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TX HVAC Contractor practice questions

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Question 1Air Conditioning Systems

In a vapor-compression air conditioning system, which sequence correctly describes the path of refrigerant through the circuit?

  1. Compressor, evaporator, condenser, metering device
  2. Condenser, evaporator, compressor, metering device
  3. Evaporator, metering device, compressor, condenser
  4. Compressor, condenser, metering device, evaporator Correct answer.

Explanation

Refrigerant leaves the compressor as a high-pressure hot vapor, rejects heat and condenses in the condenser, drops in pressure at the metering device, and boils in the evaporator before returning to the compressor.

Question 2Air Conditioning Systems

Superheat is best described as which of the following?

  1. The total heat content of the refrigerant vapor
  2. The number of degrees a liquid is below its condensing temperature
  3. The temperature difference between the condensing and evaporating temperatures
  4. The number of degrees a vapor is above its saturation temperature at a given pressure Correct answer.

Explanation

Superheat is the difference between the actual vapor temperature and the saturation temperature at the same pressure. It is measured at the evaporator outlet and confirms that no liquid refrigerant reaches the compressor.

Question 3Air Conditioning Systems

In an air-cooled condenser, the refrigerant does which of the following?

  1. Rises in temperature without changing state
  2. Absorbs heat from the space and boils
  3. Drops sharply in pressure as it passes through the coil
  4. Rejects heat to the outdoor air and condenses to a liquid Correct answer.

Explanation

The condenser receives hot high-pressure vapor from the compressor and rejects heat to outdoor air or water, causing the refrigerant to condense into a liquid. The pressure drop that follows occurs at the metering device, not in the condenser coil.

Question 4Air Conditioning Systems

A properly installed condensate drain trap on an air handler serves to do which of the following?

  1. Remove biological growth from the drain pan
  2. Raise the temperature of the condensate so it drains faster
  3. Reduce the noise made by dripping condensate
  4. Keep cabinet air from escaping through the drain and block sewer gas Correct answer.

Explanation

A trap holds a column of water that balances cabinet pressure so air is neither blown out nor drawn in through the drain opening. It also prevents sewer gas from backing up into the airstream.

Question 5Air Conditioning Systems

The primary function of the compressor in a vapor-compression system is to do which of the following?

  1. Raise the pressure and temperature of refrigerant vapor so it can condense at ambient conditions Correct answer.
  2. Remove heat from the refrigerant vapor
  3. Condense refrigerant vapor into liquid
  4. Meter liquid refrigerant into the evaporator

Explanation

The compressor is the pump of the system; it raises vapor pressure so the refrigerant's condensing temperature is above the outdoor ambient, allowing heat to be rejected.

Question 6Air Conditioning Systems

A thermostatic expansion valve controls refrigerant flow in order to do which of the following?

  1. Keep condenser subcooling at a fixed value
  2. Maintain a constant evaporator pressure regardless of load
  3. Limit compressor current draw during high ambient conditions
  4. Hold a relatively constant superheat at the evaporator outlet Correct answer.

Explanation

The TXV senses evaporator outlet temperature and suction pressure and modulates flow so a small, controlled amount of superheat is maintained over a wide range of loads.

Question 7Air Conditioning Systems

In a heat pump, the reversing valve changes the unit from cooling to heating by doing which of the following?

  1. Reversing the rotation of the compressor
  2. Changing the speed of the outdoor fan
  3. Bypassing the expansion device entirely
  4. Changing the direction of refrigerant flow through the indoor and outdoor coils Correct answer.

Explanation

The reversing valve reroutes discharge and suction lines so the indoor coil becomes the condenser in heating and the outdoor coil becomes the evaporator.

Question 8Air Conditioning Systems

The primary purpose of a return air filter in a residential system is to do which of the following?

