CSLB HVAC (C-20) Trade — All Questions

Design airflow for a heat pump in heating mode is generally set how, relative to cooling?

• a.At or near cooling airflow, because the same coil must reject heat indoors✓
• b.Independently, since the same blower cannot serve both modes
• c.Well below cooling airflow to raise supply air temperature
• d.At exactly half of cooling airflow to protect the compressor

A heat pump uses the indoor coil as a condenser, and it needs comparable air mass flow to keep head pressure and capacity in range, so heating airflow is set at or near the cooling value. Cutting airflow to raise supply temperature drives head pressure up and can trip the high-pressure control. Halving the airflow is a furnace-style adjustment that does not protect a heat pump compressor. And one variable-speed or multi-tap blower routinely serves both modes.

Which situation makes an oversized air conditioner most likely to leave a house clammy?

• a.A dry inland climate where the latent load is very small
• b.A well-shaded house with very low solar gain through glass
• c.A coastal climate where a large share of the load is latent✓
• d.A house with very high daytime internal sensible gain from appliances

Oversized equipment satisfies the thermostat on short cycles, and short run times remove little moisture, which is most obvious where the latent load is large. In a dry climate there is little moisture to remove, so the defect stays hidden. Low solar gain reduces sensible load but does not by itself create the clammy complaint. And a high sensible gain lengthens run times, which actually helps dehumidification.

A commercial space is designed for outdoor air ventilation. What two factors normally set the required outdoor air rate?

• a.Floor area and the design number of occupants✓
• b.Ceiling height and lighting power density
• c.Roof insulation R-value and window orientation
• d.Supply air temperature and the number of diffusers

Ventilation standards compute outdoor air as a rate per person plus a rate per unit of floor area, so occupancy and area drive the number. Ceiling height and lighting affect load and dilution volume but are not the rate basis. Supply temperature and diffuser count are distribution choices made after the rate is known. And envelope properties change the thermal load rather than the required fresh air.

A restaurant kitchen has a 3,000 CFM exhaust hood. What must the mechanical design provide?

• a.A return duct sized for 3,000 CFM back to the air handler
• b.A relief damper sized to exhaust an additional 3,000 CFM
• c.An outdoor air economizer locked open during cooking hours
• d.Makeup air roughly equal to the exhausted volume✓

Air removed by a hood must be replaced, or the building goes negative and pulls air backward through vents and doors. A return duct recirculates conditioned air and does not replace what left the building. A second relief path would make the negative pressure worse. And an economizer is a free-cooling control strategy, not a code-recognized substitute for engineered makeup air.

You are designing for a job site at 5,000 feet elevation. What correction does the airflow calculation need?

• a.Use the 4.5 constant in place of 1.08 for all sensible work
• b.No correction, because CFM is a volume measurement
• c.Multiply the required CFM by the ratio of sea-level pressure to local pressure
• d.Reduce the sensible constant because air is less dense at altitude✓

The 1.08 constant assumes sea-level air density, and thinner air carries less heat per cubic foot, so the constant must be reduced at altitude. Ignoring the correction overstates the capacity a given CFM can deliver. There is no single pressure-ratio multiplier applied to the CFM target itself. And 4.5 is the total-heat constant used with enthalpy, which does not substitute for the sensible constant.

A new duct system is being installed in a vented attic in California. Which requirement most directly affects the duct design?

• a.Flexible duct is prohibited anywhere outside conditioned space
• b.Ducts in unconditioned space must meet a minimum insulation R-value✓
• c.All duct joints must be soldered rather than mechanically fastened and taped
• d.Return ducts may not exceed forty feet of developed length

California's energy standards set a minimum duct insulation level for ducts outside conditioned space, which affects sizing because insulation thickness eats into the available cavity. Ducts are sealed with mastic or listed tape, not solder. There is no blanket forty-foot cap on return length. And listed flexible duct is permitted in attics when it is properly supported and insulated.California Energy Code (Title 24, Part 6)

Which approach best reduces duct load on a new California house at the design stage?

