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ACCA Residential HVAC Design practice questions

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Question 1Residential HVAC System Design (Manual J, S, and D)

In Manual J load calculations, what does the abbreviation 'HTM' represent?

  1. Heating transmission multiplier
  2. Humidified thermal mass
  3. Hourly temperature mean
  4. Heat transfer multiplier Correct answer.

Explanation

HTM stands for heat transfer multiplier, a tabulated value per square foot of building component used with the appropriate temperature difference. It converts the U-factor and design temperature difference into a per-area load. Manual J tables provide HTMs for walls, windows, doors, and ceilings.

Question 2Residential HVAC System Design (Manual J, S, and D)

What design condition does the Manual J cooling load calculation use for outdoor conditions?

  1. The 99% winter dry-bulb with mean wet-bulb
  2. The 1% design dry-bulb with coincident wet-bulb Correct answer.
  3. The 50% average summer dry-bulb
  4. The record highest dry-bulb of the last decade

Explanation

Manual J uses the 1% summer design dry-bulb temperature together with its coincident wet-bulb for sensible and latent cooling loads. This represents a condition exceeded about 1% of summer hours. Winter heating uses the 99% design temperature instead.

Question 3Residential HVAC System Design (Manual J, S, and D)

A room has a sensible load of 9,000 Btu/h and a latent load of 3,000 Btu/h. What is the room's sensible heat ratio (SHR)?

  1. 0.75 Correct answer.
  2. 0.25
  3. 0.33
  4. 1.33

Explanation

SHR is the sensible load divided by the total load. Total load is 9,000 + 3,000 = 12,000 Btu/h, so SHR = 9,000 / 12,000 = 0.75. The SHR indicates how much of the load is sensible versus latent.

Question 4Residential HVAC System Design (Manual J, S, and D)

What is the primary purpose of a Manual D duct design?

  1. To determine the correct filter MERV rating
  2. To select the furnace firing rate
  3. To size ducts so the blower delivers required airflow within available static pressure Correct answer.
  4. To size the refrigeration charge in the outdoor unit

Explanation

Manual D sizes the duct system so the air handler can deliver the required airflow against the available external static pressure. It matches duct friction loss to the blower's capability. Equipment capacity and charge selection are handled by Manuals J and S.

Question 5Residential HVAC System Design (Manual J, S, and D)

Which Manual J input most directly accounts for air leakage into the conditioned envelope?

  1. Window U-factor
  2. Infiltration rate based on envelope tightness Correct answer.
  3. Internal gain from occupants
  4. Duct insulation R-value

Explanation

Infiltration is estimated from envelope tightness (construction type and climate) and expressed as air changes or cfm, adding both sensible and latent load. Window U-factor addresses conduction through glass, not air leakage. Duct insulation and occupant gains are separate load components.

Question 6Residential HVAC System Design (Manual J, S, and D)

In Manual S equipment selection, what does the total cooling capacity of the selected unit need to do relative to the Manual J total load?

  1. Exactly equal the load with no tolerance
  2. Be about half the load to allow reserve
  3. Meet or slightly exceed the load at design conditions Correct answer.
  4. Be at least double the load

Explanation

Manual S requires the equipment's capacity at design conditions to meet or slightly exceed the calculated load. Gross oversizing harms humidity control and efficiency. Capacity must be checked at the actual entering conditions, not just nominal ratings.

Question 7Residential HVAC System Design (Manual J, S, and D)

What does the abbreviation 'CFM' represent in airflow calculations?

  1. Coil face moisture
  2. Condensate flow measure
  3. Cubic feet per minute Correct answer.
  4. Cooling factor multiplier

Explanation

CFM stands for cubic feet per minute, the volumetric airflow rate. Manual D and Manual S use it to express blower delivery and room airflow requirements. It is the standard airflow unit in US HVAC practice.

Question 8Residential HVAC System Design (Manual J, S, and D)

Which factor in Manual J reduces the cooling load contribution of a wall?

