CSLB C-20 HVAC Contractor flashcards

A standardized scale (1–16 typically for residential) rating a filter's efficiency at capturing particles of various sizes.

• Higher MERV = smaller particles captured
• Higher MERV usually = higher pressure drop
• Residential systems commonly use MERV 8–13

Memory trick: More MERV, More capture, More resistance.

AHRI industry convention assigns specific cylinder colors to common refrigerants for quick identification, though labels must always be verified before use.

• R-410A = rose/pink
• R-22 = light green
• R-134a = light blue
• Color is a convention, not a legal guarantee — always check the label

Memory trick: Pink cylinder, 410A inside.

SRE indicates the percentage of the sensible temperature difference between outgoing exhaust air and incoming fresh air that the heat exchanger core recovers and transfers to the incoming airstream.

• Supply Temp = Outdoor Temp + (SRE × ΔT)
• HRV recovers sensible heat only; ERV recovers sensible + latent (moisture)
• Typical residential HRV/ERV SRE ranges 60–80%

Memory trick: 'Recovery steals back some of the temperature difference.'

An alternate method allowing one opening near the ceiling to supply combustion air, sized at 1 sq in per 3,000 BTU/hr, provided the space has adequate infiltration or an additional outdoor air connection.

• Opening must be within 12 inches of ceiling
• Sizing factor: 1 sq in per 3,000 BTU/hr
• Two-opening method uses 1 sq in per 4,000 BTU/hr (outdoor communicating) per opening

Memory trick: 'Single opening near the ceiling: three-thousand per inch is the dealing.'

When ducts run through unconditioned spaces, delivered capacity is reduced by heat gain/loss through duct walls; equipment must be sized larger to compensate, calculated as required load ÷ (1 − loss fraction).

• Attic ducts commonly lose 10-25% of capacity without proper insulation
• Formula: Equipment Output = Delivered Load ÷ (1 − loss %)
• Properly insulating/sealing ducts reduces this compensation need

Memory trick: What the attic steals, the equipment must reveal — divide by what remains.

Devices such as fan cycling switches or flooded condenser control valves maintain adequate head pressure during cold-weather operation by limiting condenser heat rejection.

• Low ambient reduces head pressure, starving the TXV
• Fan cycling switches turn off condenser fan at low pressure
• Flooded condenser valves back up liquid to reduce effective coil surface

Memory trick: Cold outside means the fan needs to slow down.

When exhaust airflow exceeds available makeup air, the home depressurizes, which can reverse the natural draft of atmospheric combustion appliances and pull combustion gases indoors.

• Exhaust > makeup air = negative house pressure
• Naturally vented water heaters/furnaces are most vulnerable
• Backdrafting can introduce carbon monoxide into living space

Memory trick: Suck out more than you let in, and the water heater's draft caves in.

A statistically derived temperature (99% heating / 1% cooling) used in Manual J instead of record extremes to properly size equipment.

• Based on ASHRAE climate data
• Prevents oversizing from rare extreme weather
• Different values for heating vs cooling

Memory trick: Design for the usual heat wave, not the once-a-century scorcher.

Title 24 requires new residential duct systems to be tested for leakage using a CFM25 test, with total leakage typically limited to 6% of total system airflow.

• Percent leakage = (leakage CFM ÷ total system CFM) × 100
• CFM25 test pressurizes ducts to 25 Pascals to measure leakage
• Excess leakage wastes energy and requires sealing before passing inspection

Memory trick: Divide leakage by total airflow, then check against the 6% ceiling.

SHR is the fraction of total heat load that is sensible (temperature) versus latent (humidity). Multiplying total load by SHR isolates the sensible load for CFM calculations.

• SHR = sensible load ÷ total load
• Sensible CFM formula uses only the sensible portion
• Typical residential SHR ranges from 0.70 to 0.80

Memory trick: Split the load first, then send it through the sensible formula door.

Flexible duct must be fully extended during installation because compression increases internal corrugation exposure, raising friction loss and reducing system airflow.

• Compressed flex duct increases friction rate significantly
• Manual D friction charts assume fully stretched duct
• Sagging or kinks compound the airflow reduction

Memory trick: Squished flex duct chokes the airflow.

A method of isolating refrigerant charge in the outdoor unit by closing the liquid line valve and running the compressor until suction pressure drops, then closing the suction valve.

• Requires service valves on both liquid and suction lines
• Avoids need for external recovery equipment
• Prevents atmospheric release during minor repairs

Memory trick: Pump it down, lock it in the condenser.

