unit 48, unit 49, unit 50, unit 51, unit 52
How many volts are sent to a furnace spark igniter?
10,000 volts
The approximate heating value of methane i
1000 BTUs per cubic foot
The most common manifold pressure for propane furnaces is
11 inches wc
What size should the combustion air openings be for a room with an 80,000 BTU/hr input furnace and a 35,000 BTU/hr water heater if the openings are opening directly into a ventilated attic and a ventilated crawlspace?
115,000 / 4,000 = 28.75 square inches free area each
How much time should it take the ½ ft3 to turn per revolution if the furnace was using 142.8 ft3/hr of gas?
12.6 seconds per revolution
Hot surface igniters typically operate on
120 volts
Combustion air for gas appliances should be
14 to 15 cubic feet of combustion air per 1000 BTU of firing rate
What is the usual setting for the limit on the fan/limit switch of a warm air furnace?
180°F to 200°F
The control circuit in most gas furnaces operates on
24 volts
The approximate heating value of propane is
2500 BTUs per cubic foot
The most common manifold pressure for natural gas furnaces is
3.5 inches wc
How long is an ignition trial on a direct spark ignition system?
4 to 21 seconds
The normal amount of excess air in the flue gas of a properly operating category I gas furnace is approximately
50%
A technician measures a return air temperature at the furnace of 70°F and a supply air temperature of 180°F on a furnace whose temperature rise is rated at 40°F
60°F. The technician should-Increase the airflow to reduce the temperature rise
The normal oxygen and carbon dioxide percentages in the flue gas of a properly operating gas furnace are approximately
7.5%
What size should the combustion air ducts be for a room with an 60,000 BTU/hr input furnace and a 16,000 BTU/hr water heater if the air is traveling through horizontal ducts?
76,000 / 2,000 = 38 square inches free area each
Beginning in May, 2013, the minimum AFUE rating for a new furnace in the southern part of the United States is
80%
How does venting compare on 80 percent and 90 percent efficient heating units?
80% furnaces use type B vent and 90% furnaces use PVC pipe for venting material
What is required when installing a 90% furnace that is not required for an 80% furnace?
A drain for the condensate from the recuperative coil
What is an intermittent pilot ignition system?
A gas pilot that is only ignited when there is a call for heat. When there is no call for heat, the gas pilot is out.
How does a power burner differ from an atmospheric burner?
A power burner uses air forced into the combustion area by a fan.
What components are added for to make a two stage furnace?
A two stage gas valve, two draft proving switches, and a two speed induced draft fan
When is the igniter energized on an induced draft furnace that uses hot surface ignition?-
After the draft switch closes to prove the draft
How does certification affect heating equipment in the U.S.?
All heating equipment must be tested, certified, or listed by an accredited testing agency.
A precisely drilled hole that controls the amount of gas entering a burner is called
An orifice
How do water and carbon dioxide relate to combustion?
Both are stable and safe products of combustion.
Compared to blue flames, yellow flames
Burn at a lower temperature
What type of gas production is a definite indication of incomplete combustion?
CO
Agencies that set equipment standards and test equipment include
CSA and AHRI
Why is carbon monoxide production undesirable in furnace operation?
Carbon monoxide can cause illness.
The two most common categories of gas fired furnaces are
Category I and Category IV
Once a technician works on a system, what else should he or she do?
Check for gas leaks.
The difference between vent sizing for natural draft appliances and fan assisted appliances is that
Columns for fan assisted appliances list both a minimum and a maximum appliance capacity for each vent size
The typical start up sequence for a direct ignition system is:
Combustion blower pre-purge, igniter and gas valve energized, flames sensed, indoor blower energized after time delay
Which government agency is involved is setting gas fired equipment standards?
DOE
How was natural gas created?
Decomposition of organisms converted into gas through pressure and heat
Why do gas furnace efficiencies jump from 80% to 90%?
Efficiencies over 80% cause condensation but the flue gas is still too hot for PVC vent pipe until the efficiency reaches 90%
What is the purpose of excess air in furnace combustion?
