the condensing temperature
  3. fall Correct answer.
  4. rise

Explanation

Boiling point and pressure rise and fall together for a given refrigerant. Lowering the pressure reduces the saturation temperature, which is why the metering device can create a cold evaporator from warm liquid. This relationship is read directly from a pressure-temperature chart.

Question 7Refrigeration Principles and System Components

Subcooling is found by subtracting the liquid line temperature from the:

  1. evaporator saturation temperature
  2. condensing saturation temperature Correct answer.
  3. suction line temperature
  4. compressor discharge temperature at the service port

Explanation

Subcooling is the number of degrees the liquid is below the saturation temperature corresponding to the condensing pressure. It is calculated as condensing saturation temperature minus liquid line temperature. Subcooling confirms that the refrigerant has fully condensed before reaching the metering device.

Question 8Refrigeration Principles and System Components

The compressor in a vapor-compression system:

  1. removes heat from the refrigerant to condense it
  2. adds heat to the refrigerant to boil it
  3. raises vapor pressure from evaporator to condenser level Correct answer.
  4. meters liquid refrigerant into the evaporator

Explanation

The compressor takes low-pressure vapor from the evaporator and compresses it to a pressure whose saturation temperature is above the condensing medium. That pressure rise also raises the vapor temperature, permitting heat rejection in the condenser. The compressor moves vapor only; liquid refrigerant would damage it.

Question 9Refrigeration Principles and System Components

A system holds a suction pressure whose saturation temperature is 40°F, and the suction line temperature measures 52°F. The superheat is:

  1. 12°F Correct answer.
  2. 92°F
  3. 40°F
  4. 52°F

Explanation

Superheat equals the measured suction line temperature minus the saturation temperature of the suction pressure. Here 52°F minus 40°F gives 12°F of superheat. The saturation temperature, not the suction line temperature, is the reference point.

Question 10Refrigeration Principles and System Components

The condensing saturation temperature is 110°F and the liquid line temperature is 95°F. The subcooling is:

  1. 15°F Correct answer.
  2. 205°F
  3. 5°F
  4. 110°F

Explanation

Subcooling is the condensing saturation temperature minus the liquid line temperature. Subtracting 95°F from 110°F gives 15°F of subcooling. A positive value shows the liquid has fully condensed and cooled below saturation.

Question 11Refrigeration Principles and System Components

Flash gas that forms before the refrigerant reaches the evaporator reduces capacity because it:

  1. takes up coil volume that liquid refrigerant could occupy Correct answer.
  2. raises the condensing temperature
  3. increases the mass flow through the metering device
  4. lowers the latent heat of the refrigerant

Explanation

Vapor entering the coil occupies internal volume and surface area that could otherwise be wetted by liquid, so less liquid is available to absorb heat. The heat that vaporized that gas was absorbed upstream in the liquid line, where it does no useful cooling. The result is a coil that is starved of effective liquid feed.

Question 12Refrigeration Principles and System Components

The remote bulb of a thermostatic expansion valve is clamped to the:

  1. suction line at the evaporator outlet Correct answer.
  2. condenser coil return bend
  3. compressor discharge line
  4. liquid line at the receiver outlet

Explanation

The bulb senses the temperature of vapor leaving the evaporator, and the pressure it develops acts on the valve diaphragm to position the needle. This lets the valve hold a set superheat by matching flow to the load. Mounting the bulb elsewhere would feed the wrong signal to the valve.

Question 13Refrigeration Principles and System Components

Compared with a reciprocating compressor of similar capacity, a scroll compressor:

  1. uses a piston and reed valves in each cylinder
  2. uses sliding vanes inside a round housing
  3. compresses gas continuously with few moving parts Correct answer.
  4. uses a high-speed centrifugal impeller

Explanation

In a scroll compressor, two intermeshed spiral wraps create moving pockets that compress gas continuously rather than in discrete strokes. With no suction or discharge valves and only one orbiting member, it has fewer moving parts and less vibration. This continuous action is why scrolls are common in modern residential equipment.

