CSLB C-20 HVAC Contractor flashcards

A combustion air method requiring one opening within 12 inches of the ceiling, sized at 1 square inch of free area per 3,000 Btu/h of total appliance input, connecting directly or via duct to outdoor air.

• Total input of all appliances in confined space must be combined
• Opening must be within 12 in. of ceiling
• Differs from the two-opening method (1 sq in per 4,000 Btu/h from outdoors)

Memory trick: Single opening: divide total Btu by 3,000.

The sum of static pressures on the supply and return sides of an air handler, used to verify system airflow against blower ratings.

• TESP = |supply| + |return|
• Compared against manufacturer's rated max ESP
• High TESP often indicates restricted duct or dirty filter

Memory trick: Add the pushes, ignore the signs, get your TESP.

The G terminal on a low-voltage thermostat energizes the blower relay, allowing the indoor fan to run independently of heating or cooling calls.

• G = Green wire = fan/blower relay
• Used for 'Fan On' switch position
• Works alongside Y (cooling) and W (heating) calls

Memory trick: G is for 'Go, fan, Go!'

Standard pressure-temperature charts assume sea-level atmospheric pressure; at higher elevations, gauge readings must be corrected (typically subtracted) by the atmospheric pressure deficit before referencing the chart.

• Standard atmosphere = 14.7 psia at sea level
• At altitude, local atmosphere is lower, so gauge readings run higher for the same absolute pressure
• Subtract the atmospheric deficit from the gauge reading before using a standard sea-level P-T chart

Memory trick: High up, air is thin — subtract before you dive in (the chart)

TEL is the sum of actual straight duct length plus the equivalent lengths of all fittings along the longest duct run, used to calculate friction rate in Manual D.

• Fittings add resistance equivalent to extra straight duct feet
• Longest run (critical path) determines system TEL
• Used with available static pressure to find friction rate

Memory trick: Straight duct plus every twist and turn adds up the length you earn.

The Y terminal on a low-voltage thermostat sends a 24VAC signal to energize the outdoor unit's compressor contactor, starting the cooling cycle.

• R = 24V hot from transformer
• Y = cooling call to contactor
• G = fan relay call
• C = common return leg

Memory trick: 'Y' is for 'Yes, cool it now' — Y calls the compressor.

A device (start capacitor + relay) added to a single-phase compressor circuit to boost starting torque during startup, then disengage automatically.

• Used for compressors with high starting loads (e.g., high head pressure restarts)
• Relay disconnects start capacitor after motor reaches running speed
• Does not affect normal running performance or capacitance

Memory trick: Hard Start = Extra Push, then Steps Away

When a required CFM falls between two duct sizes on a friction rate table, always round up to the next larger duct size that meets or exceeds the required airflow.

• Never round down below required CFM
• Rounding up prevents excess velocity and noise
• Duct sizing tables pair friction rate with CFM capacity per size

Memory trick: When in doubt, size it out — always round up on the duct chart.

The condenser coil rejects heat from hot compressed refrigerant vapor to outdoor air, condensing it into a high-pressure liquid.

• Located outdoors on split systems
• Refrigerant enters as hot vapor, leaves as subcooled liquid
• Heat rejection = evaporator heat absorbed + compressor work

Memory trick: Condenser dumps the heat outside.

A diagnostic test that measures the pressure differential at each register while the house is depressurized by a blower door, used to locate and quantify duct leakage to outside.

• House is depressurized to a reference pressure (commonly -25 Pa)
• Low pan readings (near 0 Pa) indicate tight ducts
• High pan readings (several Pa) indicate leakage to outside

Memory trick: Low number, tight tunnel — high number, hole in the wall.

A mechanical switch in split-phase motors that disconnects the start winding once the motor reaches about 75% of synchronous speed.

• Located inside motor housing
• Failure to open causes start winding burnout
• Failure to close prevents motor from starting
• Common failure point in older split-phase motors

Memory trick: Start winding gets the boost, then the switch kicks it loose at 75%.

Venting classification for appliances with positive vent pressure and flue gas temperatures above the dew point, preventing condensation.

• Requires positive-pressure rated vent material like stainless steel
• Common on some induced-draft 80-90% furnaces
• Different from Category IV which also condenses

Memory trick: Positive & dry = III; Positive & wet = IV.

A safety control that opens the compressor circuit when discharge pressure exceeds a factory-set limit, protecting the compressor and system from overpressure damage.

• Wired in series with the compressor contactor coil
• Trips on high discharge pressure (e.g., dirty condenser, overcharge, fan failure)
• Manual-reset types require investigation before resetting

Memory trick: High pressure hits the brakes before things blow.

