HVAC Excellence Gas Heat Certification cheat sheet

Complete combustion of methane: CH4 + 2O2 -> CO2 + 2H2O; each cubic foot of natural gas needs about 10 ft3 of air for stoichiometric (perfect) combustion, roughly 15 ft3 in practice with excess air.

Stoichiometric (theoretical) air for natural gas is about 10 ft3 air per ft3 gas; typical furnaces run 25-50% excess air (about 12.5-15 ft3 air per ft3 gas), verified by an O2 reading of roughly 4-9% in the flue.

Heating (gross) value of natural gas is about 1,000-1,050 Btu/ft3; LP gas about 2,500 Btu/ft3. Input = gas rate (ft3/hr) x heating value (Btu/ft3).

Flame color signals: steady blue flame = proper primary air and combustion; yellow, lazy, or floating flame = insufficient primary air, dirty burners, or misaligned orifice; lifting flame = too much primary air or excess draft.

Carbon monoxide (CO) is produced by incomplete combustion: insufficient oxygen, dirty or blocked burners, overfiring, or a cracked heat exchanger quenching the flame; CO is colorless and odorless, so it must be measured with an instrument.

Dew point of flue products: about 130°F for natural gas; a flue-gas temperature below roughly 140°F risks condensation and corrosion in a Category I (non-condensing) vent.

Limits of flammability of natural gas in air: about 4-5% lower to 15% upper; a gas concentration in this band can ignite. LP gas limits are roughly 2.2% to 9.6%.

Primary air is mixed with gas at the burner before ignition; secondary air is supplied around the flame from the combustion-air supply. Adjust primary air shutters for a stable blue flame with slight yellow tips.

Standard induced-draft sequence: thermostat calls for heat -> inducer motor energizes -> pressure switch proves draft -> igniter energizes (hot surface) or pilot proves -> gas valve opens -> flame sensor proves flame -> blower starts after heat-on delay -> thermostat satisfied -> gas valve closes -> inducer runs post-purge -> blower runs off-delay -> cycle ends.

Standing-pilot (millivolt) sequence: thermostat closes -> gas valve main operator opens -> main burner lights from pilot -> fan/limit switch closes on temperature rise to start blower.

Pressure switch proves inducer airflow; it is normally open and closes on negative pressure. A blocked flue, blocked intake, water in the tubing, or a weak inducer prevents it from closing, and the gas valve will not open.

Hot-surface ignition (HSI) igniters draw roughly 2-4 amps at 120 V and glow orange; a cracked or open igniter reads infinite resistance and prevents ignition.

Flame sensor (flame rectification) proves flame: the control applies a small AC voltage to the sensor; the flame conducts current in one direction, producing a microamp DC signal (typically 1-6 microamps). A dirty sensor lowers the reading and causes short cycling.

Limit switch is a normally closed, temperature-actuated safety in the heat exchanger/supply air path; it opens on overheat and interrupts the gas valve. The blower/fan switch (normally open) starts the blower on temperature rise in standing-pilot designs.

Gas valve has a redundant and a main operator; the redundant operator opens only after the main operator and safety circuit are proven. Manifold (outlet) pressure for natural gas is typically 3.5 in. w.c.; LP about 10-11 in. w.c.

In-shot or inshot burners and the heat exchanger: burners deliver flame into the heat exchanger cells; a cracked heat exchanger can allow flue gases into supply air and is a CO hazard requiring shutdown and replacement.

Category I appliances are non-condensing, negative-pressure vented, and use a vertical vent or chimney; Category IV appliances are condensing, positive-pressure vented, and use PVC/CPVC or other listed plastic vent.

Natural-draft (Category I) vent sizing depends on appliance input, vent height, and connector rise; follow the appliance's venting tables. A single appliance may use a Type B vent; a common vent requires careful sizing and a properly sized manifold.

Combustion air: the standard rule is 50 ft3 of space per 1,000 Btu/hr of input for an unconfined space. A confined space needs two permanent openings (high and low) or a listed combustion-air duct sized to the appliance input.

Two-permanent-opening rule (confined space, indoor air): each opening free area = 1 in2 per 1,000 Btu/hr of total input, with a minimum 100 in2 per opening; openings must be within 12 in. of the top and bottom of the enclosure.

Direct-vent (two-pipe) appliances take combustion air from outdoors and vent outdoors; keep intake and exhaust terminations separated per the manufacturer, typically at least 12 in. above grade and clear of snow and recirculation zones.

Gas piping: use black steel pipe, corrugated stainless steel tubing (CSST), or listed copper; never use galvanized pipe for gas. Use approved pipe dope on male threads only, leaving the first two threads bare.

