Unit 63 HVAC Flashcards

Questions and Answers

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

Amperage draw, resistance check, visual inspection

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

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

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

Infinite ohms, would show OL on most digital meters

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 open Signup and view all the answers

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

Voltage drop, current drop, tattle tale Signup and view all the answers

What should always be checked when replacing a limit?

Airflow and air filter Signup and view all the answers

What should always be checked when replacing a transformer?

Check control circuit for shorts all 24 volt coils Signup and view all the answers

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?

611 CFM Signup and view all the answers

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

Oil specifically made for motor bearings Signup and view all the answers

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

The fan points on the sequencer stick as well as the points to the heater. Signup and view all the answers

What is the correct remedy for a wire with one end that has overheated?

Replace the entire wire Signup and view all the answers

Why should relay slide-on posts not be sanded to clean off oxidation?

The base metal under the outer coating does not resist oxidation well and is also not as good a conductor. Signup and view all the answers

How can over lubricating a blower motor be detrimental?

Oil that spills out will collect dust blocking airflow through the motor. Oil can also collect on the windings and slowly erode the varnish causing a short. Signup and view all the answers

What are the two general types of sequencers used on electric furnaces?

Time delay relays that control individual strips and all in one sequencers that control all the strips and the blower motor. Signup and view all the answers

Describe two ways that electric furnaces control the blower motor.

One way is by having an extra set of contacts individual sequences and the other way is with a blower relay. Signup and view all the answers

What is the purpose of the white ceramic pieces on the strip heaters?

They insulate the heater wire from the metal frame supporting it. Signup and view all the answers

Why is a visual check of the heating strips on an electric furnace difficult to accomplish?

The heaters must be removed from the furnace for inspection. Signup and view all the answers

Why are terminal and wire resistance values critical in electric furnaces?

The large current draw from the strip heaters turns even small resistances into heat that accelerates oxidation. Signup and view all the answers

What are two types of safety most electric furnaces utilize?

Thermal overload and fusible link Signup and view all the answers

Why are the strips on an electric furnace staged to turn on in sequence instead of being energized all at one time?

To prevent a large current draw at one time when initially starting. Signup and view all the answers

Study Notes

Electric Furnace Heating Elements

Check heating elements using amperage draw, resistance check, and visual inspection.
A clamp-on ammeter identifies operational strips and measures their current draw.

Resistance and Voltage Readings

An open heating element reads infinite ohms (OL on digital meters).
A 240-volt reading across the thermal limit indicates the limit is open.

Overload and Limit Checks

Check for overloads through voltage drop, current drop, and a tattle tale.
Always check airflow and air filters when replacing a limit.

Transformer and Airflow Measurements

Inspect control circuits for shorts in all 24-volt coils when replacing a transformer.
Calculate airflow using the formula: Voltage x Current x 3.41 = CFM, with temperature difference (120°F - 72°F) factored in.

Lubrication and Maintenance

Use oil specifically for motor bearings for lubrication.
Over-lubricating a blower motor can lead to dust accumulation and short circuit risks.

Sequencers and Fan Control

Electric furnaces use time delay relays for individual strips and all-in-one sequencers for combined control.
Blower motor control is achieved through an extra set of contacts and blower relays.

Insulation and Visual Checks

White ceramic pieces insulate heater wire from the supporting frame.
Conducting a visual check of heating strips requires removal from the furnace.

Resistance and Safety Features

Terminal and wire resistance values are crucial due to high current draw, which increases heat and oxidation.
Safety features in electric furnaces include thermal overload and fusible links.

Staging and Current Draw

Strip staging is employed to prevent excessive current draw during initial activation.

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Description

Test your knowledge of electric furnace components and functionality with these flashcards. Learn about the methods to check heating elements, the use of a clamp-on ammeter, and resistance readings. Perfect for HVAC students and professionals looking to solidify their understanding.