Manual J estimates internal sensible heat gain from occupants using a default occupancy count of bedrooms plus one, multiplied by a standard per-person Btu/hr allowance.

• Default occupancy = bedrooms + 1
• Standard sensible gain ≈ 230 Btu/hr per person
• Occupant gain is added to internal gains, separate from appliance loads

Memory trick: Bedrooms plus one, times 230, sums the sun (people) load.

A component that traps liquid refrigerant and oil returning from the evaporator, preventing compressor flooding and slugging, common on heat pumps.

• Common on heat pumps due to defrost cycle liquid floodback
• Meters liquid back slowly via bleed hole
• Protects against compressor mechanical damage

Memory trick: Accumulator holds the liquid hostage until it's safe.

Combustion equipment input capacity must be reduced above 2,000 ft elevation because thinner air reduces oxygen available for combustion, typically at a rate of about 4% per 1,000 ft above 2,000 ft.

• Threshold begins at 2,000 ft elevation
• Standard derate ~4% per 1,000 ft above threshold
• Affects heating input capacity and combustion efficiency

Memory trick: Above two grand, cut four percent per thousand you stand.

A system low on refrigerant charge typically shows low suction pressure, high superheat, low subcooling, and reduced capacity simultaneously.

• Low suction pressure due to reduced refrigerant mass flow
• High superheat from early vapor formation in evaporator
• Low subcooling from insufficient liquid refrigerant in condenser
• Common root cause is a refrigerant leak

Memory trick: Low charge = low everything except superheat, which climbs high.

O and B terminals control the reversing valve solenoid; O-type valves energize in cooling, B-type valves energize in heating, depending on manufacturer.

• O = Energized in Cooling (common convention)
• B = Energized in Heating (opposite convention)
• Wiring mismatch causes system to run in wrong mode

Memory trick: 'O' is for 'On in cOOling'; mismatched wiring flips the season.

Higher-MERV filters have finer media that increase resistance to airflow, raising static pressure if the system was not designed for it.

• Same-size filter, higher MERV = more restriction.
• Can cause reduced CFM, coil freezing, or blower strain.
• Media surface area and depth can offset restriction in properly designed filters.

Memory trick: Finer filter, harder to breathe through.

Thermostat/control wire gauge must increase with run distance to limit voltage drop, since thinner wire has higher resistance per foot.

• 18 AWG: up to ~100 ft
• 16 AWG: ~100–200 ft
• 14 AWG: ~200–300 ft
• Excessive voltage drop causes erratic relay/thermostat operation

Memory trick: Longer run, thicker wire—resistance adds up like tolls on a highway.

Single-wall metal vent connectors require 6 inches of clearance to combustible materials, while listed Type B double-wall connectors only require 1 inch due to their insulating air space.

• Single-wall: 6 in. clearance minimum
• Type B double-wall: 1 in. clearance (when listed)
• Clearance reduction possible only with listed protective shielding

Memory trick: Single-wall needs six; Type B only needs one.

A TXV design using a separate equalizer line connected near the evaporator outlet to sense actual outlet pressure, compensating for pressure drop across long or multi-circuit coils.

• Used when coil pressure drop exceeds about 2-3 psi
• Internal equalizers only sense valve outlet pressure, not coil exit
• Improper equalizer connection causes erratic superheat control

Memory trick: External equalizer 'checks the exit' to keep superheat honest.

SEER2 uses the updated DOE M1 test procedure with higher, more realistic external static pressure than the older SEER test, resulting in lower numerical ratings for similar equipment.

• SEER2 test uses about 0.5 in. w.c. static pressure vs. 0.1 in. w.c. for SEER
• Reflects real-world duct restriction more accurately
• SEER2 and SEER numbers are not directly comparable

Memory trick: SEER2 turns up the duct resistance dial, so the number takes a dial-down.

In Manual D design, trunk duct size decreases progressively downstream as each branch takeoff removes airflow, allowing smaller duct sizes further from the plenum.

• Trunk CFM = total minus all upstream branch takeoffs
• Reducing trunk size downstream saves material and maintains velocity
• Oversized downstream trunk can cause excessive noise/turbulence

Memory trick: Every branch takes a bite out of the trunk's CFM.

The industry standard specifying chemical purity requirements that recovered refrigerant must meet before being reclaimed and resold for reuse in another owner's system.

• Reclaimed refrigerant must be processed by an EPA-certified reclaimer
• AHRI 700 sets limits on moisture, acidity, high-boiling residue, and non-condensables
• Recycled refrigerant (not reclaimed) can only be reused by the same owner without meeting AHRI 700

Memory trick: To sell it clean, hit the AHRI 700 scene.