  1. A higher outdoor design temperature
  2. A lower wall R-value
  3. Shading from a permanently attached overhang Correct answer.
  4. A darker exterior wall color

Explanation

Permanent shading such as an overhang reduces solar gain through and on the assembly, lowering the cooling load. Higher outdoor temperature and lower R-value increase load. Darker color raises solar absorption.

Question 9Residential HVAC System Design (Manual J, S, and D)

A room requires 400 cfm and is served by a single supply register. Using the typical Manual D practice of selecting a register with a face velocity around 500 fpm, which register face area is closest to appropriate?

  1. 0.8 sq ft Correct answer.
  2. 4.0 sq ft
  3. 0.4 sq ft
  4. 2.0 sq ft

Explanation

Face area equals airflow divided by face velocity: 400 cfm / 500 fpm = 0.8 sq ft. Selecting a register of about this free area keeps velocity and noise within reasonable limits. Manual D guides register selection to balance throw and pressure drop.

Question 10Residential HVAC System Design (Manual J, S, and D)

Friction rate in Manual D is expressed in which units?

  1. Pounds per square inch per foot
  2. Cubic feet per minute per ton
  3. Inches of water column per 100 feet of duct Correct answer.
  4. Btu per hour per square foot

Explanation

Friction rate is the pressure loss per unit length, expressed as inches of water column per 100 feet of effective length. It is used with the duct sizing charts to select duct dimensions. Manual D calculates an available friction rate from blower static pressure and total effective length.

Question 11Residential HVAC System Design (Manual J, S, and D)

A duct run has an available static pressure of 0.30 in. w.c. and a total effective length of 150 ft. What is the available friction rate?

  1. 0.45 in. w.c. per 100 ft
  2. 0.20 in. w.c. per 100 ft Correct answer.
  3. 2.00 in. w.c. per 100 ft
  4. 0.02 in. w.c. per 100 ft

Explanation

Friction rate = (available static pressure x 100) / total effective length = (0.30 x 100) / 150 = 0.20 in. w.c. per 100 ft. This rate is then used with duct sizing tables to pick dimensions. Total effective length includes straight length plus equivalent lengths of fittings.

Question 12Residential HVAC System Design (Manual J, S, and D)

Why does Manual J distinguish between sensible and latent loads?

  1. Because equipment must handle both moisture and temperature to maintain comfort Correct answer.
  2. Because latent load determines duct size only
  3. Because sensible load only applies in winter
  4. Because latent load is ignored in humid climates

Explanation

Sensible load affects dry-bulb temperature while latent load reflects moisture removal. Equipment must have adequate total and sensible capacity to control both temperature and humidity. Manual S compares the equipment's SHR to the load SHR to ensure proper selection.

Question 13Residential HVAC System Design (Manual J, S, and D)

In Manual J, what is the effect of a lower window U-factor on the heating load?

  1. It reduces the heating load Correct answer.
  2. It increases the heating load
  3. It has no effect on heating load
  4. It only affects the cooling load

Explanation

Heating transmission load equals U-factor times area times temperature difference. A lower U-factor (better insulating window) reduces conduction and therefore reduces the heating load. The same principle applies to cooling conduction gains.

Question 14Residential HVAC System Design (Manual J, S, and D)

What is 'total effective length' (TEL) in Manual D?

  1. The total length of all ducts in the system combined
  2. The straight-line length of the longest duct
  3. The length of duct buried in unconditioned space
  4. The sum of straight lengths plus equivalent lengths of all fittings on the critical path Correct answer.

Explanation

TEL is measured along the critical (longest resistance) path and adds equivalent lengths for fittings to straight-run lengths. It is used with available static pressure to compute friction rate. Using the critical path ensures the worst-served terminal receives adequate airflow.

Question 15Residential HVAC System Design (Manual J, S, and D)

An air handler is rated to deliver 1,200 cfm at 0.5 in. w.c. external static pressure. If the duct system's total pressure drop exceeds this available static, what is the likely result?

