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Gas and Electric Furnaces

features of an automatic combination gas valve
pressure regulator, pilot safety shutoff, and redundant shutoff feature
warm air blower's off-delay timing
letting the fan have a 60-,100-,140-,or 180-second on-time period after the burner's shut off to cool down the heat exchanger
Inshot burners
usually used with induced draft systems; pull or suck combustion gases through the heat exchanger
bimetallic flame-proving system
Heat from the pilot light warps a bimetal switch closing the contacts to the main burner solenoid. When the pilot light goes out, the switch changes to the cold position and shuts off the main gas burner.
servo pressure regulator
monitors gas outlet pressure when gas flow rates and inlet pressures vary widely
flame rectification flame-proving system
The flame is located between two electrodes of different sizes. The electrodes are fed with an alternating current from the furnace's electronic module or controller. Current will flow in one direction greater than the other because of the different size electrodes. The flame actually acts as a switch. When the flame is present, the switch is closed and will conduct electricity.
specific gravity of natural gas
liquid-filled flame-proving system
Heat from the pilot light heats a liquid-filled element (mercury) and positions a set of contacts to the hot position which allows power to the main burner gas valve.
device that causes a rise in a column of water with a rise in gas pressure
dual in-line pair (DIP) switches
can be switched to re-program the furnace to make it more versatile; are two positional switches.
flame roll-out safety forced draft system switch
When touched or heated by the flame, the switch opens the circuit to the main gas valve or furnace controller.
purpose of the gas regulator in a gas burning appliance
decrease the pressure to burner
four types of gas furnaces
up-flow, down-flow, horizontal, and low-boy
excess air
to ensure that there is enough air for complete combustion
forced draft system
have a combustion blower motor on the inlet of the heat exchanger. They push or blow combustion gases through the heat exchanger and cause a positive pressure in the heat exchanger.
induced draft system
have a combustion blower motor on the outlet of the heat exchanger. They suck or draw combustion gases through the heat exchanger and cause a negative pressure in the heat exchanger.
intermittent pilot gas valve system
the pilot will be lit every time there is a call for heat by the thermostat. Once the pilot is lit and proved, usually by flame rectification, the pilot will light the main burner. Once the call for heat ends, the pilot will be extinguished and will not be re-lit until the next call for heat. In other words, there is automatic and independent control of the gas flow for both the pilot and main burner
auxiliary limit switches
sense room temperature, interrupt the burner operation when the temperature becomes too hot. They are usually mounted on blower housings. Down-flow furnaces all have auxiliary limit switches.
integrated furnace controller (IFC)
to control the operation of the furnace. These controllers often have dual-in-line pair (DIP) switches that can be switched to re-program the furnace to make it more versatile.
draft limit switch
normally closed switches located near the exhaust vent pipe. They monitor temperature and open when there is any spillage of flue gases.
thermocouple, bimetallic and liquid-filled systems
safety devices; they only allow gas to the main burner when there is a pilot flame to ignite the gas. This helps eliminate escaping gas
two types of pilot lights
aerated or non-aerated
aerated pilot light
mixes the air with the gas before burning
non-aerated pilot light
mixes the air with the gas at the burner outlet
direct burner gas valve system
the electronic module or integrated furnace controller lights the main burner directly without a pilot flame. A spark, hot surface igniter, or a glow coil usually accomplishes ignition.
typical manifold pressure for propane gas furnace
11" WC
typical manifold pressure for natural gas
3.5" WC
redundant gas valve
has two or three valve operators physically in series with one another, but are wired in parallel with one another
dilution air
excess air after combustion; usually enters at the end of the heat exchanger
smart valve system
gas valve and electronic control module in one package
used when the furnace does not light or does not sense a flame. The heat exchanger must be purged of residual unburned gas or combustion byproducts from the previous unsuccessful ignition attempt.
variable output programmable thermostat
send a proportional signal instead of an on-off signal to the furnace controller of a modern modulating furnace
heat exchanger
used to transfer heat from the hot gas to the airstream to heat the structure
a drilled hole in the spud
four factors that determine the efficiency of a furnace
type of draft, the amount of excess air, the temperature difference of the air or water entering versus leaving the heating medium side of the heat exchanger, and the flue stack temperature
consists of combustion air and dilution air
excess air
combustion air
primary air and/or secondary air
100% shutoff system
A system in which when there is a failure in the flame-proving device system and both the pilot valve and main gas valve are shut down (closed).