Question 14Refrigeration Principles and System Components

The liquid line filter drier is installed mainly to:

  1. separate oil from the discharge gas
  2. trap moisture and solid particles Correct answer.
  3. lower the pressure of the liquid refrigerant
  4. store refrigerant during low-load operation

Explanation

The filter drier combines a desiccant that adsorbs moisture with a screen that catches debris, protecting the metering device from ice and blockage. Moisture must be removed because it can freeze at the metering device and form acids with oil. It is placed in the liquid line so it handles liquid refrigerant rather than flash gas.

Question 15Refrigeration Principles and System Components

Adding subcooling at the condenser outlet, with all other conditions unchanged, will:

  1. increase the refrigerating effect per pound Correct answer.
  2. reduce the refrigerating effect per pound of refrigerant
  3. leave system capacity unchanged
  4. increase compressor discharge superheat

Explanation

Subcooling lowers the enthalpy of the liquid entering the metering device, and because expansion is essentially isenthalpic the evaporator inlet enthalpy drops as well. The enthalpy difference across the evaporator therefore grows, so each pound of refrigerant absorbs more heat. The practical result is slightly greater capacity and a more solid liquid column at the valve.

Question 16Refrigeration Principles and System Components

Which refrigerant type boils over a range of temperatures rather than at a single saturation temperature?

  1. a single-component refrigerant such as R-22
  2. a zeotropic blend such as R-407C Correct answer.
  3. an azeotropic blend such as R-507A
  4. a pure compound such as R-134a

Explanation

Zeotropic blends have components with different boiling points, so they evaporate and condense over a temperature range called glide. Azeotropic blends behave like a single substance and boil at one temperature. Glide affects how saturation temperatures are read from pressure during charging and superheat checks.

Question 17Refrigeration Principles and System Components

Air that enters a refrigeration system during service will:

  1. raise head pressure and discharge temperature Correct answer.
  2. improve heat transfer in the condenser
  3. lower head pressure and reduce compressor amperage
  4. freeze inside the metering device

Explanation

Air is non-condensable at normal condensing temperatures, so it collects in the condenser and takes up volume that refrigerant vapor would otherwise occupy. The compressor must push against this higher pressure, which raises head pressure, discharge temperature, and current draw. Non-condensables are removed by proper evacuation before charging.

Question 18Refrigeration Principles and System Components

A suction line accumulator is used to:

  1. store liquid refrigerant during the off cycle
  2. keep liquid refrigerant out of the compressor Correct answer.
  3. increase the pressure drop along the suction line
  4. remove moisture from the suction gas

Explanation

The accumulator is a vessel on the suction line that lets liquid droplets separate and boil off before vapor continues to the compressor. It is common on systems such as heat pumps where liquid migration or floodback is possible. Preventing liquid slugging protects compressor valves and bearings.

Question 19Refrigeration Principles and System Components

Continuous bubbles in a liquid line sight glass most often indicate:

  1. an overcharged system
  2. a restriction or low charge causing flash gas Correct answer.
  3. air trapped in the condenser
  4. excessive subcooling at the condenser

Explanation

Bubbles mean the liquid is flashing in the line, which happens when the liquid is at or near saturation because of a shortage of charge or a pressure drop from a restriction. A properly charged system with adequate subcooling shows a clear glass. Condenser subcooling and system pressures are checked to confirm which cause applies.

Question 20Refrigeration Principles and System Components

Adequate refrigerant velocity in the suction line matters mainly because it:

  1. carries oil back to the compressor Correct answer.
  2. reduces pressure drop through the evaporator
  3. cools the compressor motor windings
  4. prevents flash gas at the metering device

Explanation

Oil circulates with the refrigerant, and in horizontal suction lines it must be pushed along by sufficient gas velocity or it will pool and starve the compressor. Line sizing balances enough velocity for oil return against excessive pressure drop. Vertical risers with double risers or traps address this balance at part load.