Excess air insures that all the gas is burned by making sure there is plenty of oxygen available.
What type of flame proving do direct spark ignition controls use?
Flame rectification
The four categories of gas fired furnaces are based on
Flue gas temperature and pressure
If the drain for a 90% efficient furnace and the drain for the air conditioning coil are run with a common drain, the drain should be sized
For the furnace drain, which is larger
Compared to furnaces installed at lower altitudes, furnaces installed at higher altitudes
Have a lower firing rate
What part does heat play in combustion?
Heat is the catalyst of combustion.
Propane is
Heavier than air
How does altitude affect the firing rate of a furnace?
Higher altitudes have lower firing rates.
Furnace gas manifold pressure is measure in
Inches water column
What does the absence of O2 in the flue gas indicate?
Incomplete combustion
What condition does too much O2 in the flue gas cause?
Inefficiency
What type of burner do most modern furnaces use?
Inshot
What effect does slowing down the blower speed have on a heating unit?
It increases the air temperature rise.
How does negative pressure affect the air supply of a furnace?
It increases the need for outside air.
How does the presence of 7 to 9 percent CO2 in flue gas reflect heating unit performance?
It indicates that the unit is running efficiently.
Which of the following statements accurately describes silicon carbide?
It is brittle and will break easily.
What safety function does the limit switch perform?
It prevents the furnace from overheating.
LP gas is considered more hazardous than natural gas because
LP tends to puddle in low areas but natural gas tends to dissipate
Natural gas is
Lighter than air
Which of the following gases are most commonly mixed to form natural gas?(check all that apply)
Methane,Ethane
Which of the following is true of the fan switch?
Modern fan switches cycle the fan based on time.
The burner assembly and furnace cabinet should be grounded for safety and because
Most control boards for direct spark ignition or hot surface igniters will not operate correctly unless the unit is grounded
As a safety measure, when starting an LP gas furnace, you should position yourself
Off to the side and off the ground
What type of vent is used with 90% Category IV furnaces?
PVC
What type of vent is used for category IV gas appliances?
PVC vent
How are Category IV furnaces generally vented?
PVC venting
Combination gas valves used standing pilot furnaces combine
Pressure regulation, automatic gas control, and pilot safety
What is the difference between primary air and secondary air?
Primary air is mixed with the gas before ignition and secondary air is drawn in after ignition
What is the difference between natural gas and propane? (check the 2 that apply)
Propane has a higher heat content per cubic foot than natural gas, Propane is heavier than natural gas
Differences between propane vs natural gas: (check all that apply)
Propane is heavier and tends to accumulate in low places making it more of an explosion hazard. Propane is stored in a tank that needs to be filled by truck periodically.
How is using propane different than using natural gas?
Propane requires a self-contained tank.
If the temperature rise of a furnace is too low, how can it be increased?
Reducing the air flow
Hot surface igniters are made of
Silicon carbide
Why do solenoid type valves pose a problem for some types of burners?
Solenoids open and close too quickly.
What is the net stack temperature?
Stack temperature - ambient temperature = net stack temperature
What pieces of information are necessary to use a manufacturer's gas manifold pressure chart?
The altitude and gas heating value
What is normally used for the ground terminal on a spark igniter?
The burner assembly
If carbon monoxide is a product of a reaction resulting in combustion, what can be said about the combustion?
The combustion was incomplete.
Why must condensate pumps used with condensing furnaces be approved for furnace condensate use?
The condensate from a furnace is mildly acidic and can damage the materials in some condensate pumps
Annual Fuel Utilization Efficiency, AFUE, is
The efficiency of the furnace for an entire heating season
What does a certification done by the Canadian Standards Association (CSA) signify?
The equipment meets ANSI, UL, CSA America, or NSF standards.
How does the flame rod sense the flames in a flame rectification circuit?
The flame changes the AC voltage to pulsed DC current
Why are draft inducer fans necessary in today's furnaces?
