CSLB C-20 HVAC Contractor flashcards

Trunk ducts are sized so airflow (CFM) divided by cross-sectional area (sq ft) does not exceed a target velocity (FPM) to control noise and pressure loss.

• Area (sq ft) = CFM ÷ Velocity (FPM)
• Typical trunk duct design velocity: 500–700 FPM
• Undersized ducts increase velocity, noise, and static pressure

Memory trick: 'Divide the flow by the speed limit to get the size.'

Residential cooling systems are typically designed to deliver about 400 CFM of airflow per ton of cooling capacity to achieve proper sensible/latent split and coil performance.

• 1 ton = 12,000 BTU/hr
• Standard design airflow ≈ 400 CFM/ton
• Humid climates may target 350 CFM/ton for better dehumidification

Memory trick: '400 for cooling, keep it flowing.'

A bimetal safety switch that senses supply plenum temperature and shuts off the gas valve if the furnace overheats due to restricted airflow.

• Located in the supply air plenum near the heat exchanger
• Auto-resets once temperature drops to safe range
• Prevents heat exchanger damage from blower failure or dirty filters

Memory trick: Too hot? High-limit shuts it down.

Cal/OSHA requires testing a confined space's atmosphere for oxygen content, flammable gases, and toxic gases before entry, along with continuous monitoring throughout the work.

• Test order: oxygen, flammability, then toxicity (e.g., CO, H2S)
• Acceptable oxygen range: 19.5%–23.5%
• Permit-required spaces need an attendant and entry permit

Memory trick: Test before you step into the pit.

The air velocity passing through a filter's net free area, calculated as CFM divided by filter area in square feet; excessive velocity increases pressure drop and reduces filter effectiveness.

• Face velocity = CFM / filter area (sq ft)
• Recommended max is generally around 300 fpm
• Undersized filters cause high static pressure and reduced airflow

Memory trick: Small filter, big rush — velocity crushes the airflow.

A valve installed in the liquid line before the metering device that closes when the thermostat is satisfied, allowing the compressor to pump down refrigerant before shutting off on a low-pressure control.

• Prevents liquid refrigerant migration to the compressor during off cycles
• Closes on thermostat satisfaction, not on pressure directly
• Compressor shuts off via low-pressure control after pump-down completes

Memory trick: Thermostat says stop, solenoid shuts the liquid door, compressor mops up before it's off.

Gas pipe capacity depends on both pipe diameter and total developed length; longer runs reduce capacity, requiring larger diameters to deliver the same Btu/hr rating.

• Capacity decreases as pipe length increases due to friction loss
• Always select the smallest pipe size meeting or exceeding the appliance Btu/hr input
• Code-based sizing tables (e.g., CMC) list capacity by diameter and length

Memory trick: Match the pipe to the pressure drop over distance, not just the Btu rating alone.

Capacitors wired in parallel have their capacitance values added together, unlike resistors, which is the opposite of how they combine in series.

• Parallel: C_total = C1 + C2
• Series: C_total = (C1×C2)/(C1+C2)
• Substituted capacitance must stay within manufacturer tolerance (commonly ±6%)

Memory trick: Parallel capacitors just add up, like teammates.

Cal/OSHA requires fall protection systems (guardrails, warning lines, or fall arrest) for low-slope roof work when the fall distance is 7.5 feet or more per Title 8 CCR §3212.

• Applies specifically to low-slope roofs in general industry
• Differs from the general 4-foot unprotected edge rule
• Options include guardrails, warning line systems, or personal fall arrest
• Rooftop HVAC techs commonly work near this hazard

Memory trick: Seven and a half feet, don't take the roof-edge bet.

ECM (electronically commutated) motors use a built-in microprocessor to maintain constant programmed airflow despite static pressure changes, offering higher efficiency than PSC motors.

• ECM = DC brushless motor with onboard control module
• Maintains constant CFM as filters load or ducts restrict
• PSC motors lose airflow as static pressure rises

Memory trick: ECM thinks, PSC just spins.

Natural-draft gas appliances should maintain a slightly negative pressure at the draft hood to safely vent combustion gases; positive pressure indicates spillage/backdrafting and CO hazard.

• Normal draft: approximately -0.02 in. w.c.
• Positive reading = spillage into living space
• Common causes: blocked flue, negative house pressure, undersized chimney

Memory trick: Negative draft pulls danger up and out; positive pushes it back in.

