NATE Gas Heating Service cheat sheet

Complete combustion of methane: CH4 + 2O2 → CO2 + 2H2O; each volume of methane needs 2 volumes of oxygen (about 9.5 volumes of air).

Stoichiometric (perfect) air for natural gas is roughly 10 ft³ air per ft³ gas; typical setup targets 25–50% excess air for safety margin.

The three elements of the combustion triangle are fuel, oxygen, and an ignition source; removing any one stops combustion.

Flame color diagnosis: stable blue cone = good combustion; lazy yellow tip = insufficient primary air; lifting flame = excessive primary air or high gas pressure.

Heating value: natural gas about 1,000–1,050 Btu/ft³ HHV; propane about 2,500 Btu/ft³; LP is heavier than air (about 1.5–2.0 specific gravity) and pools at low points.

CO is produced by incomplete combustion: insufficient air, overfiring, blocked flue, dirty burners, or flame impingement on a cool surface.

Lower flammability limit for natural gas is about 4–5% gas in air; upper limit about 15%; within this band an ignition source causes ignition.

Gas-specific gravity vs air: natural gas about 0.60 (rises and vents high), propane about 1.5 (sinks); leak detection and ventilation strategy follow this difference.

Draft is created by temperature difference (stack effect) and by the vent connector height; a Category I appliance needs a negative pressure in the flue at all times.

Category I: non-condensing, negative flue pressure, must vent vertically or with approved slope; Category IV: condensing, positive pressure, uses PVC/CPVC and must be pitched back to the appliance.

Category III is positive pressure, non-condensing (usually stainless); Category II is negative pressure, condensing - rarely encountered.

A single-wall metal vent connector needs 6 in. clearance to combustibles; a Type B double-wall vent needs 1 in. to combustibles in most listings.

Vent connector slope: at least 1/4 in. per foot rise toward the chimney for natural draft appliances; horizontal runs should be as short as possible.

Standard (fan-assisted) category I appliances commonly use double-wall Type B vent; a listed vent cap or termination is required at the outlet.

Combustion air by the standard method: 50 ft³ per 1,000 Btu/hr input for an unconfined space; a confined space needs two permanent openings (high and low).

Two-opening rule: each opening sized at 1 in² free area per 1,000 Btu/hr of total input when communicating directly with outdoors, or 1 in² per 2,000 Btu/hr through vertical ducts.

A blocked flue or reversed draft spills combustion products into the space; a draft hood spill test (match flame at the relief opening) confirms proper draft.

Condensing appliance venting: PVC flue gas is acidic (condensate pH about 3–5); condensate must drain to an approved neutralizer or drain, and vent slope is back to the appliance.

Gas piping sizing uses longest-length method or branch-length method with tables based on Btu/hr load, specific gravity, and pressure drop; undersizing causes pressure drop and underfiring.

Sediment trap (drip leg) is required at the appliance connection: a tee with a capped nipple below, installed so debris falls out before reaching the gas valve.

A manual shutoff valve must be installed in the supply line within 6 ft of the appliance and be readily accessible; a union allows service removal.

Leak test: pressurize the new piping with air or inert gas (commonly 10–15 psig for a short duration or per local code); never use a flame, and isolate the appliance gas valve.

Manifold gas pressure for natural gas is typically 3.5 in. w.c.; propane typically 10–11 in. w.c.; supply pressure is higher and is reduced by the appliance regulator.

Combination gas valve functions: manual shutoff, pressure regulation, safety shutoff (automatic), and manifold pressure adjustment; some include a redundant operator.

Burner components: orifice (meters gas by size), mixer/venturi (entrains primary air), manifold, burner ports, and flame spreader; a dirty orifice changes input rate.

Orifice sizing: input Btu/hr = orifice capacity factor × manifold pressure correction; drilling out or swapping orifices changes firing rate and must follow the rating plate.

A gas valve that fails to open, a stuck redundant operator, or a bad thermocouple/pilot generator all present as no heat with the inducer running.