dew point temperature (DPT)
temperature at which the condensation process begins in a condensing furnace; varies depending on the composition of the flue gas and the amount of excess air; increases as excess air decreases
two-stage gas furnace
uses a two-stage gas valve and a two-speed combustion blower motor with two pressure switches to prove the draft; controls the temperature in the conditioned space much closer to the set point of the thermostat. Because of this, room temperature swings are smaller and comfort levels are increased.
modulating furnace
uses a modulating gas valve instead of a staged gas valve. They also come with a variable speed warm air blower to vary the speed and amount of warm air. Two-stage combustion blower motors are usually standard. An advanced intelligent integrated furnace controller (IFC) monitors and controls the furnace operations to vary the heating input and warm airflow.
distributes gas to the various burners
positive pressure venting system
can be vertically or sidewall vented. It is a system whose flue pipe pressure is positive the entire distance to its terminal end.
allows the combustion blower motor to run for a certain time period to clear the heat exchanger of any unwanted flue gases, household fumes, or dust that may have accumulated during the last heating cycle.
types of fan switches
a temperature on-temperature off fan switch, a time on-temperature off fan switch, and a time on-time off fan switch.
direct-vented furnace
has a sealed combustion chamber. This means that the combustion air is brought in from the outside through an air pipe, which is usually made of PVC plastic.
difference between a single rod and dual rod system
Flame rectification systems can be classified as either single rod (local sensing) or dual rod (remote sensing). Single-rod systems consist of an igniter and a sensor all in one rod. Dual rod systems use a separate igniter and flame sensing rod.
four types of gas burners
Cast iron, ribbon, stamped steel, and inshot
difference between a hard and soft lockout
Hard lockout is a time period built into the module that allows for a certain time to light or re-light the pilot or main burner. If this time period is exceeded, the module will go into a hard lockout or shutdown. Soft lockout is a time period built into the module that allows for a certain time to light or re-light the pilot or main burner. If this time period is exceeded, the module will go into a semi-shutdown for a certain time period, but will eventually keep trying to re-light the system.
limit switch
normally closed temperature switch when opened, opens the circuit to the gas valve
non-direct vented furnace
uses indoor air for combustion
hot-surface ignition system
uses either 24 V or 115 V to ignite an igniter. This igniter then either lights the pilot flame or the main burner flame.
draft diverter
induces air from the room while the furnace is operating to cool and dilute the flue gases. Should there be a downdraft from the flue, it allows air to spill out into the room rather than impinging on the burner flame.
vent damper shut-off
when the burners shut off, the vent damper blocks the flue gases from going up the chimney to prevent room air from being drawn up the vent during the off cycle
What is the first step a service technician should take when systematically troubleshooting a furnace that contains an IFC?
obtain the electrical diagram of the furnace and sequence of operation of the IFC
Flame current is measured in what unit of measurement?
heating value of methane
995 Btu/cu. Ft.
propane molecule
three carbon and eight hydrogen atoms
specific gravity of a fuel gas
the ratio of the weight of one cubic foot of gas to the weight of one cubic foot of air
high boiling points of propane and butane
make it possible to store them in a liquid form
three elements necessary for combustion
fuel, oxygen and heat
In the combustion reaction, one part of methane combines with two parts of oxygen to form ____________.
one part of carbon dioxide, two parts of water vapor and heat
When a gas burns in air, the nitrogen in the air _____________.
is not involved in the reaction, but does heat up to the flame temperature
The flammability limits of a fuel gas indicate __________.
the upper and lower concentration of mixtures of gas in air which are combustible
not a component of an automatic combination gas valve
The natural gas pressure regulator in the supply line into a building is usually set by the utility for:
7" WC
The draft safety control is ______.
used to sense hot flue gas and shut off the main gas valve in the event of a downdraft
thermostat location
centrally located on an inside wall about 60 inches above the floor
A condensing heat exchanger lowers the temperature of the combustion product below the dew point in order to condense water vapor and recover its latent heat. This process releases ______________.