• a.Locating the ducts and air handler inside conditioned space✓
• b.Adding a bypass duct to relieve static pressure at low load
• c.Using rigid metal duct instead of insulated flexible duct
• d.Increasing the supply air temperature difference across the coil

Ducts inside the thermal envelope lose heat and leak air into the conditioned space, so both the conduction and leakage penalties nearly disappear. Changing the coil temperature split does not change what the ducts lose on the way to the register. A bypass increases mixing losses rather than reducing duct load. And material choice affects friction and durability, but a metal duct in a hot attic still conducts and leaks.

A homeowner wants a ductless mini-split for a 400 square foot converted garage. What determines the head capacity you select?

• a.The rated airflow of the indoor head at its highest fan speed
• b.The nominal capacity of the outdoor unit divided evenly by the number of heads
• c.A load calculation for that room including the uninsulated slab and door✓
• d.The maximum line set length the manufacturer permits for the model

The head must match the room's own calculated load, and a converted garage has envelope details, a large door, and slab conditions that a general rule would miss. Splitting the outdoor unit's rating evenly ignores what each room actually needs. Fan airflow describes distribution capability, not thermal capacity. And line set length is an installation limit that affects performance derating, not the capacity requirement.

What happens to a fixed-capacity air conditioner's total capacity as outdoor temperature rises from 85 F to 105 F?

• a.Capacity falls only if the indoor wet bulb also rises
• b.Capacity falls because condensing pressure and temperature rise✓
• c.Capacity rises because the compression ratio increases
• d.Capacity stays constant because the compressor displacement is fixed

Hotter outdoor air raises condensing temperature and head pressure, which reduces mass flow and refrigerating effect, so capacity drops just as the building load peaks. A higher compression ratio hurts volumetric efficiency rather than helping capacity. Fixed displacement does not mean fixed capacity, because the density of the vapor entering the cylinder changes. And the drop happens on the outdoor side regardless of what the indoor wet bulb does.

A designer must choose between a single-stage and a two-stage condenser for a house with a wide load swing. What is the main comfort argument for two-stage?

• a.It delivers more total cooling capacity from the same physical cabinet size
• b.It eliminates the need for a variable-speed indoor blower
• c.It removes the need for an accurate Manual J load calculation
• d.It runs longer at low capacity, improving humidity control and evenness✓

Two-stage equipment spends most hours at reduced output, producing long run times that even out temperatures and pull more moisture from the air. It does not increase peak capacity for the same cabinet size. Accurate load calculation matters more with staged equipment, not less, because the low stage must match the part-load condition. And a matched variable-speed blower is usually what makes the low stage work properly.

During design you find the return grille face velocity will be about 800 feet per minute. What is the likely outcome?

• a.Objectionable noise at the grille and elevated return static✓
• b.Improved mixing that reduces temperature stratification in the room
• c.Lower static pressure because the air moves through quickly
• d.Excellent filtration because dust is captured at high velocity

Return grilles are normally kept in the range of a few hundred feet per minute, and 800 FPM produces roar and a large pressure drop. High velocity does not improve filtration; it increases the pressure drop across the media and can drive dust deeper into it. Static pressure rises rather than falls when air is forced through a small free area. And mixing is a supply-side diffuser function, not a return-grille one.

Which room would you expect to need the highest CFM per square foot in a residential design?

• a.An interior hallway with no exterior walls or windows
• b.A west-facing sunroom with large single-pane glazing✓
• c.A carpeted den under a well-insulated attic
• d.A north-facing bedroom on the ground floor

Airflow is distributed in proportion to load, and a sunroom with large west glass carries by far the highest gain per square foot. An interior hallway has almost no envelope load at all. A north-facing ground-floor bedroom sees modest conduction and diffuse light. And a den under deep attic insulation has one of the lowest gains in the house.

A technician measures 20 F temperature split across an evaporator on a 78 F day with 50% indoor humidity. How should that be interpreted?

