HVACR Flash Cards

A good vent draft reading on a category I natural gas furnace is:

0.02 to -0.04 in WC A good vent draft pressure for a category I natural gas furnace is -.02 to -.04 inches of water column.

The specific heat of air is:

0.24 BTU Specific heat is the quantity of heat required to change 1 pound of a particular substance 1°F. Every substance has a different specific heat, i.e. the specific heat of water is 1. The specific heat of air is .24.

The specific gravity of natural gas is:

0.60 Specific gravity of a gas is the ratio of density compared to air at a standard temperature and pressure. The specific gravity of air is 1 and natural gas is .55 to .80.

Water has a specific gravity of 1. What is the specific gravity of number 2 fuel oil?

0.84 to 0.87 Number 2 fuel oil has a specific gravity between .84 and .87 compared to water which has a specific gravity of 1. This makes fuel oil lighter than water.

Find the heat gain for the exposed knee-wall. The building is 40 ft. wide with an exterior wall height of 10 ft. The roof has a 9 on 12 pitch. Insulation R-11 Outdoor Summer design 92°F Indoor design 72°F Attic temperature 130°F

1,583 BTUh The first step is to find the area of the exposed knee wall; this would be calculated by taking the pitch and one half of the wall (20 x 9) 180 inches or 15 feet then multiply (15 x 40) divided by 2 which would equal 300 ft2. Next find the u value of the insulation: 1/11 =.091; then find the ΔT (130 - 72) = 58°. U factor times ΔT times square footage equals Btu (.091 x 58 x 300) = 1,583.4.

Good field practice requires insulating the condensate line for at least the first ____ feet of the run from the indoor unit installed in a hot attic.

10 Many southern codes state that the condensate line must be insulated for the first 10 feet to prevent sweating, which could create a moisture problem.

What is one "therm"?

100,000 BTU The term "therm" is used to describe an energy volume of 100,000 BTU most often associated with natural gas.

What is the flash point temperature of #2 fuel oil?

100° F. 100°F is the minimum flash point for fuel oil #2 as specified by ASTM Standard D396-02. Flash point is the minimum temperature at which a sufficient quantity of ignitable vapor forms on the surface of a heated liquid fuel.

When testing the efficiency of a water cooled condenser, the refrigerant condensing temperature should be no more than _______ higher than the leaving water temperature.

10° F Water medium is much more efficient than air cooled condensers, therefore a higher than 10° F split will cause high head pressure and the system would not operate efficiently.

What is the approximate manifold gas pressure for a propane furnace?

11 inches of water column Most manufacturers specify 11 to 13 inches of water column gas pressure as the pressure setting for propane fuelled residential furnaces.

A parallel circuit consists of three resistors: R1 = 24 ohms R2 = 60 ohms R3 = 40 ohms What is the total resistance of the circuit?

12 Ohms The smallest resistor is 24 ohms, therefore the total must be less than 24. One of the parallel circuit laws state that the total resistance is always less than the smallest resistor in a parallel circuit. There are two formulas for finding total resistance in a parallel circuit:

An R-410A heat pump operating in the cooling mode has a suction pressure of 118 psig. Adding 2 psig for pressure drop in the coil, the true outlet pressure is 120.5 psig. The temperature of the suction line at the outlet of the evaporator is 53° F. What is the superheat?

12 degrees. Saturation temperature for R410A at 120.5 psig is 41°F and suction line temperature is 53°F. Therefore, 53 minus 41 equals 12 degrees.

How much resistance must be placed in a 240 volt circuit in order to limit the power to 480 watts?

120 Ohms 480 watts divided by 240 volts will equal 2 amps, therefore Ohm's law will apply voltage divided by amperage equals resistance:240 ÷ 2 = 120 OhmsOther formulas which can be used are as follows:W ÷ I2 = R or E2 ÷ W = R

Refrigerant R-134a vapor enters the condenser at 124 psig, 100° F. It leaves the condenser at 80° F. What will be the approximate pressure?

124 psig. The pressure should not change entering or leaving the condenser. The condenser's primary design is to expel the heat from the refrigerant that was absorbed in the low side of the system. The compressor will maintain the pressure and only the temperature will change.

