HVAC final exam, unit 14

List four reasons why low voltage controls are usually found on residential air conditioning systems?

1) Economy 2) safety 3) more precise control 4) doesn't need a electrical license to work on low voltage

What is the control voltage on a package unit or ducted split air conditioner?

24 volts

If the suction pressure of a system is 60 psi and the oil pump is 85 psig, what is the net oil pressure?

25 psig

Where is the low voltage transformer typically located on a ducted split a/c?

Air handler unit/ Fan coil unit

An oil pressure safety control usually has a 90 second time delay built int it. Why?

Allow system to build up positive pressure before start up

High pressure control contacts are normally ______ type

Closed

The heater that functions in a thermostat in the cooling mode is called the

Cold anticipator

The heater in the thermostat control circuit in the cooling mode is active when the

Cooling contact is open

T/F: A dual pressure control is actually a low pressure control and a high pressure control connected in parallel with each other.

False

T/F: The advantage of using a thermistor-type thermometer over a mercury-filled thermometer for measuring refrigerant line temperature is that the thermistor-type does not have to be installed.

False

When a water regulating valve is used to control the head pressure of a water-cooled system, the valve's spring is acting against which pressure to modulate the water flow?

Head pressure of the system

The heater that functions in a thermostat in the heating mode is called the

Heat anticipator

The heat anticipator in the thermostat control circuit in the heating mode is active when the ___.

Heating contact is closed

A low pressure control has a "high event" and a "differential" setting. The technician wants the system to turn on at 28 psig and shut down at 2 psig. What would be the proper settings?

High event: 28 psig, differential: 26 psig

Where is the thermistor motor overload device usually positioned?

Inside within the motor windings

What type of control would be used to cycle compressors on and off in response to evaporator pressures that indirectly control box temperatures?

Low pressure control

To control ice build-up on a heat pump outdoor coil in the heating cycle, a ____ is attached to a large diaphragm.

Microswitch

The cold anticipator is wired in _____ with the mercury bulb cooling contacts, and is energized during the off cycle.

Parallel

What is meant by the term "detent" ?

Rapid making breaking of a contact

The purpose of a heat anticipator is to_____.

Reduce the temperature swing of a conditioned space

To prove that the combustion blower motor is operating before ignition, a ______ would be used.

Sail switch

What device may be used to ensure that air is flowing through a duct before an electric strip heater is energized?

Sail switches

Explain why the thermistor type sensor is normally embedded in the motor windings.

So when the temp of the motor gets to hot, it will brake the electrical circuit to the motor, saving the motor.

The difference between system overshoot and system lag is called?

Temperature swing

Oil pressure safety controls measure the difference in pressure to establish ______.

That positive oil pressure is present

T/F: A helix coil is used when in some applications to sense temperature in a moving air stream.

True

T/F: Natural gas fired heating units operate at a manifold pressure of 3.5" water column

True

T/F: The cold anticipator starts the cooling system early to allow the air-conditioning system to get up to capacity when needed.

True

T/F: The heat anticipator in a thermostat prevents the temperature in the conditioned space from rising above the thermostat settings

True

To obtain an accurate reading of a fluid in a large pipe, a technician should____.

Use a permanently mounted thermometer in a well

Where would a water regulating valve that is controlling head pressure be installed?

Water circuit at inlet of condenser

Explain the most accurate way to measure the current through the heat anticipator.

Wrap the wire 10 times around the ammeter, and divide by 10

Line voltage thermostats have heavier and use which type of sensing element?

all of the above

The ______ must determine what the current flow is through the heating control circuit.

heating technician

The term used to describe when the temperature of a heated room falls below the thermostat's set point is called

system lag

The term used to describe when the temperature of a heated room rises above the thermostat's set point is called

system overshoot