• a.As evidence of a restricted metering device
• b.As a normal split that still needs airflow and charge verified separately✓
• c.As proof the refrigerant charge is correct
• d.As a sign the system is significantly oversized for the house

A split near 20 F is within the usual range for those conditions, but split alone cannot separate an airflow problem from a charge problem, so superheat, subcooling, and CFM still must be checked. It is not proof of correct charge, since low airflow with low charge can produce a normal-looking split. A restricted metering device would usually show a much larger split with very low suction. And sizing is determined by run time and load calculation, not by split.

What does a psychrometric chart let a designer determine that a thermometer alone cannot?

• a.The friction rate needed to size a duct run
• b.The static pressure the blower must overcome
• c.The moisture content and enthalpy of the air at a given state✓
• d.The saturation temperature of the refrigerant at a given pressure

A psychrometric chart plots dry bulb against humidity and yields wet bulb, dew point, grains of moisture, and enthalpy, which is what the latent and total heat formulas need. Static pressure is a duct measurement read with a manometer. Refrigerant saturation comes from a pressure-temperature chart for the specific refrigerant. And friction rate is calculated from available static and effective length in Manual D.

Air enters a coil at 80 F dry bulb and 67 F wet bulb and leaves at 57 F dry bulb and 56 F wet bulb. What is happening to the air?

• a.It is being heated and humidified at the same time
• b.It is being dehumidified without any temperature change
• c.It is being cooled and dehumidified simultaneously✓
• d.It is being cooled without any moisture removal

Both dry bulb and wet bulb fall, and a falling wet bulb means enthalpy and moisture content have dropped, so the coil is doing sensible and latent work together. Sensible-only cooling would drop dry bulb while moisture content stayed put. Nothing here is being heated, since both readings decrease. And the dry bulb fell 23 degrees, so this is clearly not a constant-temperature process.

In a light commercial rooftop design, why is the return air path through a ceiling plenum a design concern?

• a.Materials exposed in the plenum must meet flame and smoke limits✓
• b.Plenum returns eliminate the need to calculate return static pressure
• c.Plenum returns always require a fire damper at every wall
• d.Plenum returns cannot be used with packaged rooftop equipment

When a ceiling cavity is used as a return plenum, wiring, insulation, and other materials in that space must be rated for flame spread and smoke development. Packaged rooftops are commonly applied with plenum returns. Fire dampers are required where the assembly penetrates a rated barrier, not at every wall. And return static pressure must still be measured and included in the fan's external static.

A load calculation shows a heating load of 48,000 BTU/hr and a cooling load of 18,000 BTU/hr in a mountain California climate zone. What equipment strategy fits best?

• a.Average the two loads and select one system to match the average
• b.Size both the furnace and the condenser to match the heating load for simplicity
• c.Size the furnace to the heating load and the condenser to the cooling load✓
• d.Size the condenser to the heating load and add electric strip heat

A split system has independent heating and cooling components, so each is selected against its own calculated load. Matching the condenser to the 48,000 BTU/hr heating number would put a 4-ton unit on an 18,000 BTU/hr cooling load and cause severe short cycling. Sizing on the heating load and adding strip heat compounds the same oversizing. And averaging two unrelated loads leaves both seasons mismatched.

You are asked to justify why the new system is smaller than the 5-ton unit being replaced. What is the most defensible explanation?

• a.Manufacturers now rate equipment at a lower outdoor temperature
• b.The original unit was sized by rule of thumb before the house was upgraded✓
• c.Smaller equipment is required by the state on every change-out
• d.Newer refrigerants deliver more capacity per nominal ton

Replacement loads routinely come in smaller because the original was guessed at, and windows, insulation, and air sealing have improved since. Refrigerant choice affects operating pressures and efficiency, not the tonnage a house needs. Rating conditions have not been moved to a lower outdoor temperature. And no state rule mandates downsizing; the load calculation is what justifies the selection.

A 100,000 BTU/hr furnace is being installed in a closet that draws combustion air from inside a tight house. What is the fundamental problem?