  1. Protect the blower and coils from dirt accumulation Correct answer.
  2. Improve the efficiency of the compressor
  3. Control the humidity of the supply air
  4. Remove odors from the supply air

Explanation

The filter protects the blower wheel, coil, and ductwork from dirt. A loaded filter also reduces airflow and capacity as a side effect, but protection of equipment is its design purpose.

Question 9Air Conditioning Systems

A split system equipped with a TXV is being charged in cooling mode. The technician should do which of the following?

  1. Charge to the subcooling value specified on the rating plate Correct answer.
  2. Charge to a fixed 10 degrees F superheat at the service valve
  3. Add refrigerant until compressor amperage equals nameplate RLA
  4. Add refrigerant until the suction line begins to sweat

Explanation

Because the TXV holds superheat nearly constant at any charge, superheat is not a valid charging indicator in cooling. Subcooling changes with charge, so it is compared to the rating plate value at the specified indoor and outdoor conditions.

Question 10Air Conditioning Systems

A system using a fixed orifice (piston) metering device should normally be charged in cooling mode by doing which of the following?

  1. Adjusting charge to the superheat listed on the rating plate for the measured indoor wet-bulb and outdoor dry-bulb Correct answer.
  2. Adding charge until suction pressure reaches a fixed value
  3. Weighing in the nameplate charge plus an additional 10 percent
  4. Adjusting charge to a target subcooling

Explanation

Fixed-orifice systems change superheat with load, so manufacturers publish a target superheat based on indoor wet-bulb and outdoor dry-bulb. Charge is adjusted until measured superheat matches that target.

Question 11Air Conditioning Systems

Restricted indoor airflow across a direct-expansion evaporator coil most directly causes which of the following?

  1. Lower head pressure with a warmer suction line
  2. Lower suction pressure, reduced capacity, and possible coil icing Correct answer.
  3. Unchanged suction pressure with higher capacity
  4. Higher suction pressure and higher superheat

Explanation

Less air over the coil removes less heat, so the refrigerant boils at a lower pressure and temperature. This lowers suction pressure and capacity and can frost the coil.

Question 12Air Conditioning Systems

A fixed-orifice system operating at a normal load shows 30 degrees F of superheat at the evaporator outlet. This most likely indicates which of the following?

  1. A condenser fan failure
  2. A high outdoor ambient temperature
  3. An undercharge or a restricted metering device Correct answer.
  4. An overcharge of refrigerant

Explanation

High superheat means the refrigerant is fully boiled well before the coil outlet, so the coil is starved. That pattern is typical of low charge or a restriction at the metering device.

Question 13Air Conditioning Systems

Under current federal rules, which statement about R-22 is accurate?

  1. R-22 is the required refrigerant for new split systems sold in Texas
  2. R-22 may no longer be used in any existing equipment
  3. Production and import for new equipment have ended, and only recovered or reclaimed R-22 may be used for service Correct answer.
  4. R-22 may still be manufactured in unlimited quantities for residential equipment

Explanation

Under the Montreal Protocol and EPA rules, U.S. production and import of HCFC-22 for new equipment ended in 2020. Only recovered, recycled, or reclaimed R-22 may be used to service existing equipment.

Question 14Air Conditioning Systems

When filling a refrigerant recovery cylinder, the maximum fill level is generally which of the following?

  1. 100 percent of the water capacity
  2. No limit applies if the cylinder is DOT approved
  3. 90 percent of the cylinder's gross weight
  4. 80 percent of the cylinder's water capacity Correct answer.

Explanation

DOT and EPA rules limit recovery cylinders to 80 percent of water capacity to leave room for liquid expansion. Overfilling can cause hydrostatic rupture of the cylinder.

Question 15Air Conditioning Systems

A system has a return air dry-bulb of 75 degrees F and a supply air dry-bulb of 55 degrees F. The temperature split is which of the following?

  1. 130 degrees F, obtained by adding the two readings
  2. 5 degrees F, indicating low airflow
  3. 55 degrees F, indicating excessive airflow
  4. 20 degrees F, which is within the typical range Correct answer.

