CSLB C-20 HVAC Contractor flashcards

A water-seal trap installed in the condensate drain line of condensing furnaces to prevent positive-pressure flue gases from venting through the drain.

• Required on all Category IV (condensing) furnaces
• Must be primed with water before startup
• Failure to install can allow flue gas leakage or drain blockage from soot

Memory trick: No trap, gas escapes — think sink P-trap.

An electronic ignition device that heats a ceramic element to a high temperature to ignite the gas-air mixture directly, replacing standing pilots.

• Made of silicon carbide or silicon nitride
• Very fragile — avoid touching with bare skin (oils cause failure)
• Common on modern 80%+ and 90%+ AFUE furnaces

Memory trick: Hot rod glows, gas flows.

Gas pipe sizing tables require flow demand in cubic feet per hour (cfh), calculated by dividing the appliance's Btu/h input by the fuel's heating value (typically ~1,000 Btu/cf for natural gas).

• Natural gas heating value averages ~1,000-1,100 Btu per cubic foot
• Propane has a much higher heating value (~2,500 Btu/cf), requiring different cfh calculations
• Pipe size also depends on total developed length and allowable pressure drop (commonly 0.5 in w.c.)

Memory trick: Divide Btu load by heating value to get flow.

A hinged damper installed in a Category I vent connector that admits room air to regulate and stabilize draft pressure in the venting system.

• Used only on natural-draft, negative-pressure (Category I) appliances
• Swings open under excess draft to admit dilution air
• Not used on induced-draft or condensing (Category III/IV) systems

Memory trick: Swinging door lets air in to calm the draft.

The optimal location for an air separator in a hydronic system is where the water is hottest and under the lowest pressure, typically on the boiler supply, to maximize the removal of dissolved and entrained air.

• Prevents noise, corrosion, and inefficient heat transfer.
• Highest temperature, lowest pressure point is ideal.
• Often paired with the expansion tank connection.

Memory trick: Boiler's hot, air bubbles float, separator's there, to keep them out of the boat.

For a confined space with gas appliances, a single opening connecting directly to an adjacent unconfined space requires a minimum free area of 1 square inch per 1,000 Btu/h of total input, but not less than 100 square inches.

• Used when direct outdoor air isn't feasible.
• Adjacent space must be unconfined.
• Calculated area must be at least 100 sq inches.

Memory trick: One opening to unconfined, 1 per 1000, never less than a hundred you'll find.

The high-limit switch is a safety device in a furnace that monitors the temperature within the heat exchanger compartment and shuts off the burner if the temperature exceeds a safe limit.

• Prevents overheating of the heat exchanger.
• Typically resets automatically once temperature drops.
• Commonly trips due to restricted airflow (e.g., dirty filter, closed vents).

Memory trick: Hot air needs a clear path, or the furnace will halt.

A condition where a compressor struggles to initiate rotation, often due to a failed starting component, leading to excessive current draw and potential circuit breaker trips.

• Often caused by a failed start capacitor or relay.
• Results in high locked-rotor amperage (LRA).
• Can damage compressor windings if not addressed.

Memory trick: Breaker goes 'snap,' if the start cap's a trap.

A valve that blends two fluid streams of different temperatures to achieve a desired, lower output temperature for a specific zone or application within a hydronic system.

• Commonly used in radiant floor heating to protect tubing.
• Can be thermostatic or motorized.
• Ensures optimal and safe operating temperatures for zones.

Memory trick: Boiler's too hot, radiant's not; a mixing valve hits the right spot.

An electrical component that stores and releases electrical energy to provide a momentary boost of current to the compressor motor, helping it overcome inertia and begin rotating.

• Essential for starting single-phase AC motors, including compressors.
• Failure often results in a 'humming' sound from the compressor without it starting.
• Common failure point due to heat and electrical stress.

Memory trick: Compressor's hum, but no pump, points to the start's slump.

The maximum allowed distance between supports for horizontal PVC vent piping in condensing furnace installations to maintain structural integrity and proper slope for condensate drainage.

• Prevents sagging and water traps.
• Critical for proper condensate removal.
• Typically 4 feet for Schedule 40 PVC.

Memory trick: PVC vent, 4 feet, no sag, no defeat.

Carbon monoxide (CO) levels in flue gas, particularly undiluted, that exceed specific thresholds (e.g., 400 ppm) are considered immediately hazardous and require the appliance to be shut down.

• CO is a colorless, odorless, toxic gas.
• High CO indicates incomplete combustion or flue gas recirculation.
• Industry standards define dangerous levels requiring immediate action.

Memory trick: When CO hits four hundred, safety's call cannot be blundered.

Furnace sizing involves matching the appliance's heat output to the building's heat loss, accounting for the furnace's efficiency (AFUE) to determine the necessary input rating.

• Heat loss calculation determines required output.
• AFUE is the efficiency rating.
• Input = Output / AFUE.

Memory trick: What you lose, divided by what you use, gives the input you choose.

Two-stage furnaces operate at two different heating capacities (low and high) to provide more consistent temperatures and better efficiency, controlled by a two-stage thermostat or the furnace's internal timer.

• Low stage for most heating needs, high stage for colder conditions.
• Requires a compatible thermostat or proper furnace board configuration.
• Improper wiring or settings can prevent second-stage activation.

Memory trick: Two stages to climb, if W2's not connected, it's always first-stage time.

A circulator pump is a type of centrifugal pump used in hydronic systems to circulate water or other heat transfer fluids through a closed loop, ensuring heat distribution.

• Designed for closed-loop systems.
• Overcomes system resistance (friction).
• Essential for distributing heat from boiler to emitters.

Memory trick: To make the hot water flow, a circulator pump must go.

The minimum upward slope required for a vent connector serving a natural draft appliance to ensure proper removal of combustion byproducts by natural convection.

• Crucial for safety and efficient venting.
• Typically 1 inch per foot of horizontal run.
• Ensures hot flue gases rise effectively.

Memory trick: For natural draft's rise, one inch per foot is wise.

A graphical representation showing the relationship between a fan's airflow (CFM), static pressure, power consumption, and efficiency at various operating points.

• Shows how CFM changes with static pressure.
• As static pressure increases, CFM typically decreases.
• Used to select the correct fan for specific system requirements.

Memory trick: Fan pushes air, but resistance slows the flow.