• a.The closet door must be louvered only on new construction
• b.Closet installations always require a listed vent damper
• c.Interior air is too dry to support a stable flame
• d.The interior volume may not supply enough air for complete combustion✓

Combustion air requirements are based on the volume of the space and the total input of the appliances, and a tight house cannot supply the needed volume without engineered openings to outdoors. Vent dampers are an efficiency device, not a combustion-air solution. Dry air burns fine; the issue is oxygen quantity, not humidity. And louvers or openings sized to the code method are required whenever the space is confined, in new and existing work alike.California Mechanical Code (Title 24, Part 4), Chapter 7

When the two-opening method is used to bring combustion air directly from outdoors, where must the openings be located?

• a.One within 12 inches of the top and one within 12 inches of the bottom✓
• b.One in the enclosure and one in an adjoining bedroom
• c.Both within 12 inches of the floor of the enclosure
• d.Both within 12 inches of the ceiling of the enclosure

The high opening admits or relieves warm air and the low opening supplies denser cool air, so the pair must straddle the enclosure top and bottom. Two high openings leave the lower part of the space without supply. Two low openings give no path for the buoyant air. And a bedroom is specifically not an acceptable source for appliance combustion air.California Mechanical Code (Title 24, Part 4), Chapter 7

You are replacing an 80% furnace with a condensing model on an existing Type B vent that also serves a natural-draft water heater. What must be evaluated?

• a.Whether the water heater alone can still vent safely in the oversized common vent✓
• b.Whether the water heater needs a larger gas supply line
• c.Whether the Type B vent can be reused for the condensing furnace flue
• d.Whether the vent must be relined with single-wall connector pipe

Removing the furnace from a common vent leaves an orphaned water heater in a vent sized for two appliances, which can fail to establish draft and spill flue gases. The Type B vent cannot serve the condensing furnace, whose acidic condensate requires listed plastic venting, so reuse is not the open question. Gas line sizing is unrelated to the venting change. And single-wall connector is not a lining material for a masonry or B-vent system.California Mechanical Code (Title 24, Part 4), Chapter 8

Why must a Category IV condensing furnace be vented with listed plastic pipe rather than Type B vent?

• a.Flue gases condense and the resulting condensate is corrosive✓
• b.Plastic pipe is lighter and easier to route through framing
• c.Type B vent cannot be routed horizontally through an exterior sidewall
• d.Plastic pipe resists the higher flue temperatures produced

A condensing appliance cools flue gas below its dew point, and the acidic liquid that forms would destroy metal vent. Ease of routing is a convenience, not the code reason. Type B vent can run horizontally within listed limits, so that is not the distinction. And the flue temperature of a condensing furnace is low, not high, which is exactly why it condenses.

A single-wall vent connector runs horizontally from a natural-draft furnace to the vertical vent. How should it be installed?

• a.With a downward trap at the appliance to catch soot
• b.Level, with no rise at all, to keep the run as short as possible
• c.Sloping downward toward the appliance to drain condensate
• d.Sloping upward toward the vent at least a quarter inch per foot✓

Buoyant flue gas needs continuous upward slope toward the vent so draft develops and gases do not stall in the connector. A level run offers no rise and invites spillage at the draft hood. Sloping toward the appliance sends flue gas and any moisture back into the equipment. And a trap in a vent connector would obstruct flow and create a condensate and soot pocket.California Mechanical Code (Title 24, Part 4), Chapter 8

A direct-vent sealed-combustion furnace is installed in a garage workshop. Why is this arrangement well suited to that space?

• a.It allows the return air to be taken from the garage itself
• b.It removes the requirement for a service disconnect
• c.It eliminates the need for a condensate drain
• d.It draws combustion air from outdoors, isolating it from garage vapors✓

A sealed-combustion appliance pipes its own air from outdoors, so the burner never breathes garage air that may carry gasoline or solvent vapors. It still condenses in most cases and still needs a drain. An electrical disconnect is required regardless of combustion type. And return air must never be taken from a garage, since that would draw contaminants into the living space.

A gas furnace is being set on the floor of an attached residential garage. Which installation requirement generally applies?