Gaseous indoor pollutants such as formaldehyde cannot be captured by particulate air filters; the most effective mitigation is increasing outdoor air ventilation to dilute contaminant concentrations.

• Filters (even high MERV) do not remove gases/VOCs
• Activated carbon media, not MERV filters, target gaseous pollutants
• Increased outdoor air ventilation is a primary dilution strategy per IAQ guidelines

Memory trick: 'Gases slip past filters — dilute them with fresh air.'

Polyol ester (POE) oil is the required synthetic lubricant for R-410A systems due to its miscibility with HFC refrigerants.

• Mineral oil is incompatible with R-410A
• POE oil is hygroscopic and absorbs moisture quickly
• Keep POE oil containers sealed to prevent moisture contamination

Memory trick: R-410A needs POE, not the old MO.

A2L refrigerants like R-32 are classified as lower toxicity (A) and lower flammability (2L) under ASHRAE Standard 34, requiring special handling precautions beyond nonflammable A1 refrigerants.

• Letter = toxicity (A=lower, B=higher)
• Number = flammability (1=none, 2L=lower, 2=moderate, 3=high)
• A2L refrigerants require ignition-source precautions during handling and installation

Memory trick: A is safe from poison, number tells you about fire

The recommended maximum face velocity for residential return grilles is about 500 FPM to control noise.

• Higher velocity = more noise at the grille.
• Grille free area, not duct area, determines face velocity.
• Undersized grilles are a common cause of noise complaints.

Memory trick: Five hundred keeps it quiet.

When TESP significantly exceeds a blower's rated ESP, first check for the most common cause: a dirty or undersized air filter restricting airflow.

• Dirty filters are the #1 cause of high static pressure
• High static pressure reduces airflow and can overheat equipment
• Always troubleshoot cheapest, most common causes first

Memory trick: Filter first before you tear the ductwork apart.

Infiltration airflow is estimated from the building volume and air changes per hour (ACH), converted to CFM using CFM = (Volume × ACH) ÷ 60.

• ACH represents how many times the house air volume is replaced per hour
• Divide by 60 to convert hourly volume exchange to CFM
• Used as input to sensible/latent infiltration load formulas

Memory trick: Volume times ACH, then divide by the hour's minutes (60).

ACCA methodology for matching HVAC equipment capacity (sensible and total) to the Manual J load within specified tolerances, rather than simply rounding nominal tonnage.

• Total capacity typically 90%-130% of design load
• Sensible capacity should closely match sensible load
• Prevents oversizing and poor humidity control

Memory trick: Match the sensible split, not just the ton.

Mechanical code requirement for bathroom exhaust when no operable window exists: 50 CFM intermittent or 20 CFM continuous.

• Based on ASHRAE 62.2 residential ventilation standard
• Continuous fans run at lower CFM than intermittent
• Must exhaust directly to outdoors, not into attic

Memory trick: Fifty if it stops, twenty if it never drops.

The distance air travels from a diffuser before its velocity decays to a specified terminal velocity, primarily controlled by discharge velocity from CFM and effective outlet area.

• Throw increases with higher discharge velocity
• Discharge velocity = CFM / effective free area
• Manufacturer throw/terminal velocity charts guide diffuser selection

Memory trick: Faster jet, farther yet — velocity drives the throw.

A timed/temperature-based control sequence that reverses the heat pump to cooling mode briefly to melt outdoor coil frost, while auxiliary heat warms supply air.

• Uses a defrost sensor (coil temp) and timer (compressor run time)
• Reversing valve energizes to send hot gas to outdoor coil
• Auxiliary/emergency heat often engages to offset cool supply air during defrost

Memory trick: Time plus temp equals thaw — the board decides the defrost law.

NEC 440.12 requires a motor-compressor disconnecting means to be rated at least 115% of the motor's nameplate rated-load current (RLA), rounded up to the nearest standard size.

• Formula: Minimum disconnect rating = RLA × 1.15
• Standard disconnect sizes: 30, 60, 100A, etc.
• Applies to horsepower-rated or ampere-rated disconnect switches

Memory trick: 115% keeps the switch safely ahead of the load.

Bypass dampers relieve excess static pressure in zoned systems when small zones are active, but an oversized bypass recirculates too much air, starving coil airflow and causing overly cold supply air.

• Bypass duct routes excess supply air back to return
• Oversized bypass reduces net coil airflow
• Symptoms: cold discharge air, turbulent noise, possible coil icing risk

Memory trick: 'Too much bypass steals air from the coil.'