Gas pressure: supply (line) pressure for natural gas is typically 7 in. w.c. (range about 5-10.5 in. w.c.); manifold pressure typically 3.5 in. w.c. natural gas, 10-11 in. w.c. LP. Test with a manometer, not a gauge.

Leak test: pressurize the piping and check joints with a non-corrosive leak-detection solution or electronic detector; never use an open flame. A drip leg (sediment trap) must be installed at the appliance connection.

24 V control circuit: transformer steps 120 V down to 24 V; the thermostat, gas valve, pressure switch, limit, and flame sensor all operate on the 24 V circuit. A blown low-voltage fuse (typically 3-5 A) kills the whole control circuit.

Flame rectification: the control board supplies AC to the flame sensor; the flame acts as a rectifier and returns a DC microamp signal (typically 1-6 microamps). Below the board's threshold (often about 0.5-1 microamp) the board locks out.

Hot-surface ignition boards include an ignition trial period (often 15-30 seconds) followed by a flame-proving period; failure to prove flame causes lockout or retry, depending on the control.

Direct-spark ignition (DSI) uses a spark electrode to light the burner; the spark gap is typically about 1/8 in. (3 mm). A cracked ceramic insulator or a grounded electrode prevents spark.

Safety devices that interrupt the gas valve: limit switch (high temperature), rollout switch (flame rollout from the burner compartment, manual reset), pressure switch (draft), and flame sensor (flame proving).

Rollout switches are manual-reset, normally closed, and located at the burner opening; they trip if flame rolls out due to a blocked heat exchanger, sooted burners, or a bad inducer.

Inducer motor proving: the pressure switch must close before the gas valve is energized; the control board often checks the switch is open before starting the inducer to detect a stuck-closed switch.

Thermostat and control wiring: R (24 V hot), W (heat call), C (common), G (fan), Y (cooling). A heat call is R to W; the control board sequences the inducer, igniter, gas valve, and blower.

Combustion analysis instruments measure O2, CO, CO2, flue temperature, and draft; calculate excess air from O2 and check CO (air-free) against the appliance limit (commonly under 100 ppm air-free for clean combustion).

Set manifold pressure with a manometer at the gas valve outlet tap: natural gas typically 3.5 in. w.c., LP about 10-11 in. w.c. Adjust only to the manufacturer's rating plate value, then recheck gas input by clocking the meter.

Clock the gas meter to verify input: input (Btu/hr) = (3600 / seconds per revolution) x dial size (ft3) x heating value (Btu/ft3). Compare to the rating plate within about 5%.

Temperature rise = supply air temperature minus return air temperature; it must fall within the rating plate range (commonly 35-65°F). A low rise indicates excess airflow; a high rise indicates low airflow or overfiring.

Static pressure: measure supply and return static with a manometer; total external static pressure should be within the blower table for the required airflow. High static pressure reduces airflow and raises temperature rise.

Draft check: verify negative draft in the vent connector (about -0.02 to -0.04 in. w.c. for natural draft) and confirm no spillage at the draft hood with a smoke or flame test.

Startup steps: verify gas supply pressure and piping, check vent and combustion air, set manifold pressure, confirm ignition and flame proving, measure temperature rise and static pressure, then run a combustion analysis and record results.

A CO reading that rises when the blower starts suggests a cracked heat exchanger or a flue leak drawing into the supply air; shut down and inspect before returning the appliance to service.

No heat call, inducer runs but gas valve never opens: check pressure switch (blocked flue, water in tubing, weak inducer), then flame sensor and igniter, then gas valve and 24 V at the valve.

Igniter glows but no flame: verify gas supply and manifold pressure, check the gas valve operator (24 V at valve terminals), and confirm the igniter is positioned in the burner gas stream.

Burner lights then shuts off after a few seconds: dirty or misaligned flame sensor (low microamps), poor flame grounding, or a flame that lifts off the sensor. Clean the sensor with steel wool and recheck microamps.

Repeated lockout with a good flame: check the control board's flame-proving circuit, the sensor wire, and the burner ground. Many boards lock out after a set number of retries and require a power reset.

Short cycling on limit: restricted airflow (dirty filter, blocked coil, undersized duct), overfiring (high manifold pressure), or a failing blower. Verify temperature rise and static pressure before replacing the limit.

CO safety: if CO is detected or suspected, shut off the appliance, ventilate, and evacuate as needed; never return a unit with a cracked heat exchanger or a blocked vent to service. Use a CO detector and combustion analyzer.

Gas leak response: shut off the gas at the meter or appliance valve, ventilate, do not operate electrical switches, and call the gas utility. Use leak-detection solution or an electronic detector to locate the leak.

Service checks: verify supply pressure, manifold pressure, temperature rise, static pressure, draft, flame signal, and combustion readings; record all values and compare to the rating plate before signing off.