• a.The ignition source must be elevated unless the unit is listed for floor installation✓
• b.The furnace must be converted to propane before garage installation
• c.The furnace must be placed within 3 feet of the vehicle door
• d.The furnace must be enclosed in a fire-rated closet with a locked door

Gasoline vapors are heavier than air and pool near the slab, so appliances that generate a spark or flame are elevated above the floor unless they carry a listing that permits floor mounting. A locked rated closet is not the code remedy and would create service and combustion-air problems. Fuel conversion has nothing to do with vapor ignition risk. And placing the unit near the vehicle door increases physical impact exposure rather than reducing hazard.California Mechanical Code (Title 24, Part 4)

An appliance is installed in a garage where a vehicle can be driven near it. What additional protection is required?

• a.A dedicated smoke detector directly above the unit
• b.A sheet metal shroud installed around the whole burner compartment
• c.A barrier such as bollards to protect it from vehicle impact✓
• d.A secondary drain pan under the entire cabinet

Where an appliance is subject to vehicle damage, physical protection such as pipe bollards or a curb must be provided. A shroud around the burner does nothing to stop a car. A drain pan addresses condensate overflow, a different failure. And smoke detection is an alarm function that does not prevent the impact.California Mechanical Code (Title 24, Part 4)

You are brazing a suction line with nitrogen flowing through the tubing. What flow condition is correct?

• a.Full cylinder pressure with the regulator removed for speed
• b.No flow during heating and a purge afterward to clear smoke
• c.A high-pressure blast to cool the joint while it solidifies
• d.A low, steady trickle just sufficient to displace the air✓

A gentle continuous flow sweeps oxygen out so no copper oxide scale forms inside the tube, without blowing the molten alloy out of the joint. A high-pressure blast chills the joint and prevents the filler from flowing. Purging only after the fact leaves scale already formed inside the line. And running unregulated cylinder pressure into thin-wall tubing risks rupture and injury.

Why does copper oxide scale inside a refrigerant line matter after the system is running?

• a.It causes the compressor windings to short to the shell
• b.It reacts with the refrigerant to raise system operating pressure
• c.It raises the copper's thermal conductivity and overcools the line
• d.It breaks loose and plugs the metering device and filter drier✓

Flakes of scale travel with the refrigerant and lodge in the smallest passages, restricting the metering device and loading the drier. Scale insulates rather than improving conductivity. It is chemically inert in the refrigerant and does not raise pressure by reaction. And a winding short is an electrical failure that loose scale in the refrigerant circuit does not cause.

A new line set has been brazed. What is the correct order of the next two steps?

• a.Pressure test for leaks, then evacuate to a deep vacuum✓
• b.Release the factory charge, then pressure test the joints
• c.Evacuate to a deep vacuum, then pressure test for leaks
• d.Start the system briefly, then evacuate while it runs

Leaks are found with pressure because a vacuum cannot push anything out for a detector to find, so testing precedes evacuation. Evacuating first wastes the pump run if a joint leaks. Releasing refrigerant to test is venting and is prohibited. And running a compressor under vacuum can damage the motor and pull air and moisture into the system.

What gas should be used to pressure test a newly brazed refrigerant line set?

• a.Dry nitrogen through a regulator with a relief valve✓
• b.A small charge of system refrigerant boosted with air
• c.Compressed shop air through an inline dryer
• d.Oxygen from the brazing cart at reduced pressure

Dry nitrogen is inert and moisture free, and a regulator with a relief device keeps test pressure below the weakest component's limit. Shop air carries moisture and oil even through a dryer. Oxygen under pressure with oil residue can ignite violently and must never touch a refrigeration system. And mixing refrigerant with air creates a mixture that cannot be recovered and can become combustible.

Where should the TXV sensing bulb be mounted on a suction line larger than 7/8 inch?

• a.On the side of the horizontal line at about four or eight o'clock✓
• b.On the underside of the line at the six o'clock position
• c.Anywhere on the vertical suction riser above the evaporator coil outlet
• d.On the top of the horizontal line at the twelve o'clock position

On larger lines the bulb goes on the side, clear of the oil that runs along the bottom and clear of the vapor stratification at the top, so it reads a representative temperature. The top position reads vapor that may not represent the true suction condition on a large line. The bottom position reads the oil film and gives a false low reading. And a vertical riser can hold refrigerant and oil unpredictably, so the bulb belongs on a horizontal run.

