CSLB HVAC (C-20) Trade — All Questions

A homeowner asks whether a higher AFUE furnace will lower their bill in a mild coastal climate. What is the honest technical answer?

• a.AFUE only matters when the furnace is oversized for the load
• b.The savings depend on how many heating hours the house actually runs✓
• c.AFUE applies to the blower's electrical use rather than the fuel
• d.AFUE gains apply equally in every climate because efficiency is fixed

AFUE is a percentage of fuel converted to useful heat, so the dollar savings scale with how much fuel is burned across the season. The percentage is fixed, but the size of the savings is not, because it depends on run hours. Oversizing hurts efficiency in practice but does not determine whether AFUE matters. And AFUE addresses fuel conversion, while blower electricity is accounted separately.

A two-stage furnace never leaves low fire even on the coldest days. What should be checked?

• a.The staging control wiring and the second-stage thermostat call✓
• b.The condensate neutralizer for saturation
• c.The vent termination clearance from the wall
• d.The flame sensor microamp reading

Second-stage operation depends on a thermostat that can call for it and control wiring that carries the call, so those are the first things to verify. A saturated neutralizer affects drainage, not staging. The flame sensor proves flame at either stage and does not select one. And vent termination clearance is an installation dimension unrelated to staging logic.

An air conditioner short cycles, running four minutes and stopping for three all afternoon. Which cause fits best?

• a.The condensate trap has lost its water seal
• b.The blower motor is failing intermittently on its start winding
• c.The equipment is significantly oversized for the load✓
• d.The supply registers are open too far

Rapid on-off cycling under a steady load is the classic behavior of equipment with far more capacity than the house needs. A failing blower would produce airflow and overload symptoms rather than clean short cycles. A dry trap causes drainage and air-leakage problems, not cycling. And registers that are open too far would extend run time rather than shorten it.

A thermostat's cooling setpoint is satisfied but the indoor blower keeps running continuously. What is the first thing to check?

• a.Whether the compressor contactor has welded itself closed
• b.Whether the evaporator coil is frozen solid
• c.Whether the fan switch is set to ON rather than AUTO✓
• d.Whether the outdoor unit's capacitor has failed

A fan switch left in the ON position runs the blower continuously by design, and it is the simplest explanation to rule out first. A welded contactor would keep the compressor running, not just the blower. A frozen coil affects capacity and can extend run time but does not command continuous fan operation. And an outdoor capacitor failure affects the condenser, not the indoor blower.

You measure 197 volts at a condenser rated 208/230 volts while the compressor is trying to start. What is the concern?

• a.Low voltage will make the compressor run cooler and last longer
• b.Low voltage has no effect because the compressor is single phase
• c.Low voltage raises current draw and can overheat the windings✓
• d.Low voltage only matters on three-phase equipment

A motor delivering the same mechanical work at reduced voltage draws more current, and that extra current heats the windings. Low voltage does not make a motor run cooler. Single-phase compressors are just as vulnerable as three-phase, arguably more so at starting. And the effect is a property of induction motors generally, not of phase count.

A technician measures 240 volts across an open contactor's line terminals with the unit off and 0 volts across the contacts when it is pulled in. What does that indicate?

• a.The line voltage supply is unstable
• b.The contactor coil is open
• c.The contacts are burned and adding resistance
• d.The contactor is operating normally✓

Full voltage across an open contactor and no measurable drop across the closed contacts is exactly what a healthy contactor does. An open coil would prevent it from pulling in at all. Burned contacts would show a measurable voltage drop while closed. And these readings show a stable supply rather than an unstable one.

A technician performs annual maintenance on a residential split system. Which task most directly protects capacity on hot days?

• a.Replacing the thermostat batteries
• b.Tightening the electrical lugs at the disconnect
• c.Cleaning the outdoor coil and clearing the surrounding area✓
• d.Lubricating the blower motor's sealed bearings

A clean condenser with unobstructed airflow keeps condensing temperature and head pressure down, which is exactly what protects capacity when it is hottest. Tight electrical connections prevent failures but do not improve heat rejection. Sealed bearings are not field lubricated. And fresh thermostat batteries prevent nuisance outages but add no capacity.

During maintenance you find the outdoor coil packed with cottonwood. What is the correct cleaning approach?

• a.Brush the fins vigorously across their face with a wire brush
• b.Rinse from the inside out with low-pressure water after disconnecting power✓
• c.Spray a caustic cleaner and leave it on the coil to dry
• d.Blast the outside face of the coil with a pressure washer set at full pressure

Rinsing from the inside outward pushes debris back the way it came, and low pressure with the power off protects the fins and the technician. High-pressure washing folds fins flat and drives debris deeper. A wire brush across the face bends and tears the aluminum. And leaving a caustic cleaner to dry on the coil corrodes fins and tubing.

A customer replaced a 1-inch pleated filter with a high-MERV version and now complains the house is not cooling. What is the likely mechanism?

• a.Higher MERV filters remove humidity from the return air
• b.The added pressure drop cut airflow, reducing capacity✓
• c.The filter media has changed the refrigerant charge requirement
• d.The denser filter causes the compressor to short cycle on high pressure

A dense filter in a slot sized for a low-resistance one raises static pressure and drops airflow, which lowers capacity and can freeze the coil. Filter selection has no bearing on how much refrigerant the system holds. Filters capture particles, not water vapor. And the resulting problem shows up as low suction and possible icing rather than a high-pressure trip.