A return grill, not using restrictive filters, should be sized to provide a minimum of:

144 unrestricted square inches per ton of capacity. It is recommended that return grills have a minimum of 144 unrestricted square inches per ton of capacity.

How much of all the available water on the planet is fresh water?

2.5% Of all the water available on our planet less than 2.5% is fresh water, three quarters of which is frozen or otherwise not attainable. Only 0.3 percent is available for use in the form of rivers and lakes, most of which are polluted in some way, requiring treatment before drinking.

If a pressure control has been adjusted to cut in at 30 psig, in order to have a 10 psig cut out, the differential setting of the control must be:

20 Psig The difference between cut in and cut out is known as differential setting. In this problem, cut in is 30 psig and cut out is 10, therefore 30 minus 10 equals 20 psig.

Showers can use as much as:

22% of the water used in a home. Showers can use as much as 22% of the water used in homes. High efficiency shower-heads use approximately 1.5 to 2.3 gallons of water per minute.

How many watts of power are used by a heater operating at 480 volts and drawing 5 amps?

2400 watts Power laws P = I x E. 480 x 5 = 2400 watts.

A furnace is using 25,800 watts of power. What is the approximate KW rating?

25.8 kW Kilo watt is equal to wattage divided by 1000. Therefore, 25,800 ÷ 1000 = 25.8KW

What is equivalent to one PSI?

27.7 inches of water column To convert inches of water column to PSI it is necessary to multiply the inches of water column by .0361:27.7 x .0361 = 0.99997 PSITo convert pascals to PSI multiply pascals by 0.000145037738:35 x 0.000145037738 = .00507632083

What is the minimum vertical drop from the outlet of the condensate drain pan to the inlet of the P trap when the evaporator is ahead of the blower?

3 inches The water level in the trap must be low enough that a pressure higher than the static pressure of the air-handler would be required to make it flow back into the drain pan. The higher the air-handle static pressure, the more vertical drop is required.

What is the minimum clearance for access panels on an outdoor condensing unit?

30 inches Most manufacturers recommend 30 inches of clearance from the structure, bushes, hedges and any other obstructions.

A 800 CFM furnace operating with a 10 kW heater at 230 volt AC would have a temperature rise of_____________ °F.

39.5 916;T is equal to BTUs divided by (1.08 x CFM).Therefore, 10,000 x 3.413 = 34,13034,130 ÷ (1.08 x 800) = ΔT34,130 ÷ 864 = 39.5

When installing a roof-mounted unit on a flat roof, what is the minimum distance above the highest water level expected that it should be installed?

4 inches Most manufacturers recommend 4 inches of height above the highest water level.

What is the recommended CFM across the evaporator coil of a heat pump?

400 to 450 CFM. Manufacturers recommend 400 to 450 CFM per ton should be used to limit the head pressure when operating in the heating cycle. Too little or too much air flow can cause severe problems.

High-temperature refrigeration applications normally provide box temperatures of:

47F to 60F High temperature refrigeration applications normally are 47 to 60 degrees box temperature. The storage of such things as flowers, candy and some fruit and vegetables must be maintained at these temperatures.

A compressor is operating with a discharge pressure of 235.3 psig and a suction pressure of 35.3 psig. What is the compression ratio (pumping ratio)?

5:1 Compression ratio is found by dividing the absolute discharge pressure by the absolute suction pressure. PSIG is converted to absolute by adding 14.7 to the pressure.In this problem, discharge pressure 235.3 plus 14.7 equals 250 PSIA and the suction pressure 35.3 plus 14.7 equals 50 PSIA.250/50 = 5The ratio is 5 to 1

When setting a high pressure cut-out on a R-410A system which has an air cooled condenser, the setting should be approximately _______.

610 psig. The pressure must be set above the working pressure for an R-410A system. The design condensing temperature is from 65°F to 120°F. The pressure is 417 psi for 120 degree condensing temperature. The cutout must be set approximately 45% - 50% above the operating pressure.

41.65 lbs. of refrigerant passing through a water-cooled condenser will give up 10 Btu's of heat per lb. If 5 gals. of water pass through with an entering temperature of 80°F, what would be the temperature of the water leaving the condenser?

