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Unit 61 Exam

Description

Unit 61- Troubleshooting Electric Heat
Neil Revollo
Quiz by Neil Revollo, updated more than 1 year ago
Neil Revollo
Created by Neil Revollo over 8 years ago
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Resource summary

Question 1

Question
Why is it important to cycle the furnace on and off several times when replacing a sequencer?
Answer
  • It is important to make sure the heating elements and the blower are switching on and off in the correct cycle.
  • It is important to make sure the heating elements and the blower are turning on at the same time.
  • It is important to make sure the power is working correctly.
  • It is important to make sure that only the heating elements are coming on and not the blower.

Question 2

Question
What could cause a false high reading for the supply-air temperature?
Answer
  • If the temperature is taken with the blower on at the same time the heating elements are on.
  • If the temperature is taken after the unit has been running for too long.
  • If the temperature is taken with a probe inserted upside down.
  • If the probe is inserted in the supply plenum in a direct line of sight with the heating elements.

Question 3

Question
If the furnace has one or more broken heating elements or one or more fusible links open, what should the technician do?
Answer
  • The technician should check the furnace airflow.
  • The technician should check the power source to the furnace.
  • The technician should replace all the heating elements.
  • The technician should replace all the fusible links.

Question 4

Question
The two general types of sequencers used on electric furnaces are
Answer
  • High current sequencers and low current sequencers
  • Fast acting sequencers and slow acting sequencers
  • Mechanical and electronic
  • Individual time delay relays and all-in-one sequencers that control all the strips and the blower motor

Question 5

Question
The most effective method of checking the heating elements of an electric furnace during operation is
Answer
  • A visual inspection
  • Checking the Amp draw of each heat strip
  • Measuring the temperature of each heat strip
  • Checking the resistance of each heat strip

Question 6

Question
What is the purpose of the white ceramic pieces on the strip heaters?
Answer
  • They insulate the heater wire from the metal frame supporting it
  • They make it easier to identify where the heat strips are installed
  • They concentrate the heat of the strips
  • They seal the heat produced by the strips inside the furnace

Question 7

Question
An electric furnace that runs continuously, regardless of the thermostat setting probably has
Answer
  • An open strip heater
  • An open fusible link
  • A bad transformer
  • A stuck sequencer

Question 8

Question
If a voltmeter shows a reading of 240 volts across the thermal limit to a strip heater on an electric furnace
Answer
  • The limit is closed, breaking the circuit to the heater
  • The limit is open, breaking the circuit to the heater
  • The limit is open, completing the circuit to the heater
  • The limit is closed, completing the circuit to the heater

Question 9

Question
Whenever a thermal limit or fusible link needs replacing, the technician should always check
Answer
  • The power wire for voltage drop
  • The transformer control voltage
  • The thermostat calibration
  • The air filter and airflow

Question 10

Question
Overheating can cause a terminal to discolor or the wiring insulation to melt. What should you consider when repairing the wire or the terminal?
Answer
  • Replace the terminal end only. It is okay to cut off the end and repair the wire. If the terminal is on a relay, sequencer or fuse block, the component should also be replaced in addition to the wire.
  • Replace the terminal end only. It is okay to cut off the end and repair the wire. If the terminal is on a relay, sequencer or fuse block, only the wire needs to be replaced. The component does not need to be replaced.
  • It is recommended to replace the entire wire with another wire of the same or larger gauge. If the terminal is on a relay, sequencer, or fuse block, the component should also be replaced in addition to the wire.
  • It is recommended to replace the entire wire with another wire of the same or larger gauge. If the terminal is on a relay, sequencer, or fuse block, only the wire needs to be replaced. The component does not need to be replaced.

Question 11

Question
Given the following readings from an electric furnace, use the formula to calculate airflow. CFM = (V x A x 3.412)/(∆T x 1.08) What is the airflow in CFM and does this furnace have the recommended air flow? Voltage at strips = 240 V Amp draw of strips = 22.6 A Return-air temperature = 65 ºF Supply-air temperature = 120 ºF
Answer
  • The airflow is 336 CFM; No, the air flow is too low.
  • The air flow is 312 CFM; No, the air flow is too low.
  • The air flow is 312 CFM; Yes, the air flow is at the recommended level.
  • The air flow is 336 CFM; Yes, the air flow is at the recommended level.

Question 12

Question
A technician finds that a wire in an electric furnace is brittle and discolored from overheating. The technician should
Answer
  • Cut off the overheated end, strip the remaining wire end, and reinstall it
  • Cut off the overheated end and splice on a short length of similar wire using high quality electrical tape
  • Replace the entire wire
  • Cut off the overheated end and add a short length of similar wire using a wire nut

Question 13

Question
A technician takes the following readings while checking the operation of an electric furnace with 2 heat strips: 220 volts, 36 amps, 70ºF return air temperature, and 120ºF supply air temperature. What is the airflow through the furnace?
Answer
  • (120 - 70) x 1.08 x 10 = 540 CFM
  • (220 x 36 x 3.41)/(120 - 70) = 27,002/50 = 540 CFM
  • (220 x 36)/(120 - 70) = 7,920/50 = 158 CFM
  • (220 x 36 x 3.41)/((120 - 70) x 1.08)) = 27,002/54 = 500 CFM

Question 14

Question
When replacing a transformer, the technician should always check
Answer
  • The 24 volt control circuit for shorts
  • The air filter and airflow
  • The temperature rise across the furnace
  • The continuity of the electric strip heaters

Question 15

Question
What problems can be caused by overlubricating a blower motor?
Answer
  • This could eventually cause the motor windings to short out.
  • The motor would run more effectively with the extra lubrication.
  • It is not possible to overlubricate the motor.
  • The blower motor could run faster than specifications recommend.
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