HVAC Excellence Gas Heat Certification Exam flashcards
HVAC Excellence Gas Heat · Build recall with a focused review session.
32 cards. Read the term and the definition here. The deck at the top of this page is optional.
- Stoichiometric air for methane
- Complete combustion of methane requires about 2 cubic feet of oxygen, roughly 10 cubic feet of air, per cubic foot of gas; excess air above this dilutes flue CO2.
- Limits of flammability
- The gas-in-air concentration range in which ignition occurs; natural gas is roughly 4 to 15 percent, and below the lower limit the mixture is too lean to burn.
- Incomplete combustion produces CO
- Carbon monoxide forms when oxygen is insufficient or the flame is quenched against a cool surface, so CO readings rise while CO2 may stay stable.
- Gas input rate formula
- Input in Btu per hour equals gas flow in cubic feet per hour multiplied by the heating value in Btu per cubic foot; clocking the meter gives actual input.
- Sediment trap purpose
- A drip leg installed ahead of the gas valve that collects dirt, scale, and pipe dope so debris does not reach the valve or burner orifices.
- Pipe sizing factors
- Sizing depends on total connected Btu load, allowable pressure drop, developed length including fittings, gas specific gravity, and the material's capacity table.
- Pipe dope application
- Apply gas-rated pipe dope to male threads only, leaving the first two threads bare; never apply dope to flare fittings or the flare seating surface.
- CSST bonding requirement
- Corrugated stainless steel tubing must be bonded to the building grounding electrode system per manufacturer and code, and protected from sharp edges and physical damage.
- Category I versus IV venting
- Category I appliances are non-condensing and vent under negative pressure with metal pipe; Category IV appliances are condensing and vent under positive pressure, often with plastic pipe.
- Combustion air rule of thumb
- An unconfined space needs about 50 cubic feet per 1,000 Btu per hour of input; confined spaces need permanent high and low openings or ducted outdoor air.
- Draft measurement
- A manometer or draft gauge at the flue collar shows negative draft confirming flue gases leave; positive pressure indicates spillage or a blocked vent.
- Flue gas analyzer readings
- The analyzer reports oxygen, carbon dioxide, carbon monoxide, and stack temperature; excess oxygen shows excess air and rising CO shows incomplete combustion.
- Manifold pressure settings
- With the burner at full fire, natural gas manifold pressure is typically 3.5 inches water column and LP about 10 to 11, unless the rating plate states otherwise.
- Primary versus secondary air
- Primary air mixes with gas before the burner ports and shapes the flame; secondary air feeds the flame from around the burner for complete combustion.
- Redundant gas valve
- The combination valve contains two series solenoid operators so a single stuck-open valve cannot pass gas when the valve is de-energized.
- Orifice and altitude derating
- Higher altitude reduces available oxygen, so manufacturers require smaller orifices or reduced manifold pressure per their altitude chart to maintain proper combustion.
- Thermocouple function
- A standing pilot heats the thermocouple, producing a small millivolt signal that holds the pilot operator open; a weak or dirty pilot drops the signal and shuts the gas off.
- Hot surface igniter
- A silicon carbide or nitride element that heats to ignition temperature on a call for heat; an open element reads infinite resistance and must be replaced.
- Flame rectification
- The flame conducts ions and rectifies alternating current into a small direct current, measured in microamps, that proves flame to the ignition control.
- Direct spark ignition
- The control sparks the main burner directly on a call for heat and proves flame with a remote sensor or the spark electrode itself, depending on design.
- Furnace sequence of operation
- Thermostat calls, inducer runs and proves draft, ignition energizes, gas valve opens, flame proves, blower starts after delay, then gas closes and blower off-delays.
- High-limit switch
- A temperature-actuated safety in the supply air path that opens on overtemperature and cuts the gas valve; it must never be bypassed or jumpered.
- Rollout switch
- A manual-reset safety near the burner that opens when flame rolls out of the combustion chamber, indicating a blocked flue or failed heat exchanger.
- Pressure switch function
- Proves the inducer is moving air by closing on negative pressure; the ignition control will not allow ignition until this switch closes.
- Ladder diagram reading
- Read from the power source on the left toward the loads on the right and top to bottom; the sequence of operation follows the order components energize.
- Flame sense and polarity
- Flame rectification requires correct line polarity and a solid ground; reversed polarity or a floating ground causes flame loss and lockout despite a good sensor.
- Safety devices in series
- Limit, rollout, and pressure switches are wired in series in the gas valve circuit so any open safety de-energizes the gas valve and stops gas flow.
- Temperature rise formula
- Temperature rise equals supply air temperature minus return air temperature and must fall within the rating plate range, commonly 35 to 65 degrees Fahrenheit.
- CFM from temperature rise
- CFM equals Btu per hour divided by 1.08 times the temperature rise; low airflow raises the rise and can trip the high limit.
- External static pressure
- Total external static pressure measured in both plenums with a manometer; readings above the blower's rated maximum indicate duct or coil restriction.
- Heat exchanger crack test
- Watch for a CO rise or flame disturbance when the blower starts, plus visual inspection; a cracked heat exchanger is a safety hazard and must be replaced.
- Ignition failure diagnosis
- With spark present but no flame, verify gas supply, manifold pressure, orifice condition, and air in the line before replacing the ignition control.