CSLB HVAC (C-20) Trade — All Questions

A whole-house exhaust fan removes 100 CFM continuously from a tight new home. How should the load calculation treat that air?

• a.As a latent-only load with no sensible component
• b.As irrelevant because mechanical ventilation is outside Manual J
• c.As additional outdoor air that must be conditioned✓
• d.As a reduction in load because stale air is removed

Air exhausted from a house is replaced by outdoor air entering somewhere, and that air arrives at design temperature and humidity, so it becomes load. It cannot reduce the load, because the incoming replacement air is exactly what the equipment must condition. It carries both a temperature difference and a moisture difference, so restricting it to latent is wrong. And deliberate ventilation air is specifically accounted for in a modern load calculation.

A two-story house cools unevenly, with the upstairs several degrees warmer every afternoon. Which design remedy addresses the cause rather than the symptom?

• a.Lower the thermostat setpoint until the upstairs is comfortable
• b.Install a larger condenser to raise total system capacity
• c.Calculate loads floor by floor and redistribute airflow or zone the system✓
• d.Increase the refrigerant charge to lower the supply air temperature upstairs

Upstairs carries more roof and solar gain, so the fix is to match delivered CFM to each floor's actual load, by redistribution or by zoning. Dropping the setpoint overcools the downstairs while the upstairs still lags. Overcharging raises head pressure and can flood the compressor without moving more air upstairs. And a larger condenser adds capacity to a system that is already delivering it to the wrong floor.

What is the main design risk of using a bypass duct with a zoned residential system?

• a.It prevents the zone dampers from ever closing fully
• b.It makes the outdoor unit short cycle on high head pressure
• c.It raises static pressure so high that the blower stalls
• d.It recirculates cold air to the coil and can freeze it during small zone calls✓

A bypass sends already-cooled supply air back to the return, dropping entering air temperature until the coil ices when only one small zone calls. It actually relieves static pressure rather than raising it, which is why it is installed in the first place. It has no mechanical effect on damper travel. And the low-side starvation it creates tends to lower head pressure, not raise it into a high-pressure trip.

Return air paths are being designed for bedrooms that have no dedicated return grille. What is the design objective?

• a.Keep the bedroom at a measurable negative pressure while occupied
• b.Ensure each bedroom receives more return CFM than supply CFM
• c.Limit the pressure the closed door creates between the room and the hall✓
• d.Route the transfer air path through the attic to gain extra effective length

Transfer grilles, jump ducts, or undercuts exist to keep room pressurization small when the door closes so supply air can actually enter. Deliberately returning more than is supplied would depressurize the room and pull in attic or garage air. A measurable negative pressure is the defect being avoided, not the goal. And running the path through an unconditioned attic adds heat gain and leakage to a transfer that should stay inside the envelope.

You measure total external static pressure of 1.1 in. w.c. on a system whose blower is rated for 0.5 in. w.c. What is the first consequence for the design?

• a.The compressor will draw amperage above its rated load amps
• b.The condensate trap will siphon dry on every cycle
• c.The furnace will fail to ignite on a call for heat
• d.Airflow will fall below the design CFM, cutting capacity✓

A blower operating far beyond its rated external static moves much less air than its table value, so delivered capacity and temperature split both suffer. Compressor amperage is set by refrigerant load and voltage, and low airflow typically lowers it rather than raising it above RLA. Ignition depends on the gas train and flame proving, not on duct static. And trap siphoning is caused by blower suction at the drain, a separate and much smaller pressure issue.

Which pair of measurements is used to determine total external static pressure on a residential air handler?

• a.Velocity pressure at the register and velocity pressure at the grille
• b.Supply plenum pressure and outdoor barometric pressure
• c.Filter pressure drop and coil pressure drop added together
• d.Supply plenum pressure and return pressure at the equipment✓

Total external static is the pressure the blower works against outside the cabinet, read as supply-side positive plus return-side negative at the equipment. Barometric pressure is an ambient reading and is not part of a duct measurement. Filter and coil drops are internal component losses that many manufacturers exclude from external static. And velocity pressure measures air speed, which is not the static resistance the fan table uses.