Service factor (SF) is a multiplier indicating how much above the nameplate FLA a motor can be safely operated continuously without overheating.

• Formula: Max current = FLA × SF
• Common SF values: 1.0, 1.15, 1.35
• Running above the SF-adjusted amperage shortens motor life

Memory trick: Multiply FLA by SF for the safe ceiling.

Method of dividing a hydronic system into independently controlled areas using zone valves or zone circulators triggered by individual thermostats.

• Zone valves are motorized and open/close based on thermostat calls
• Some systems use a dedicated circulator per zone instead of valves
• End switches on zone valves signal the boiler/circulator to run

Memory trick: One thermostat, one valve, one warm room — others stay closed till their turn.

Standard refrigeration gauge sets use blue for low side, red for high side, and yellow for the utility/center port.

• Blue = low/suction side
• Red = high/liquid/discharge side
• Yellow = neutral utility hose for charging/recovery

Memory trick: Red is hot (high side), Blue is cool (low side)

California energy code requirement mandating diagnostic testing of new duct systems to confirm leakage does not exceed 5% of total fan airflow.

• HERS rater performs required verification
• Applies to new construction duct systems
• Duct sealing with mastic or UL-181 tape ensures compliance

Memory trick: Five percent or less keeps Title 24 happy.

VA (volt-amps) describes a transformer's power capacity; dividing VA by secondary voltage gives the maximum safe continuous current.

• Formula: I = VA ÷ V
• Overloading causes overheating and nuisance transformer failures
• Common HVAC control transformers are rated 40 VA at 24V

Memory trick: VA over Volts gives you Amps.

Solar heat gain through glazing is calculated by multiplying window area by its SHGC and an orientation-specific peak solar factor from Manual J tables.

• Gain = Area × SHGC × Solar Factor
• Lower SHGC glass reduces solar heat gain
• Solar factor varies by window orientation and latitude

Memory trick: Area times shading coefficient times sun strength equals glare's heat.

On TXV-equipped systems, technicians verify and adjust refrigerant charge primarily using subcooling measurements compared against manufacturer-specified targets.

• Low subcooling relative to target = undercharge, add refrigerant
• High subcooling relative to target = overcharge, recover refrigerant
• TXV systems use subcooling as primary charge check (not superheat)
• Always verify against manufacturer's specific target values

Memory trick: Low subcooling, low charge — add gas to balance the scale.

Run capacitors are rated with a manufacturer tolerance (commonly ±6%); a measured capacitance outside this range indicates a failing capacitor.

• Calculate range: rated µF × (1 ± tolerance)
• Weak capacitors cause hard starting, overheating, tripped overloads
• Always discharge a capacitor safely before testing

Memory trick: Out of range, time to change.

Commercial ventilation requirements combine a per-person outdoor air rate and a per-area outdoor air rate, both of which must be added together to determine minimum outdoor air CFM.

• Rp = CFM per person (varies by space type)
• Ra = CFM per sq ft (varies by space type)
• Total OA = (Rp × people) + (Ra × area)

Memory trick: 'People plus space, both get a place in the sum.'

The sum of all branch CFM values in a duct system must equal the total system design airflow; any unmeasured branch can be found by subtraction.

• Total CFM = sum of all branch CFMs
• Missing branch CFM = Total − sum of known branches
• Balancing dampers fine-tune actual field measurements to match design

Memory trick: What's left over is what the last branch must deliver.

A confined space has less than 50 cubic feet of volume per 1,000 Btu/h of appliance input; an unconfined space has more, allowing natural infiltration for combustion air.

• Rule of thumb: 50 cu ft per 1,000 Btu/h input
• Confined spaces need engineered combustion air openings
• Determines whether ducted or direct openings are required

Memory trick: Tight closet, tight air — that's confined.

A table on mechanical blueprints listing tagged equipment with performance specifications such as CFM, capacity, and electrical requirements.

• Uses unit tags like AHU-1, FAU-1
• Cross-referenced with floor plan symbols
• Essential for verifying design intent before installation

Memory trick: Tag it, table it, trust the schedule.

The filter drier is installed in the liquid line ahead of the metering device to remove moisture, acid, and debris before refrigerant reaches the TXV or orifice.

• Protects TXV/orifice from clogging
• Removes moisture to prevent acid formation
• Should be replaced after compressor burnout (suction line drier used temporarily)

Memory trick: Clean the liquid before it meets the tiny valve.