Question 21Refrigeration Principles and System Components

A packaged unit delivers 36,000 Btu/h of cooling while the compressor draws 3.0 kW. Using 3.412 Btu/h per watt, the COP is approximately:

  1. 12.0
  2. 1.0
  3. 0.3
  4. 3.5 Correct answer.

Explanation

Convert the power input to matching units: 3.0 kW is 3000 W, or about 10,236 Btu/h. COP is the cooling output divided by the energy input, so 36,000 divided by 10,236 gives roughly 3.5. COP is dimensionless because both terms are expressed in the same units.

Question 22Refrigeration Principles and System Components

A system with a TXV shows low subcooling, high superheat, low suction pressure and steady bubbles in the sight glass. The most likely cause is:

  1. an overcharge of refrigerant
  2. an undercharge of refrigerant Correct answer.
  3. a condenser fan failure
  4. a restricted suction line screen

Explanation

Too little charge means the condenser holds a small liquid seal, so subcooling falls while the evaporator is starved and superheat rises. Starving also drops suction pressure, and the flashing liquid shows bubbles at the sight glass. Adding charge until subcooling reaches design value corrects all four symptoms together.

Question 23Refrigeration Principles and System Components

On a system using a zeotropic blend, the evaporating temperature used to calculate superheat should be read at the:

  1. bubble point of the blend
  2. dew point of the blend Correct answer.
  3. critical temperature of the blend
  4. midpoint between bubble and dew point

Explanation

Superheat concerns the vapor leaving the coil, and that vapor is at the dew-point temperature for the suction pressure. Using the bubble point would understate the true superheat and could lead to overcharging. Manufacturers publish both bubble and dew columns on pressure-temperature charts for this reason.

Question 24Refrigeration Principles and System Components

Volumetric efficiency of a reciprocating compressor drops as the pressure ratio increases mainly because:

  1. trapped clearance gas re-expands on the downstroke Correct answer.
  2. the suction valves flutter at high lift
  3. the refrigerant becomes less dense at high head pressure
  4. oil is pushed out of the crankcase

Explanation

Gas left in the clearance volume at the end of discharge must re-expand before the suction valves can open, so a portion of each stroke produces no intake. Higher pressure ratios increase that re-expansion, reducing the fraction of the stroke that actually draws in vapor. This is why capacity falls sharply as evaporating temperature drops.

Question 25Refrigeration Principles and System Components

On a pressure-enthalpy diagram, the refrigerating effect per pound of refrigerant is the enthalpy difference between the:

  1. suction line and the discharge line
  2. compressor inlet and the compressor outlet
  3. condenser inlet and the condenser outlet
  4. evaporator outlet and the expansion valve inlet Correct answer.

Explanation

Refrigerating effect is the heat absorbed in the evaporator, found by subtracting the enthalpy of liquid entering the expansion valve from the enthalpy of vapor leaving the evaporator. Because expansion is essentially isenthalpic, that inlet value equals the evaporator inlet state. It is the horizontal distance across the evaporator on the chart.

Question 26Refrigeration Principles and System Components

One ton of refrigeration is equivalent to a heat removal rate of:

  1. 12,000 Btu/min
  2. 1,000 Btu/h
  3. 12,000 Btu/h Correct answer.
  4. 144,000 Btu/h

Explanation

One ton of refrigeration is defined as the heat required to melt one ton of ice in 24 hours, which works out to 12,000 Btu per hour, or 200 Btu per minute. It is a rate of heat transfer, not a quantity of refrigerant or a temperature.

Question 27Refrigeration Principles and System Components

Heat added to a substance that raises its temperature without changing its state is called:

  1. sensible heat Correct answer.
  2. specific heat
  3. superheat
  4. latent heat

Explanation

Sensible heat produces a measurable temperature change in a single phase. Latent heat, by contrast, is absorbed or released during a phase change while the temperature stays constant.

