HVAC Excellence Master Combustion cheat sheet

Theoretical (stoichiometric) air for natural gas is about 10 ft³ air per ft³ gas (≈9.5:1 by volume); typical appliances run 25–50% excess air for safety.

Complete combustion of methane: CH₄ + 2O₂ → CO₂ + 2H₂O. Each cubic foot of methane needs 2 ft³ of oxygen, supplied by roughly 10 ft³ of air.

O₂ in flue gas maps to excess air: excess air % ≈ O₂ / (20.9 − O₂) × 100 when measured on a dry basis.

CO₂ falls as excess air rises. Natural gas flue CO₂ near 9–10% indicates low excess air; 7–8% indicates high excess air.

CO is a product of incomplete combustion: causes include insufficient air, poor mixing, flame impingement, cold surfaces, and overfiring.

Stack (gross) efficiency = 100% − stack loss. Stack loss rises with flue temperature and with excess air (more mass to heat).

Net efficiency subtracts latent heat of vaporization from gross; condensing appliances can exceed 90% net because they recover latent heat.

Air-free (corrected) CO = measured CO × (20.9 / (20.9 − measured O₂)). Use air-free CO to compare readings at different excess-air levels.

Dew point of natural gas flue gas is about 130–140°F; below that, condensate forms and latent heat is recovered.

Category I: negative draft, non-condensing. Category II: negative draft, condensing. Category III: positive pressure, non-condensing. Category IV: positive pressure, condensing.

Draft hood (dilution device) relieves stack effect and provides a stable draft; it also dilutes flue gases with room air.

Natural draft depends on stack height and temperature difference: taller, hotter flue = more draft. A blocked or cold flue loses draft.

Type B vent is double-wall, listed for negative-pressure gas appliances; it requires clearance to combustibles per listing (commonly 1 inch).

Single-wall vent connectors need 6-inch clearance to combustibles; Type B connectors typically 1 inch.

Vent connector slope: rise toward the chimney at least 1/4 inch per foot for natural-draft appliances; avoid long horizontal runs.

Common-venting rules: appliances must be on the same floor, compatible categories, and connector sized to the combined input per the venting tables.

Spillage test: with the appliance firing and the door closed, hold a mirror or smoke near the draft hood; flue gas should be drawn in, not spilled out.

Draft pressure in a Category I vent typically runs about −1 to −3 in. w.c. at the appliance outlet; positive-pressure venting (Cat III/IV) requires sealed, listed venting.

Gas piping sizing uses the longest-length method: total developed length plus fittings allowance, then table look-up for capacity at the given inlet pressure and pressure drop.

Typical allowable pressure drop for natural gas piping is 0.5 in. w.c.; for LP gas the tables often allow 0.5 in. w.c. as well, but inlet pressures differ.

Manifold pressure: natural gas commonly 3.5 in. w.c.; LP commonly 10–11 in. w.c. Always verify the appliance rating plate.

Gas line sediment trap (drip leg) is required at the appliance inlet, installed so debris falls into the cap, not into the burner.

Combustion air: the standard rule of thumb is 50 ft³ of space per 1,000 Btu/h input for unconfined spaces.

Confined space (less than 50 ft³ per 1,000 Btu/h): provide two permanent openings, one near the top and one near the bottom, sized per the applicable code (often 1 in² per 1,000 Btu/h each, with allowances for grilles).

Outdoor combustion air openings may be sized at 1 in² per 2,000 Btu/h when using vertical ducts, or 1 in² per 4,000 Btu/h for horizontal ducts, depending on code edition.

Appliance clearances to combustibles come from the listing label; never reduce them because the installation 'looks safe.'

Flexible appliance connectors must be listed, not concealed in walls, and not used to pass through floors or ceilings.

Manifold gas pressure is set with a manometer at the manifold pressure tap; adjust the regulator with the burner firing.

Clock the meter to check input: input (Btu/h) = (3600 × gas metered in ft³) / time in seconds × heating value (Btu/ft³).

Flame signal for a typical flame rectification system is in the microamp range; a weak signal often points to a dirty flame sensor, poor ground, or wrong sensor position.

Ignition sequence: call for heat → inducer/pressure switch proves → ignition source energizes → gas valve opens → flame proven → blower starts (on delay).

Hot-surface igniters are fragile; check for cracks and proper glow before condemning the control board.

Direct-spark ignition requires a spark gap and clean electrodes; a weak spark can be caused by a bad ground or cracked insulator.

Gas valve troubleshooting: verify 24 V at the operator, inlet pressure present, and manifold pressure responds; no gas flow with voltage present suggests a failed operator or stuck valve.

Burner flames should be steady blue with slight yellow tips on natural gas; lifting, floating, or yellow lazy flames indicate air/gas imbalance or dirty burners.

Pressure switches prove inducer airflow; a switch that won't close may indicate a blocked flue, water in the tubing, or a failing inducer.

CO alarm thresholds commonly reference UL 2034: alarm at 70 ppm within 60–240 minutes, 150 ppm within 10–50 minutes, 400 ppm within 4–15 minutes.

Ambient CO in a living space should read 0 ppm; any sustained reading above 9 ppm warrants investigation.

Air-free CO above about 100 ppm in undiluted flue gas is a red flag; above 400 ppm indicates a serious problem requiring shutdown.

Draft measurement: use a digital manometer at the flue; negative draft should be confirmed under worst-case conditions (doors closed, exhaust fans on).

Worst-case depressurization test: operate all exhaust fans and dryers, close doors, and measure the pressure difference across the appliance; excess negative pressure causes spillage.

Combustion efficiency tuning: adjust excess air toward 25–50% for natural gas; too little air raises CO, too much air lowers efficiency.

Check CO before and after any adjustment; also record O₂, CO₂, flue temperature, and draft to document the tune-up.

A CO reading that climbs while the blower runs may indicate a cracked heat exchanger; confirm with a combustion analyzer and visual inspection.

Document baseline and post-service readings; trend data reveals degrading heat exchangers, dirty burners, or drifting gas pressure.