HVAC Excellence Master Combustion flashcards
HVAC Excellence Master Combustion · 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 volume needed for complete combustion, about 10 cubic feet of air per cubic foot of natural gas, or roughly 9.5 to 1 by volume.
- Excess air formula
- Percent excess air approximately equals measured O₂ divided by (20.9 minus measured O₂), multiplied by 100, using dry-basis oxygen readings from the analyzer.
- Air-free CO
- Measured CO multiplied by 20.9 divided by (20.9 minus measured O₂). It normalizes CO to zero excess air so readings taken at different dilution levels can be compared.
- Methane combustion equation
- CH₄ plus 2O₂ yields CO₂ plus 2H₂O. Each cubic foot of methane requires two cubic feet of oxygen, supplied by about ten cubic feet of air.
- Stack loss
- The portion of input energy carried up the flue. It increases with higher flue temperature and with greater excess air because more mass must be heated.
- Gross vs net efficiency
- Gross efficiency ignores latent heat in water vapor. Net efficiency credits latent heat recovery, so condensing appliances can exceed 90 percent net efficiency.
- CO₂ as excess air indicator
- Natural gas flue CO₂ near 9 to 10 percent suggests low excess air, while 7 to 8 percent suggests high excess air. Readings fall as dilution increases.
- Category I appliance
- A gas appliance that operates with negative vent pressure and is non-condensing, typically vented with Type B vent or a lined masonry chimney.
- Category IV appliance
- A gas appliance that operates under positive vent pressure and is condensing. It requires sealed, listed venting such as PVC or CPVC approved by the manufacturer.
- Draft hood function
- A dilution device that stabilizes draft by mixing room air with flue gases and relieving stack effect, preventing excessive draft from pulling heat up the flue.
- Type B vent clearance
- Double-wall listed gas vent commonly requires one inch clearance to combustibles per its listing. Always confirm the specific listing label on the product.
- Vent connector slope
- Natural-draft vent connectors should rise toward the chimney at least one quarter inch per foot and be kept as short and direct as practical.
- Spillage test
- With the appliance firing and doors closed, hold a mirror or smoke source near the draft hood. Flue gas should be drawn in, not pushed into the room.
- Common venting rules
- Appliances sharing a vent must be on the same floor, compatible in category, and connected with connectors sized to the combined input per the venting tables.
- Draft pressure range
- Category I natural-draft appliances typically show about negative one to negative three inches water column draft at the appliance outlet during normal operation.
- Longest length gas sizing
- Gas piping is sized using the longest developed run from meter to appliance, adding an allowance for fittings, then looking up capacity in the tables.
- Natural gas manifold pressure
- Most natural gas appliances are rated at about 3.5 inches water column manifold pressure. Verify the appliance rating plate before adjusting the regulator.
- LP manifold pressure
- Liquefied petroleum gas appliances commonly use 10 to 11 inches water column manifold pressure. Confirm the exact value on the rating plate for the specific unit.
- Sediment trap
- A drip leg installed at the appliance inlet so debris and pipe dope fall into the capped section instead of entering the gas valve and burner.
- Combustion air rule of thumb
- Unconfined spaces are generally considered adequate when they provide about 50 cubic feet of volume per 1,000 Btu/h of total appliance input.
- Confined space openings
- A confined space needs two permanent openings, one high and one low, sized per code, often one square inch per 1,000 Btu/h each with grille allowances.
- Flexible connector rules
- Listed flexible appliance connectors must not be concealed in walls or pass through floors and ceilings, and they must be installed without kinks or sharp bends.
- Clocking the gas meter
- Input equals 3,600 times the gas volume metered in cubic feet, divided by time in seconds, multiplied by the gas heating value in Btu per cubic foot.
- Flame rectification signal
- A typical flame sensor produces a microamp-range DC signal. Weak signal often means a dirty sensor, poor ground, or incorrect sensor position in the flame.
- Ignition sequence
- Call for heat, inducer proves airflow, ignition energizes, gas valve opens, flame is proven, then the blower starts after a time delay.
- Hot-surface igniter checks
- Inspect for cracks and confirm the element glows evenly. A cracked or cold igniter should be replaced before condemning the control board.
- Gas valve troubleshooting
- Confirm 24 volts at the operator, inlet pressure present, and manifold pressure response. Voltage present with no flow suggests a failed operator or stuck valve.
- Proper burner flame
- Natural gas flames should be steady and blue with slight yellow tips. Lifting, floating, or lazy yellow flames indicate air and gas imbalance or dirty burners.
- CO alarm thresholds
- Common UL 2034 alarm points: 70 ppm within 60 to 240 minutes, 150 ppm within 10 to 50 minutes, and 400 ppm within 4 to 15 minutes.
- Worst-case depressurization
- Run all exhaust fans and dryers, close doors, then measure pressure across the appliance. Excessive negative pressure can cause spillage and CO entry.
- Air-free CO red flag
- Undiluted flue air-free CO above about 100 ppm warrants investigation, and above 400 ppm indicates a serious problem requiring shutdown and repair.
- Cracked heat exchanger clue
- A CO reading that rises while the blower runs can indicate a cracked heat exchanger. Confirm with a combustion analyzer and visual inspection.