  1. Higher refrigerant capacity
  2. Increased airflow above design
  3. Reduced airflow below design Correct answer.
  4. No change in airflow

Explanation

A blower delivers airflow according to its fan curve against system resistance. If system static exceeds the available static at design airflow, the operating point shifts and delivered airflow falls below design. Reduced airflow degrades capacity and can cause coil icing or overheating.

Question 16Residential HVAC System Design (Manual J, S, and D)

Which Manual J load component is typically the largest sensible gain in a well-insulated home on a hot afternoon?

  1. Internal gain from occupants
  2. Conduction through exterior walls
  3. Infiltration through the envelope
  4. Solar gain through windows Correct answer.

Explanation

In well-insulated homes, solar gain through glass often dominates the sensible cooling load on a sunny afternoon. Wall conduction is reduced by insulation, and occupant gains are relatively small. Infiltration is controlled by tight construction.

Question 17Residential HVAC System Design (Manual J, S, and D)

In Manual S, equipment sizing must consider the manufacturer's expanded performance data because:

  1. Nominal capacity ratings assume standard conditions that differ from design conditions Correct answer.
  2. Expanded data is only needed for heating equipment
  3. Nominal ratings are always higher than actual capacity
  4. Expanded data removes the need for a Manual J load

Explanation

Nominal (ARI/DOE) ratings are based on standard test conditions that rarely match the design entering air and outdoor temperatures. Expanded performance tables give capacity at actual conditions so the unit truly meets the load. This avoids both undersizing and oversizing.

Question 18Residential HVAC System Design (Manual J, S, and D)

What does a Manual J 'block load' calculation provide compared with a room-by-room calculation?

  1. The total whole-house load without individual room loads Correct answer.
  2. The equipment's refrigerant charge
  3. The duct friction rate
  4. Airflow for each register

Explanation

A block load gives the total building load and is useful for equipment selection, but it does not provide individual room loads. Room-by-room calculations are needed for duct and register sizing. Manual D requires room loads to distribute airflow.

Question 19Residential HVAC System Design (Manual J, S, and D)

A window has an area of 40 sq ft, a U-factor of 0.35, and a design temperature difference of 40°F. What is the conduction heating load through the window?

  1. 1,400 Btu/h
  2. 140 Btu/h
  3. 560 Btu/h Correct answer.
  4. 56 Btu/h

Explanation

Conduction load = U x area x temperature difference = 0.35 x 40 x 40 = 560 Btu/h. This is the transmission portion; solar gain is not part of the winter heating load. The same formula applies to walls and doors.

Question 20Residential HVAC System Design (Manual J, S, and D)

Which condition best justifies increasing the design infiltration rate in a Manual J calculation?

  1. A home with continuous vapor barrier and sealed penetrations
  2. A home with an insulated slab
  3. A home with many unsealed recessed lights and a loose attic hatch Correct answer.
  4. A home with new low-e windows

Explanation

Infiltration estimates rise when the envelope has known leakage paths such as unsealed recessed lights and loose attic access. Sealed penetrations and vapor barriers indicate tighter construction. Windows and slab insulation affect conduction more than infiltration.

Question 21Residential HVAC System Design (Manual J, S, and D)

In Manual D, what is the purpose of balancing dampers?

  1. To increase the blower's total static pressure
  2. To reduce the refrigerant charge
  3. To filter the supply air
  4. To adjust airflow among branches so each room receives its design cfm Correct answer.

Explanation

Balancing dampers throttle individual branches so measured airflow matches design in each room. This compensates for differences in duct resistance across branches. They do not change blower capability or refrigerant charge.

Question 22Residential HVAC System Design (Manual J, S, and D)

A designer computes a room load SHR of 0.85, but the selected equipment has a rated SHR of 0.72 at design conditions. What is the most likely consequence?

  1. The space will be over-dehumidified
  2. The equipment will deliver too much sensible cooling
  3. The equipment cannot operate at all
  4. The space may be too humid because the coil removes excess moisture relative to sensible capacity Correct answer.