970 Btus per pound of water condensed
The speed of a belt drive blower can be changed by __________.
adjusting the motor pulley
The temperature of a gas flame can vary depending on which major factor(s).
type of gas and amount of primary air
burning speed
the speed of the flame traveling through the gas-air mixture
The more ________, the more heat content when burned.
methane's boiling point
-259 degrees F
propane's boiling point
-44 degrees F
butane's boiling point
31 degrees F
Heat content is measured in __________.
BTU's per cubic foot
methane's heat content
995 BTU's per cubic foot
propane's heat content
2500 BTU's per cubic foot
butane's heat content
3200 BTU's per cubic foot
methane's specific gravity
0.4 - 0.8
dry air's specific gravity
propane's specific gravity
butane's specific gravity
_______ is lighter than air.
_______ is heavier than air
Propane and butane
L.P. gases
propane and butane
determines how much gas will flow through an orifice
area, pressure difference, and density
rapid oxidation of fuel, resulting in heat and light
0 - 4.3% natural gas
will not burn
4.3% - 15% natural gas
will burn
15 - 100% natural gas
will not burn
lower explosive limit (L.E.L.) of natural gas
upper explosive limit (U.E.L.) of natural gas
maximum burning speed of methane
1 ft/second (this is at 10% gas/air mixture)
maximum burning speed of propane
2.78 ft/second
maximum burning speed of butane
2.85 ft/second
Burning speed is determined by _______.
gas/air mixture
stoichiometric combustion
exact amounts of gas and air for complete combustion
To be sure complete combustion will occur ____% excess air must be present.
In combustion _______ is produced in trace amounts.
NOx (nitrous oxides)
ignition temperature of methane
1170 degrees F
ignition temperature of propane
932 degrees F
ignition temperature of butane
896 degrees F
flammability (gas/air ratio) of methane
4.3 - 15%
flammability (gas/air ratio) of propane
2.4 - 9.5%
flammability (gas/air ratio) of butane
1.9 - 8.4%
air requirements for combustion for methane
10 cubic feet of air (requires 10 cubic feet of air per 1000 BTU's)
air requirements for combustion for propane
25 cubic feet of air (requires 10 cubic feet of air per 1000 BTU's)
air requirements for combustion for butane
32.5 cubic feet of air (requires 10 cubic feet of air per 1000 BTU's)
incomplete combustion can produce _________.
carbon monoxide, aldehydes and soot
Incomplete combustion can be caused by __________.
flame impidgement
have sharp odor, irritate nose, are accompanied by carbon monoxide, will kill
anything that contains chlorine or fluorine when burned can create:
hydrochloric acid, chlorine gas, hydrofluoric acid, phosgene
2-5 parts per million of contaminants from bleach and detergents in air will cause _______.
rust in heat exchanger
Condensation in flue pipe can cause _________.
corrosion of heat exchangers
Induced draft or natural draft furnaces use __________ vent pipe.
double wall type B
Condensing furnace uses ________ vent pipe.
efficiency =
% efficiency =
output/input x 100
A condensing furnace is ____% efficient (AFVE).
A natural draft furnace is ____% efficient (AFVE).
An induced draft furnace is ____% efficient (AFVE).
A natural draft furnace will have ____ air intakes.
An induced draft furnace will have ____ air intakes.
A condensing furnace will have ____ air intakes.
annual fuel utilization efficiency
AFUE requires efficiency of at least ____%.
heating load
difference between designed indoor temperature and projected outdoor temperature.
The greater the temperature difference between outside and inside, the ______________.
faster the heat transfer from inside to outside
The _______ houses the components of the furnace.
A(n) _________ prevents the blower motor from operating when the blower door is removed.
interlock switch
If out of warranty, it is cheaper to replace the entire furnace than it is to replace the _________.
heat exchanger
A _______ furnace has primary heat exchanger as well as a condensing heat exchanger.
Natural draft heat exchangers have _____ resistance to air flow.
flue baffle
slows down the flow of combustion products to get maximum efficiency
induced draft furnace has _______ pressure in heat exchanger
cut-off plate
on blower to prevent recirculation of air and to insure proper air flow.
gas regulator
maintains constant outlet gas pressure (set by gas company)
1 psi = ____" WC
gas valve
controls the flow of gas; distributes gas to the manifold and to the pilot
An automatic combustion gas valve consists of _______________.
manual shutoff, pilot shutoff, and redundant safety valves
redundant valve
two valves in series with coils wired in parallel
pressure tap
component on gas valve which regulates gas pressure in manifold (turn clockwise to increase pressure and counter-clockwise to decrease pressure)
mixes air and gas and delivers mixture to combustion chamber.
types of ignition systems
standing pilot, direct pilot, and re-ignition pilot
A standing pilot furnace uses a ___________.
