HVAC Excellence Gas Heat Certification Exam cheat sheet

Complete combustion of methane: CH4 + 2O2 -> CO2 + 2H2O; each cubic foot of methane needs about 2 ft3 of oxygen, roughly 10 ft3 of air (air is ~21% O2).

Stoichiometric (perfect) combustion of natural gas yields about 9.4 ft3 air per ft3 gas and a flue CO2 near 11.7-12%; measured CO2 well below that signals excess air dilution.

The three requirements for combustion are fuel gas, oxygen (air), and an ignition source; removing any one stops combustion.

Natural gas heating value is about 1,000-1,050 Btu/ft3; LP gas is about 2,500 Btu/ft3, so LP orifices are smaller for the same input rate.

Natural gas is lighter than air (specific gravity ~0.60) and vents upward; LP gas is heavier (~1.5-2.0) and pools in low areas.

Limits of flammability in air: natural gas roughly 4-15% gas by volume; LP roughly 2-10%. Below the lower limit the mixture is too lean to ignite.

Complete combustion products are CO2 and water vapor; incomplete combustion produces CO, which is produced when oxygen is insufficient or flame impingement/quenching cools the flame.

Gas input rate (Btu/hr) = gas flow rate (ft3/hr) x heating value (Btu/ft3); clocking the meter and correcting for altitude and time gives actual input.

Sizing depends on total connected load (Btu/hr), allowable pressure drop, pipe length including fittings, pipe material, and gas specific gravity; longer runs and higher load require larger pipe.

Use the appropriate gas capacity table for the material (black steel, CSST, copper) and pressure range (e.g., low pressure under 0.5 psi vs. elevated 2 psi systems).

Sediment trap (drip leg) is installed on the supply line ahead of the appliance gas valve, with the leg extending downward so debris collects below the gas flow.

A manual shutoff valve must be installed upstream of the appliance, in the same room, and accessible; a union allows removal of the appliance or valve.

Use pipe dope rated for gas on male threads only, leaving the first two threads bare; never use it on flare fittings.

Leak testing uses air or inert gas pressure with a manometer or gauge; soap bubbles locate leaks, and a bubble-free standing pressure test for the specified period confirms tightness. Never pressure-test with gas or with the appliance valve open.

CSST must be bonded to the electrical grounding system per manufacturer and code requirements, and must be protected from physical damage and sharp edges.

Piping must be supported, protected from corrosion, and not used as a ground; concealed piping must be accessible or installed per code, and drip legs and proper slope prevent condensate traps.

Category I appliances are non-condensing, negative vent pressure; Category IV are condensing, positive vent pressure; Categories II and III are the less common positive-pressure non-condensing and negative-pressure condensing types.

Natural draft (atmospheric) venting relies on a hot flue gas column; the vent connector rises toward the chimney, and a barometric draft diverter stabilizes draft at the appliance.

Combustion air: the standard rule of thumb is 50 ft3 of space per 1,000 Btu/hr of input for an unconfined space; confined spaces need two permanent openings (high and low) sized per code, or outdoor air ducted in.

A flue gas analyzer measures O2, CO2, CO, and stack temperature; excess air (from O2) and CO readings together diagnose air/fuel problems.

Draft measurement uses a manometer or draft gauge at the flue collar or test port; negative draft confirms flue gases are leaving, positive pressure indicates spillage or a blocked/incorrect vent.

CO in the flue of a properly adjusted natural gas appliance should be very low (single-digit to low double-digit ppm air-free); rising CO with stable CO2 indicates incomplete combustion.

Condensing furnaces require PVC/CPVC or approved plastic venting sloped back to the furnace with a condensate drain and trap; non-condensing appliances need metal vent and cannot share a plastic vent.

Spillage, rust at the draft hood, or a back-drafting water heater indicates depressurization, blocked flue, or inadequate combustion air; test with doors closed and exhaust fans running.

The combination gas valve contains a manual shutoff, pressure regulator, redundant and main solenoid valves, and often a slow-opening feature and pressure taps (inlet and manifold).

Manifold pressure is set with the appliance running at full fire; natural gas typically 3.5 in. w.c., LP typically 10-11 in. w.c., unless the rating plate states otherwise.

Inlet pressure for natural gas is typically 5-7 in. w.c. at the valve; if inlet pressure is below the minimum, the regulator cannot maintain manifold pressure.

Orifice sizing controls input: larger orifice = more gas = higher input; altitude requires derating by changing orifices and/or manifold pressure per the manufacturer's altitude chart.

Burner flame characteristics: a steady blue flame with a short inner cone indicates good primary air; yellow tips, lifting, or floating flames indicate too much primary air, and lazy yellow flames indicate too little.

Primary air is mixed with gas before the burner ports; secondary air is drawn around the flame. Insufficient primary air causes yellow, carbon-forming flames.

The manifold distributes gas to individual burner orifices; a partially blocked orifice causes uneven flame and hot spots on the heat exchanger.

Gas valve checks: verify 24 V at the operator, inlet pressure, manifold pressure, and that the valve opens and closes with the thermostat; a valve that does not close is a safety hazard and must be replaced.

Standing pilot: a continuously burning pilot heats a thermocouple (millivolt output, roughly 250-750 mV) or thermopile that holds the gas valve open; a dirty or mispositioned pilot weakens the signal.

Intermittent pilot (IP): an ignition control sparks the pilot only on a call for heat, proves flame with a flame sensor, then opens the main valve; the pilot goes out after the call ends.