The flue gasses cannot make their way through the new heat exchangers on their own.
What controls the amount of gas entering the burner?
The gas manifold pressure and the burner orifice size
Why should the heating element of a hot surface igniter not be touched when installing a replacement igniter?
The igniter will burn out where you touched it
Which of the following is true of piping tables?
The length of piping helps to find cubic foot capacity.
How is a gas meter used to check gas furnace input?
The number of times the gas dial rotates in a minute and the gas heating capacity are used to calculate the firing rate
How did serpentine heat exchangers compare to older heat exchangers?
The serpentine heat exchangers required a heat-induced fan, causing negative pressure.
How does the specific gravity of propane compare with the specific gravity of natural gas?
The specific gravity of propane is greater than that of natural gas.
The temperature rise of a furnace is
The temperature difference between the air entering the furnace and leaving the furnace
What is a vent connector?
The vent connector is the pipe running from the furnace to the vent or chimney.
Why are the 90 percent condensing furnaces unique?
The venting of this type of furnace is made of PVC and is easy to seal airtight.
For what reason are Category II and Category III furnaces rare?
The venting system is made of a material that is no longer available.
What is the purpose of the gas burner ventui?
The venturi mixes the gas and primary air by making them swirl as they pass through
What effect do volatile liquids stored near the furnace have on its performance?
They can produce flammable vapors.
Why are standing pilot natural draft furnaces no longer manufactured?
They cannot provide the minimum 78% AFUE required by DOT
What is the reason for the door switch on the furnace?
To keep the blower from sucking combustion gases out of the vent into the living space.
What is the purpose of the heat exchanger?
To separate the combustion gas from the circulating air while transferring heat from the combustion gas to the circulating air
Soot forming in the heat exchanger can be caused by
Too much gas in the gas air mixture
What type of vent is used with 80% Category I furnaces?
Type B double wall gas vent
What type of vent is designed specifically for category I gas appliances?
Type B vent
Available furnace cabinet configurations include
Up-flow, down-flow, horizontal, low boy, multi-position
Flue gas condensation can be prevented by
Using double wall vent
Why is it possible to use smaller pipes in higher efficiency units?
Volume and temperature of flue-gases are lower.
The products of complete combustion of a hydrocarbon gas are
Water and carbon dioxide
The products of incomplete combustion of a hydrocarbon gas are
Water, carbon, carbon dioxide, and carbon monoxide
Carbon monoxide is formed
When the gas is not burning completely.
What type of flame is characteristic of incomplete combustion?
Yellow flames
A furnace that is composed of a heat exchanger compartment and a blower compartment is considered
a gas forced-warm-air furnace.
What is a redundant gas valve?
a gas valve with two valves mechanically in series for safety in case one fails
A Category I furnace operates with
a high temperature, negative pressure vent
A Category IV operates with
a low temperature, positive pressure vent
What is a gas pilot light?
a small flame that is used to light the main burners
What is a draft pressure switch?
a switch that senses pressure differential when the draft inducer is producing the proper draft on the exhaust vent
Which of theese technologies is NOT used in 80% furnaces?
adding a condensing recuperative coil
How does the location of the supply
air thermometer affect the results of the air temperature rise test?-The temperature should be taken in the duct system as close to the furnace as possible, but not so close that the radiant heat of the heat exchanger affects the reading.
Heat exchanger designs from oldest to newest are
barrel shaped, clamshell, serpentine, tubular
How are transformers in the HVACR field rated?
by secondary power output
How does the induced draft blower inprove furnace efficiency of a natural draft furnace?
combustion gasses do not have to be as hot to correctly vent
What did the removal of the pilot light, improvement of the heat exchanger, and the addition of an induced
draft blower do for the mid-efficiency furnace? -It improved unit efficiency to 80 percent.
How do technicians increase the blower speed in a belt
driven motor?-They decrease pulley spread.
How can a technician determine a major component failure on a mid
efficiency furnace?- The technician can use the self-testing feature on the control box.