Manual D recommends maximum air velocities in occupied spaces to prevent noise: about 600 fpm for branch ducts and 700-900 fpm for trunk ducts.

• Branch ducts near occupants need lower velocity
• Trunk ducts can run faster since farther from occupants
• Excessive velocity causes register whistling/noise

Memory trick: Slow it down near the bed, speed's fine in the trunk instead.

A start capacitor provides a large capacitance boost to increase starting torque in single-phase motors and is disconnected by a relay once the motor reaches running speed.

• Wired in series with the start winding
• Disconnected by potential or current relay near 75% speed
• Not rated for continuous duty, unlike a run capacitor

Memory trick: Start gives the push, then takes a rest.

Sones measure perceived loudness of exhaust fans; continuous whole-house ventilation fans must be quiet (≤1.0 sone) so occupants do not disable them.

• 1 sone ≈ quiet refrigerator hum
• Continuous whole-house fans: max 1.0 sone
• Intermittent local exhaust fans allow higher sone ratings (up to 3.0)

Memory trick: 'Quiet fans keep running, loud fans get turned off.'

Air velocity in a duct equals CFM divided by the cross-sectional area of the duct in square feet.

• Convert duct dimensions from inches to square feet before dividing.
• Velocity = CFM ÷ Area (sq ft).
• Excess velocity causes noise; too little causes poor mixing and low delivery.

Memory trick: CFM over Area gives you the Air-speed Affair.

Total Leakage testing seals all registers and measures all duct leaks; Leakage to Outside testing leaves registers open and uses a blower door to isolate only leaks that escape the building envelope.

• Total leakage test: registers sealed, duct blaster only
• Leakage to outside test: registers open, blower door + duct blaster together
• Title 24 compliance often requires leakage to outside ≤ 6% of fan flow

Memory trick: 'Sealed registers seal in ALL the leaks — total test.'

Manual T evaluates air distribution devices by matching rated throw (distance air travels before reaching terminal velocity) to room dimensions for proper air mixing.

• Terminal velocity is typically defined at 50 fpm
• Throw shorter than room depth causes poor mixing and comfort issues
• Register selection must match both CFM and room geometry

Memory trick: If the throw falls short, comfort goes wrong.

For a fixed duct system, static pressure varies with the square of the airflow ratio, while horsepower varies with the cube.

• CFM ratio ∝ RPM ratio.
• Pressure ratio = (CFM ratio)².
• Horsepower ratio = (CFM ratio)³.

Memory trick: Pressure squares, power cubes, speed stays linear.

A four-way solenoid valve in a heat pump that redirects refrigerant flow, allowing the system to switch between heating and cooling modes.

• Also called a four-way valve
• Energized position typically corresponds to cooling mode in most designs
• Failure can leave unit stuck in one mode

Memory trick: Flip the valve, flip the season.

EPA Section 608 requires technicians to hold one of four certification types based on the equipment they service.

• Type I: small appliances (≤5 lbs refrigerant, factory sealed)
• Type II: high-pressure/very high-pressure systems (e.g., split AC)
• Type III: low-pressure chillers
• Universal: covers all three types

Memory trick: Small appliance, small number — Type I is for the little guys.

In Manual D duct design, each room's branch airflow is allocated proportionally to its share of the whole-house heat load relative to total system airflow.

• Room CFM = (Room Load / Total Load) × Total System CFM
• Ensures each space receives conditioning matched to its load
• Prevents over- or under-conditioning individual rooms

Memory trick: Give each room its fair share of the total air pie.

Zeotropic refrigerant blends (like R-407C) contain components with different boiling points and must be charged as a liquid to prevent the mixture from separating (fractionating) during charging.

• Zeotropic blends exhibit temperature glide
• Vapor charging removes lighter components first, altering blend ratio
• Near-azeotropic blends like R-410A have minimal glide but manufacturers still recommend liquid charging

Memory trick: Blends stay true only charged in liquid brew.

A manufacturer-provided chart used to determine correct superheat for fixed-orifice systems based on outdoor dry-bulb temperature and indoor return air wet-bulb temperature.

• Only used for fixed-orifice (non-TXV) systems
• Indoor wet-bulb reflects humidity load on the evaporator
• TXV systems use subcooling charging method instead

Memory trick: Outdoor dry, indoor wet — that's the superheat chart you get.

Recovery cylinders must never be filled beyond 80% of their rated weight capacity to allow for liquid thermal expansion and prevent overpressure rupture.