Hot surface ignition (HSI): a silicon carbide or silicon nitride igniter heats to ignition temperature; the control proves flame by flame rectification (microamp current through the flame to the sensor).

Direct spark ignition (DSI): the spark ignites the main burner directly; flame is proved by a remote flame sensor or by the spark electrode itself, depending on design.

Flame rectification requires a clean sensor, proper flame contact, and correct polarity/ground; a sensor reading below the control's minimum microamp threshold causes lockout.

Igniter resistance checks: HSI elements typically read a few ohms to a few tens of ohms cold; an open element reads infinite resistance and must be replaced.

Spark gap and electrode position must match the manufacturer's specification; a cracked ceramic insulator or grounded electrode prevents reliable spark.

Ignition controls have trial-for-ignition timing and lockout; repeated lockout with a good igniter and gas supply points to flame sensor, grounding, or gas valve problems.

Sequence of operation: thermostat calls for heat -> inducer starts (draft proved) -> ignition source energizes -> gas valve opens -> flame proved -> blower starts after a time delay -> thermostat satisfied -> gas valve closes -> blower off-delay.

The pressure switch proves inducer airflow; it closes on negative pressure from the inducer and must be closed before the ignition control allows ignition.

The high-limit switch is a temperature-actuated safety in the supply air path; it opens on overtemperature, cutting the gas valve, and resets automatically (or manually on some designs).

The rollout switch is a manual-reset safety near the burner/heat exchanger that opens if flame rolls out of the combustion chamber, indicating a blocked flue or heat exchanger problem.

The flame rollout and high-limit devices are safety controls; never bypass or jumper them.

The control board manages inducer, ignition, gas valve, blower, and fault codes; blinking LED codes indicate specific lockouts such as pressure switch open, flame sensed without call, or ignition failure.

A blocked-vent or condensate pressure switch on condensing furnaces protects against flue blockage and condensate backup; a stuck-closed switch prevents ignition.

Thermostat anticipator or cycle-rate settings affect cycle timing; a misadjusted anticipator can cause short cycling or long cycles, and modern thermostats use cycle-rate adjustment.

Control voltage is typically 24 VAC from the transformer; line voltage is 120 VAC for the blower and inducer on many furnaces.

Read ladder diagrams from left (power source) to right and top to bottom; the sequence of operation follows the order in which components are energized.

The transformer secondary supplies R and C; the thermostat switches R to W (heat), G (fan), Y (cooling), and Rc/Rh as applicable.

A call for heat energizes W, which starts the inducer through the control board; the pressure switch then proves draft and allows ignition.

Continuity checks find open switches and blown fuses; voltage checks confirm power is present; resistance checks find open windings and sensors.

Flame rectification depends on correct polarity and a good ground; reversed line and neutral or a floating ground causes flame-sense failure and lockout.

The blower motor may be PSC (with a capacitor and speed taps) or ECM (electronically commutated, with low-voltage control and programmed airflow); ECM faults often set diagnostic codes.

Safety devices are wired in series in the gas valve circuit so that any open safety (limit, rollout, pressure switch) de-energizes the gas valve.

Temperature rise = supply air temperature minus return air temperature; it must fall within the range on the rating plate (commonly 35-65 F for furnaces, but always use the nameplate).

Sensible heat output (Btu/hr) = 1.08 x CFM x temperature rise; rearranged, CFM = Btu/hr / (1.08 x temperature rise).

Low airflow raises temperature rise, overheats the heat exchanger, and can trip the high limit; high airflow lowers temperature rise and can cause condensation in the flue on non-condensing units.

Static pressure is measured with a manometer in the supply and return plenums; total external static pressure above the blower's rated maximum indicates duct restriction.

A dirty filter, dirty blower wheel, or restricted evaporator coil reduces airflow and raises temperature rise; check the filter first on any high-limit complaint.

Heat exchanger integrity is checked visually for cracks, with a combustion analyzer for CO rise when the blower starts, and with approved leak-detection methods; a cracked heat exchanger is a safety hazard and must be replaced.

Flame distortion or rollout when the blower starts indicates a breached heat exchanger or blocked flue; shut down and inspect before returning the unit to service.

Combustion air and return air must be separated; a return duct drawing combustion air from the furnace room can cause back-drafting and CO spillage.

Start with the complaint and the sequence of operation: does the inducer run, does the igniter heat, does the gas valve open, does flame prove, does the blower start?

No heat with a good thermostat call: check 24 V at the board, the pressure switch, the igniter, the flame sensor, and the gas valve in sequence.

Ignition failure with spark present: verify gas supply, manifold pressure, orifice condition, and air in the gas line; purge the line if newly installed.

Flame proves then drops out: check flame sensor microamps, grounding, and the sensor's position in the flame; clean or replace the sensor.

Short cycling: check the high limit (airflow, filter, blower), the thermostat anticipator, and the pressure switch tubing for condensate or blockage.

CO complaints or headaches: perform a combustion analysis, check for spillage and back-drafting, inspect the heat exchanger, and verify combustion air; treat as an urgent safety issue.

Use a manometer for gas pressures, a multimeter for voltage/continuity/microamps, and a combustion analyzer for O2, CO, CO2, and stack temperature; record readings before and after repair.

After any gas valve, orifice, or pressure adjustment, verify manifold pressure, temperature rise, and combustion readings, and leak-test all connections before leaving.