The static pressure remaining for the duct system after subtracting all equipment component pressure drops from the blower's rated external static pressure.

• ESP = rated blower capability at design airflow
• Subtract coil, filter, and other component drops
• Remaining value budgets supply + return duct friction losses

Memory trick: Start with total ESP, then peel away every drop in the way.

A time-delay relay that prevents a compressor from restarting for several minutes after shutdown, protecting it from damaging inrush current and pressure imbalance.

• Typical delay: 3-5 minutes after any shutdown
• Prevents motor winding damage from rapid on/off cycling
• Distinct from crankcase heaters and hard start kits, which solve different problems

Memory trick: Short cycling? Slow it down with a Timer

A blueprint scale converts a measured distance on paper into an actual field dimension using a fixed ratio.

• 1/4"=1' means each 1/4 inch equals 1 actual foot
• Divide drawing measurement by scale factor to get real length
• Architect's scale rulers simplify this conversion

Memory trick: Quarter inch, whole foot — measure small, build big.

The differential is the temperature swing between on/off points; the heat anticipator adjusts cycle timing by simulating current draw to prevent overshoot or short cycling.

• Anticipator should match actual circuit amperage
• Too low anticipator setting causes short cycling
• Too high anticipator setting causes long cycling
• Narrower differential means tighter temperature control but more cycles

Memory trick: Match the anticipator to the amp draw, or cycling goes haywire.

Compressor discharge temperature should generally stay below about 225°F for R-410A scroll compressors; excessive temperatures indicate faults and risk oil breakdown.

• High discharge temp causes oil carbonization and acid formation
• Common causes: low charge, dirty condenser, failing valves
• Measured with a clamp-on thermocouple on the discharge line

Memory trick: Too hot at discharge cooks the oil.

The available static pressure divided by total effective duct length (expressed per 100 ft), used with duct sizing charts to select proper duct diameters.

• Formula: (Available Static ÷ Effective Length) × 100
• Available static = Total ESP − fitting/coil/filter losses
• Lower friction rate generally requires larger ducts

Memory trick: Leftover pressure divided by length, times 100, gives your strength.

The CMC sets a default maximum clothes dryer exhaust duct length of 14 feet with two 90-degree elbows, with length reductions for additional elbows unless otherwise justified.

• Default max is 14 feet with two 90° elbows
• Additional elbows reduce allowable length
• Manufacturer instructions or engineering calculations can permit longer runs

Memory trick: Fourteen feet, two turns — that's the dryer duct's default rule.

Headache, dizziness, and nausea that improve when leaving a building are classic signs of carbon monoxide exposure from a malfunctioning combustion appliance.

• Symptoms improve away from the source
• Cracked heat exchangers are a common cause
• CO is colorless and odorless
• Always test combustion appliances after symptom reports

Memory trick: Symptoms fade when you leave? Blame the flame (combustion).

A formula-based standard requiring continuous mechanical ventilation calculated from floor area and number of bedrooms to maintain indoor air quality.

• Formula: CFM = 0.03×floor area + 7.5×(bedrooms+1)
• Applies to whole-house continuous ventilation systems
• Adopted by reference in Title 24 residential IAQ requirements

Memory trick: Point-oh-three times area, plus seven-point-five per body.

A furnace's usable heating output is found by multiplying its input Btu/hr rating by its AFUE efficiency percentage; this output is compared to the Manual J heating load for proper sizing.

• Output = Input × AFUE
• Output should meet or slightly exceed the design heating load
• Manual S limits acceptable oversizing to avoid short-cycling

Memory trick: Input times efficiency equals what actually warms the house.

A scroll compressor uses two interleaved spiral scrolls—one stationary, one orbiting—to compress refrigerant vapor through continuous rotary motion.

• Fewer moving parts than reciprocating compressors
• Smoother, quieter operation with less vibration
• Common in residential and light commercial split systems
• Tolerant of small amounts of liquid refrigerant slugging

Memory trick: Scroll spirals swirl; reciprocating pistons pump.

'FD' on mechanical drawings marks the required location of a fire damper where ductwork penetrates a fire-rated wall, floor, or ceiling assembly.

• Fire dampers maintain fire-resistance rating of the assembly
• Required by Mechanical Code at rated penetrations
• Must be accessible for inspection and testing after installation

Memory trick: FD stops fire from spreading through the duct.

