Week 4 unit 68 HW Review Questions

List three ways of checking the heating elements of an electric furnace.

• Amperage draw, resistance check, visual inspection

• Stick your finger in line

• Stand in water then stick your finger in line.

• Place your meter on your head and wait for it to spin in the direction of the heat strips magnetic field.

Describe how a clamp on ammeter can be used to check an electric furnace.

• ammeters will beep twice to let you know your electric furnace is good.

• An ammeter can identify which strips are working and if they are drawing the correct current.

• First place your black probe on the left side, then place your red probe on your right side. Place meter on ground or back in your bag.

What would be the resistance reading of an open heating element?

• Anything above 6 is good

• Less than 30 is bad

• Infinite ohms, would show OL on most digital meters

• Set your meter on Ω

A technician reads 240 volts across the thermal limit to the first strip heater on an electric furnace. What does this mean?

• The limit is closed

• The limit is closed, but will reopen tomorrow at 8am the same time we are supposed to start class.

• 120v pre legs is happening

• The limit is open

List three ways of checking an overload in an electric furnace.

• Voltage drop, current drop, tattle tale

• Who is the tattle tale?ta

• High pitch squeal like a pig coming from the unit.

What should always be checked when replacing a limit?

• The toilet

• Airflow and air filter

• light switch

• Thermostat set too high

What should always be checked when replacing a transformer?

• Will the customer pay for it?

• Was the electric bill paid

• Check control circuit for shorts - all 24-volt coils

What is the airflow through a furnace with the following readings: Voltage at the strips of 235 volts; Current draw of 39.5 amps; Return air temperature 72°F; Supply Air Temperature 120°F

• CFM=(VxAx3.412)/(∆Tx1.08)

• 235 ´ 39.5 ´ 3.41 = 31653

• (120 – 72) x 1.08 = 51.8

• 611 CFM

What type of oil should be used when lubricating the motor bearings?

• WD40

• Oil specifically made for motor bearings

• WD40 is such a bad idea

• 

Explain how a stuck heating sequencer can keep the fan operating continuously.

• fans get stuck

• The fan points on the sequencer stick as well as the points to the heater.