A designer wants to reduce the friction loss in a long branch run without changing the duct diameter. What is the most effective option?

• a.Wrap the duct with a higher R-value insulation blanket
• b.Replace sharp fittings with long-radius elbows and smooth boots✓
• c.Add a manual balancing damper near the takeoff
• d.Increase the blower speed to push more air through the branch duct

Fittings contribute most of the effective length in a branch, so long-radius elbows and better boots cut pressure loss substantially without resizing. Insulation changes heat transfer, not friction. A balancing damper deliberately adds resistance to reduce flow, which is the opposite of the goal. And more blower speed does not lower friction; it raises pressure loss further while burning more fan power.

Flexible duct is installed with visible sag and slack between supports. How does this affect the Manual D design?

• a.Effective friction rises well above the tables, so delivered CFM drops✓
• b.It matters only on return ducts, since supply ducts are pressurized anyway
• c.Capacity improves because the longer path increases heat exchange
• d.Only noise increases, because friction depends on diameter alone

Manual D friction values for flex assume the duct is pulled taut, and sagging inner liner adds turbulence that can cut delivered airflow sharply. Friction depends on interior surface condition and path shape, not diameter alone. Extra path length in an unconditioned space adds losses rather than useful heat exchange. And the friction penalty applies equally to return ducts, which are just as sensitive to added resistance.

Which measurement pair lets a technician determine the latent capacity a coil is actually producing?

• a.Suction pressure and suction line temperature
• b.Entering and leaving wet-bulb temperature✓
• c.Supply plenum static and return plenum static
• d.Entering and leaving dry-bulb temperature

Wet-bulb readings translate to enthalpy, and the drop in enthalpy beyond the sensible portion is the latent work the coil is doing. Dry-bulb alone captures only sensible heat and says nothing about moisture removed. Suction pressure and line temperature give superheat, a refrigerant-side charge check. And static pressures describe duct resistance, not the moisture content of the air.

A house has a design cooling load of 22,000 BTU/hr. Available equipment comes in 1.5, 2, 2.5, and 3 ton sizes. Which selection best follows Manual S?

• a.1.5 ton, because smaller equipment runs longer and dehumidifies much better
• b.2 ton, verified against the expanded performance data at design conditions✓
• c.2.5 ton, to leave headroom for hotter than design summers
• d.3 ton, because the next size up protects against future additions

A 2-ton unit sits closest to the 22,000 BTU/hr load, and Manual S then confirms its sensible and latent split at the actual design conditions. A 1.5-ton unit is roughly 4,000 BTU short and will not hold setpoint on a design day. A 2.5-ton unit exceeds the Manual S oversizing allowance and short cycles. And sizing for a hypothetical future addition penalizes comfort and efficiency for every year until that addition exists.

Two houses have the same floor area, but one has 12 air changes per hour at 50 pascals and the other has 3. What does that tell you about their loads?

• a.The loads are equal because area drives infiltration
• b.The tighter house has the higher heating load
• c.The difference shows up only in the summer cooling load
• d.The leakier house has the higher infiltration load in both seasons✓

A blower-door result of 12 ACH50 means far more air exchange than 3 ACH50, and that unconditioned air is load in both winter and summer. Tightness reduces load, so the tighter house cannot be the higher one. Floor area is only one input; leakage area is what drives infiltration. And infiltration is driven by temperature and wind differences year round, not summer only.

Why does Manual J use a cooling load temperature difference rather than the plain indoor-outdoor difference for roofs and walls?

• a.To convert the BTU per hour result into tons of refrigeration directly
• b.To correct the calculation for elevation above sea level
• c.To include the latent gain absorbed by the roof sheathing
• d.To account for solar absorption and the mass delay of the assembly✓

Sunlit opaque surfaces run hotter than the air and release that heat on a lag governed by mass, so an adjusted temperature difference represents the real driving force. Converting to tons is a simple division at the end and needs no such adjustment. Elevation corrections apply to air density in the airflow constants, not to wall conduction. And roof sheathing does not add moisture to the space, so no latent term is involved.

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.