Question 28Refrigeration Principles and System Components

A refrigerant gauge reads 68 psig. The corresponding absolute pressure is approximately:

  1. 68 psia
  2. 100 psia
  3. 83 psia Correct answer.
  4. 53 psia

Explanation

Absolute pressure equals gauge pressure plus atmospheric pressure, about 14.7 psi at sea level. Adding 68 and 14.7 gives roughly 83 psia.

Question 29Refrigeration Principles and System Components

The purpose of a crankcase heater is to:

  1. raise discharge temperature during cold weather operation
  2. prevent refrigerant from condensing in the oil while the compressor is off Correct answer.
  3. warm the suction gas before it enters the compressor
  4. hold the oil at its normal running temperature while the compressor operates

Explanation

During the off cycle the crankcase is the coldest part of the system, so refrigerant migrates there and condenses into the oil, thinning it. The crankcase heater keeps the oil warm enough that refrigerant stays vaporized and does not dilute the lubricant.

Question 30Refrigeration Principles and System Components

Why must a system using a capillary tube be critically charged?

  1. The compressor is unable to pump vapor
  2. The condenser is oversized for the connected load
  3. The evaporator must remain flooded at all times
  4. There is no receiver to hold excess refrigerant Correct answer.

Explanation

A capillary tube is a fixed restriction with no storage volume downstream of the condenser besides the tubing and coil themselves. Charge therefore has to be matched to the system so that the condenser holds the right amount of liquid at all expected conditions; too much or too little shifts the balance point of the system.

Question 31Refrigeration Principles and System Components

Turning the adjusting screw of a thermostatic expansion valve clockwise (inward) normally:

  1. lowers evaporator pressure by increasing refrigerant flow
  2. decreases superheat by opening the valve wider
  3. has no effect once the valve has been set at the factory
  4. increases superheat by restricting refrigerant flow Correct answer.

Explanation

The adjusting screw changes the spring force that opposes the bulb pressure. Turning it in adds spring force, so a higher bulb pressure is needed to open the valve, which means more superheat is required before the valve admits refrigerant.

Question 32Refrigeration Principles and System Components

A liquid receiver is normally installed:

  1. in the discharge line near the condenser inlet
  2. in the suction line ahead of the compressor
  3. inside the evaporator coil assembly
  4. between the condenser and the metering device Correct answer.

Explanation

The receiver stores liquid refrigerant leaving the condenser so that a solid column of liquid is available to the metering device over a range of loads and conditions. It is placed on the high-pressure liquid side, ahead of the expansion device.

Question 33Refrigeration Principles and System Components

Hot gas bypass is used mainly to:

  1. speed up defrost cycles on outdoor coils
  2. raise condensing pressure during low-ambient operation
  3. purge non-condensable gases from the condenser
  4. prevent low evaporator pressure and coil freeze-up at light load Correct answer.

Explanation

At light load the evaporator can be starved of heat, dropping suction pressure until the coil frosts or the low-pressure control trips. Bypassing discharge vapor to the low side keeps suction pressure and mass flow up so the compressor stays loaded and the coil stays above freezing.

Question 34Refrigeration Principles and System Components

Polyolester (POE) oil is specified for HFC refrigerants mainly because:

  1. it contains chlorine, which improves boundary lubrication
  2. it is miscible with HFCs, where mineral oil is not Correct answer.
  3. it has the highest viscosity index of any lubricant type
  4. it does not absorb moisture from the surrounding air

Explanation

HFC refrigerants are not miscible with mineral oil, so oil would not circulate and return to the compressor. POE oils are miscible with HFCs and carry oil through the circuit as intended.

Question 35Refrigeration Principles and System Components

Under ASHRAE Standard 34, a refrigerant classified as A1 has:

  1. lower toxicity and no flame propagation Correct answer.
  2. lower toxicity and high flammability
  3. higher toxicity and no flame propagation
  4. higher toxicity and flame propagation

Explanation

The letter designates toxicity class and the number designates flammability class. Class A means lower toxicity and class 1 means no flame propagation at standard test conditions, making A1 the least hazardous combination.