Explanation

Temperature split, or delta T, is return dry-bulb minus supply dry-bulb. A split of about 15 to 20 degrees F is typical for residential equipment at design airflow; readings far outside that range point to airflow, charge, or duct problems.

Question 16Air Conditioning Systems

A single-stage residential air conditioner cools the house quickly on mild days but short-cycles repeatedly. The most likely cause is which of the following?

  1. The system is oversized for the load Correct answer.
  2. The system is undersized for the load
  3. The filter is restricted
  4. The system is undercharged

Explanation

An oversized unit removes sensible heat too quickly, satisfies the thermostat before it can dehumidify, and then cycles off. The result is short run times and often clammy indoor conditions.

Question 17Air Conditioning Systems

A technician measures 0.9 in. w.c. of total external static pressure on an air handler rated at 0.5 in. w.c. This reading most likely indicates which of the following?

  1. A blower that should be slowed down to reduce the reading
  2. Excessive duct restriction or an undersized duct system Correct answer.
  3. Normal operation, since anything below 1.0 in. w.c. is acceptable
  4. A clean filter and an unrestricted coil

Explanation

Most residential air handlers are rated near 0.5 in. w.c. of total external static pressure. A higher reading means the blower is working against excessive resistance from undersized ducts, a dirty coil, or a restrictive filter.

Question 18Air Conditioning Systems

Which condition produces the greatest latent heat removal at an evaporator coil?

  1. Low indoor humidity with a large sensible load
  2. High indoor humidity with a coil temperature above the dew point
  3. Low indoor relative humidity with a warm coil
  4. High indoor humidity with a coil temperature well below the dew point Correct answer.

Explanation

Latent heat is removed only when the coil surface is below the air's dew point, allowing moisture to condense. High humidity plus a cold coil maximizes the moisture removed from the airstream.

Question 19Air Conditioning Systems

A system shows abnormally high subcooling along with high head pressure. The most likely cause is which of the following?

  1. An undercharge of refrigerant
  2. A failed evaporator blower motor
  3. An overcharge or a liquid line restriction Correct answer.
  4. A TXV stuck wide open

Explanation

Subcooling rises when liquid backs up in the condenser, which happens with too much charge or with a restriction downstream that limits liquid flow. Both conditions also elevate head pressure.

Question 20Air Conditioning Systems

During a heat pump defrost cycle, the unit does which of the following?

  1. Switches to cooling mode with the indoor fan on high speed
  2. Shuts off the compressor and energizes auxiliary heat only
  3. Runs the indoor and outdoor fans together to speed melting
  4. Reverses to cooling mode with the outdoor fan off and energizes backup heat Correct answer.

Explanation

In defrost, the reversing valve switches the unit to cooling so hot discharge gas warms the outdoor coil and melts frost. The outdoor fan is stopped, and backup electric heat is energized so the space does not get cold.

Question 21Air Conditioning Systems

A suction line at the evaporator outlet measures 52 degrees F, and the suction pressure corresponds to a saturation temperature of 40 degrees F. What is the superheat?

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

Explanation

Superheat equals the actual suction line temperature minus the saturation temperature at the same pressure: 52 minus 40 equals 12 degrees F. The saturation value must come from the pressure-temperature chart for the specific refrigerant.

Question 22Air Conditioning Systems

An electric heater rated 10 kW (34,130 Btu/h) produces a 30 degree F temperature rise. Using cfm = Btu/h divided by (1.08 times temperature rise), the airflow is approximately which of the following?

  1. 3,413 cfm
  2. 633 cfm
  3. 1,895 cfm
  4. 1,053 cfm Correct answer.

Explanation

Substituting gives 34,130 divided by (1.08 times 30), or 34,130 divided by 32.4, which is about 1,053 cfm. The 1.08 constant applies to standard air at sea level.

