Forced-Air Heating Systems 030203b

Question 1

What are the two most common materials used for electric heating elements and what are they generally encased in?

Answer 1

Nichrome & Tungsten.

Encased in a protective insulating material like ceramic, mica or glass.

Question 2

What is the control circuit voltage of a 120V heating system?

Answer 2

Control circuit power rating of 120V. Requires no 24V transformers. Gas valve is designed to operate at 120V.

Question 3

What is the purpose of a step open gas valve?

Answer 3

Limits the initial flow of gas to the ignition source to prevent a large explosion.

Question 4

What is the device called that stops the gas flow to the pilot flame as well as opens the circuit to the main gas valve?

Answer 4

100% shutoff valve, safety gas valve, or automatic pilot valve.

Question 5

What is the valve called that controls the flow of gas directly by means of an electrically operated plunger?

Answer 5

Solenoid valve.

Question 6

Where should solenoid and diaphragm valves be installed?

Answer 6

Downstream of the safety gas valve.

Question 7

What is the most common gas control device currently available?

Answer 7

Combination gas valve.

Includes a gas regulator, safety gas valve, power unit, main gas valve, and a manual gas shutoff all in one unit.

Question 8

What are the two types of fans in a forced air heating system?

Answer 8

Belt-driven type.

Directly coupled drive type.

Question 9

What is the range that the high limit switch is usually set to?

Answer 9

82 degrees Celsius to 93 degrees Celsius.

Resets when the temperature drops 11 degrees Celsius below the thermostat set point.

Question 10

What is the range setting for the Fan-on and fan-off function?

Answer 10

Fan-On - 49 degrees Celsius to 60 degrees Celsius.

Fan-Off - 32 degrees Celsius to 38 degrees Celsius.

Question 11

What is a down flow furnace?

Answer 11

Air is blows downward and force heated air through ductwork in or below the concrete slab floor.

Question 12

What is the electrical energy that comes from the thermocouple used for?

Answer 12

Operating the power unit.

Question 13

What safety pilot device provides 100% gas shutoff?

Answer 13

Safety gas valve.

Question 14

What is the AFUE (Annual Fuel Utilization Efficiency) rating of a high efficiency furnace?

Answer 14

90% to 98%

Question 15

What is the Annual Fuel Utilization Efficiency (AFUE) rating of a mid efficiency furnace?

Answer 15

80% to 83%

Question 16

What is the minimum Annual Fuel Utilization Efficiency (AFUE) rating for a new standard furnace?

Answer 16

78%

Question 17

What are the four main air combustion components?

Answer 17

Combustion Air Blower.
Combustion Air Prove Switch (Air proving switch).
Heat Exchanger.
Primary High Limit Safety Switch.

Question 18

What are the 3 main primary components needed for a condensing furnance?

Answer 18

Primary heat exchanger.
Secondary heat exchanger.
Condensate drain line.

Question 19

What is the purpose of the primary heat exchanger?

Answer 19

Removes the heat from the combustion flue gases.

Made of a specially coated steel tube system.

Question 20

What is the purpose of the secondary heat exchanger also known as the condenser coil?

Answer 20

Extracts more heat from exhaust gases passing through the primary heat exchanger.
Cools gases to the point that they condense into water and carbon dioxide which creates carbonic acid.

Question 21

What is the purpose of the condensate drain line?

Answer 21

Drains the carbonic acid away from the furnace. Uses a PVC pipe to resist corrosion.
Critical to ensure integrity of all connections and proper sloping.

Question 21

Why must the secondary heat exchanger be made of stainless steel?

Answer 21

To resist corrosion from the carbonic acid it creates.

Question 22

What is the main purpose of the combustion air blower (CAB)?

Answer 22

Moves air through the heat exchanger during the heating cycle.
May also be used to purge the combustion chamber and heat exchanger of gases and products of combustion before and after the lighting of the main burners.

Two options: Forced draft fan or induced draft fan.

Question 23

What is the purpose of the air proving switch?

Answer 23

N/O pressure switch that ensures combustion air is flowing through the heat exchanger.
Prevents burner operation during unsafe conditions.
Connects to the heat exchanger with a flexible rubber hose.

Question 24

What is the purpose of the high limit safety switch?

Answer 24

Monitors the temperatures of the heat exchanger, furnace cabinet and circulation fan housing to prevent excessive temperatures and damage. Could be up to 3 installed in a high efficiency furnace and are usually sense temperatures with bimetal disks or strip sensors.

Question 25

What is the general set point for high limit switches?

Answer 25

82 degrees to 94 degrees Celsius.

Question 26

What are the two igniter options?

Answer 26

Direct Spark Igniter (DSI) Hot Surface Igniter (HSI)

Question 27

What temperature does a hot surface igniter get up to?

Answer 27

1370 degrees Celsius.

Question 28

What are igniters sensitive to?

Answer 28

moisture, dirt, and grounding issues.

Question 29

What are hot surface ignitors (HSI) made from?

Answer 29

Silicon carbide or silicon nitride.

Question 30

Why should you never handle a hot surface igniter with bare hands?

Answer 30

oil from your hands may contaminate the ceramic surface and cause a premature burnout of the device.

Question 31

What is flame rectification?

Answer 31

When AC current passes through flames and is rectified to DC. Allows the electronic control board to sense the presence of a flame.

Question 32

What is a flame roll-out condition?

Answer 32

Where flame and flue gases exit the combustion chamber by a path other than the flue.

Question 33

What are some problems that can cause a flame roll-out condition?

Answer 33

Inadequate building static pressure. Plugged or obstructed heat exchanger. Misaligned burners. Cracked burner tube. Gas leak in manifold, piping, or valve. Dirty or faulty combustion air fan. Cracked heat exchanger.

Question 34

What type of blower do most efficient furnaces use?

Answer 34

Direct drive blower. Modern ones have sealed fan bushings or bearings that do not require lubrication and maintenance.

Question 35

What does the electronic control board (ECB) control?

Answer 35

Ignition sequence. Blower fan speed and operation. Optional remote air conditioning condensing unit. Optional humidifier. Optional heat recovery ventilator (HRV). Optional electronic filter.

Question 36

What can happen if the wrong ECB or incorrectly configured ECB is installed?

Answer 36

Could result in a violent gas explosion.

Question 37

What are the two main purposes of a sealed combustion chamber?

Answer 37

Prevents already heated air from being used for combustion air. Separates home air and combustion air which is an important safety feature.

Question 38

What are the three types of electronic ignition systems?

Answer 38

Pilot flame. Hot surface ignitor. Direct spark ignitor.

Question 39

What voltage does an intermittent pilot ignition system require?

Answer 39

a very high stepped-up voltage.

Question 40

What does the national building code require houses to have?

Answer 40

mechanical ventilation. Requires that makeup and exhaust air be interlocked so that when the main exhaust fan is operated, a supply fan is automatically turned on as well.

Question 41

What is the purpose of the heat recovery ventilator?

Answer 41

To recover heat from the air being exhausted from the space.