A condition where liquid refrigerant returns to the compressor due to insufficient superheat, risking mechanical damage from incompressible liquid entering the compression chamber.

• Zero or near-zero superheat indicates floodback risk
• Slugging can break valves, reeds, or scroll sets instantly
• Suction line accumulators help protect against floodback

Memory trick: Zero superheat, liquid's a threat — slug the compressor, cause regret.

After isolating and locking an energy source, Cal/OSHA requires the worker to verify de-energization by testing the circuit with a meter or attempting to operate the equipment.

• Steps: identify, notify, isolate, lock/tag, verify (try-out/test)
• A meter reading of zero volts confirms de-energization
• Skipping verification is a leading cause of electrical injuries

Memory trick: Lock it, tag it, then TEST it.

Capacitors store electrical energy even when disconnected from power and must be safely discharged before handling to prevent shock injury.

• Use an insulated resistor, not a screwdriver, to discharge
• Capacitors can hold charge for hours after power-off
• Always verify zero charge with a multimeter before touching terminals
• Shock hazard exists even with the disconnect open

Memory trick: Power off doesn't mean charge is gone — discharge before you carry on.

An ACCA-approved method for calculating a building's heating and cooling loads room-by-room, based on construction, climate, orientation, and occupancy.

• Produces Btu/hr heating and cooling loads
• Required before Manual S equipment selection
• Considers insulation, windows, infiltration, and internal gains

Memory trick: J is for 'Just how much heat does this house need?'

NFGC classification of gas appliance venting based on vent pressure (positive/negative) and flue gas condition (condensing/non-condensing).

• Category I: negative pressure, non-condensing (standard furnace)
• Category III: positive pressure, non-condensing (power vented)
• Category IV: positive pressure, condensing (high-efficiency PVC vent)

Memory trick: I is old and lazy (negative draft, dry gas); IV is new and wet (positive, condensing).

Bubbles in the liquid line sight glass indicate flash gas forming in the liquid line, typically caused by low refrigerant charge or a restriction.

• Clear sight glass = properly subcooled liquid
• Bubbles = flash gas from undercharge or restriction
• Some systems have no sight glass on zeotropic blends

Memory trick: Bubbles mean trouble — low charge or blockage.

The direction of rotation of a three-phase motor is determined by phase sequence; swapping any two of the three line leads reverses rotation direction.

• Three-phase motors have no capacitors for starting
• Rotation depends on phase sequence (e.g., L1-L2-L3 vs L1-L3-L2)
• Swap only two leads—swapping all three has no effect

Memory trick: Swap two, spin true.

Cal/OSHA establishes a permissible exposure limit of 25 ppm for carbon monoxide as an 8-hour time-weighted average, stricter than the federal 50 ppm PEL.

• California PEL: 25 ppm (8-hr TWA)
• Federal OSHA PEL: 50 ppm (8-hr TWA)
• CO is colorless, odorless, and produced by incomplete combustion

Memory trick: California cuts it to twenty-five.

In zoned systems, a bypass duct is sized to handle the difference between total blower airflow and the airflow delivered when only partial zones are open, preventing excessive static pressure.

• Bypass CFM = Total system CFM − Open zone CFM
• Prevents blower overload when zones close
• Oversized bypass can cause temperature control issues

Memory trick: Bypass carries what the closed zones refuse.

A thermostatic expansion valve modulates refrigerant flow to maintain constant superheat, while a fixed orifice has a set opening optimized for only one condition.

• TXV uses a sensing bulb to respond to evaporator outlet temperature
• Fixed orifice is a non-adjustable, fixed-size opening
• TXV performs better across variable load and outdoor temperature swings
• Both require proper charge for accurate superheat/subcooling readings

Memory trick: TXV = Tracks and adjusts; fixed orifice = fixed and forgetful.

The process of removing air and moisture from a refrigeration system using a vacuum pump until the system reaches 500 microns or below, verified with a micron gauge.

• Standard compound gauges cannot accurately measure deep vacuum levels
• 500 microns is the widely accepted industry target before charging
• A rising micron reading after pump shutoff (decay) indicates a leak or moisture

Memory trick: Five hundred microns or bust — dry system is a must.

A repeatedly blown transformer fuse immediately upon reconnecting thermostat wiring usually indicates a short circuit in the low-voltage wiring or a shorted component.

• Test transformer output before connecting wires
• Bare wires touching grounded metal cause shorts
• Isolate wires one at a time to find the short
• Never bypass a fuse with foil or jumper

Memory trick: Fuse pops the instant wires connect? Somewhere there's a short circuit connect.