A split system has the condenser 20 feet above the evaporator. What is the main piping consideration?

• a.The suction line must be reduced one size to raise velocity
• b.Vertical lift adds pressure loss in the liquid line, reducing subcooling✓
• c.The line set must include a receiver at the base of the riser
• d.The liquid line must be insulated to prevent flash gas

Lifting liquid refrigerant costs static head, and that pressure drop consumes subcooling and can produce flash gas at the metering device. Insulation on a liquid line is sometimes used in hot locations but does not counteract static lift. Reducing suction line size raises velocity but also raises pressure drop and hurts capacity. And residential split systems do not normally carry a receiver, so adding one is not the standard remedy.

The evaporator sits above the condenser and the suction riser is 25 feet tall. What piping detail helps oil return?

• a.A properly formed trap at the base of the riser✓
• b.A check valve installed at the midpoint of the riser
• c.Oversizing the suction riser by two pipe sizes
• d.A liquid line trap at the top of the riser

A trap at the bottom of a suction riser collects oil until refrigerant velocity carries it up as a slug. A trap belongs in the vapor line, not the liquid line, where liquid flows continuously anyway. Oversizing the riser lowers velocity and makes oil return worse, not better. And a check valve blocks reverse flow but does nothing to lift oil against gravity.

Which line-set error most commonly causes low suction pressure and poor capacity on a long run?

• a.Using long-radius bends instead of tight elbows
• b.Insulating the suction line with too thick a wall
• c.Running the line set inside conditioned space
• d.Using a suction line one size smaller than the manufacturer specifies✓

An undersized suction line adds pressure drop over the whole run, lowering pressure at the compressor and cutting mass flow and capacity. Thicker insulation reduces heat gain, which helps rather than hurts. Long-radius bends reduce pressure loss compared with tight elbows. And routing inside conditioned space lowers heat gain to the suction line, again a benefit.

A liquid line filter drier is installed backwards on a cooling-only split system. What is the likely result?

• a.Filtered debris can wash off the outlet screen and reach the metering device✓
• b.The drier will not absorb any moisture from the refrigerant
• c.The system will operate normally, because refrigerant flow direction does not matter
• d.The compressor will run backwards on the next start

A drier's screen and filter media are arranged for one direction; reversed, trapped particles are flushed downstream toward the metering device. The desiccant still adsorbs moisture regardless of direction, so that is not the failure. Compressor rotation is set by the motor and the electrical supply, not by drier orientation. And a bidirectional drier exists precisely because ordinary driers are direction sensitive.

An air handler is installed in an attic above finished ceilings. What condensate provision is normally required?

• a.A single primary drain sloped at least one inch per foot
• b.A gravity drain discharging onto the roof surface directly beneath the unit
• c.An auxiliary drain pan with its own drain or a water-level shutoff device✓
• d.A condensate pump discharging into the nearest plumbing vent

Equipment above a finished space needs secondary protection, provided by an auxiliary pan with a separate drain or by a switch that shuts the equipment off when water rises. A pump may be used for the primary drain but does not by itself satisfy the secondary protection requirement. A single primary drain provides no backup when it clogs, and a one-inch-per-foot slope is far steeper than the usual requirement. And discharging condensate onto a roof is not an approved point of disposal.California Mechanical Code (Title 24, Part 4)

Why does a draw-through cooling coil require a trap in its condensate drain?

• a.The trap is required only when the drain runs uphill to a sink
• b.The trap raises the water level so the pan drains faster
• c.The trap prevents refrigerant vapor from escaping through the drain
• d.The drain is under negative pressure, and without a trap air is pulled in instead of water out✓

In a draw-through arrangement the pan sits on the suction side of the blower, so the drain outlet is negative and a water seal is needed for the pan to actually empty. A trap does not increase drainage speed; it enables it. Refrigerant is inside sealed tubing and never reaches the condensate pan. And a drain line should never run uphill under gravity, trapped or not.

A condensate drain line is run with several sags where it crosses the attic. What symptom should you expect?