90°F Five gallons of water is equivalent to 41.65 lbs, therefore the BTUs absorbed by water is equal to the BTUs given up by the refrigerant. Since the water will absorb 10 BTUs of heat, the temperature will rise 1 degree per lb. Water enters at 80°F and would leave at 90°F.

What should be used to light an acetylene torch?

A striker OSHA proclaims that a striker is the only safe lighting device for acetylene torches. Propane lighters used for lighting cigarettes are considered very hazardous and can explode. Matches may put fingers in jeopardy due to close proximity of intense heat.

On a gas manifold assembly, where is the "air-fuel mixing zone" located?

After the orifice inside the burner. On a gas manifold assembly. the air fuel mixing zone is after the orifice inside the burner. The gas velocity creates a slight negative pressure inside the burner tube. This negative pressure sucks the air for combustion into the burner tube where the air and fuel mix prior to exiting the burner head.

What is an energy audit?

An assessment of energy used. An energy audit is an accounting system and analysis for energy usage and energy conservation in a structure or building.

What two flame detection devices are used in oil fired primary controls?

Bimetal and cadmium sulfide The bimetal stack safety control and the cadmium sulfide cell are the primary flame detection devices for oil burners.

What is a possible result of charging a high fractionation refrigeration system until the sight glass is clear?

Both a and b Technicians attempting to clear the sight glass when charging with high fractionation refrigerant can lead to overcharging and have poor system performance or possible compressor damage.

What is the rapid oxidation of any material classified as combustible matter called?

Combustion Combustion is the rapid oxidation of any material classified as combustible matter.

What is "Tidal and Ocean Energy"?

Electrical power produced from ocean waves, tides and currents by propelling submerged turbines. By using the oceans or rivers current power can be produced by the use of submerged turbines.

What type of system uses the earth as both a heating and cooling medium?

Geothermal system Geothermal heat pumps uses the earth as both heating and cooling medium for absorbing and releasing heat to the ground from the refrigerant.

What does carbon monoxide bond with when it is inhaled into the lungs?

Hemoglobin in the blood to form carboxyhemoglobin. When carbon monoxide is inhaled into the lungs, it bonds with hemoglobin in the blood to form carboxyhemoglobin.

What can an overcharged system cause?

High side pressure increase An overcharge of refrigerant can cause pressure to increase by over feeding the evaporator and refrigerant backing up in the condenser.

An increase of gas pressure will cause a (an):

Increase of gas velocity and an increase of primary air. Increasing the gas pressure increases the venturi affect created by the flow of gas into the burner. This increase will increase the volume of air pulled into the burner to mix with the fuel. If the pressure is increased too much gas spillage may occur as the burner venturi may not be large enough to accommodate the increased volume of fuel and air.

If non-condensables are trapped in a condenser, what symptoms will be present?

Increased heat build-up, high amperage and high discharge pressure. Non-condensables in an air conditioning system are gases that will not condense at the same temperature and pressure in which the refrigerant will condense. Non-condensables take up valuable space in the condenser which is used for removing the heat from the refrigerant allowing it to condense into a liquid. If the refrigerant cannot condense into a liquid taking up less space, the gas will cause the temperature and pressure to rise causing high amperage draw.

What is used to check how well a building retains the conditioned air from heating or cooling?

Infrared camera test An infrared camera indicates temperature ranges in different colors. Warm areas of a building are indicated by shades of red and cool areas by shades of blue.

What must a technician do to convert a propane gas furnace to natural gas?

Install larger orifices. To convert from propane to natural gas it is necessary to change the orifices to larger ones because natural gas has less BTU per cubic foot and is operated at a much lower pressure.

Why can an R-410A system be topped off after a leak?

It behaves like a pure compound. R-410A is a near azeotropic blend and has a temperature glide of less than 0.3°F.

What is the purpose of a delayed oil valve?

It delays the oil pressure to the nozzle. The delayed oil valve delays pressure to the nozzle until the pump has reached full RPM. The delayed oil valve is not pressure sensing only time delayed by electrical resistance. The delay of oil helps prevent smoking on burner start up.

What is heat, which causes a change in the state of a material without a change in temperature, called?