Question 23Air Conditioning Systems

A dual-run capacitor is rated 45/5 microfarads. The compressor will not start; the HERM section measures 0 microfarads and the FAN section measures 5 microfarads. The technician should do which of the following?

  1. Replace the capacitor, because the HERM section has failed Correct answer.
  2. Replace the compressor because the windings are open
  3. Install a hard-start kit and leave the capacitor in place
  4. Check the thermostat and contactor coil before replacing anything

Explanation

A dual-run capacitor has separate HERM and FAN sections, and a failed section reads near zero microfarads on a capacitance meter. After discharging the capacitor and confirming the reading, the correct repair is replacement with one of the same ratings.

Question 24Air Conditioning Systems

Two airstreams have the same dry-bulb temperature but different wet-bulb temperatures. The airstream with the lower wet-bulb temperature has which of the following?

  1. A higher relative humidity
  2. A lower relative humidity and a lower dew point Correct answer.
  3. More latent heat per pound of dry air
  4. A higher dew point temperature

Explanation

At the same dry-bulb, a lower wet-bulb means less moisture in the air. That corresponds to lower relative humidity, a lower dew point, and less latent heat per pound of dry air.

Question 25Air Conditioning Systems

A pump-down cycle controlled by a low-pressure switch is used in a system to do which of the following?

  1. Remove refrigerant vapor from the evaporator before shutdown to prevent liquid migration into the compressor Correct answer.
  2. Raise suction pressure before the compressor restarts
  3. Purge non-condensables from the condenser during operation
  4. Maintain a minimum condensing pressure during cold weather

Explanation

Pump-down runs the compressor until the low-pressure switch opens, pulling vapor out of the evaporator and suction line. This keeps liquid refrigerant from migrating into the compressor crankcase during the off cycle.

Question 26Air Conditioning Systems

An air-cooled condenser in a residential split system changes the refrigerant from a high-pressure vapor to a high-pressure liquid by doing which of the following?

  1. Absorbing heat from the outdoor air as the refrigerant passes through the coil
  2. Rejecting heat to the outdoor air as the refrigerant passes through the coil Correct answer.
  3. Adding heat to the refrigerant to raise its saturation temperature
  4. Dropping the refrigerant pressure as it passes through the coil

Explanation

Condensing occurs at a constant pressure on the high side, with heat being rejected from the refrigerant to the surrounding air. Removing that heat collapses the vapor into a liquid, which then leaves the coil slightly subcooled before traveling to the metering device.

Question 27Air Conditioning Systems

A residential split system has a nominal capacity of 3 tons. Using the usual 400 cfm per ton rule, the blower should move approximately how much air?

  1. 800 cfm
  2. 400 cfm
  3. 1,000 cfm
  4. 1,200 cfm Correct answer.

Explanation

The standard rule of thumb for residential direct-expansion cooling is about 400 cfm of supply air per ton of capacity. Multiplying 3 tons by 400 cfm per ton gives 1,200 cfm, which is also the airflow the equipment nameplate typically expects.

Question 28Air Conditioning Systems

A technician is commissioning a new residential split system whose nameplate specifies R-410A. Which statement about this refrigerant is accurate?

  1. It is a single-component refrigerant that may be topped off with R-22.
  2. It operates at higher pressures than R-22 and is paired with polyolester oil. Correct answer.
  3. It is a low-pressure refrigerant that is paired with mineral oil.
  4. It must always be charged as a vapor into the low side of the system.

Explanation

R-410A is an HFC blend that runs roughly 50 to 60 percent higher pressures than R-22 and requires synthetic polyolester lubricant, which is hygroscopic. It is a near-azeotropic blend, so it is removed and charged as a liquid to maintain the intended composition.

Question 29Air Conditioning Systems

An air handler is installed in an attic where the blower pulls return air through the cabinet, placing the drain pan under negative pressure. Why does the condensate drain need a trap?

