MCC HVAC MIDTERM EXAM

How much auxiliary heat will be required on a design day when a home's heat loss is 35,600 Btuh and the heat pump output is 17,200 Btuh?

(35,600 Btuh - 17,200 Btuh) / 3,413 Btuh/kW = 5.39 kW ((Ref: 54/1))

Absolute zero on the Fahrenheit scale is _____ ^0F

-460 degrees F ((Ref: 7/2))

List three heat sources and heat sinks, and explain the main advantages of each.

1) Air. It is abundant and allows for use of equipment that is economical to install and operate. 2) Water. Large amounts of heat can be transferred, and the volume of water needed is much less than the volume that would be required if air were used. 3) Earth. Ground-source equipment provides for consistent temperatures of heat transfer fluids, which often makes the use of auxiliary heating components unnecessary. ((Ref: 32/3-34/6))

List at least three of the ways in which heat pumps are classified.

1) Heat source and sink 2) Heating and cooling distribution fluid 3) Thermodynamic cycle 4) Building structure 5) Capacity and configuration 6) Limitation of the source and sink ((Ref: 35/4))

List four possible causes of a compressor motor overload condition.

1) High current draw caused by a refrigerant overcharge 2) High system loads on either the evaporator or the condenser 3) Excessively high return air temperatures entering the evaporator 4) High suction superheat caused by a refrigerant undercharge ((Ref: 117/1))

What is the synchronous speed of a six-pole motor operating on power supplied at a frequency of 60 Hz?

1,200 rpm ((Ref: 116/1))

Typical superheat values measured at the evaporator outlet of a properly operating heat pump system is about how many degrees?

10 to 12 degrees ((Ref: 124/2))

Outdoor thermostats should be set to activate supplemental heat when the outdoor ambient temperature is ____°F higher than the calculated balance point.

10. 2 to 3°F ((Ref: 49/1))

One atmosphere of pressure (atmospheric pressure at sea level) is equivalent to _____ psia.

14.696 psia ((Ref: 8/2))

What indoor air volume (expressed in cfm) would be required for a heating unit that has a 45°F temperature rise and an output of 70,000 Btuh?

1440 cfm (CFM=BTU÷(∆T×1.08) ((Ref: 51/2))

A heat pump thermostat is set so that there will be a dead-band range of about ________ °F between the heating and cooling cycles. A closer setting might result in _________ .

2 to 4 °F, frequent cycling. ((Ref: 95/1))

Rotary compressors generally are used in heat pumps with capacities of less than __________ tons.

2 tons ((Ref: 111/3))

The control voltage used in typical residential heat pump systems is how many volts?

24 volts ((Ref: 147/2))

Convert 25 degrees Celsius to an equivalent Fahrenheit temperature.

25 times 1.8, plus 32 ((Ref: 7/2))

Reversing valves can withstand a temperature of approximately how many degrees during installation?

250 degrees ((Ref: 139/2))

At what head pressure does a pressure activated defrost control typically terminate the defrost cycle?

250 to 300 psig ((Ref: 91/2))

The typical compression ratio for a heat pump operating in the cooling mode is approximately _______

3 to 1 ((Ref: 96/7))

At what liquid-line temperature does a temperature-activated defrost control typically terminate the defrost cycle?

50 to 65°F ((Ref: 91/3))

Convert 68 degrees F to an equivalent Celsius temperature.

68 degrees minus 32, divided by 1.8 = 20 degrees Celsius ((Ref: 7/2))

The physical properties of a refrigerant may be represented as a curve on a pressure enthalpy diagram. Plot points that fall outside and to the right of the curve indicate that the refrigerant is a(n) _______ . Plot points that fall outside and to the left of the curve indicate that the refrigerant is a(n) ________ . Plot points that fall inside the curve itself indicate that the refrigerant is a(n) ___________

7. Superheated vapor, sub-cooled liquid, mixture of saturated liquid and vapor. ((Ref: 71/2))

One horsepower (1 hp) of mechanical energy is equivalent to ______watts of electrical energy, and one watt (1 W) of electrical energy is equivalent to ________ Btu of heat energy.

746 Watts, 3.413 Btu ((Ref: 3/4, 5))

The start capacitor in a capacitor-start, capacitor-run motor is switched out of the circuit when the motor has achieved ___________ % of full speed.

75 to 80% ((Ref: 113/4))

The supplemental heating capacity of a heat pump system should be equal to _____ % of the design heat loss to provide the heat needed during defrost and emergency heat periods.

75 to 80% ((Ref: 56/6))

What is the coefficient of performance (COP) of a heat pump?