• Applies to all DOT-approved refrigerant recovery cylinders
• Leaves 20% vapor space for expansion
• Overfilling can cause dangerous hydrostatic pressure buildup

Memory trick: Eighty percent, no more — leave room, or it'll roar.

CFM = Btu/hr ÷ (1.08 × ΔT) is used to calculate the airflow needed to deliver a given sensible heating or cooling load.

• 1.08 is the standard air sensible heat constant at sea level
• ΔT is the temperature difference between supply and return air
• Formula is foundational to duct design (Manual D)

Memory trick: 1.08 times Delta T tells you how much air you need to see.

Cal/OSHA requires arc-rated PPE and insulated tools rated for the specific voltage and incident energy when work on energized equipment cannot be de-energized.

• Energized work requires a documented justification/permit
• PPE rating must match calculated incident energy
• Insulated tools prevent direct contact shock
• De-energization is always preferred when feasible

Memory trick: Can't kill the power? Suit up in arc-rated armor.

Positive-pressure, condensing appliance venting requiring corrosion-resistant plastic pipe (PVC/CPVC/polypropylene) per manufacturer's listing.

• Condensate is acidic and will corrode metal vents
• Vent operates under positive pressure, requiring sealed joints
• Common in 90%+ AFUE furnaces

Memory trick: Acidic sweat needs plastic coat — PVC keeps the vent afloat.

A device installed in a closed hydronic loop that removes entrained air bubbles from circulating water to prevent noise, corrosion, and reduced heat transfer.

• Typically installed near the boiler and expansion tank
• Uses reduced velocity or coalescing media to release air
• Air is vented via an automatic air vent at the top

Memory trick: Bubbles out, heat flows better.

A fixed-length, small-diameter tube used to restrict and meter refrigerant flow in smaller or simpler systems, relying purely on pressure drop with no moving parts.

• Common in window units and small refrigeration equipment
• Cannot adjust for varying load — charge must be precise
• Failure mode is typically restriction from debris or moisture

Memory trick: Capillary: thin and fixed, never adjusts, never mixed.

Return grille capacity is found by multiplying the recommended face velocity by the grille's effective free area (gross area × free area percentage), not gross area alone.

• Free Area = Gross Area × Free Area %
• CFM = Velocity × Free Area
• Typical return grille max velocity ~500 fpm for noise control

Memory trick: Only the open holes carry the air — apply the free area factor first.

A closed-system component containing a cushioned air or diaphragm chamber that absorbs the volume increase of heated water, preventing dangerous pressure rise.

• Water expands roughly 4% in volume when heated from 40°F to 200°F
• Diaphragm-type tanks separate air charge from system water
• Undersized tanks cause relief valve weeping or nuisance pressure trips

Memory trick: Hot water swells, the tank absorbs and quells.

The TXV remote sensing bulb must be mounted on the suction line at a position that avoids oil pooling and vapor stratification for accurate superheat control.

• Small suction lines (under ~7/8 in): mount at 12 o'clock
• Large suction lines: mount at 4 or 8 o'clock to avoid oil (bottom) and stratification (top)
• Bulb must be insulated and have good thermal contact with the line

Memory trick: Big pipe? Go to the side (4 or 8), not top or bottom

Overload protection devices for continuous-duty motors without integral thermal protection are typically sized at 125% of the motor's full-load amperage.

• Formula: Overload setting = FLA × 1.25
• Prevents motor damage from sustained overcurrent
• Different from short-circuit/branch-circuit protection sizing
• Motors with built-in protection may use different sizing rules

Memory trick: Quarter more than full load — that's the overload code.

Manual volume dampers in branch ducts allow technicians to redistribute airflow among rooms without changing total system CFM.

• Balancing dampers are adjusted after verifying total system airflow is correct.
• Over-restricting one branch increases flow to others.
• Balancing improves comfort without altering blower speed.

Memory trick: Total is fine, dampers refine.

A thermocouple produces a small millivolt current when heated by the pilot flame, holding the gas valve's safety solenoid open to allow gas flow.

• Generates ~20-30 millivolts when properly heated
• Loss of flame breaks the circuit and shuts gas valve
• Common failure point in standing-pilot furnaces

Memory trick: No heat on the tip, no juice, no gas.

Vented under-floor crawlspaces require net free ventilation area typically equal to 1/150 of the floor area, reduced to 1/1,500 when an approved ground vapor retarder is installed.