  1. To keep the drain line from freezing during cold weather
  2. To stop refrigerant from migrating into the drain pan
  3. To increase the pitch of the drain line inside the attic
  4. To offset the negative cabinet pressure so condensate can flow out Correct answer.

Explanation

When the coil is on the negative-pressure side of the blower, the cabinet pressure pulls air inward through the drain opening. A properly sized trap provides a water seal that balances that negative pressure, allowing condensate to drain and preventing air and dirt from being drawn into the pan.

Question 30Air Conditioning Systems

What is the purpose of a crankcase heater on a compressor?

  1. Driving refrigerant out of the crankcase oil to limit migration Correct answer.
  2. Raising the discharge pressure during cold-weather operation
  3. Keeping the motor windings dry while the compressor is off
  4. Warming the suction gas so that superheat at the compressor stays high

Explanation

During the off cycle, refrigerant can condense and dissolve into the compressor oil, reducing lubricity and causing oil foaming and slugging at startup. The crankcase heater warms the oil so that liquid refrigerant is vaporized and driven out, keeping the oil as pure as possible.

Question 31Air Conditioning Systems

In a heat pump, the balance point is best described as which of the following?

  1. The outdoor temperature at which heat pump capacity equals the heat loss of the house Correct answer.
  2. The outdoor temperature at which the defrost cycle is initiated by the coil sensor
  3. The indoor temperature at which the thermostat switches to emergency heat
  4. The outdoor temperature at which compressor lockout occurs and strip heat carries the full load

Explanation

The balance point is where the building heat loss curve crosses the heat pump capacity curve, meaning the unit can just maintain indoor temperature by itself. Below that outdoor temperature, supplemental heat must make up the shortfall.

Question 32Air Conditioning Systems

A blower moves 800 cfm through a duct system at 0.25 in. w.c. of external static pressure. If airflow through the same ductwork is increased to 1,600 cfm, the static pressure will be closest to which value?

  1. 0.50 in. w.c.
  2. 1.00 in. w.c. Correct answer.
  3. 0.35 in. w.c.
  4. 2.00 in. w.c.

Explanation

For a fixed duct system, static pressure varies with the square of the airflow (the fan law relationship). Doubling the airflow multiplies the static pressure by 2 squared, or 4, so 0.25 in. w.c. becomes about 1.00 in. w.c.

Question 33Air Conditioning Systems

A hermetic compressor has worn or broken suction and discharge valves. Which set of operating readings is most typical?

  1. Lower-than-normal suction pressure with higher-than-normal head pressure and reduced capacity
  2. Higher-than-normal suction pressure with lower-than-normal head pressure and reduced capacity Correct answer.
  3. Low suction pressure and low head pressure with a normal supply-to-return temperature split
  4. Normal suction pressure with abnormally high subcooling at the condenser outlet

Explanation

Damaged valves allow discharge gas to leak back into the cylinder and suction side, so the compressor cannot build normal head pressure and cannot pull the suction pressure down to its usual level. The result is poor pumping, high suction, low head, and a loss of capacity.

Question 34Air Conditioning Systems

A thermostatic expansion valve loses its remote bulb charge because the capillary tube breaks. The valve will most likely respond in which way?

  1. It will modulate on suction pressure alone without affecting superheat.
  2. It will hold its last position and continue to feed the coil normally.
  3. It will close and starve the evaporator, producing low suction pressure and high superheat. Correct answer.
  4. It will open fully and flood the evaporator with liquid refrigerant.

Explanation

The bulb charge supplies the opening force on the diaphragm, while the evaporator pressure and the spring oppose it. Losing the charge removes the opening force, so the spring drives the valve closed, starving the coil and driving superheat and suction pressure to abnormal extremes.