A comparison of how much heat the heat pump is producing compared to how much heat an electric furnace would produce using the same amount of electricity. ((Ref: 95/4))

If a system operates correctly in one cycle and pumps down when switched to the other cycle, what problem is indicated?

A defective TEV (in such cases, the power element could be defective) or a stuck check valve. ((Ref: 126/3))

Define work.

A force moving an object in the direction of the force. ((Ref: 3/3))

Describe in simple terms the basic process by which a refrigerant transfers heat energy.

A refrigerant is used to transfer heat when it changes states. When liquid refrigerant evaporates into a vapor, heat is absorbed by the vapor. When refrigerant vapor is condensed into a liquid, heat is rejected from the refrigerant. ((Ref: 2/3))

In your own words, state the second law of thermodynamics.

A temperature difference must exist between two materials in order for heat energy to travel. ((Ref: 3/7))

What is the most common type of heat pump used in residential applications?

Air-to-air split system. ((Ref: 36/1))

What is the main difference between an open compressor and a Hermetic compressor?

An open compressor does not have the motor in the same housing as the compressor. A hermetic compressor has the motor and compressor housed in the same welded shell. ((Ref: 102/1))

A pressure drop equivalent to how many degrees across the evaporator of a heat pump system is considered to be of consequence and would require a expansion valve with an external equalizer?

At or above 2 degrees Fahrenheit. Example: R22 equals 1.5 psi per degree. A 3 psi pressure drop (2 x 1.5 = 3) would require a external equalizer. ((Ref: 124/4))

What component may be used to allow a conventional refrigerant distributor to function in a heat pump system?

Auxiliary side connector tees permit the conversion of conventional refrigerant distributors to reverse-cycle service. ((Ref: 128/2))

Why does a heat pump require three-function control?

Because a heat pump performs three functions-it provides cooling, it provides heating, and it must defrost the outdoor coil when ice forms on it. ((Ref: 88/4))

Where in the refrigerant circuit is an accumulator located?

Between the reversing valve and the compressor inlet. ((Ref: 137/2))

How is the compression ratio of an operating heat pump calculated?

Both the high-side and low-side refrigerant pressures must be converted to absolute pressure values by adding 14.7 psi to each gauge reading. The absolute high-side (discharge) pressure is divided by the absolute low-side (suction) pressure to determine the compression ratio. ((Ref: 103/1))

When multiple switches are connected in series with each other, they all must be in the position in order for the circuit to be energized.

Closed ((Ref: 148/4))

What type of heat pump systems are well suited for applications in which heating is required in some zones while cooling is required in others?

Closed-loop water-source heat pumps ((Ref: 43/2))

What effect does compression have on the temperature and density of refrigerant vapor?

Compressing the vapor increases both its temperature and density. ((Ref: 102/3))

The latent heat of _____ causes a change in phase from vapor to liquid. The latent heat of _____ causes a change in phase from liquid to vapor.

Condensation, vaporization. ((Ref: 6/1))

SEER is a measure of a heat pump's __________ . It is expressed in units of _________

Cooling efficiency, Btu/watt. ((Ref: 97/3))

Unless otherwise noted, all wiring diagrams depict switches and contacts in what mode?

Deenergized ((Ref: 149/3))

In your own words, state the first law of thermodynamics.

Energy can neither be created nor destroyed, but can be changed from one form into another. ((Ref: 3/2))

The indoor coil of a heat pump system serves as the _____ during the cooling season, and as the _____ during the heating season.

Evaporator, condenser. ((Ref: 16/6))

How is factory-installed wiring differentiated from field-installed wiring on a schematic diagram?

Factory-installed wiring normally is indicated with solid lines, and field-installed wiring with dashed lines. (Power circuits may be indicated with thick or heavy lines, and control wiring with thinner or lighter lines. ((Ref: 148/3))

Define pressure

Force per unit of area. ((Ref: 8/2))

Why are suction-line accumulators and crankcase heaters often required to protect heat pump compressors?

Heat pump compressors are subject to collecting liquid refrigerant in the compressor crankcase during the "off" cycle. Liquid refrigerant in the crankcase results in poor lubrication on start-up and increases wear on the compressor bearings. ((Ref: 64/1))

Heat always travels from a(n) _____ material to a(n) ______ material.

Higher-temperature, lower-temperature. ((Ref: 37-4/1))

What is the equation for converting watts to horsepower?

Horsepower = watts + 746 ((Ref: 114/1))

What guidelines should be followed when the cooling capacity of a heat pump is oversized to reduce the amount of auxiliary heat that will be required in the winter?

In colder climates, the cooling capacity may be oversized by 25%. In milder climates, the cooling capacity may be oversized by 15%. ((Ref: 49/2))

In the cooling mode, which coil (indoor or outdoor) of a heat pump system functions as the condenser? Which coil functions as the condenser in the heating mode?