• Standard ratio: 1/150 of crawlspace floor area
• Reduced ratio (1/1,500) allowed with Class I vapor retarder
• Ventilation controls moisture buildup and prevents wood decay

Memory trick: Divide by one-fifty to keep the crawlspace dry and breezy.

NC ratings quantify acceptable background noise levels in occupied spaces; duct-generated noise is reduced primarily by lowering air velocity and using sound-attenuating duct linings or silencers.

• Lower velocity reduces turbulence noise
• Duct lining absorbs sound before it reaches the space
• Elbows and fittings close to outlets increase noise generation

Memory trick: Slow the air, line the walls — quiet halls.

Commercial kitchen grease exhaust ducts require a minimum 18-inch clearance to combustible construction unless enclosed in a rated shaft or a listed reduced-clearance enclosure system per NFPA 96 and the CMC.

• Standard clearance is 18 inches for unenclosed ducts
• Listed enclosure systems can reduce required clearance
• Grease ducts pose fire risk from accumulated grease residue

Memory trick: Eighteen inches keeps the grease fire from the frame.

Installing cooling equipment larger than the Manual J load causes short cycling, reduced dehumidification, increased wear, and higher energy costs.

• Short cycles satisfy thermostat before full dehumidification
• Increases compressor wear from frequent starts
• Manual S limits oversizing to roughly 15% above calculated load

Memory trick: Bigger isn't better — it just cycles and shivers wetter.

The minimum outdoor air damper setting needed to meet ventilation code requirements, calculated as required outdoor air CFM divided by total system supply CFM.

• OA% = required OA CFM / total supply CFM
• Damper must maintain at least this minimum position during occupied hours
• Economizers can exceed minimum OA% for free cooling but not go below it

Memory trick: Divide the fresh need by the total feed to find the damper's minimum speed.

A restricted filter drier causes a pressure drop that lowers refrigerant saturation temperature downstream, producing a measurable temperature drop (often with frost) across the component.

• Restriction causes pressure drop = temperature drop across component
• Frost may form at/after the restriction
• Confirmed by comparing inlet vs outlet temperatures with a thermometer

Memory trick: A cold spot after the drier means it's clogged.

A megohmmeter (megger) measures insulation resistance between motor windings and ground; readings below 1 megohm typically indicate insulation breakdown and unsafe operating conditions.

• Minimum acceptable: usually 1-2 megohms depending on motor size/voltage
• Low readings indicate moisture intrusion or winding insulation failure
• Test performed with motor de-energized and disconnected

Memory trick: Low megohms = leaky insulation = 'mega' problem.

VD on a mechanical blueprint marks a manually operated damper placed in ductwork to balance airflow between branches during system commissioning.

• VD = Volume Damper for balancing
• FD = Fire Damper at rated assemblies
• BD = Backdraft Damper preventing reverse airflow

Memory trick: VD Very Delicately balances the air.

A condition where a TXV repeatedly overfeeds and underfeeds the evaporator, causing superheat and suction pressure to oscillate, usually due to an oversized valve or poor bulb installation.

• Oversized TXV reacts too aggressively to small temperature changes
• Bulb must be properly clamped and insulated on the suction line for accurate sensing
• Correct bulb placement and properly sized valve prevent hunting

Memory trick: An oversized valve overreacts like a hunter chasing a moving target.

California Electrical Code (per NEC 210.8) requires GFCI protection on 125-volt, 15- and 20-amp receptacles installed outdoors or on rooftops, including those provided for HVAC equipment servicing.

• Applies to outdoor, rooftop, and damp/wet location receptacles
• Protects against ground-fault shock hazard, not overcurrent
• AFCI addresses arc faults, a different hazard type

Memory trick: Outdoors and wet = GFCI must protect the tech.

Oxygen is never used for pressure testing refrigeration systems due to explosion risk when combined with compressor oil under pressure.

• Only dry nitrogen (sometimes with trace refrigerant) is approved for pressure testing
• Oxygen + oil under pressure = explosion hazard
• OSHA and industry standards prohibit oxygen use in this application

Memory trick: Oxygen feeds fire — never let it near your refrigeration wire.

A ratio comparing a heat pump's heating output to its electrical energy input, both expressed in the same units, indicating heating efficiency.

• Formula: COP = heat output (Btu/h) ÷ (watts × 3.412)
• Typical residential heat pump COP ranges from 2.0 to 4.0
• Higher COP means more efficient heat delivery per unit of electricity

Memory trick: Watts times 3.412 gives Btu; divide output by that.