Latent heat Heat, which causes a change of state without a change in temperature, is known as latent heat. An example of latent heat is water changing to ice or vice a versa. Latent heat is the heat released or absorbed by a body or a thermodynamic system during a process that occurs without a change in temperature.

What does a technician need to do to measure temperature rise?

Measure supply and return air temperatures and subtract the return air temperature from the supply air temperature. Temperature rise is the differential temperature measured across the heat exchanger. The temperature change across the heat exchanger is measured by measuring the supply air temperature and subtracting the return air temperature. The value remaining is the temperature rise.

Most heating elements are mounted on which type of insulator?

Mica or ceramic Ceramic or mica insulators are used when electrical insulation is necessary for high heat applications.

The electric heat element is usually made of what material?

Nickel and chromium Heat elements are made of Ni-chrome which is nickel and chromium. They are combined to create a resistance wire.

If the combustion process is complete, how much carbon monoxide should be measured in the flue gases?

No measurable quantity If combustion is complete there should be minimal or no carbon monoxide measured in the flues gases of the appliance.

What does a temperature differential across a filter-drier indicate?

Restricted filter-drier When a system has a restricted filter drier, it causes a pressure drop across the drier, thus restricting the flow of refrigerant, and the difference in pressure results in a temperature drop across the drier at the point of the restriction.

What is the major difference between a heat pump and an air conditioner?

Reversing valve The reversing valve is the component that will reverse the flow of refrigerant when changing from air conditioning to heating.

A technician takes the temperature rise of an electric furnace. What is being measured?

Sensible heat only Sensible heat is being measured since there is no moisture being added or removed.

What should a heat pump that uses a reciprocating compressor be equipped with because of the possibility of liquid flood-back?

Suction accumulator Since the heat-pump operates in adverse conditions, the compressor must be protected by a suction line accumulator to prevent any liquid refrigerant from entering the compressor.

What are two types of thermal storage?

Tank and cell Thermal energy storage comprises a number of technologies that store thermal energy in energy storage reservoirs for later use. A system that uses ice directly is usually called a tank system, and a system using plastic balls for storage is called cell system.

What is the first step in the heating sequence of operation?

Thermostat calls for heat The first step in the heating sequence of operation is the thermostat calls for heat. Nothing else happens until the thermostat calls for heat.

What is an advantage of ester-based oils such as Polyolester?

They have a wax-free composition. POE lubricants are completely wax free and have a proven track record of mixing with HFC refrigerants.

What are some wind pathways for air penetration during heating season?

Through connections at a foundation/wall, floor/wall, wall/window, wall/roof and through materials such as concrete blocks and mortar joints . Wind may enter a building during the heating season through slab openings for pipes, cracks or openings in walls, windows and door leaks, holes in wall plates for electrical, pipe and entertainment conductors, as well as roof openings.

Why must an oil separator be kept warm?

To prevent refrigerant from condensing in the separator during the off cycle. An oil separator is a device installed in the discharge line between the compressor and condenser to help remove oil from the gaseous refrigerant. The oil is returned to the compressor by the use of a float or some other means. If during the off cycle the oil separator is allowed to cool, some gaseous refrigerant may condense and displace the oil and flow back to the compressor causing damage on start-up.

What is the main function of an air condition condenser?

To remove heat from the refrigerant. The main function of any air condition condenser is to remove both sensible and latent heat from the refrigerant by causing the gas from the compressor to go through three stages; de-superheat, change of state and sub-cool.

What is the purpose of a flame rod?

To sense flame as a safety device. The flame rod is the smaller of two electrodes in the control circuit. The control emits an AC current which completes its circuit through either the pilot flame or main flame as a rectified DC current to the flame rod.

What is a liquid receiver used for?

To store liquid refrigerant. A receiver is used to store liquid refrigerant for varying loads, multi evaporators and pump-down.

Fuses are sized by:

Volts and Amps Fuses are rated in voltage and amps.

What is a sling psychrometer used to measure?

Wet and dry bulb temperature A sling psychrometer is a device that consist of two thermometers fitted in a hinged bracket used for spinning the two thermometers. One of the bulbs is kept moist with distilled water on a sock or wick placed over the thermometer's tip. As the thermometers are rotated, evaporation of water from the wick lowers the temperature indicating a lower temperature than that of the dry-bulb thermometer. This is the Wet bulb temperature which will be lower than the dry bulb and can never be above dry bulb measurement. The lower the relative humidity, the lower the wet bulb temperature. The difference in the two bulbs readings can be applied to a psychrometric chart to calculate relative humidity and other properties of air.

