HVAC Unit 31 Gas Heat EXAM STUDY GUIDE
2026 COMPLETE QUESTIONS WITH CORRECT
DETAILED ANSWERS || 100% GUARANTEED PASS
<LATEST VERSION>
1. Venturi - ANSWER ✔ Internal restriction in a passageway that causes fluid
to speed up when it passes through, creating low pressure
2. Combustion Fuels - ANSWER ✔ Natural gas, liquid petroleum (LP), fuel oil
3. Stoichiometric Combustion - ANSWER ✔ Perfect combustion;
unachievable
4. Complete Combustion - ANSWER ✔ When fuel is burned in the presence of
excess oxygen to produce water and CO2
5. Ignition Temperature - ANSWER ✔ The temperature that a fuel must reach
before combustion can begin; provided by pilot light or heated surface
6. Oxygen to Methane Ratio - ANSWER ✔ 2:1
7. Air to Fuel Ratio - ANSWER ✔ 10:1
,8. Flammability Limits - ANSWER ✔ Range of a fuel's concentrations that the
fuel is capable of igniting in
9. Primary Air - ANSWER ✔ Air mixed with fuel prior to ignition
10.Secondary Air - ANSWER ✔ Air added to the flame to support combustion
11.Hydrocarbons - ANSWER ✔ Compounds composed of only carbon and
hydrogen
12.Incomplete Combustion - ANSWER ✔ When a flame doesn't receive
enough oxygen to finish combustion process; leads to yellow flame tip
because of I burned carbon
13.what are the types of furnaces - ANSWER ✔ upflow, low-boy, downflow,
horizontal & multipoise or multipositional
14.describe an upflow furnace - ANSWER ✔ stands vertically & needs
headroom. Takes in cool air from the rear, bottom, or sides near the bottom
& discharges it from the top. Designed for first floor installation with
ductwork in the attic, or basement installation with ductwork between or
under the first floor joists.
,15.describe a low boy furnace - ANSWER ✔ about 4 ft high. Used primarily in
basement installations with low headroom where the ductwork is located
under the first floor. Air intake & discharge are at the top.
16.describe a downflow furnace - ANSWER ✔ referred to sometimes as a
counterflow furnace & looks like an upflow furnace. Might be in a crawl
space. Air intake is at the top and discharges at the bottom.
17.describe a horizontal furnace - ANSWER ✔ positioned on its side &
installed in crawl spaces, attics or suspended from floor joists in basements.
Air intake is at one end & it discharges at another.
18.describe a multipositional furnace - ANSWER ✔ can be installed in any
position
19.What does combustion require & what is the reaction between them? -
ANSWER ✔ fuel, oxygen & heat. reaction is known as rapid oxidation, or
the process of burning
20.describe a solenoid valve - ANSWER ✔ a normally closed valve. The
plunger in the solenoid is attached to the valve or is in the valve. When an
electric current is applied to the coil, the plunger is pulled into the coil. This
opens the valve. The plunger is spring loaded so that when the current is
turned off, the spring forces the plunger to its NC position, shutting off the
gas
, 21.what is a limit switch - ANSWER ✔ a safety device that senses heat
22.To test for an obstructed flue pipe on an induced draft furnace , a technician
should use ; - ANSWER ✔ a manometer to test the negative pressure
generated by the induced draft motor
23.What is used to ignite the gas burners on most conventional gas furnaces. -
ANSWER ✔ Pilot flames.
24.What is the difference between aerated and nonaerated pilot burners? -
ANSWER ✔ In an aerated pilot, the air is mixed with the gas before it enters
the pilot burner.
In a nonaerated pilot, only secondary air is used for combustion.
25.T or F: a pilot is technically just a burner with an orifice that gas passes. -
ANSWER ✔ True.
26.T or F: a standing pilot is only lit when needed, it goes out to conserve gas
when it is not lighting a burner. - ANSWER ✔ False- a standing pilot burns
continuously.
27.What is the technical term to verify that a standing pilot is lit? - ANSWER
✔ It is called flame-proving.