How is a concentric vent used in a high
efficiency furnace?-It allows a single outdoor exit point for the vent and combustion air pipe.
Which of the following is true about the installation of condensate pumps on high
efficiency furnaces? -A pump must be approved for use with condensate.
A stainless steel coil that is built like a refrigeration coil and allows a high
efficiency unit to move into the 90+ efficiency range is called-a recuperative coil.
In all furnace cabinet configurations, the return air
enters at the fan housing end and exits the heat exchanger end of the furnace
The three things necessary for combustion are
fuel, oxygen, and heat
The difference between 80% efficiency furnaces and a 90% efficiency furnaces is that 90% efficiency furnaces
have an extra recuperative heat exchanger that condenses water out of the flue gas
A combination of two elements makes up common fuel gases. Which of the following elements make up these fuel gases?
hydrogen and carbon
What happens if a technician comes into contact with the spark
ignition circuit while it is energized?- The technician will experience great discomfort.
If pulsed DC current is not present in the flame rod, what is the effect on the gas valve in a direct spark
ignition system?- The valve will remain closed.
Which type of burner port should be used for maximum efficiency?
in-shot
Why did the development of 80 percent efficient furnaces using draft
inducer fans present a problem for venting tables?-The inducer fan cooled the air and a smaller vent was required to prevent condensation.
The correct operating sequence for an induced draft 80% furnace is
inducer starts, igniter glows, gas valve opens, indoor fan starts
The different types of atmospheric gas burners include
inshot, drilled port, slotted, and ribbon
How does a thermocouple provide pilot safety?
it generates voltage to hold in the pilot safety solenoid
What is the type of gas that is created as a byproduct of other manufacturing processes?
manufactured gas
Three broad categories of fuel gasses include
natural, manufactured, liquid petroleum (LP)
What does a roll
out switch do? -shuts of the gas valve if flames are sensed outside the combustion area
What is the purpose of the roll
out switches?-to shut the gass valve off if flames roll out of the combustion area
How does the standing
pilot system provide a safety check for the continually lit pilot? - The pilot flame impinges on the thermocouple.
How does air flow in a standing
pilot, natural-draft system?- It is relies on heat to increase the buoyancy of the air to pull air into the combustion chamber and force flue-gases into the proper vent.
Which is mixed with fuel in the burner prior to combustion?
primary air
Air that is mixed with the fuel before it is ignited is referred to as
primary air.
What is the purpose of a flame
proving device in a system with a standing pilot?- It shuts off the gas flow in case the pilot goes out.
Some furnaces use two automatic shutoffs that are mechanically in series to provide additional safety. What type of valves are these
redundant valves
What is the difference between a a 90% furnace with a sealed combustion system and a typical 90% furnace?
sealed combustion units require combustion air to be piped in from the outside.
How do technicians adjust in
shot burners in high-efficiency furnaces?-Gas pressure only is adjusted.
How does a thermocouple work?
temperature difference between the hot and cold junction generates voltage
Beginning in 2013, which region in the United States will have the highest minimum AFUE rating?
the North
What causes flue gas condensation?
the flue gas is cooled below the dewpoint of the combustion gas
The main limit switch shuts the gas valve down when
the heat exchanger area becomes overheated
What controls the indoor blower motor on an 80% induced draft furnace?
the integrated furnace control board
The maximum required length in a piping table considers
the length of the pipe, including all fittings, valves, or obstructions.
How is the limit wired on a furnace that uses a line voltage limit?
the limit is wired in series with the transformer primary
How do gas furnaces ensure the production of a blue flame?
the use of gas spuds and a manifold
What is the purpose of the draft switch?
to verify that the draft is working before energizing the igniter or energizing the gas valve
How is the Annual Fuel Utilization Efficiency (AFUE) efficiency rating determined?
using the NIST equation for 100 percent efficiency minus losses
Which of the following is a product of complete combustion?
water vapor
When will installation technicians most likely install an upflow furnace?
when there is little floor space