In an _______________, the pilot is lit with each cycle of the furnace.
intermittent pilot
intermittent pilot
uses a spark or hot surface ignitor to light the pilot
A hot surface ignitor is also called a ______________.
silicon carbide ignitor
six safety controls
high temperature limit switch, pilot safety, combustion air flow, draft safety, flame roll-out, blower door interlock
two causes for the limit switch to shut off the gas valve
not enough air flow or over-firing the furnace
auxiliary limit switch
protects furnace from overheating
gas burners
burn products of combustion under control in order to produce usable heat
yellow flame
does not burn cleanly; only has secondary air
blue flame
burns cleanly; has primary and secondary air
final products of combustion
CO2, H2O, and N2
Combustion is completed in __________.
outer cone
In-shot burners are used in _____________ furnaces.
induced-draft and condensing
Spoiler screws are ____________.
not adjustable
negative pressure draws in primary air
When burning speed is reduce you get ________.
flame lifting
When burning speed is increased you get ________.
disadvantages of primary aerated pilot
subject to blocking of primary air openings by dirt, dust and lint
heat anticipator
prevents overshoot
Complete combustion is accomplished when _____________.
one part methane is combined with two parts oxygen
Air is approximately __________.
21% oxygen and 79% nitrogen
The proper air and fuel mixture for complete combustion using natural gas is ______________.
1 part fuel and 10 parts air
To ensure complete combustion, ________________.
an additional 5 parts of excess air is added to the fuel-air mixture
The maximum allowable CO in flue gases is ____________.
400 PPM
The maximum allowable concentration of CO for continuous 24-hr exposure, according to ASHRAE is ___________.
.009 PPM
When testing a gas-fired furnace for CO, measurements should be taken _____________.
in the vent pipe leaving the furnace and around the burner area and draft diverter
A common percentage of CO2 in the flue gases of a natural gas furnace ranges from _________.
A power-pile produces ________.
750 mV
What three components make up a spark ignition system?
an ignition transformer, a cable, and an electrode
An intermittent pilot system uses ___________.
a spark or hot surface igniter to light the pilot
The quietest ignition system is a ____________.
hot surface igniter
The voltage of a hot surface igniter can be _______.
50 or 24 volts
What is the setting of the spark gap on a spark ignition system?
The voltage created by an ignition control module is commonly in the range of ________.
7,000 to 15,000 volts
If gas burners are not installed properly, what problem could this cause?
delayed ignition
Which part of a gas furnace allows the air within the furnace location to blend with the products of combustion?
the draft diverter
The purpose of the fan control is to __________.
start and stop the blower
What type of fan control uses an electric heater as a means of operating the blower motor?
a timed delay fan control
A pressure switch on a non-condensing furnace is used to __________.
prove the operation of the induced-draft blower
A limit control is designed to ___________.
de-energize the gas valve if the furnace overheats
Which control is commonly found on down-flow and multi-positional furnaces but is not normally used on up-flow furnaces?
an auxiliary limit control
What control is commonly used on high-efficiency, non-condensing furnaces to protect against high stack temperatures?
a vent safety control
What control is commonly used on high-efficiency, non-condensing furnaces to protect against a plugged heat exchanger?
a flame roll-out control
Two common methods used to seal a sectional heat exchanger are _____________.
welded seam and mechanical crimp
A flame rod sensor ______________.
proves the presence of a pilot or main burner flame
A normally open float switch should be wired to control _________.
the condensate pump
Which control is used to disconnect all electrical power supply to the furnace, which will prevent the products of combustion from mixing with supply air?
the interlock switch
What control is used to start the operation of the induced draft blower of a high-efficiency non-condensing furnace?
the thermostat
What control is used to energize the ignition control on the start-up of a high-efficiency non-condensing furnace?
the pressure switch
dilution air
air that mixes with combustion gases in a natural draft furnace
How much combustion air is required for every one cubic foot of natural gas used on a gas-fired appliance?
15 cu. ft.