• a.The blower will draw excessive amperage
• b.The compressor will trip on high head pressure
• c.Water will back up and the safety switch will shut the unit off✓
• d.The coil will freeze on long cooling cycles

Standing water in a sagging line restricts flow and eventually backs up into the pan, tripping the float switch or overflowing. Coil freezing is caused by low airflow or low charge, not by drain routing. Blower amperage responds to static pressure and wheel condition. And head pressure is a refrigerant-side condition unaffected by where the drain sags.

Where may condensate from a residential cooling coil generally be discharged?

• a.Onto the ground directly beneath the outdoor condenser
• b.Into the crawl space provided a vapor barrier is already present
• c.Into a vent pipe serving a plumbing fixture
• d.To an approved location such as an indirect waste receptor✓

Condensate is treated as an indirect waste and must go to an approved receptor or point of disposal that prevents contamination and nuisance. Discharging into a plumbing vent obstructs the vent and violates its function. Random ground discharge under equipment creates a nuisance and can undermine the pad. And crawl-space discharge introduces continuous moisture under the building regardless of a ground cover.California Mechanical Code (Title 24, Part 4)

A furnace in an attic requires an unobstructed passageway to reach it. What else is normally required at the equipment?

• a.A ceiling-mounted smoke detector inside the enclosure
• b.A window or roof hatch within six feet of the appliance
• c.A fire extinguisher mounted within reach of the platform
• d.A level working platform, permanent lighting, and a receptacle outlet✓

Attic equipment must be reachable and serviceable, which means a solid working platform in front of the unit plus a light and a convenience receptacle for service tools. Smoke detection in the attic enclosure is not the servicing requirement. A mounted extinguisher is a good practice but is not what the access provisions call for. And no window or hatch adjacent to the appliance is required by the access rules.

You are setting an outdoor condensing unit on a new concrete pad. How should the pad be prepared?

• a.On loose topsoil so the pad can settle evenly with the surrounding ground
• b.Elevated on wood blocking to isolate it from ground moisture
• c.On compacted, well-drained soil so the unit stays level over time✓
• d.Sloped at least two inches toward the house for drainage

A pad on firm, drained subgrade keeps the cabinet level, which protects compressor oil return and proper condensate and fan operation. Loose topsoil settles unevenly and tips the unit. Sloping toward the house directs runoff at the foundation and tilts the equipment. And untreated wood blocking rots and shifts, leaving the unit unsupported.

A condenser is being installed 6 inches from a solid fence on the coil side. What is the practical consequence?

• a.The unit will short cycle on the low-pressure control
• b.Recirculated hot discharge air raises head pressure and cuts capacity✓
• c.The compressor will lose oil because of restricted suction
• d.The fan motor will overheat from a lack of ventilation air

Blocking airflow to a condenser coil forces the unit to re-ingest its own hot air, driving condensing temperature and head pressure up and capacity down. Fan motors on these units are cooled by the air they move and rarely fail first from tight clearance. Oil return is a piping and velocity issue, not a clearance issue. And low-pressure trips come from loss of charge or airflow on the indoor side.

Where must the disconnecting means for an outdoor air-conditioning condenser be located?

• a.Directly below the unit, within 12 inches of the pad
• b.Within sight of and readily accessible from the equipment✓
• c.Inside the building at the electrical panel serving the circuit
• d.Anywhere on the property provided it is padlocked

A technician working on the unit must be able to see and reach the disconnect, so it is installed within sight and readily accessible. A panel inside the house may be out of sight of the outdoor unit, allowing someone to re-energize it. A remote lockable disconnect alone does not satisfy the within-sight rule for this equipment. And there is no requirement fixing it a foot below the unit, which would also expose it to water and debris.California Electrical Code (Title 24, Part 3)

Low-voltage thermostat wire is being run alongside the line-voltage feeder in the same attic. What practice should be followed?