A homeowner asks why closing registers in unused rooms did not lower their bill. What is the correct explanation?

• a.Closed registers cause the compressor to run at reduced capacity
• b.Closing registers has no measurable effect on any other part of the duct system
• c.Closing registers raises system static pressure and can reduce total airflow✓
• d.Closed registers redirect air to the return and shorten run times

Closing outlets raises static pressure, which reduces total airflow and can push a system toward coil icing or high limit trips rather than saving energy. Compressor capacity is set by refrigerant conditions, not by damper position. Air blocked at a register does not usefully return to the equipment. And the effect is measurable, which is exactly why the practice causes problems.

A technician suspects duct leakage is hurting performance in an existing home. What test provides the evidence?

• a.An amp draw reading on the blower motor
• b.A combustion analysis at the furnace flue
• c.A temperature split reading at the coil
• d.A pressurized duct leakage test with a calibrated fan✓

A duct pressurization test measures leakage in CFM at a reference pressure, which is the direct evidence of how much conditioned air is escaping. Temperature split reflects the coil's performance, not the duct's tightness. Combustion analysis addresses the burner. And blower amperage responds to total system resistance without isolating leakage.

An older thermostat has a heat anticipator setting. What does it do?

• a.Adds a small heat source that shuts the burner off slightly early to limit overshoot✓
• b.Runs the blower before the burner to warm the ducts
• c.Prevents the compressor from starting within five minutes of shutdown
• d.Delays the burner start until the room has dropped a full degree

The anticipator warms the bimetal slightly so the contacts open before the room reaches setpoint, letting residual heat finish the job and reducing temperature swing. It does not add a starting delay based on room temperature. Pre-purging air through the ducts is a fan control function. And short-cycle protection for the compressor is a separate timing control.

A smart thermostat is installed on a system with no common wire, and it resets intermittently. What is the standard fix?

• a.Replace the thermostat with one rated for line voltage
• b.Run a common conductor or install a listed adapter at the equipment✓
• c.Increase the transformer's secondary voltage to 30 volts
• d.Move the thermostat to an exterior wall to improve its wireless signal

These thermostats need continuous 24-volt power, and either a dedicated common conductor or a manufacturer's adapter supplies it without power stealing. Transformers are not field adjustable and raising voltage would damage controls. Wall location affects temperature accuracy, not power supply. And residential systems use low-voltage controls, so a line-voltage thermostat is not an option.

A technician needs to confirm actual airflow on an installed system without a flow hood. Which method is most practical?

• a.Measure temperature rise on the furnace and calculate CFM from the output✓
• b.Estimate from the register face velocity multiplied by the grille free area
• c.Compare the amp draw to the blower motor's rated amps
• d.Read the nameplate CFM for the equipment model

With a known firing rate and efficiency, measuring the temperature rise and applying the sensible heat formula gives a reliable airflow figure with tools every technician carries. Register velocity estimates are crude and ignore the effective free area of the grille. Nameplate CFM is a rated value, not what the installed system delivers. And motor amperage varies with static in ways that do not translate directly to CFM.

A rooftop unit's economizer damper is stuck fully open on a 100 F day. What symptom will the building experience?

• a.The supply air will be colder than design
• b.The system will fail to hold setpoint as it conditions hot outdoor air✓
• c.The compressor will short cycle on low pressure
• d.The building will go strongly negative in pressure

An economizer stuck open in high ambient forces the coil to cool outdoor air at 100 F instead of return air, so capacity is consumed and the space drifts above setpoint. Supply air will be warmer, not colder. Suction pressure will be high with that hot air over the coil rather than low. And bringing in extra outdoor air pressurizes the building rather than making it negative.

A commercial rooftop unit trips its high-pressure switch every afternoon. Which cause should be checked first?

• a.An undersized return air filter rack
• b.A failed condensate overflow switch
• c.Condenser airflow, including a dirty coil or recirculated discharge air✓
• d.A shorted low-voltage thermostat cable

High-pressure trips that follow the outdoor temperature point to heat rejection, so a fouled coil, a failed condenser fan, or hot air recirculating from an adjacent unit lead the list. A shorted control cable would cause erratic operation or transformer failure. A restrictive filter rack lowers head pressure by starving the evaporator. And a condensate switch interrupts the call without producing a high-pressure trip.

How should a technician document a diagnosis that requires equipment replacement rather than repair?

• a.By noting only the model and serial number of the failed unit
• b.By recording the recommendation without the supporting measurements
• c.In writing, with the measured readings that support the conclusion✓
• d.Verbally, so the customer is not overwhelmed with technical detail

A written record of the readings, what they mean, and the recommendation protects the customer and the contractor and lets a second opinion be evaluated on the same evidence. A verbal explanation leaves nothing to review later. Nameplate data alone does not establish why the unit failed. And a recommendation without measurements is an opinion the customer cannot verify.

During a service call you find a heat exchanger with a visible crack. What is the appropriate immediate action?

• a.Seal the crack with high-temperature furnace cement and retest
• b.Shut the appliance down, tag it, and inform the customer in writing✓
• c.Reduce the manifold pressure to lower the exchanger temperature
• d.Leave it operating and schedule the replacement for the following week

A cracked heat exchanger can put combustion products into the supply air, so the appliance is taken out of service, tagged, and the condition documented for the customer. Patching a heat exchanger is not an approved repair. Lowering firing rate does not close the crack or stop the leakage path. And leaving it running for another week keeps a carbon monoxide hazard in operation.