Question 35Heating Systems

Temperature rise across a forced-air gas furnace is calculated by subtracting the return-air temperature from the:

  1. outdoor design temperature
  2. thermostat setpoint temperature
  3. supply-air temperature in the plenum Correct answer.
  4. flue-gas temperature at the draft hood

Explanation

Temperature rise is the difference between the air entering the furnace and the air leaving it, with the supply reading taken in the plenum outside line of sight of the heat exchanger to avoid radiant error. The result is compared against the data-plate range to judge airflow.

Question 36Heating Systems

The AFUE rating printed on a furnace rating plate represents the:

  1. steady-state combustion efficiency at full fire
  2. percentage of excess air needed for clean combustion
  3. annual fuel utilization efficiency over a heating season Correct answer.
  4. maximum allowable supply-air temperature rise

Explanation

AFUE is a seasonal efficiency that accounts for cycling, standby, and jacket losses in addition to steady-state performance, so it is always lower than combustion efficiency measured at steady state.

Question 37Heating Systems

In a standing-pilot gas furnace, the thermocouple performs which function?

  1. It steps 24 volts down to millivolts for the thermostat circuit
  2. It measures manifold pressure and modulates the burner
  3. It generates a small voltage that holds the gas valve open Correct answer.
  4. It senses main burner flame and locks out the igniter

Explanation

A thermocouple heated by the pilot flame produces roughly 250 to 750 millivolts, which energizes the gas valve magnet. If the pilot goes out, voltage collapses and the valve closes on its own.

Question 38Heating Systems

The high-limit switch on a forced-air furnace opens the circuit to the gas valve when:

  1. the blower motor exceeds its rated amp draw
  2. the thermostat is satisfied and stops calling for heat
  3. the bonnet temperature rises above its setpoint Correct answer.
  4. flue draft falls below the pressure switch setpoint

Explanation

The limit is a temperature safety that opens on excess bonnet temperature, shutting off the burners while the blower keeps running to remove residual heat. It resets automatically once temperature drops to the reset point.

Question 39Heating Systems

The typical manifold pressure for a natural gas appliance is:

  1. 13.5 in. w.c.
  2. 3.5 in. w.c. Correct answer.
  3. 9.0 in. w.c.
  4. 0.5 in. w.c.

Explanation

Natural gas appliances are normally set to about 3.5 in. w.c. at the manifold, with supply pressure upstream at roughly 7 in. w.c. Liquefied petroleum appliances run near 10 to 11 in. w.c. at the manifold.

Question 40Heating Systems

A properly adjusted natural gas burner flame should appear:

  1. orange and lifting away from the burner ports
  2. bright yellow with visible floating tips
  3. blue and stable with a soft, slightly yellow tip Correct answer.
  4. short, hard, and blue-white with a loud roar

Explanation

A correct flame is blue, attached to the ports, and stable, with a slight yellow tip acceptable. Yellow, floating, or lifting flames signal improper primary air, draft, or gas pressure.

Question 41Heating Systems

A sediment trap installed in the gas piping ahead of the appliance controls is intended to:

  1. collect dirt and scale before it reaches the gas valve Correct answer.
  2. provide a test port for leak detection
  3. absorb pressure surges from the gas meter
  4. increase gas velocity at the burner orifice

Explanation

The trap is a capped dirt leg installed at a change of direction so debris settles out before it can foul the valve or plug orifices. It must be located downstream of the shutoff valve and ahead of the controls.

Question 42Heating Systems

A single-wall vent connector from a natural-draft furnace to a chimney must be installed to slope:

  1. downward toward the chimney at 1/2 in. per foot
  2. downward toward the appliance at 1/4 in. per foot
  3. level, with no slope required
  4. upward toward the chimney at 1/4 in. per foot Correct answer.

Explanation

Vent connectors slope upward toward the chimney at a minimum of 1/4 in. per foot so buoyant flue gases rise and any condensate drains back toward the appliance.