• a.Twist the two cables together so they support each other across the joists
• b.Run both inside the same flexible conduit for neatness
• c.Splice the low-voltage cable to the equipment ground for shielding
• d.Keep the classes separated and support the low-voltage cable independently✓

Class 2 control wiring is separated from line-voltage conductors and supported on its own so induced noise and insulation damage are avoided. Twisting them together puts the two classes in continuous contact. Sharing a raceway mixes voltage classes in a way the electrical code restricts. And connecting a control conductor to the equipment ground short-circuits the control circuit rather than shielding it.

On a conventional residential low-voltage control system, which terminal energizes the compressor contactor on a cooling call?

• a.W
• b.R
• c.Y✓
• d.G

The Y terminal carries the cooling call from the thermostat to the contactor coil at the condensing unit, which then closes and starts the compressor and condenser fan. W energizes the heat source, whether that is the gas valve circuit on a furnace or the sequencers on electric heat. G energizes the indoor blower independently of any heating or cooling call. And R is the transformer hot side that feeds all of the other calls rather than being a call itself.

A heat pump is installed with electric strip heat. How should the strip heat be controlled to protect efficiency?

• a.Wired directly to the W terminal in place of the compressor
• b.Energized with every heating call so recovery is fast
• c.Staged on only when the heat pump cannot satisfy the call✓
• d.Controlled by the outdoor fan relay so it follows defrost only

Auxiliary heat is a supplement, so controls bring it on only when the compressor falls behind or during defrost. Energizing it on every call wastes the heat pump's efficiency advantage entirely. Wiring it to W in place of the compressor turns an expensive heat pump into an electric furnace. And tying it only to the outdoor fan relay leaves the house without backup on the coldest hours.

Where should a wall thermostat be mounted in a typical residence?

• a.On an interior wall in a frequently used room, away from drafts and sun✓
• b.In the kitchen so cooking heat is detected quickly
• c.Directly above a supply register so it senses delivered air
• d.On an exterior wall near a window so it senses outdoor temperature swings

A thermostat should read average conditions in the living space, which means an interior wall in a used room and clear of sun, drafts, and appliance heat. An exterior wall conducts outdoor temperature into the sensor and gives false readings. A kitchen sees cooking heat that has nothing to do with the rest of the house. And mounting over a register makes the thermostat measure supply air and short cycle the equipment.

A furnace is installed in a downflow configuration on a combustible floor. What is required?

• a.Sheet metal flashing extending 12 inches beyond the cabinet
• b.A listed floor base or subbase approved for that furnace✓
• c.A two-inch air gap between the cabinet and the finished floor
• d.A layer of unfaced fiberglass insulation under the cabinet

Downflow furnaces discharge hot air through the floor, so the manufacturer's listed base or an approved subbase is required to protect the combustible construction. An arbitrary air gap is not a listed assembly. Loose flashing does not carry a listing and does not create the required clearance. And fiberglass under the cabinet is neither listed nor structurally suitable for the equipment.

What does the clearance to combustibles marked on a furnace rating plate represent?

• a.The minimum tested separation between the cabinet and combustible material✓
• b.The recommended distance for airflow around the cabinet
• c.The space needed in front of the unit to service the blower
• d.The distance the vent must be kept from the roof deck

Clearance to combustibles is a listed, tested distance that keeps adjacent wood and other combustible materials below their ignition temperature. Service clearance is a separate dimension driven by access to components. Vent clearances are specified for the vent system, not the cabinet. And it is a minimum safety requirement, not an airflow recommendation the installer may adjust.

Gas piping to a new furnace is being sized. Which set of inputs governs the pipe size?

• a.The total connected input BTU, the developed length, and the gas pressure✓
• b.The number of elbows and the diameter of the existing meter
• c.The furnace efficiency rating and the total number of appliances in the house
• d.The furnace output BTU and the length of the vent run

Gas pipe sizing tables are entered with the demand in BTU input, the developed length to the furthest outlet, and the supply pressure and allowable pressure drop. Output BTU understates the fuel actually drawn, and the vent run is irrelevant to gas sizing. Fittings are usually accounted for in the length allowance rather than counted individually against a meter diameter. And efficiency affects output, not the input the pipe must deliver.California Mechanical Code (Title 24, Part 4), Chapter 13