Question 36Refrigeration Principles and System Components

A liquid line solenoid valve used with a pump-down cycle serves to:

  1. stop liquid flow to the evaporator so the compressor can pump it down Correct answer.
  2. modulate refrigerant flow to match the evaporator load
  3. relieve excess head pressure when the compressor stops
  4. bleed refrigerant into the evaporator during the off cycle

Explanation

On shutdown the solenoid closes and blocks the liquid line, so the compressor continues running briefly and pulls the low side down into the receiver or condenser. This leaves the evaporator and suction line largely free of liquid refrigerant.

Question 37Refrigeration Principles and System Components

A discharge line oil separator is installed to:

  1. strip non-condensable gases from the discharge gas
  2. filter acidic products out of the refrigerant
  3. cool the discharge gas before it reaches the condenser
  4. remove oil from the discharge gas and return it to the crankcase Correct answer.

Explanation

Some oil always leaves the compressor with the hot discharge gas. An oil separator knocks that oil out of the stream and meters it back to the compressor sump, protecting the evaporator from oil logging and keeping the crankcase level up.

Question 38Refrigeration Principles and System Components

On a pressure-enthalpy diagram, the refrigerant's passage through a metering device is drawn as:

  1. a curve lying along the saturated vapor line
  2. a line of constant entropy sloping to the right
  3. a vertical line at constant enthalpy Correct answer.
  4. a line of constant temperature and constant pressure

Explanation

The expansion process is treated as a throttling process in which no work or heat transfer occurs, so enthalpy stays essentially constant. Pressure falls and temperature drops along a vertical line between the liquid and low-pressure regions of the diagram.

Question 39Refrigeration Principles and System Components

Above the critical temperature of a refrigerant:

  1. its latent heat of vaporization reaches a maximum value
  2. it behaves as a zeotropic mixture with a wide glide
  3. it condenses at a lower pressure than normal
  4. it cannot be condensed regardless of how high the pressure is raised Correct answer.

Explanation

The critical temperature is the point above which the distinction between liquid and vapor disappears. A refrigerant hotter than its critical temperature will not condense no matter how much pressure is applied, so condenser performance collapses in that region.

Question 40Refrigeration Principles and System Components

With evaporator conditions held constant, lowering the condensing temperature will:

  1. increase capacity and reduce compressor power Correct answer.
  2. reduce both capacity and compressor power
  3. reduce capacity and increase compressor power
  4. have no measurable effect on capacity, power, or compression ratio

Explanation

A lower condensing temperature reduces the pressure ratio the compressor must work against, so less work is required per pound of refrigerant. The enthalpy difference across the evaporator also grows slightly, so refrigerating capacity increases while power input falls.

Question 41Heating Systems: Gas, Oil, Electric and Hydronic

For a natural gas furnace, the manifold gas pressure specified by most manufacturers is approximately:

  1. 7.0 in. w.c.
  2. 11.0 in. w.c.
  3. 3.5 in. w.c. Correct answer.
  4. 1.5 in. w.c.

Explanation

Natural gas appliances are normally set to a manifold pressure near 3.5 in. w.c. while the burner is operating, with the exact value taken from the rating plate. The gas valve's built-in regulator reduces the higher supply pressure down to this manifold value.

Question 42Heating Systems: Gas, Oil, Electric and Hydronic

In a standing-pilot gas appliance, the thermocouple serves to:

  1. ignite the main burner
  2. sense flue gas temperature
  3. prove the pilot flame Correct answer.
  4. modulate manifold pressure

Explanation

A thermocouple heated by the pilot generates a small millivolt signal that energizes the gas valve operator and holds it open. If the pilot goes out, the signal collapses and the valve closes to stop unburned gas flow.