Question 43Heating Systems

A confined space contains a furnace rated at 100,000 BTU/hr input. Using the two-permanent-opening method, each opening must have a free area of at least:

  1. 100 square inches Correct answer.
  2. 200 square inches
  3. 50 square inches
  4. 1,000 square inches

Explanation

Each of two permanent openings is sized at 1 sq in of free area per 1,000 BTU/hr of total input, so 100,000 BTU/hr requires 100 sq in at each opening. One opening is near the top of the space and one near the bottom.

Question 44Heating Systems

On an induced-draft furnace, the pressure switch functions to:

  1. regulate manifold gas pressure at the burner
  2. measure static pressure in the supply duct
  3. limit bonnet temperature during a call for heat
  4. prove draft before the ignition sequence continues Correct answer.

Explanation

The pressure switch closes when the inducer develops enough draft, permitting ignition to proceed. If it does not close, the control locks out before any gas is admitted.

Question 45Heating Systems

A flame rollout switch that has tripped indicates:

  1. normal operation after an extended heating cycle
  2. the thermostat is calling for cooling
  3. flames escaping the burner area, often from a blocked flue Correct answer.
  4. low gas supply pressure at the manifold

Explanation

Rollout switches are manual-reset devices mounted near the burner box. They trip when flame rolls out of the combustion chamber, typically because of a blocked flue, inadequate draft, or a failed heat exchanger.

Question 46Heating Systems

A condensing furnace reaches efficiency above 90% chiefly because it:

  1. recirculates flue gas into the combustion air
  2. reduces excess air to nearly zero at the burner
  3. removes latent heat from flue gas in a secondary exchanger Correct answer.
  4. fires at a higher manifold pressure than standard furnaces

Explanation

The secondary heat exchanger cools flue products below the dew point, releasing the latent heat of the water vapor formed during combustion. The resulting condensate must be trapped and drained.

Question 47Heating Systems

When converting a natural gas furnace to LP gas operation, the technician must:

  1. enlarge the burner orifices for greater gas flow
  2. raise manifold pressure to 3.5 in. w.c.
  3. remove the regulator spring from the gas valve
  4. install smaller orifices and reset the valve for LP Correct answer.

Explanation

LP gas carries roughly 2,500 BTU per cubic foot compared with about 1,000 for natural gas, so smaller orifices are needed to hold the rated input. The gas valve regulator is also adjusted or converted to LP manifold pressure.

Question 48Heating Systems

A gas furnace installed at 5,000 ft elevation typically requires:

  1. smaller orifices, derated about 4% per 1,000 ft Correct answer.
  2. larger orifices and higher manifold pressure
  3. higher manifold pressure with the original orifices
  4. no adjustment, since the gas valve compensates

Explanation

Lower atmospheric pressure at altitude reduces the mass of gas flowing through a fixed orifice, so input is derated, commonly about 4% per 1,000 ft above sea level. The derate is normally achieved with smaller orifices per the manufacturer's tables.

Question 49Heating Systems

The fan-on setting of a combination fan/limit control is always set above the fan-off setting in order to:

  1. prevent rapid cycling of the blower Correct answer.
  2. deliver air before the plenum has warmed
  3. start the blower before the burners ignite
  4. reset the limit automatically after a trip

Explanation

The spread between fan-on and fan-off temperatures provides a differential so the blower does not chatter on and off as plenum temperature fluctuates. The blower comes on only after the heat exchanger has warmed.

Question 50Heating Systems

In a typical induced-draft furnace with hot surface ignition, the correct sequence at a call for heat is:

  1. blower starts, inducer starts, gas valve opens, igniter heats
  2. gas valve opens, igniter heats, inducer starts, blower starts
  3. inducer starts, pressure switch closes, igniter heats, gas valve opens Correct answer.
  4. igniter heats, gas valve opens, inducer starts, blower starts

Explanation

Draft must be established and proven before ignition is attempted. The inducer runs, the pressure switch closes, the igniter heats, the valve opens, flame is proven, and the blower starts after a timed delay.