Standard Furnace Components Flashcards
Max temp of a furnace by code
250f°
Components of a low efficient furnace
Heat exchangers
Gas valve
Draft hood
Burners
Hi-limit
Combination fan limit control
Pilot assembly
Transformer
Thermostat
Flame rollout switch
Draft spill switch
Motor
Manifold orifices
Capacitors
What is the fan on temp range
What is the fan off temp range
What is differential temp
120f°(49c°)-140f°(60C°)
90f°(32C°)-100f°(38C°)
Min 35f° (19c°)
How many millvolts are used in a thermocouple
20-30milvolts
What changes the rotation of a belt motor
Reverse the wires under the cover plate
Type of capacitors
What do they do
Run capacitor
Start capacitor
Hold electricity and starts the furnace process
Furance sequence of operation
- T-stat calls for heat (R and W closes)
- gas valve opens/energized
- Pilot light ignites burners
- Fan control turns on
- Blower motor energized
- T stat satisfied
- R and w opens
- Gas valve de energized
- Burners extinguish
- fan-off temp reached
- Blower motor de energized
- Stand by
Manifold pressure
3.5” W.C.
Configurations of furnaces
Up flow, down flow, horizontal, all positions
What switch measures temp once heat exchanger is warmed up
Fan switch or fan control
When T-stat is satisfied, what happens
Turns off electrical circuit to gas valve
“De-energized”
Furance operation process is called what?
Heating cycle
Controls used to operate furance are called. What are they
Operating controls
Thermostat, fan control,transformer,gas valve, and motor
Controls to make sure the operation is safe are called?
What are they?
Safety controls
Limit switches and thermocouple/ therompile
What is a standing pilot
Variations of pilot
Is a small flame that burns 24/7 located near main burner
Aerated, post-aerated (most common), and non aerated
If the pilot is out of alignment, what can happen
What is it called
Gas flows to burner to long and accumulates. When it does ignite causes small explosion.
Delayed ignition
What is a thermocoupler
Thermal electric device made of two dissimilar metals (iron and constantan) produces a small voltage 20 to 30 millvolts AC
How much should the flame encompass the tip of a thermocouple.
⅜” to ½”
What is a thermopile, also known as
Thermocouples wired in a series
Power piles and pilot generators
how does a power unit work
If the power unit isn’t getting power from the thermocouple/pile, the electromagnet in it will close the plunger, preventing gas flow.
What is the setting on the heat anticipator attached too
Amp draw of low voltage circuit (current draw of control unit to which the Thermostat is installed)
What class equipment is a transformer
Type 2
What do transformers operate by
Magnetic induction
If the secondary has less windings then the primary it will have more or less voltage? What is this Transformer called
Less voltage, a step down Transformer
2 types of blower motors
Belt drive, direct drive
What do Draft diverters or draft hoods do
Provide a pressure balanced flue. Breaks the stack action while at the same time prevents down drafts.
Furnace Rating plates will have what information
Orifices sizes, information on approval agency,altitude rating (listed as sea level) input and out rating of appliance and fuel type
3 bimetal temperature sensing devices
Strip, spiral and disc
Main purpose of a limit switch
To shut down the gas supply, should the furnace get too hot
List the different furnace configurations
Upflow,downflow, horizontal, and all position
Why is a Tstat better for controlling the furnace than a light switch
They can monitor the temperature and respond by turning off or on equipment.
Once there is a call for heat in a furnace, what happens
Complete an electrical circuit that energizes the gas valve allowing gas to flow.
A combination gas valve consists of?
Regulator
Manual ON/OFF/pilot valve
Control circuit connections
Pilot gas adjustment
Thermocouple connection
Power unit
What is CW and CCW stands for what in regards to belt driven motors?
Clockwise and counterclockwise
Regardless of hp rating, all belt drive motors rotate at what?
1725 rpm
While most belt driven motors are single speed(1725rpm), there are two speed motors. What rpm is the low speed
1140rpm
Different between belt and direct motors
Direct drive doesn’t have a belt it is mounted directly to the fan unit
Direct drive motors have up to how many speeds
5
What are most motors in furances called
Squirrel cage induction motors gets name from fan wheel
Fans used in furnaces are ____ type fans with____curving blades
Centrifugal, forward
If the fan wheel is installed backwards what can happen
Furnace may overheat and be shut off by limit switch
Shaft size of a belt driven fan
¾”
What happens when adjusting the pulley of a belt drive fan
If the pulley is adjusted in, the diameter becomes larger, and fan speed increases.
If the pulley is let out, the diameter will be smaller, resulting in decreased air
Direct drive fan shift size
½”
What do heat exchangers do
Transfer heat from burners to the air stream.
Heat exchangers can be what 3 styles
Clam style, barrel style, or tubural
What happens if down draft enter venting system?
Could interfere with the burners or extinguish pilot light
What is the venturi effect
the reduction in fluid pressure that results when a moving fluid speeds up as it flows through a constricted section (or choke) of a pipe
What happens when manifold pressure is increased or reduced
If increased will cause more air to enter burner resulting in flames lifting off burner
If reduced, then air gas mixture is reduced, and flashback will occur
The venturi tube is also called? What does it do
The mixing tube
Mixes the gas and air for the burner through high speed low pressure
Inside the heat exchanger, heat from burners transfer through what process?
The heat from burners transfers through convection and radiation up the metal surface of the
What is the job of a burner
Transforming a combustible fuel into heat
Two types of burners
Upshot and In-shot
Can you just resize the orifice size? What about converting to a different gas
Under no circumstances should you. If furance is to be converted from one gas to another, you can install a factory conversion kit only.
Purpose of the flame rollout switch
The flame rollout switch is a safety feature that shuts off the gas supply to the burner if the flame blows out.
What common voltage, used in furnaces, is found on the secondary side of a transformer?
24V