The evaporator coil absorbs heat from indoor air by boiling low-pressure liquid refrigerant into vapor.

• Located on the indoor (or low-pressure) side of the system
• Refrigerant enters as a low-pressure liquid/vapor mix and leaves as superheated vapor
• Heat absorption here is the actual 'cooling' effect felt indoors

Memory trick: Evaporator = Absorbs heat, like a sponge soaking up warmth

A safety test that measures pressure in the combustion appliance zone under worst-case exhaust conditions, compared against manufacturer spillage limits for atmospherically vented appliances.

• Typical safe limit for atmospheric appliances is around -3 to -5 Pa depending on manufacturer/testing protocol.
• Exceeding the limit indicates backdraft/spillage risk of carbon monoxide.
• Corrective actions include makeup air, reducing exhaust CFM, or appliance venting changes.

Memory trick: Too much suck, appliance backdraft luck runs out.

Recycled refrigerant is cleaned on-site for reuse by the same owner; reclaimed refrigerant is processed off-site to AHRI 700 purity standards for resale to other owners.

• Recycling: on-site, same system/owner, minimal processing
• Reclaiming: certified facility, AHRI 700 purity, required for resale/change of ownership
• EPA requires reclamation whenever refrigerant changes ownership

Memory trick: Recycle for yourself, Reclaim to sell

Appliances with 50+ lbs of refrigerant charge exceeding a specified annual leak rate must be repaired within 30 days under EPA regulations.

• Comfort cooling/other appliances: 15% threshold
• Industrial process refrigeration: 30-35% threshold
• Applies to appliances with 50 lbs or more full charge

Memory trick: 50 lbs, 15% — fix it fast, or face the blast (of fines)

Friction rate = (Available Static Pressure ÷ Total Effective Length) × 100, expressed in inches of water column per 100 feet of duct, and used to select duct sizes from Manual D tables.

• TEL includes straight duct plus fitting equivalent lengths
• Lower friction rate generally means larger required duct size
• Result is used with airflow (CFM) on a duct sizing chart

Memory trick: Divide the pressure by the path, then scale to a hundred.

NEC 440.22(A) limits the maximum branch-circuit protective device for a single motor-compressor to 175% of its rated-load current (RLA), unless that size is insufficient to handle starting current, in which case it may be increased up to 225%.

• Formula: Max breaker = RLA × 1.75
• Round down to nearest standard size unless 175% isn't sufficient
• May increase up to 225% if needed for starting current

Memory trick: 175% keeps the breaker from tripping on startup surge.

CMC requirement that kitchen exhaust systems rated 400 CFM or greater must be provided with a makeup air source to prevent house depressurization.

• Threshold is 400 CFM per CMC
• Prevents backdrafting of gas water heaters/furnaces
• Makeup air can be passive or mechanically supplied and interlocked with the hood

Memory trick: Four hundred's the line — bring in makeup air time.

SEER2 is an updated efficiency rating standard using a revised test procedure (M1) with higher external static pressure to better reflect real-world duct system resistance.

• Effective for equipment manufactured starting January 1, 2023
• Uses 0.5 in. w.c. static pressure vs. 0.1 in. w.c. under old SEER
• SEER2 values are typically slightly lower than equivalent SEER values
• Applies to central air conditioners and heat pumps

Memory trick: SEER2 pressure tests reality, not a lab fantasy.

Title 24 requires certain energy features, including duct leakage, to be field-verified by a certified HERS rater and documented on the CF3R compliance form, separate from the contractor's CF2R installation certificate.

• CF2R = installation certificate (contractor)
• CF3R = verification certificate (HERS rater)
• Duct leakage is a HERS-verified measure under Title 24

Memory trick: 'Contractor installs (CF2R), rater verifies (CF3R).'

Manual J calculates conductive heat loss through building assemblies using Q = U × A × ΔT, where U is the assembly's U-factor, A is area, and ΔT is the design temperature difference.

• Q = U × A × ΔT
• U-factor is the inverse of total R-value
• ΔT uses 99% heating design temperature, not average winter temp

Memory trick: U times Area times Delta-T equals heat that flees.

Carbon monoxide detectors should be installed near sleeping areas, away from vents and dead-air pockets, since CO mixes with air rather than settling at floor or ceiling level.

• CO density ~ same as air (slightly lighter)
• Avoid corners, humid areas (bathrooms), and near supply registers
• Install near each sleeping area per most jurisdictions

Memory trick: CO mixes like fog—spread evenly, avoid dead air corners.

The outdoor temperature at which a heat pump's output equals the building's heat loss; below this point, supplemental heat is needed to maintain comfort.

