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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

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