Heating I Section#1 Furnace - Gas and Oil Quiz#8

This study session reviews the components of conventional efficiency gas-fired forced air furnaces. A sequence of operation of these furnaces is explained. Inspection and testing procedures for conventional gas furnaces are outlined, and mid and high-efficiency furnaces are introduced.

The estimated completion time for this study session is sixty minutes.

Objectives
At the end of this study session, you should be able to:

• List twelve components of conventional gas-fired furnace
• Describe the sequence of operation of a gas-fired furnace
• Outline the four basic components of the inspection and testing procedure for a conventional gas furnace
• Describe the two areas where improvements in efficiency take place when we move from conventional to mid-efficiency furnaces.

Describe the 2 main advantages of an induced draft fan.

1. Prevent off cycle losses by blocking chimney when the system is at rest
2. Reduces amount of house air needed for combustion, since no dilution air is needed, and allows a more restrictive (efficient) heat exchanger

Describe the AFUE rating in two words.

Seasonal efficiency

Describe the sequence of operation of a conventional gas furnace from when the thermostat calls for heat to when the thermostat is satisfied. Try to include 12 steps.

1. The thermostat in the living room space calls for heat

2. The thermocouplesee that pilot flame is on and allows gas valve to open. If the thermocouple does not sense pilot flame, the gas valve will not open

3. The burners are ignited and a flame is established

4. The burner side of heat exchanger gets heated

5. Exhaust fases flow through the furnace flue passanges, past the draft hood (or via induced fan), through the vent connector and up pass chimney or vent

6. The fan control senses the air getting warmer on the house side of the heat exchanger

7. The house-air fan (blower fan) is started when the fan control sensor sess roughly 120F

8. The fan pulls 70F air from return ducts, through the filter and pushes the air past the other side of heat exchanger

9. Warm (140F) air comes out of the top of the furnace and is pushed through plenum into supply duct and pushed into rooms via registers

10. The thermostat is satisfied

11. The burner shuts off, but the pilot stays on

12. The fan control turns off blower fan when the house-air side of the heat exchanger cools to about 90F

Describe two methods mid-efficiency furnaces may use to eliminate or reduce off cycle losses.

1. Intermittent ignition
2. Induced draft fan or vent damper

List 6 of the components that would be found inside the cabinet of a conventional gas-fired forced air furnace.

1. Gas valve
2. Burner(s)
3. Heat exchanger
4. Exhaust flue passages
5. Draft hood
6. Fan/limit control
7. Blower
8. Air filter

What is the purpose of air proving switch in a furnace which is equipped with an induced draft blower?

The air proving switch proves that the induced draft fan is working.

What is the purpose of the spillage switch which is located on the heat shield of a mid-efficiency furnace?

The spillage switch proves that the vent connector and vent are not obstructed. It shuts off the furnace if exhaust products backdraft through the burner.

Why would an existing B vent possibly not be adequate for a new mid-efficiency furnace? (List 2 reasons).

1. An existing B vent can be oversized since the new furnace doesn't use dilution air, or since the new furnace is smaller capacity than the old one.
2. This may cause condensation and corrosion.

The off-cycle losses of a conventional efficiency furnace total __% of the fuel consumed.

20

Our standards require us to check the heat exchanger by:

Looking at visible components with a mirror and flashlight

Reducing off cycle losses improves steady state efficiency.

False

The post purge cycle on an induced draft fan ensures all the products of combustion are exhausted.

True