Unit 51 Review Flashcards
List the four categories of gas-fired furnaces based on flue-gas temperature and pressure.
Category I: Negative pressure, low-temperature vent.
Category II: Negative pressure, low-temperature, condensing.
Category III: Positive pressure, high-temperature vent.
Category IV: Positive pressure, low-temperature, condensing.
Explain the difference between a Category I furnace and a Category IV furnace.
A Category I furnace operates with a negative pressure vent and does not condense moisture, while a Category IV furnace has a positive pressure vent and condenses flue gases for higher efficiency.
List the five furnace cabinet configurations.
Upflow, downflow (counterflow), horizontal, lowboy, and outdoor.
Why are standing-pilot natural-draft furnaces no longer manufactured?
They are inefficient, waste energy by constantly burning a pilot light, and do not meet modern energy efficiency standards.
What changes were made in furnace design to achieve 80 percent efficiency?
Improved heat exchanger designs, better burner efficiency, and the use of induced draft fans to control combustion airflow.
What is the difference between an 80 percent efficient furnace and a 90 percent efficient furnace?
A 90% efficient furnace has a secondary heat exchanger to extract more heat from combustion gases, allowing it to condense moisture and recover additional energy.
Discuss the evolution of the heat exchanger.
Heat exchangers have evolved from simple steel tubes to high-efficiency stainless steel and aluminized heat exchangers designed for maximum heat transfer and corrosion resistance.
Discuss the operation of an atmospheric burner.
An atmospheric burner relies on natural gas pressure and primary air induction to mix fuel and air before ignition, creating a stable flame.
Explain why furnace efficiencies jump from 80 percent to 90 percent.
This jump is due to the addition of a secondary heat exchanger that captures and condenses moisture from flue gases, recovering more heat.
What is the purpose of the orifice?
The orifice meters the correct amount of gas into the burner to ensure proper combustion.
Why is it not a good idea to drill out orifices in the field?
It can alter gas flow, leading to improper combustion, higher emissions, and safety hazards.
What components are needed for a furnace to operate as a two-stage furnace?
A two-stage gas valve, a variable-speed blower motor, and a compatible thermostat.
Why are some condensate pumps not approved for furnace condensate?
Furnace condensate is acidic and can damage standard pumps that are not designed to handle corrosive liquids.
Can the furnace drain and the air-conditioning drain be run with a common drain?
Yes, but only if the drain is properly trapped and vented to prevent backflow issues.
List the different types of gas burners.
Inshot burners, upshot burners, ribbon burners, and shotgun burners.
Why do most furnaces today use burners?
Burners efficiently mix fuel and air for controlled combustion, ensuring proper heating and efficiency.
What is the temperature rise of a furnace?
The difference between the return air temperature and the supply air temperature, typically 30°F to 70°F.
What part of the furnace does the air travel through first in all furnace cabinet configurations?
The air filter, which removes dust and debris before air enters the heat exchanger.
Why should technicians be familiar with standing-pilot, natural-draft furnaces, since they are no longer manufactured?
Many older homes still use them, and technicians need to know how to service and replace them safely.
Why are draft-inducer fans necessary in today’s furnaces?
They improve combustion efficiency by controlling airflow, reducing heat loss, and ensuring proper venting of combustion gases.
