Forced-Air Heating Systems Flashcards
What are the two common materials electric heating elements are made from?
Tungsten and Nichrome
What is the primary function of the indoor fan motor in an electric furnace?
To move air from the return air ducts to the space being warmed.
What device does not provide 100% gas shutoff in a gas fired furnace?
Pilot Safety Switch
What efficiency rating classifies a furnace as mid-efficient?
80-83%
Where would the sensing portion of a secondary high limit device be placed?
Blower Compartment
What is the purpose of a heat recovery ventilator?(HRV)
To recover heat from an exhaust fan.
What is the most likely faulty component if a high efficient furnace is not running correctly and the error code indicates a flame proving failure?
Gas Valve
What is the preferred range for the fan on setting of the combination fan controller?
49-60C
What device is used to ignite the main burner in efficient gas fired furnaces?
Hot surface ignitor.
What safety feature ensures gas is not released if the flame is not detected?
FLAME SENSOR
What efficiency do mid to high efficiency units operate at?
90%-96%
What are the components of an electric heating unit?
Heating element, thermostat, reflector, fan
Describe a Reflector in an electric heating unit
Located behind the heating element to direct radiant heat forward
Usually made of polished aluminum or another highly reflective material.
What is the purpose of the indoor fan motor?
To move air from the return airducts across the heat exchanger and through the space being warmed.
Where is a 120v heating system commonly used?
Commercial and industrial applications.
(Warehouses, workshops, garages, larger buildings where temp differential is not as important as in residential application.)
Why does a 120v heating system thermostat have no anticipator?
Temperature differential is not as important as in a personal residence.
What are the Fan-ON and fan-OFF settings?
ON - 49-60C
OFF- 32-38
How do step open gas valves work?
Step open gas valves open in two stages to reduce the initial rush of gas into the burner, thus reducing the size and intensity of the ignition explosion in the heat exchanger.
What is the purpose of flame proving systems?
To shut off the flow of fuel through the gas valve if the pilot flame goes out.
(thermocouples are used to prove the existence of a pilot flame)
What are the two reasons for pressing the manual reset when starting a gas fired furnace?
1) To allow pilot gas to pass through the valve to the pilot burner
2) To hold the armature up into the power unit.
What is the most common type of valve used in modern heating units?
Combination Gas Valve
Why were pilot safety switch dangerous?
It would not prevent gas from flowing into the extinguished pilot burner.
What are other names for a safety gas valve?
100% shutoff valves, safety gas valves, automatic pilot valves
Solenoid Valves control the flow of gas directly by:
By mean of an electrically operated plunger.
The Solenoid Valve is a fast acting primary control device.
True or False
True
What does a diaphragm valve do?
Turns the supply of gas to the main burner on or off.
Typically smaller than the solenoid types
Gas pressure operates the valve
Which is faster to respond to a change in control circuit status?
Diaphragm Valve or Solenoid Valve
Solenoid Valve
(Takes time to bleed gas from the diaphragm)
What is incorporated in a combination gas valve?
The gas regulator
Safety gas valve
Power unit
Main gas valve
Manual gas shutoff
What is the preferred valve for both natural gas and propane fired systems?
Combination Gas Valve
What are the two types of fans?
Belt driven type
Directly Coupled drive
The fractional horsepower motors typically range from:
1/4hp to 3/4 hp
What are the components of a 24v system?
Thermostat
Gas Valve
Transformer Secondary
What are the components of the 120v circuit?
Fan control switch and transformer primary
Where is the high limit connected
24v or 120v circuit?
The high limit may be connected into either the 24v or 120v circuits
When does the HL/Fan control switch reset?
When it drops 11C below the setpoint
What are the benefits of two stage heating?
Higher efficiency and better temperature control
How can fan cycling be prevented?
By maintaining proper settings and by keeping at least 17C to 22C differential between the settings.
What is the efficiency of condensing gas furnaces?
90% - 98%
In a high efficient gas furnace, PVC exhaust piping can be routed vertically or horizontally as it exits the structure.
True or False?
True
What are the major components of an efficient gas fired forced air furnace?
Combustion Air components
Burner assembly
Fan blower assembly
Electronic printed circuit control board (ECB)
What is the minimum AFUE for new furnaces?
78% AFUE
What are the 4 main combustion air components?
Combustion Air Blower (flue gas vent blower/system purge blower)
Combustion Air Prove Switch
Heat Exchanger
Primary high limit safety switch
What does the heat exchanger do?
Transfers heat from the combustion chamber to the circulating air and separates the products of combustion from the circulating air.
How can the effectiveness of the heat exchanger be improved?
By increasing the heat exchanger surface area by using thinner metals that transfer the heat more effectively to the circulating air.
What are high limit safety controls?
Safety controls that prevent mid-efficient furnace burners from operating during a dangerously high temperature event.
What is the set point for high limit safety controls?
Between 82-94C
The DSI control board provides the spark electrode with a high voltage potential of up to ________V
Up to 5000V
What are Hot Surface Ignitors made from?
Silicon Carbide or Silicon Nitride
Under certain conditions when a flame is present between two points of electrical potential difference, current flows through the flame. This is called:
Flame Rectification
Roll Out Limit control prevents the furnace from firing while:
a flame roll out condition exists.
(flame and flue gasses exit the combustion chamber by a path other than the flue)
Source Voltage over a control device indicates:
An open switch
Source Voltage over a load device will indicate:
That all control devices ahead of it are closed
What are the 5 steps for troubleshooting a Forced-Air Heating System?
1.) Observe the cycle of operation.
2.)Measure Voltages
3.)Confirm Fault.
4.) Fix the fault.
5.) Confirm Operation
The high limit control is usually located in the:
Heat Exchanger