Standing pilot: thermocouple generates about 25–30 mV open circuit; the gas valve needs roughly 10–15 mV under load to hold the pilot operator open.

Intermittent pilot (IP) ignition: spark ignites pilot, flame rectification proves it, then main valve opens; a dirty flame sensor causes lockout after a short trial.

Direct spark ignition (DSI) and hot surface ignition (HSI) prove flame by flame rectification (microamp signal, typically 1–5 µA); a weak signal causes nuisance lockout.

Flame rectification requires the flame to touch the sensor and the burner ground to be intact; reversed polarity or a poor ground prevents proving.

Sequence of operation: call for heat → inducer proves (pressure switch) → ignition source energizes → gas valve opens → flame proved → blower on delay → heat rise → blower off delay.

Pressure switch proves inducer airflow; a blocked port, water in the tubing, or a weak inducer prevents the switch from closing and the sequence stops before ignition.

Limit switch (high limit) opens on excess temperature and breaks the thermostat circuit; rollout switch is a manual-reset device that trips on flame rollout.

Control board lockout: after a set number of failed trials the board locks out; reset by cycling power or thermostat, and the LED fault code identifies the failed step.

24 V control circuit: thermostat R–W call energizes the gas valve and safeties in series; a blown low-voltage fuse or open safety anywhere in the string stops the call.

Temperature rise = supply air temperature minus return air temperature; compare to the rating plate range (commonly 35–65°F for residential furnaces).

CFM ≈ output Btu/hr ÷ (1.08 × temperature rise); 1.08 is the sensible heat constant for standard air at sea level.

High temperature rise: low airflow, dirty filter, restricted evaporator coil, dirty blower wheel, or wrong blower speed - check static pressure and filter first.

Low temperature rise: overfiring, oversized blower speed, or duct leakage; verify manifold pressure and gas input before changing blower speed.

External static pressure: measure with a manometer across the furnace; typical residential design is 0.5 in. w.c. total; excessive static means duct restriction.

Heat exchanger inspection: look for cracks at the cell seams, discoloration, water stains, and flame distortion; a cracked heat exchanger can allow CO into supply air.

Blower motor types: PSC (permanent split capacitor) speed set by taps; ECM/X13 (constant torque) and variable-speed ECM maintain airflow against static changes.

Combustion air and dilution air must not be blocked by a dirty filter or a closed louver; verify the inducer wheel and flue are clear before diagnosing airflow.

A heat exchanger with a restricted secondary passage shows high rise, high flue temperature, and a yellow flame - a combustion analysis confirms before replacement.

Combustion analyzer readings for natural gas: O2 about 4–9%, CO2 about 8–9.5%, CO under 100 ppm air-free (target under 50 ppm), stack temperature per rating.

Air-free CO = measured CO × (20.9 ÷ (20.9 − measured O2)); air-free CO corrects for dilution and is the number compared to limits.

Excess air % = (20.9 ÷ (20.9 − O2)) − 1, then ×100; 25–50% excess air is typical for a properly set natural gas appliance.

Draft check: measure draft in the flue with a manometer or draft gauge; a Category I appliance should show negative draft (about −0.02 to −0.04 in. w.c. at the relief opening).

Startup checklist: verify gas supply pressure at the valve inlet, manifold pressure, flame appearance, temperature rise, static pressure, and combustion readings.

Flame signal (microamps) below the manufacturer's minimum (often 1–2 µA) causes intermittent lockout; clean the sensor and verify ground before replacing the board.

Troubleshooting order: confirm thermostat call, check 24 V at the board, confirm inducer and pressure switch, confirm ignition, then confirm gas valve and flame proof.

Gas leak at the manifold or valve shows as a pressure drop with the appliance off; test with a manometer or leak detector solution, never an open flame.

Short cycling causes: high limit trip (low airflow), thermostat location, oversized unit, or a failing flame sensor dropping the call - verify rise and static first.