In the cooling mode, the outdoor coil functions as the condenser. In the heating mode, the indoor coil functions as the condenser. ((Ref: 66/1, 67/5))

Which pressure scale has the greater resolution, one calibrated in inches of mercury or one calibrated in inches of water column?

Inches of water column. ((Ref: 8/4-9/2))

One of the most important principles of refrigeration is that an increase in pressure on a liquid results in a(n) _________in the liquid's boiling temperature.

Increase. ((Ref: 68/3))

The vapor line from the indoor section to the outdoor section of a split System must be ___________ to prevent unwanted heat transfer.

Insulated. ((Ref: 38/5))

Temperature is a measurement of heat _____. British thermal units and calories are measurements of heat _____.

Intensity, quantity ((Ref: 5/1))

What is the difference between latent heat and sensible heat?

Latent heat causes a change in the state of a substance without a change in its temperature. Sensible heat causes a change in the temperature of substance without a change in its state. ((Ref: 6/1))

State at least three of the five laws of heat transfer.

Law 1: Heat exists in the air at all temperatures. Law 2: Heat flows from a region of higher temperature to a region of lower temperature, regardless of how slight the temperature difference might be. Law 3: All gases become warmer when they are compressed. Law 4: Most matter can exist as a solid, liquid, or gas. Law 5: The temperature at which a material changes from a liquid to a gas, or from a gas to a liquid, depends on the pressure to which it is subjected. ((Ref: 32/2))

TEVs that limit the pressure created by an expanding bulb charge are called what?

Maximum operating pressure (MOP). ((Ref: 124/5-125/1))

What is meant by "excessive" supplemental heat?

More heat than would be required to neutralize the cooling effect that occurs during the defrost cycle. ((Ref: 87/2))

Rotary compressors are marginally ________ efficient and _____ noisy than similarly sized reciprocating compressors.

More, less ((Ref: 112/2))

What is the best type of filter-drier for general use?

One that has the ability to remove moisture, acid, and sludge, and to filter solid particles. ((Ref: 131/6))

The indoor section of an air-to-air split system, when installed in an attic, should be placed in a(n)

Overflow pan. ((Ref: 38/2))

List four components that are typically part of the power circuits in a heat pump system.

Power circuits include the components that require line voltage, such as the compressor, condenser fan motor, indoor blower motor, and electrical resistance heaters. ((Ref: 147/2))

13. A numerical value (always less than 1) that describes the relative amount of true electric power, in watts, that is actually delivered to a motor is called the motor's _______

Power factor. ((Ref: 114/1, 2))

Wiring diagrams generally are divided into two parts, what are they?

Power, control. ((Ref: 147/1))

What factors determine the proper size of a TEV?

Proper TEV size is determined by the Btuh load requirement, the pressure drop across the valve, and the evaporator temperature. ((Ref: 122/3))

What three types of compressors are used in residential heat pump applications?

Reciprocating, scroll, and rotary. ((Ref: 102/1))

What other names are used for reversing valves?

Reversing valves also are called switchover valves or four-way valves. ((Ref: 138/1))

10. A permanent split-capacitor motor can be identified by the presence of a(n) ______ in the motor circuit.

Run capacitor. Ref: 112/6-113/1

Are safety switches connected in series or parallel?

Series ((Ref: 148/4))

What is specific heat?

Specific heat is the energy required to raise the temperature of 1 lb of a substance by 1 degree F. ((Ref: 6/3))

The upper scroll in a scroll compressor is the __________scroll, and the lower scroll is the ________________scroll.

Stationary (or fixed), driven (or orbiting). ((Ref: 108/1))

The motor in a hermetic compressor is cooled by?

Suction vapor. ((Ref: 105/4))

A typical reversing valve is a cylinder with three tubes on one side and a single tube.on the other side. The center tube of the three tubes is always the compressor _________ connection. The single tube on the opposite side of the cylinder is always the compressor ________ connection.

Suction, discharge. ((Ref: 93/2))

Heat pump systems with line sets longer than 80 ft require the use of a(n) _____________ to protect the compressor from flood-back on start-up.

Suction-line accumulator ((Ref: 39/4))

What is superheat?

Superheat is energy that raises the temperature of a vapor above its saturation temperature. ((Ref: 6/4))

What is the only method of checking superheat that yields an accurate value?

Taking pressure and temperature readings at the evaporator outlet. ((Ref: 124/5))

In which component of a refrigeration system is heat removed?

The Condenser. ((Ref: 17/3))

What three forces are necessary to operate a thermostatic expansion valve (TEV)?

