HVAC Excellence Gas Heat Certification flashcards
HVAC Excellence Gas Heat · Build recall with a focused review session.
32 cards. Read the term and the definition here. The deck at the top of this page is optional.
- Stoichiometric air for natural gas
- The theoretical air needed for complete combustion of natural gas, about 10 cubic feet of air per cubic foot of gas; practical furnaces supply 25-50% excess air above this.
- Complete combustion products of methane
- Burning methane with enough oxygen yields carbon dioxide and water vapor plus heat: CH4 + 2O2 gives CO2 + 2H2O. Insufficient oxygen instead produces carbon monoxide and soot.
- Heating value of natural gas
- The gross heating value of natural gas is about 1,000 to 1,050 Btu per cubic foot. Appliance input equals gas flow rate in cubic feet per hour times this heating value.
- Lower flammability limit
- The minimum gas concentration in air that will support ignition, about 4-5% for natural gas and about 2.2% for LP gas. Below this limit the mixture is too lean to burn.
- Primary versus secondary air
- Primary air mixes with gas at the burner before ignition and is adjusted by the air shutter. Secondary air surrounds the flame and is supplied from the combustion-air space.
- Flue gas dew point
- For natural gas products, about 130 degrees Fahrenheit. Flue temperature below roughly 140 degrees Fahrenheit in a non-condensing vent risks condensation and corrosion.
- Induced-draft sequence of operation
- On a heat call the inducer starts, the pressure switch proves draft, the igniter heats, the gas valve opens, flame is proven, and the blower starts after a delay.
- Pressure switch function
- A normally open switch that closes when the inducer creates sufficient negative pressure, proving draft. It must close before the gas valve is energized; a blocked flue keeps it open.
- Flame sensor operation
- A rod in the burner flame that rectifies alternating current into a small direct-current signal, typically 1-6 microamps. The control board requires this signal to keep the gas valve open.
- Hot-surface igniter
- A silicon carbide or nitride element that glows orange when energized at 120 volts and lights the burners. A cracked element reads infinite resistance and prevents ignition.
- Limit switch
- A normally closed, temperature-actuated safety in the heat exchanger or supply air path. It opens on overheating and interrupts the gas valve circuit to stop combustion.
- Manifold pressure natural gas
- The gas valve outlet pressure for natural gas is typically 3.5 inches water column, while LP gas is about 10 to 11 inches. Set it to the rating plate value with a manometer.
- Category I venting
- Non-condensing appliances vented under negative pressure, using a vertical Type B vent or chimney. Flue temperature stays above the dew point to prevent condensation.
- Category IV venting
- Condensing appliances that vent under positive pressure through listed plastic pipe such as PVC or CPVC. The flue gas is cooled below dew point and condensate is drained away.
- Combustion air unconfined space
- An unconfined space must provide about 50 cubic feet of volume per 1,000 Btu/hr of appliance input. Smaller confined spaces need permanent openings or outdoor air ducts.
- Two-permanent-opening rule
- For a confined space using indoor air, each opening needs one square inch of free area per 1,000 Btu/hr input, minimum 100 square inches, placed near top and bottom.
- Sediment trap requirement
- A drip leg installed at the appliance gas connection that catches dirt and scale before it reaches the gas valve. It is required by code on gas piping to appliances.
- Gas supply pressure natural gas
- Line pressure at the appliance is typically about 7 inches water column, with an acceptable range near 5 to 10.5 inches. Measure it with a manometer at the inlet tap.
- Flame rectification microamps
- The direct-current signal a flame returns through the sensor rod, typically 1-6 microamps. A dirty sensor or poor ground lowers it below the board threshold and causes lockout.
- Direct-spark ignition gap
- The spark electrode gap on a direct-spark ignition system is typically about one-eighth inch. A cracked ceramic insulator or grounded electrode prevents the spark.
- Rollout switch
- A manual-reset, normally closed switch at the burner opening that trips when flame rolls out of the burner compartment. Causes include blocked heat exchanger, sooted burners, or failed inducer.
- 24-volt control circuit
- The low-voltage circuit powered by the transformer that operates the thermostat, gas valve, pressure switch, limit, and flame sensor. A blown low-voltage fuse disables the whole circuit.
- Thermostat heat call
- A heat call connects the R terminal, 24 volts hot, to the W terminal. The control board then sequences the inducer, igniter, gas valve, and blower.
- Clocking the gas meter
- Timing meter dial revolutions to calculate input: input equals 3600 divided by seconds per revolution, times dial cubic feet, times heating value. Compare to the rating plate within about 5%.
- Temperature rise
- Supply air temperature minus return air temperature, which must fall within the rating plate range, commonly 35 to 65 degrees Fahrenheit. High rise indicates low airflow or overfiring.
- Total external static pressure
- The sum of supply and return static pressures measured with a manometer. It must fall within the blower table for the required airflow; excess static reduces airflow and raises temperature rise.
- Draft verification
- Confirming negative pressure in the vent connector, about -0.02 to -0.04 inches water column for natural draft, and checking for spillage at the draft hood with a smoke or flame test.
- Air-free CO reading
- A carbon monoxide measurement corrected for excess air, allowing comparison to the appliance limit. Clean combustion is commonly under 100 ppm air-free; rising CO signals a problem.
- No ignition with glowing igniter
- When the igniter glows but no flame appears, verify gas supply and manifold pressure, check for 24 volts at the gas valve, and confirm the igniter sits in the burner gas stream.
- Short cycling on limit
- Repeated limit trips usually mean restricted airflow from a dirty filter, blocked coil, or undersized duct, or overfiring from high manifold pressure. Check temperature rise and static pressure first.
- Cracked heat exchanger response
- A cracked heat exchanger can admit flue gases including carbon monoxide into supply air. Shut down the appliance, tag it out, and replace the exchanger or furnace before returning to service.
- Gas leak response
- Shut off gas at the meter or appliance valve, ventilate the area, avoid operating electrical switches, and call the gas utility. Locate leaks with soap solution or an electronic detector, never a flame.