How much dilution air is required for every one cubic foot of gas used on a natural draft appliance?
14 cu. ft.
Combustion air
can be taken from an unconfined space if the construction of the home is loose.
commonly found in products of combustion
CO2, H, N, O2
Type "B" vent pipes
aid in helping to keep flue gas from reaching their dew-point temperature, decrease heat transfer from the flue gases, used when a vent system passes through floors and ceilings.
A drip leg should be installed ________________.
on the vertical gas line
A manual gas cock valve shut-off should be installed at which part of the furnace?
The specific gravity of natural gas is approximately _____.
All gas piping must be _________.
tested with air pressure
Leaks in gas piping should be located with __________.
a soapy solution
On a standing pilot furnace, the thermocouple must be replaced when __________.
the millivolt reading is 22 or lower
If the flame is lifting off a burner port, what should a technician do to correct this problem?
adjust primary air by closing off the air shutter
A step-down transformer is reading 0 ohms at its secondary winding. What is a possible cause for this problem?
a shorted secondary winding
Upon testing a motor run winding for continuity, the technician records 3 ohms. The winding resistance should read 8 ohms. The technician concludes that ___________.
the winding is shorted
The temperature rise of a particular furnace states that it should be between 35 and 65 degrees F. The furnace is operating at 20 degrees F temperature rise with the proper gas pressure at the manifold. What could be done to solve this problem?
decrease the indoor blower speed
What is the acceptable voltage of a thermocouple?
30 millivolts
What can cause a limit control to open?
a dirty filter, too high of supply air static pressure, and not enough return air
A direct short-to-ground of a primary wire on the transformer would most likely cause __________.
a circuit breaker to trip
When a natural gas furnace has flames that are more yellow than blue, what corrective action should a technician take?
open the primary air ports
What are the normally open set of contacts used for on a fan center relay?
high-speed operation of the blower motor
How should a limit safety check be performed on a gas furnace?
stop the blower operation for at least 10 minutes or until the limit control opens.
Where do high-efficiency condensing furnaces get their combustion air?
from outside of the home only
Low-voltage thermostats are used in central electric furnaces because they are ________.
compact, responsive and safe
Power-passing devices are wired ________ with the power-consuming devices.
in series
The heat anticipator setting is determined by __________.
adding all the current draw in the 24V circuit that passes through the thermostat control bulb and heat anticipator
Three common controls used in electric furnaces are ________.
thermostat, contactors, and sequencers
Two types of wiring diagrams used in this field are ________.
pictorial and schematic
The limit switch shuts off the heating element if ________.
high temperature occurs
________ wire is often used for the heating elements in an electric furnace.
A ________ can start and stop more than one stage of heat.
package sequencer
The formula to determine cfm of air flow in an electric furnace is ________.
cfm = BTUs X 1.08 X TD
Power-consuming devices are wired ________ with each other.
in parallel
Heat generated by baseboard heaters is transferred by ________.
A sequencer uses ________ control power to start and stop the electric heaters.
The pilot safety switch functions _________.
to shut off the main burner and prevent its reopening if the pilot is out
A flue baffle ________.
slows down the flow of combustion products
When an automatic gas valve is de-energized, it is ________.
normally closed
With a natural draft furnace, a total of ________ cu. ft. of air is required to burn one cu. ft. of natural gas.
In a gas pressure regulator, the opening force is controlled by ________.
the adjustable spring above the diaphragm
In a gas pressure regulator, the closing force is controlled by ________.
the pressure under the diaphragm
A ________ is not a component of a Bunsen-type burner.
gas valve
Time-on time-off fan switches should be ________.
mounted so heat from heat exchanger does not effect bi-metal
The sequencer is used to ________.
activate the heating elements in stages
Sequencers have ________.
bimetal strips with 24-V heaters
The fan in a central system must ________.
operate while the heating elements are on.
Systems are protected with ________.
limit switches and fusible links (temperature controlled)
Any heater designed to have air forced across the element should not be operated without the ________.
fan operating
When the furnace air filter is stopped up, the _______ will cut off the electric heat first.
limit switch
Electric baseboard heaters are classified as ________.
natural draft convectors
The _______________ thermostat is commonly used for baseboard electric heat installations.
high voltage
The control voltage for the controls on a sequencer are typically _______.
24 V
What control component starts the fan on a typical electric heat furnace?