Heating I Section#1 Furnace - Gas and Oil Quiz#9

This study session deals with condensing furnaces.

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 the components of an induced-draft condensing furnace.
• Describe the typical sequence of operations of an induced-draft condensing furnace.
• Describe the basic operation of a pulse-combination, high-efficiency furnace.
• Describe the sequence of operations in a pulse-combustion, high-efficiency furnace.
• Outline the significance of the AFUE rating.

Condensing furnace vents discharging below a building soffit will:

Rot the soffit over time.

Explain briefly how we use the latent heat of vaporization to our benefit in a condensing furnace.

We capture most of the heat released by the water vapor in the exhaust gas when it condenses. This heat is the heat put into the combustion products to turn the water to steam; the latent heat of vaporization

List 3 advantages of a sealed combustion system.

1. More efficient; you don't burn air you have already heated
2. The heat exchanger is not attacked by internal chemicals such as laundry or pool chemicals. 
3. The furnace combustion cannot affect (or be affected by)the draft of any other appliance in the house such as a fireplace.

List the 2 things we need in a condensing furnace at the point where the exhaust gases condense.

1. A heat exchanger capable of withstanding the acidic condensate
2. A condensate collection system

List the sequence of operation for an induced draft condensing furnace from the time the thermostat calls for heat until the house air fan is turned on. Try to list 6 steps.

1. The thermostat calls for heat
2. The induced draft fan starts
3. The differential pressure switch proves the induced draft fan is running
4. The ignition device ignites the pilot or burner
5. The burner warms the heat exchanger
6. The house air fan is turned on after a set time delay, or by a temperature control at 90 to 120F.

List the sequence of operations for a sealed combustion pulse furnace from the thermostat calls for heat. Try to list 11 steps.

1. The thermostat calls for heat

2. The primary contrl initiates a 30 sec pre-purge by turning on the purge blower. This happens only if the air intake vacuum switch and exhaust outlet pressure switch are both closed (i.e. there is adequate air movement)

3. After the purge, the spark plug and gas valve are energized for about 8 secs

4. The initial pulse is ignited by the spark plug

5. The flame sensor makes sure that there was ignition

6. The spark plug and purge blower are turned off

7. The combustion pulses continue without the aid of a spark plug

8. The fan contrl switch turns on the house air fan 30 to 45 secs after the combustion process has started. (Note: The house air fanis not activated by a temperature sensor, but rather is timed to come on after the comustion process has started)

9. The comustion process and house-ar fan operate until the thermostat is satisfied

10. The gas valve closes, combustion stops and the purge blower is activated for a 30 sec purge

11. When the house air temprature in the plenum drops to 90F, the house-air fan will shut off (Note: although the system used a timer to turn the house air fan on, it turns off when it senses the low temperature set point)

What is the average temperature of a high-efficiency furnace exhaust gas?

100F to 150F

What is the purpose of a differential pressure switch in a condensing gas furnace.

The differential pressure switch proves that the induced draft fan is running.

What is the average efficiency of a condensing furnace?

90%

The vent should slope up away from the furnace gently to allow combustion gases to rise by gravity.

False