Ducts in unconditioned spaces like attics must be insulated to at least R-8 under California's Title 24 Energy Code to reduce conduction losses.

• Ducts in conditioned space have lower or no insulation requirement.
• Insulation reduces energy loss through duct walls, not air leakage.
• Duct sealing and insulation are separate but complementary requirements.

Memory trick: Attic ducts wear an R-8 coat.

Subcooling is the difference between the condensing saturation temperature and the actual liquid line temperature, showing how much the liquid has cooled below its boiling point.

• Subcooling = saturation temp (from high-side pressure) − liquid line temp
• Used primarily to charge TXV systems
• Low subcooling suggests undercharge
• High subcooling suggests overcharge or restriction

Memory trick: Sub means below — subcooling is below boiling point on the liquid side.

Superheat is the difference between the actual temperature of refrigerant vapor and its saturation temperature at the measured pressure, indicating how much liquid has fully vaporized.

• Superheat = actual suction line temp − saturation temp at suction pressure
• High superheat often indicates undercharge or restriction
• Low superheat often indicates overcharge
• Measured on fixed-orifice metering device systems for charging

Memory trick: High Superheat = Hungry system (needs more charge).

When combustion air comes from outdoors via two permanent openings, each opening requires 1 sq in of free area per 4,000 BTU/hr of appliance input.

• Openings placed within 12 in. of top and bottom of enclosure.
• Indoor air two-opening method instead uses 1 sq in per 1,000 BTU/hr.
• Confined spaces need adequate combustion, ventilation, and dilution air.

Memory trick: Outdoor air: four-thousand per inch.

Federal law under Section 608 of the Clean Air Act prohibits knowingly venting refrigerants to the atmosphere during service, maintenance, or disposal.

• Applies to all refrigerants, including substitutes like R-410A
• Recovery equipment must be EPA-certified
• Violations carry civil penalties

Memory trick: Never let it fly — recover before you pry.

One ton of air conditioning capacity equals 12,000 Btu/hr, used to convert calculated loads into standard equipment sizes.

• 1 ton = 12,000 Btu/hr
• Manual S matches equipment tonnage to Manual J load
• Oversizing by more than 15% can cause poor humidity control

Memory trick: Twelve thousand Btu makes one ton — remember the golden conversion.

R-410A operates at significantly higher system pressures than R-22, requiring gauges, hoses, and recovery equipment specifically rated for its higher pressure range.

• R-410A pressures run roughly 50-60% higher than R-22 at the same temperature
• Using R-22-rated equipment on R-410A systems risks equipment failure
• Always verify equipment pressure ratings match the refrigerant in use

Memory trick: Match your gauge to the gas — mismatched pressure means glass could pass (burst).

The difference between static pressure entering and leaving the evaporator coil, used to detect airflow restrictions such as dirty coils or clogged filters when compared to manufacturer specifications.

• Pressure drop = entering pressure − leaving pressure
• Compare measured drop to manufacturer's rated maximum
• Excessive drop often indicates dirty coil, filter, or restricted airflow

Memory trick: Subtract the readings, compare to spec — restriction you'll detect.

Supply plenum cross-sectional area is calculated by dividing system airflow (CFM) by the maximum recommended velocity to prevent noise and pressure loss.

• Area (sq ft) = CFM ÷ Velocity (fpm)
• Typical max plenum velocity ~700-900 fpm
• Undersized plenums increase static pressure and noise

Memory trick: Divide the flow by the speed to find the space you need.

A small adjustable resistor inside a mechanical thermostat that must be set to the actual amperage of the heating control circuit to properly time the thermostat's cycling.

• Measured with a clamp-on ammeter in series with the control circuit
• Setting too high causes overshoot; too low causes short cycling
• Not used on electronic/digital thermostats

Memory trick: Match the amp, avoid the cramp (short cycling).

The regulated gas pressure at the burner manifold, typically 3.5 in. W.C. for natural gas and 10-11 in. W.C. for propane.

• Measured with a manometer at the gas valve tap
• Incorrect pressure causes under/overfiring and poor combustion
• Set per manufacturer nameplate specifications

Memory trick: Natural gas: three-point-five; propane: eleven.

Solar heat gain through windows varies by orientation and time of day; west-facing glass typically drives peak afternoon cooling loads.

• West windows peak with afternoon sun
• East windows peak in morning
• South gain is highest in winter, moderate in summer at low sun angle

Memory trick: West is worst for afternoon heat.

The four major components of a basic vapor-compression system operate in a fixed sequence to move heat from indoors to outdoors.