What do the terms awning, bay and bow casement refer to?

What do the terms awning, bay and bow casement refer to?

When is the installation of a "P" trap mandatory in a drain line?

When the evaporator coil is located on the negative side of the blower. Due to the negative pressure, without a P-trap air will be sucked through the drain and will block the flow of condensate.

During the cooling cycle the outdoor coil becomes;

a condenser. During the cooling cycle the outdoor coil becomes the condenser and rejects heat to the ambient outdoor air.

Short cycling during pump down can be caused by:

a leaking compressor suction valve. Short cycling is caused by discharge pressure leaking back to the low side through the compressor suction valve.

An ohmmeter should never be connected to ___________.

a live circuit Connecting an ohmmeter to live voltage can damage the meter and possibly harm the person testing with the meter.

Polyolester (POE) oils stored in plastic containers will _______.

absorb moisture through the plastic Polyolester (POE) Oils are very hygroscopic and must be stored in a metal containers. Plastics containers may be permeable to moisture.

When a cad cell is sighting flame, its resistance should be:

below 1,000 ohms of resistance. The cadmium sulfide cells resistance decreases with exposure to light from the flame.

A refrigeration system used to cool another refrigeration system is called a _______.

cascade system Cascade systems are an arrangement of two or more refrigeration systems that are used in series to achieve ultra-low temperatures. The evaporator cools the condenser of the other system.

What is one of the main reasons that reciprocating compressors are not 100% efficient?

clearance volume Efficiency of reciprocating compressors are reduced due to the re-expanding gas that is in the cylinder clearance space . The greater the clearance between the top of the piston and bottom of the valve plate, the more the gas will re-expand on the down stroke of the piston reducing the pumping efficiency .

A ___________ is commonly used on a single phase fractional horse power compressor motor to disconnect the start winding after the motor starts.

current relay Current type motor starting relays are used for assisting in the starting function of single phase AC motors of resistance start, induction run and capacitor start, induction run types. The primary function is to remove the start capacitor or start winding from the circuit.

When the defrost cycle is terminated, the evaporator fans will _______.

delay coming on The high velocity fans are delayed from coming on to prevent any water left on the evaporator from been blown into cooler space causing an ice safety hazard.

The On Demand Water Circulation System:

delivers hot water immediately without the cold water waste period. On demand water circulation systems will deliver hot water immediately without cold water waste.

When testing a fuel unit cut-off, the pressure at the nozzle port should ______.

drop and hold at approximately 80% of the pump's pressure setting With a pressure gauge connected directly to the nozzle port on the pump, the pressure on shut down should never be more than 20% below the pump set operating pressure.

The temperature rise method of determining air volume CFM is mostly used with a/an;

electric resistive heat air handler. The Btu output along with the temperature difference across the furnace can easily be calculated and applied to the sensible heat formula. CFM = Btu divided by (1.08 times the temperature difference).

The temperature of a single compound at 100% saturated vapor with no superheat is _____________.

equal to its saturation temperature for that pressure Refrigerants that are a single compound will follow the pressure temperature saturation chart.

The clearance space in a reciprocating compressor _________.

has an effect on compressor capacity Efficiency of reciprocating compressors are reduced due to the gas that re-expands in the clearance space. The greater the clearance between the top of the piston and bottom of the valve plate, the more the gas will re-expand on the down stroke of the piston. The suction valve will not open until the pressure is lower than pressure on the other side of the valve.

A pump-down system should have a (an);

liquid receiver. Depending on the size of the evaporator and liquid line, the condenser may not be large enough to hold all the refrigerant during pump down.

When current flows, a _____________ is produced around the conductor.

magnetic field When current flows through a conductor, a magnetic field is created and extends out from the conductor.

When the temperatures of a structure both inside and outside are equal, there is _____.

no heat transfer In order for heat to transfer there must be a difference in temperature. This follows the second law of thermodynamics. (Heat travels from a warmer area to cooler area.)