2026 COMPLETE QUESTIONS WITH CORRECT
DETAILED ANSWERS || 100% GUARANTEED PASS
<LATEST VERSION>
1. Venturi - ANSWER ✔ Internal restriction in a passageway that causes fluid
to speed up when it passes through, creating low pressure
2. Combustion Fuels - ANSWER ✔ Natural gas, liquid petroleum (LP), fuel oil
3. Stoichiometric Combustion - ANSWER ✔ Perfect combustion;
unachievable
4. Complete Combustion - ANSWER ✔ When fuel is burned in the presence of
excess oxygen to produce water and CO2
5. Ignition Temperature - ANSWER ✔ The temperature that a fuel must reach
before combustion can begin; provided by pilot light or heated surface
6. Oxygen to Methane Ratio - ANSWER ✔ 2:1
7. Air to Fuel Ratio - ANSWER ✔ 10:1
,8. Flammability Limits - ANSWER ✔ Range of a fuel's concentrations that the
fuel is capable of igniting in
9. Primary Air - ANSWER ✔ Air mixed with fuel prior to ignition
10.Secondary Air - ANSWER ✔ Air added to the flame to support combustion
11.Hydrocarbons - ANSWER ✔ Compounds composed of only carbon and
hydrogen
12.Incomplete Combustion - ANSWER ✔ When a flame doesn't receive
enough oxygen to finish combustion process; leads to yellow flame tip
because of I burned carbon
13.what are the types of furnaces - ANSWER ✔ upflow, low-boy, downflow,
horizontal & multipoise or multipositional
14.describe an upflow furnace - ANSWER ✔ stands vertically & needs
headroom. Takes in cool air from the rear, bottom, or sides near the bottom
& discharges it from the top. Designed for first floor installation with
ductwork in the attic, or basement installation with ductwork between or
under the first floor joists.
,15.describe a low boy furnace - ANSWER ✔ about 4 ft high. Used primarily in
basement installations with low headroom where the ductwork is located
under the first floor. Air intake & discharge are at the top.
16.describe a downflow furnace - ANSWER ✔ referred to sometimes as a
counterflow furnace & looks like an upflow furnace. Might be in a crawl
space. Air intake is at the top and discharges at the bottom.
17.describe a horizontal furnace - ANSWER ✔ positioned on its side &
installed in crawl spaces, attics or suspended from floor joists in basements.
Air intake is at one end & it discharges at another.
18.describe a multipositional furnace - ANSWER ✔ can be installed in any
position
19.What does combustion require & what is the reaction between them? -
ANSWER ✔ fuel, oxygen & heat. reaction is known as rapid oxidation, or
the process of burning
20.describe a solenoid valve - ANSWER ✔ a normally closed valve. The
plunger in the solenoid is attached to the valve or is in the valve. When an
electric current is applied to the coil, the plunger is pulled into the coil. This
opens the valve. The plunger is spring loaded so that when the current is
turned off, the spring forces the plunger to its NC position, shutting off the
gas
, 21.what is a limit switch - ANSWER ✔ a safety device that senses heat
22.To test for an obstructed flue pipe on an induced draft furnace , a technician
should use ; - ANSWER ✔ a manometer to test the negative pressure
generated by the induced draft motor
23.What is used to ignite the gas burners on most conventional gas furnaces. -
ANSWER ✔ Pilot flames.
24.What is the difference between aerated and nonaerated pilot burners? -
ANSWER ✔ In an aerated pilot, the air is mixed with the gas before it enters
the pilot burner.
In a nonaerated pilot, only secondary air is used for combustion.
25.T or F: a pilot is technically just a burner with an orifice that gas passes. -
ANSWER ✔ True.
26.T or F: a standing pilot is only lit when needed, it goes out to conserve gas
when it is not lighting a burner. - ANSWER ✔ False- a standing pilot burns
continuously.
27.What is the technical term to verify that a standing pilot is lit? - ANSWER
✔ It is called flame-proving.