Question 43Heating Systems: Gas, Oil, Electric and Hydronic

Oil burner nozzles are rated in gallons per hour at a standard pump pressure of:

  1. 300 psi
  2. 100 psi Correct answer.
  3. 50 psi
  4. 200 psi

Explanation

Nozzle capacity is published at 100 psi pump pressure, so a nozzle must be replaced with a matching rating when pump pressure is changed. Raising pump pressure above 100 psi increases flow above the stamped rating.

Question 44Heating Systems: Gas, Oil, Electric and Hydronic

Electric resistance furnaces stage their heating elements with a sequencer mainly to:

  1. limit startup current draw Correct answer.
  2. reduce the required branch-circuit ampacity
  3. raise the supply air temperature
  4. prevent the blower from running

Explanation

A sequencer brings elements on one at a time so the inrush current and the demand on the service are spread out rather than occurring all at once. The branch circuit and overcurrent protection must still be sized for all elements operating together.

Question 45Heating Systems: Gas, Oil, Electric and Hydronic

The expansion tank in a closed hot water heating system is installed to:

  1. store hot water for domestic use
  2. remove air from the system water
  3. absorb the expansion of heated water Correct answer.
  4. raise the operating pressure at the boiler

Explanation

Water expands as it is heated, and in a closed loop that added volume must go somewhere or pressure will rise sharply. The expansion tank provides a compressible cushion that accepts the added volume.

Question 46Heating Systems: Gas, Oil, Electric and Hydronic

Liquefied petroleum (LP) gas manifold pressure on a typical gas furnace is approximately:

  1. 8.0 in. w.c.
  2. 10.0 in. w.c. Correct answer.
  3. 6.0 in. w.c.
  4. 3.5 in. w.c.

Explanation

LP gas is delivered at higher manifold pressure than natural gas, generally around 10 in. w.c., because of its higher heating value per cubic foot. The appliance must be converted with the correct orifices and regulator spring before firing on LP.

Question 47Heating Systems: Gas, Oil, Electric and Hydronic

A furnace equipped with a secondary heat exchanger and a PVC flue is best described as:

  1. a power-vented non-condensing furnace
  2. a natural draft furnace
  3. an atmospheric boiler
  4. a condensing furnace Correct answer.

Explanation

A condensing furnace extracts additional heat in a secondary exchanger, cooling flue gases below their dew point so water vapor condenses. The low flue temperature allows inexpensive PVC venting, which non-condensing appliances cannot use.

Question 48Heating Systems: Gas, Oil, Electric and Hydronic

A residential hot water boiler is normally protected by a pressure relief valve set at:

  1. 30 psi Correct answer.
  2. 50 psi
  3. 75 psi
  4. 15 psi

Explanation

Residential hot water boilers are typically rated for 30 psi working pressure and carry a relief valve set at that value. The valve must be piped to a safe drain and must never be capped or plugged.

Question 49Heating Systems: Gas, Oil, Electric and Hydronic

A gas furnace lights, burns for about five seconds, and then the gas valve closes while the blower continues to run. The most likely cause is:

  1. a blower motor with a bad bearing
  2. a failed hot surface igniter element
  3. a dirty flame sensor Correct answer.
  4. a gas valve that is stuck open

Explanation

The control board must prove flame through the flame sensor within a short trial period; a coated or poorly grounded sensor cannot conduct the rectification current and the valve is de-energized. The blower then runs out its programmed delay.

Question 50Heating Systems: Gas, Oil, Electric and Hydronic

A condensing furnace runs the inducer continuously but never attempts ignition, and the flue termination is found blocked by snow. The component that most likely prevents ignition is:

  1. the blocked-vent pressure switch Correct answer.
  2. the high limit switch
  3. the gas valve regulator
  4. a rollout switch

Explanation

A blocked flue keeps the inducer from developing the required negative pressure, so the pressure switch stays open and the control never proceeds to the ignition step. Clearing the termination restores the proving signal.