• Compressor raises pressure/temperature of vapor
• Condenser rejects heat and condenses gas to liquid
• Metering device drops pressure before evaporator
• Evaporator absorbs heat and boils liquid to vapor

Memory trick: Compress, Condense, Meter, Evaporate — CCME keeps it straight.

A cracked heat exchanger allows flue gases like CO to enter the supply air; testing supply air CO levels with the blower running is the most reliable confirmation method.

• Blower operation can pull gases through the crack (negative pressure effect)
• Visual inspection alone often misses hairline cracks
• Elevated supply-air CO confirms exchanger leakage
• Furnace should be shut down and tagged if confirmed

Memory trick: Blower on, CO detector out — that's how cracks get found.

The temperature drop between return air and supply air across the evaporator coil, used as a quick diagnostic indicator of system performance.

• Normal split range is about 16–22°F
• Depends on indoor humidity and airflow (CFM per ton)
• Abnormal splits point to airflow, charge, or coil problems

Memory trick: Twenty degrees means 'twenty-twenty' airflow vision — system's fine!

Only UL 181-listed tapes and mastics are approved for sealing ductwork joints; ordinary cloth duct tape is prohibited.

• UL 181B-FX = listed tape for flex duct.
• Mastic + mesh used for larger gaps or seams.
• Cloth duct tape dries out and fails within a few years.

Memory trick: Cloth tape cracks, mastic sticks.

Latent load represents moisture removal capacity and is found by subtracting the sensible load (Total × SHR) from the total cooling load.

• Sensible = Total × SHR
• Latent = Total − Sensible
• High SHR means most load is sensible (dry heat)

Memory trick: SHR tells you the dry share; what's left is the wet share.

An overcharged system shows subcooling above target, superheat below target, and elevated head pressure due to excess liquid refrigerant backing up into the condenser.

• Subcooling = saturation temp − liquid line temp
• Overcharge raises subcooling and head pressure
• Overcharge lowers superheat toward zero

Memory trick: High sub, low super, high head — too much refrigerant instead.

ACH measures how many times the total air volume of a space is replaced in one hour, calculated as (CFM × 60) ÷ room volume.

• Higher ACH means more frequent air replacement.
• Used to evaluate ventilation adequacy and IAQ.
• Formula: ACH = (CFM × 60) / Volume (cu ft).

Memory trick: Sixty minutes times CFM, divided by the room's whole scene.

Manual S requires verifying equipment capacity using manufacturer extended performance tables when the site's design temperature differs from the standard AHRI 95°F rating condition.

• AHRI nameplate ratings are only valid at 95°F outdoor condition
• Capacity generally decreases as outdoor temperature rises above 95°F
• Extended performance tables list capacity across a range of temperatures/CFM

Memory trick: Don't trust the nameplate past its test conditions—check the extended chart.

A failure in the metal barrier separating combustion gases from circulating air, allowing carbon monoxide to enter the home's airstream.

• CO is odorless and colorless, making detection instruments essential
• Testing supply air CO levels is a standard diagnostic step
• A confirmed crack requires furnace shutdown and exchanger replacement

Memory trick: Crack in the exchanger, CO comes to visit — test before you twist (wrenches).

Passive (system-dependent) recovery uses the appliance's own compressor to move refrigerant; active (self-contained) recovery uses a dedicated recovery machine's compressor.

• Passive method fails if the system's compressor is inoperable
• Active recovery machines have independent pumping capability
• Venting to atmosphere is prohibited under EPA Section 608 regardless of compressor condition

Memory trick: No compressor, no passive — go active instead

An electric heating element wrapped around or inside a compressor crankcase that keeps oil warm during the off cycle to prevent refrigerant migration and startup damage.

• Prevents liquid refrigerant from mixing with oil during shutdown
• Reduces risk of oil foaming and compressor slugging on startup
• Should be energized for a period before initial startup or after extended power loss

Memory trick: Warm oil stays put — cold oil lets refrigerant loot.

EER (Energy Efficiency Ratio) is calculated by dividing cooling output in Btu/h by electrical input in watts at a single fixed test condition (95°F outdoor temperature).

• EER = Btu/h output ÷ Watts input
• Measured at one fixed outdoor temperature, unlike seasonal SEER/SEER2
• Higher EER indicates better efficiency at that specific condition
• Often used to evaluate peak-condition performance, important in hot climates

Memory trick: Btu over watts, straight math gets the EER shot.