For gravity fed installations where the oil tank is above the fuel pump there should be _____.

no measured vacuum On installations where the tank is installed above the fuel pump there should be no measured vacuum. A vacuum measurement would indicate there is a restriction in the fuel delivery line or the fuel filter.

Single phase motor speed is determined by the __________.

number of stator poles Alternating current motor speed is based on the number of poles in the motor.Examples:2 pole motor would be 3600 RPMs4 pole motor 1800 RPMs

When the heat pump operates in defrost mode, the outdoor fan is normally ________.

off In order for the heat-pump to melt any accumulated ice off the outdoor coil the heat-pump will reverse cycle and the outdoor fan is shut off to increase the head pressure to melt the ice.

Two 120 volt A/C incandescent light bulbs are wired in series; one is a 60 watt bulb and the other is a 4 watt bulb. The result would be _________.

only the 4 watt bulb will light The 60 watt bulb will act as a conductor and only the lower watt bulb would light. There is a small amount of voltage drop across the 60 watt bulb but usually not enough to make it light.

In a (direct expansion) evaporator, liquid refrigerant must boil away as close to the end of the coil as possible in order to:

operate at high efficiency. Direct type evaporators are designed to operate most efficiently with approximately 10 degrees superheat. In order to have superheat, all liquid must be boiled away before reaching the end of the evaporator.

An electronically commutated motor (ECM) is designed to ________.

provide proper air flow under various conditions ECM motors are designed to maintain the proper air flow based on the static pressure to a predetermined limit set by the manufacturer.

When using an electronic leak detector, the sensor probe _______.

should be moved at approximately 1 inch per second Moving the probe any faster than the recommended speed of one inch per second may result in leaks not being detected.

The high limit safety on a condensing furnace ____________.

shuts the burner down if the furnace overheats The high limit safety shuts the burner down in the event that the furnace is operating outside of its designed temperature range. The safety switch only shuts the burner down allowing the fan to continue running to cool the furnace.

Elevation changes greater than a few hundred feet can _________.

significantly change the design conditions Elevation changes can significantly change the design conditions based on temperature change, wind speed or other environmental conditions.

A properly functioning condenser should always have ______________ leaving the condenser;

sub-cooled, liquid The condenser causes the refrigerant to go through three processes: first de-superheat, second a change of state and finally the refrigerant is cooled below saturation temperature. The refrigerant is sub-cooled at this point. The amount of sub-cooling is critical to the system efficiency.

In a direct expansion (DX) evaporator system, refrigerant leaves the evaporator as ______.

superheated vapor In a direct expansion evaporator the refrigerant enters as a liquid and as it absorbs heat the liquid boils changing into a vapor. After the change to a vapor it absorbs more heat in the last rows of the evaporator and becomes a superheated gas.

R-407C has _______-.

the ability to fractionate R-407C is a near-azeotropic ternary blend of hydrofluorocarbons refrigerants. It contains difluoromethane (R-32), pentafluoroethane (R-125) and 1,1,1,2-tetrafluoroethane (R-134a) which all have different pressure temperature relationships.

Before a vapor / compression system is opened for service _____________.

the refrigerant must be recovered using approved equipment Refrigerant must be recovered with approved equipment according to EPA Section 608. Failure to recover could result in loss of certification, fines and having to appear in federal court.

Many multi-cylinder reciprocating compressors have a capacity control device called;

un-loaders Un-loaders are devices used on reciprocating compressors to control capacity. Un-loaders will disengage one or more cylinders hydraulically or electrically lifting the suction valves off its seat thus controlling capacity. When suction valve is lifted off its seat the gas enters the cylinder on the down stroke and when the compressor on the up stroke will force the gas back into the suction port. This will reduce capacity and energy consumption of a compressor.

Tube within a tube, shell and coil, and shell and tube are all types of:

water cooled condensers. All of the above condensers are classified as water cooled type condensers. Tube within a tube condensers are used in systems such as: residential water source heat pumps and shell type condensers are used in commercial applications.

Waste-heat recovery systems can be adapted to air conditioning systems for ______.

water heating A waste heat recovery unit is an energy recovery heat exchanger that recovers heat from hot gas of the refrigerant before it enters the condenser to preheat water in a water heater.