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UNIT 39 REVIEW QUESTIONS ANSWER KEY GO BACK TO INDEX
39-1) What is the difference between an open loop and a closed loop control systems?
39-2) What is the difference between a two position control and a modulating control?
39-3) List the types of control systems.
39-4) How does a pneumatic control system operate?
39-5) What is the difference between an electronic control system and a direct digital
control system?
39-6) Why are electro mechanical control systems often referred to as relay control
systems?
39-7) Why is there a potential of 24 V between the R terminal and the other terminals on
a thermostat subbase?
39-8) In Figure 39-33, what is the difference between emergency heat operation and
normal second stage heat operation?
39-9) In Figure 39-33, what happens when the defrost relay DR is energized?
39-10) In Figure 39-45, list the sequence of events that occur when the cooling relay is
energized.
motor 1.
The outdoor fan contactor NO contacts on line 8 close. If the high pressure
39-11) In Figure 39-45, what determines whether or not outdoor fan motor 2 OFM2
operates?
39-12) In Figure 39-38, what does the LPS control?
39-13) In Figure 39-33, when is the reversing valve energized: the heating cycle or the
cooling cycle?
39-14) In Figure 39-45, how does the high pressure switch, HPS, protect the compressor?
39-15) In Figure 39-23, what will happen if the limit switch opens while the furnace is
operating?
39-16) In Figure 39-33, what does the field control wire on the thermostat B terminal
connect to on the unit?
39-17) In Figure 39-45, how does the thermostat control the operation of the compressor?
39-18) In Figure 39-32, what voltage operates the potential start relay coil?
39-19) In Figure 39-32, what is the purpose of the potential start relay?
39-20) List the heating operational sequence for the unit in Figure 39-24.
39-21) In Figure 39-23, what controls the fan operation in heating?
39-22) List the operational sequence for Figure 39-23.
With the system switch in the heat position, the heating thermostat
closes when the room temperature drops. This initiates a call for heat,
16), closing its contacts and energizing the igniter, line 1. The igniter is
energized for 60 seconds before the gas valve is energized. The gas valve
and 6. Starting of the indoor fan motor is delayed to allow the furnace
time to heat up.
39-23) List the operational sequence for the second stage of Figure 39-33. Just list what
happens when the W2 terminal is energized.
The second stage heating thermostat TSH2 closes to send 24 volts from R
to W2, energizing heat sequencer HS1, line 18. The HS1 coil is a small
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the coil to the second heat sequencer HS2, lines 1 and 2. HS2 closes its
contacts after a time delay to energize the second heater HTR2, line 3.
Note that HS1 and HS2 normally open contacts in parallel with the IFR
contacts, lines 10 and 12. They insure that the indoor fan motor will
operate whenever the heat strips are operating.
