NATE Gas Heating Service flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Combustion triangle

Fuel, oxygen, and an ignition source must all be present for combustion. Removing any one element stops the reaction, which is the basis for gas valve shutoff and flame safeguard design.

Stoichiometric air

The exact air volume needed for complete combustion with no excess oxygen. Natural gas requires roughly 10 cubic feet of air per cubic foot of gas at perfect (stoichiometric) conditions.

Excess air

Air supplied beyond the theoretical requirement, usually 25 to 50 percent for natural gas appliances. Excess air ensures complete combustion but lowers efficiency by carrying heat up the flue.

Carbon monoxide formation

CO results from incomplete combustion caused by insufficient air, overfiring, blocked flue, dirty burners, or flame impingement. It is odorless and dangerous, so combustion analysis is required.

Specific gravity of gases

Natural gas is about 0.60 (lighter than air, rises); propane is about 1.5 (heavier, pools low). Leak detection and ventilation strategy depend on which gas is present.

Category I venting

Non-condensing appliance with negative pressure in the flue, vented vertically or with approved slope. Common residential furnaces use Type B double-wall vent with a listed termination cap.

Category IV venting

Condensing appliance operating under positive flue pressure, vented with PVC or CPVC. Vent must slope back to the appliance so acidic condensate drains to a neutralizer or approved drain.

Vent connector slope

Natural-draft vent connectors must rise at least one-quarter inch per foot toward the chimney. Horizontal runs should be short, and the connector must not sag or run downhill.

Combustion air openings

A confined space needs two permanent openings, high and low. Each is sized at one square inch free area per 1,000 Btu/hr of total input when communicating directly with outdoors.

Draft hood spill test

A match flame held at the draft hood relief opening should be pulled into the hood. Spillage indicates blocked flue, reversed draft, or inadequate combustion air.

Sediment trap

A tee with a capped nipple installed below the appliance gas connection so debris settles out before reaching the gas valve. Required by code at the appliance connection.

Manifold gas pressure

Pressure downstream of the appliance regulator, typically 3.5 inches water column for natural gas and 10 to 11 for propane. It sets the firing rate through the burner orifices.

Gas piping leak test

Pressurize new piping with air or inert gas per code, isolate the appliance gas valve, and check for pressure loss. Never use an open flame for leak detection.

Combination gas valve

A single unit providing manual shutoff, pressure regulation, automatic safety shutoff, and manifold pressure adjustment. Some include redundant operators for fail-safe closure.

Burner venturi

The mixer section that entrains primary air into the gas stream before the burner ports. A dirty or misaligned venturi causes yellow flame, soot, and incomplete combustion.

Manual shutoff location

The appliance shutoff valve must be installed in the supply line within six feet of the appliance and be readily accessible for service and emergency isolation.

Thermocouple output

A standing pilot thermocouple generates about 25 to 30 millivolts open circuit; the gas valve needs roughly 10 to 15 millivolts under load to hold the pilot operator open.

Flame rectification

The flame conducts ions and rectifies alternating current into a small direct current, typically 1 to 5 microamps. The control board uses this signal to prove flame and keep the gas valve open.

Pressure switch function

Proves the inducer is moving air before ignition. Blocked ports, water in tubing, or a weak inducer keep it open and stop the sequence before the gas valve opens.

High limit switch

A temperature safety in series with the thermostat that opens on excess supply air temperature. It protects the heat exchanger when airflow is low or the blower fails.

Rollout switch

A manual-reset safety at the burner box that trips if flame rolls out of the burner compartment. It signals blocked flue, restricted combustion air, or a cracked heat exchanger.

Sequence of operation

Call for heat, inducer proves through the pressure switch, ignition energizes, gas valve opens, flame proves, blower starts on delay, and the blower stops after the off delay.

Temperature rise

Supply air temperature minus return air temperature. Compare against the rating plate range, commonly 35 to 65 degrees Fahrenheit for residential furnaces, to confirm proper airflow.

CFM from temperature rise

CFM equals output Btu/hr divided by 1.08 times the temperature rise. The constant 1.08 is the sensible heat factor for standard air at sea level.

External static pressure

Pressure measured across the furnace with a manometer, reflecting duct and coil restriction. A typical residential design is about 0.5 inches water column total.

Heat exchanger inspection

Look for cracks at cell seams, discoloration, water stains, and flame distortion. A cracked heat exchanger can allow flue products into supply air and must be confirmed before replacement.

Air-free CO calculation

Air-free CO equals measured CO times 20.9 divided by (20.9 minus measured O2). It corrects for dilution and is the value compared against combustion limits.

Excess air calculation

Excess air percent equals (20.9 divided by (20.9 minus measured O2)) minus 1, times 100. Typical natural gas appliances run 25 to 50 percent excess air.

Flue draft measurement

Draft is measured with a manometer or draft gauge in the flue. A Category I appliance should show negative draft, roughly -0.02 to -0.04 inches water column at the relief opening.

Low flame signal

A microamp signal below the manufacturer minimum, often 1 to 2 microamps, causes intermittent lockout. Clean the flame sensor and verify burner ground before replacing the control board.

Troubleshooting order

Confirm the thermostat call, check 24 volts at the board, verify inducer and pressure switch, confirm ignition, then check gas valve and flame proof. Follow the sequence to isolate the failure.

Short cycling causes

High limit trips from low airflow, thermostat location, an oversized unit, or a failing flame sensor dropping the call. Verify temperature rise and static pressure before changing parts.