Forced-Air Heating Systems 030203b Flashcards
What are the two most common materials used for electric heating elements and what are they generally encased in?
Nichrome & Tungsten.
Encased in a protective insulating material like ceramic, mica or glass.
What is the control circuit voltage of a 120V heating system?
Control circuit power rating of 120V. Requires no 24V transformers. Gas valve is designed to operate at 120V.
What is the purpose of a step open gas valve?
Limits the initial flow of gas to the ignition source to prevent a large explosion.
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?
100% shutoff valve, safety gas valve, or automatic pilot valve.
What is the valve called that controls the flow of gas directly by means of an electrically operated plunger?
Solenoid valve.
Where should solenoid and diaphragm valves be installed?
Downstream of the safety gas valve.
What is the most common gas control device currently available?
Combination gas valve.
Includes a gas regulator, safety gas valve, power unit, main gas valve, and a manual gas shutoff all in one unit.
What are the two types of fans in a forced air heating system?
Belt-driven type.
Directly coupled drive type.
What is the range that the high limit switch is usually set to?
82 degrees Celsius to 93 degrees Celsius.
Resets when the temperature drops 11 degrees Celsius below the thermostat set point.
What is the range setting for the Fan-on and fan-off function?
Fan-On - 49 degrees Celsius to 60 degrees Celsius.
Fan-Off - 32 degrees Celsius to 38 degrees Celsius.
What is a down flow furnace?
Air is blows downward and force heated air through ductwork in or below the concrete slab floor.
What is the electrical energy that comes from the thermocouple used for?
Operating the power unit.
What safety pilot device provides 100% gas shutoff?
Safety gas valve.
What is the AFUE (Annual Fuel Utilization Efficiency) rating of a high efficiency furnace?
90% to 98%
What is the Annual Fuel Utilization Efficiency (AFUE) rating of a mid efficiency furnace?
80% to 83%
What is the minimum Annual Fuel Utilization Efficiency (AFUE) rating for a new standard furnace?
78%
What are the four main air combustion components?
Combustion Air Blower.
Combustion Air Prove Switch (Air proving switch).
Heat Exchanger.
Primary High Limit Safety Switch.
What are the 3 main primary components needed for a condensing furnance?
Primary heat exchanger.
Secondary heat exchanger.
Condensate drain line.
What is the purpose of the primary heat exchanger?
Removes the heat from the combustion flue gases.
Made of a specially coated steel tube system.
What is the purpose of the secondary heat exchanger also known as the condenser coil?
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.
What is the purpose of the condensate drain line?
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.
Why must the secondary heat exchanger be made of stainless steel?
To resist corrosion from the carbonic acid it creates.
What is the main purpose of the combustion air blower (CAB)?
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.
What is the purpose of the air proving switch?
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.
What is the purpose of the high limit safety switch?
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.
What is the general set point for high limit switches?
82 degrees to 94 degrees Celsius.
What are the two igniter options?
Direct Spark Igniter (DSI)
Hot Surface Igniter (HSI)
What temperature does a hot surface igniter get up to?
1370 degrees Celsius.
What are igniters sensitive to?
moisture, dirt, and grounding issues.
What are hot surface ignitors (HSI) made from?
Silicon carbide or silicon nitride.
Why should you never handle a hot surface igniter with bare hands?
oil from your hands may contaminate the ceramic surface and cause a premature burnout of the device.
What is flame rectification?
When AC current passes through flames and is rectified to DC. Allows the electronic control board to sense the presence of a flame.
What is a flame roll-out condition?
Where flame and flue gases exit the combustion chamber by a path other than the flue.
What are some problems that can cause a flame roll-out condition?
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.
What type of blower do most efficient furnaces use?
Direct drive blower.
Modern ones have sealed fan bushings or bearings that do not require lubrication and maintenance.
What does the electronic control board (ECB) control?
Ignition sequence.
Blower fan speed and operation.
Optional remote air conditioning condensing unit.
Optional humidifier.
Optional heat recovery ventilator (HRV).
Optional electronic filter.
What can happen if the wrong ECB or incorrectly configured ECB is installed?
Could result in a violent gas explosion.
What are the two main purposes of a sealed combustion chamber?
Prevents already heated air from being used for combustion air.
Separates home air and combustion air which is an important safety feature.
What are the three types of electronic ignition systems?
Pilot flame.
Hot surface ignitor.
Direct spark ignitor.
What voltage does an intermittent pilot ignition system require?
a very high stepped-up voltage.
What does the national building code require houses to have?
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.
What is the purpose of the heat recovery ventilator?
To recover heat from the air being exhausted from the space.