• Below balance point, heat pump capacity declines with dropping outdoor temp
• Auxiliary heat (electric strips or gas furnace) supplements, not replaces, the heat pump
• Emergency heat mode is a manual override that uses only auxiliary heat

Memory trick: Below the balance, call for backup — strips join in when it's cold enough to hiccup.

When combustion air is supplied via horizontal ducts to outdoors, each of two required openings must provide 1 sq in of free area per 2,000 Btu/h of total appliance input.

• Vertical ducts/openings direct to outdoors: 1 sq in per 4,000 Btu/h
• Horizontal ducts: 1 sq in per 2,000 Btu/h (double the vertical requirement)
• Applies to EACH of the two openings, not combined

Memory trick: Horizontal ducts work harder — half the ratio, double the size.

Return-side duct leaks in hot or unconditioned spaces draw outside air into the system under negative pressure, increasing cooling load and reducing delivered capacity, unlike supply-side leaks which simply waste conditioned air.

• Return ducts operate under negative pressure
• Leaks there pull in unconditioned air, not lose conditioned air
• Return leaks in hot attics significantly raise cooling load

Memory trick: Return sucks in the heat; supply just leaks it out.

Gases like air or nitrogen that remain in vapor form inside the refrigeration system and add extra pressure that doesn't correspond to refrigerant saturation temperature.

• Usually caused by improper or incomplete evacuation
• Causes head pressure higher than the PT chart predicts
• Fixed by proper deep vacuum evacuation before charging

Memory trick: Air won't condense — it just adds pressure and offends.

Total external static pressure (ESP) available from the blower must be divided among all system components (coil, filter, ductwork); remaining pressure after non-duct losses is the friction budget for the duct system.

• Available ESP = blower rated ESP
• Subtract fixed component losses (coil, filter) first
• Remaining ESP is allocated to supply + return duct friction per Manual D

Memory trick: Total pressure minus coil minus filter equals what the ducts get to spend.

LRA is the current a compressor motor draws when its rotor cannot turn (starting or stalled); sustained current at this level indicates a seized or locked compressor.

• RLA = normal running current; LRA = stall current, much higher
• Brief LRA spike at startup is normal
• Continuous LRA-level draw with humming signals mechanical lockup

Memory trick: Stuck at LRA too long means the rotor's in a locked-up song.

The formula Btu/h = 1.08 × CFM × ΔT relates furnace heat output, airflow, and air temperature rise across the heat exchanger.

• Always use OUTPUT Btu/h (Input × AFUE), not input rating
• Manufacturer's data plate specifies an acceptable temp rise range
• Rise outside the rated range indicates airflow or firing rate problems

Memory trick: Output over airflow times 1.08 tells the rise, no.

A furnace control strategy using low (~65%) and high (100%) firing rates to match heating output more precisely to building load, improving comfort and efficiency.

• Low stage handles most mild/moderate heating demand
• High stage engages during high demand or if low stage cannot maintain setpoint
• Reduces temperature swings compared to single-stage on/off cycling

Memory trick: Low and slow most days, full blast on the coldest.

A hydronic design rule stating the circulator pump should be located on the supply side, pumping away from the expansion tank's connection point, to maintain positive pressure throughout the system.

• Point of no pressure change = where expansion tank connects
• Pumping away prevents negative pressure zones and air infiltration
• Developed by hydronics engineer Dan Holohan, widely taught in trade practice

Memory trick: Pump away, pressure stays positive all day.

A safety method where a flame sensor rod uses the flame's ionized gases to allow a small one-directional DC current to flow, proving flame presence to the control board.

• Typical minimum signal is 1-3 microamps DC
• Oxidation/carbon buildup is the most common failure cause
• Clean with fine emery cloth, not sandpaper (can leave residue)

Memory trick: Dirty rod, dead current, dead flame.

A safety control on a hydronic boiler that senses water level and shuts off the burner if water drops below a safe operating level, preventing dry-fire damage.

• Required by code on most hot water and steam boilers
• Can be float-type or electronic probe-type
• Should be tested periodically per manufacturer instructions

Memory trick: No water, no fire — cutoff saves the boiler.

A code-based adjustment reducing gas furnace input rating by approximately 4% per 1,000 ft of elevation above 2,000 ft to compensate for reduced oxygen density.

• Baseline elevation for standard orifice sizing is sea level to 2,000 ft
• Reduction typically applied via smaller orifice size, not just derated rating on paper
• Manufacturer's high-altitude kit or orifice tables should be consulted for exact values

Memory trick: Every thousand feet past two thousand, shave four percent.