The bulb pressure, the evaporator pressure, and the spring force. ((Ref: 122/1))

Define the coefficient of performance (COP).

The coefficient of performance is the ratio of the heat delivered by a heat pump (output) to the energy required to operate the heat pump (input). ((Ref: 49/3))

Refrigerant distributors are sized for cooling operation. What potential problem does this create for heat pump applications, and how is it solved?

The distributor nozzle, sized for refrigerant flow during the cooling cycle, may offer too much restriction during the reverse flow of the heating cycle. Heat pump distributors have a side connection between the nozzle and the internal passages, allowing refrigerant to bypass the nozzle and thus minimizing pressure loss. ((Ref: 127/3))

List at least five factors that must be controlled if a comfortable indoor environment is to be maintained.

The factors that must be controlled include temperature, humidity, air motion, dust, bacteria, odors, toxic gases, and ionization. ((Ref: 1/1))

What is the "critical point" of a refrigerant?

The maximum saturation condition at which evaporation and condensation may occur. ((Ref: 71/4))

What is the thermal balance point of a heat pump system?

The outdoor air temperature at which the output of the heat pump equals the heat loss of the structure. ((Ref: 47/2))

What is saturated vapor pressure?

The pressure exerted by a saturated vapor above its own liquid or solid (the pressure at which vapor and its liquid, or vapor and its solid, can exist in equilibrium at a given temperature). ((Ref: 10/5))

What is meant by referring to a vapor compression system as a "closed" system?

The term "closed" indicates that the system continually repeats itself without depleting the refrigerant contained within it. ((Ref: 64/2))

14. What is meant by the term synchronous motor speed?

The theoretical speed that a motor could achieve in the absence of friction. ((Ref: 116/1))

What problems occur when flash gas enters a TEV while a system is operating?

The valve operates inefficiently, the system loses capacity, and superheat increases. ((Ref: 123/1))

What is the mathematical relationship between specific volume and density?

They are reciprocals of each other. ((Ref: 11/3))

16. How does the oil return to the compressor in a heat pump system that is equipped with a suction-line accumulator?

Through the small hole drilled in the lower portion of the U-tube. ((Ref: 137/4))

What is the function of a motor overload protector?

To de-energize the compressor if system problems cause winding temperatures to rise above an acceptable level. ((Ref: 116/5-117/1))

Why is the indoor air volume required for heat pump operation greater than the volume required for cooling-only applications?

To ensure adequate condensing of the refrigerant during the heating mode. ((Ref: 64/1))

Why are suction-line accumulators used in heat pump systems?

To prevent liquid refrigerant from entering the compressor when the system evaporator is flooded. When a heat pump changes from heating to cooling, the flooded outdoor coil would allow liquid refrigerant to reach the compressor if not protected by a accumulator. ((Ref: 137/1))

Why does the indoor coil of a heat pump need more area than the indoor coil of a cooling-only unit?

To prevent the condensing temperatures and corresponding head pressures from becoming too high during the heating cycle. ((Ref: 63/2))

How can you inspect ball-type check valves for free movement of the ball?

Valves of this type may be checked for free movement with a magnet when the system is not running. ((Ref: 136/4))

Reciprocating compressors are valved and exhibit _____ compression ratios. By contrast, scroll compressors are ported and exhibit ___________ compression ratios.

Variable, fixed. ((Ref: 109/4-110/1))

Describe the method used to free up a sticky main valve slide.

With the system operating at normal operating pressures, rapidly energize and deenergize the reversing valve solenoid several times. Before condemning the valve, perform the same procedure after raising the system head pressure. ((Ref: 139/3))

Is it possible for parallel circuits operating with the same power supply to have different currents passing through each branch?

Yes ((Ref: 149/1))

On a new or repaired heat pump unit, what steps should you take to check for contamination?

You should check the moisture indicator (if available), the acid content, the system pressure drop, and the pressure drop across the filter-drier. ((Ref: 132/4))

When using the "touch test" chart to check the operation of a reversing valve, what are the normal conditions for each of the following in the cooling mode? a. Discharge tube from compressor b. Suction tube to compressor c. Tube to inside coil d. Tube to outside coil e. Left pilot capillary tube f. Right pilot capillary tube

a. Hot b. Cool c. Cool d. Hot e. Temperature of valve body f. Temperature of valve body ((Ref: 142/Table 5-4))

When using the "touch test" chart to check the operation of a reversing valve, what are the normal conditions for each of the following int 'Heating Mode'

a. Hot b. Cool c. Hot d. Cool e. Temperature of valve body f. Temperature of valve body ((Ref: 142/Table 5-4))