Explanation

When the equipment SHR is lower than the load SHR, the coil removes proportionally more latent heat than the space needs, which can overcool the space before humidity is satisfied or, with cycling, leave humidity high. Manual S aims to match equipment SHR to load SHR. A large mismatch signals a selection problem.

Question 23Residential HVAC System Design (Manual J, S, and D)

A duct passes through an unconditioned attic. Which Manual D/J consideration most directly addresses the added load?

  1. Increasing the register face velocity
  2. Adding duct insulation and applying a duct loss multiplier Correct answer.
  3. Reducing the total effective length
  4. Lowering the blower static pressure

Explanation

Ducts in unconditioned space gain or lose heat, so Manual J applies a duct loss multiplier and Manual D specifies duct insulation. This accounts for the temperature difference between duct air and surrounding space. Register velocity, TEL, and blower static are separate design issues.

Question 24Residential HVAC System Design (Manual J, S, and D)

Two duct layouts serve the same room load. Layout A uses many short-radius elbows; Layout B uses gradual fittings with the same straight lengths. How do their friction rates and duct sizes compare at the same available static?

  1. Layout A yields a lower friction rate and requires larger ducts Correct answer.
  2. Both layouts yield identical friction rates
  3. Layout B yields a higher friction rate because gradual fittings add length
  4. Layout A yields a higher friction rate and requires smaller ducts

Explanation

Short-radius elbows have larger equivalent lengths, raising TEL and lowering the available friction rate (static x 100 / TEL). A lower friction rate requires larger ducts for the same airflow. Gradual fittings reduce equivalent length, allowing a higher friction rate and smaller ducts.

Question 25Residential HVAC System Design (Manual J, S, and D)

A homeowner reports that one distant bedroom is always warm in summer while the rest of the house is comfortable. Which Manual D diagnostic step is most appropriate first?

  1. Increase the refrigerant charge
  2. Replace the outdoor unit with a larger model
  3. Measure airflow at the bedroom register against its design cfm Correct answer.
  4. Reduce the filter MERV rating

Explanation

The first step is to verify delivered airflow at the affected register against design, since the symptom points to a distribution problem. Low airflow may result from excessive branch resistance, an undersized duct, or a closed damper. Equipment and charge changes should follow only after airflow is confirmed.

Question 26Residential HVAC System Design (Manual J, S, and D)

What does the abbreviation 'BTU' stand for in the context of HVAC load calculations?

  1. Building Thermal Unit
  2. Balanced Temperature Unit
  3. Base Temperature Unit
  4. British Thermal Unit Correct answer.

Explanation

BTU stands for British Thermal Unit, the standard unit of heat energy in HVAC. It is defined as the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit.

Question 27Residential HVAC System Design (Manual J, S, and D)

In Manual J, what does the abbreviation 'SHR' represent?

  1. Static Heat Resistance
  2. Sensible Heat Ratio Correct answer.
  3. Seasonal Heat Reduction
  4. System Heating Rate

Explanation

SHR stands for Sensible Heat Ratio, the ratio of sensible heat gain to total heat gain. It is used to match equipment latent capacity to the load.

Question 28Residential HVAC System Design (Manual J, S, and D)

Which Manual J table is typically used to determine the heating and cooling load for a window based on its construction and U-factor?

  1. Table 3A
  2. Table 2A
  3. Table 4A Correct answer.
  4. Table 1A

Explanation

Manual J Table 4A provides U-factors for windows and glass doors. It is used to calculate conduction gains and losses through fenestration.

Question 29Residential HVAC System Design (Manual J, S, and D)

What is the primary purpose of Manual D?

  1. To size ducts and select the air distribution system Correct answer.
  2. To calculate heating and cooling loads
  3. To select heating and cooling equipment
  4. To determine indoor air quality requirements

Explanation

Manual D is the ACCA standard for residential duct design, covering duct sizing, layout, and air distribution. It ensures proper airflow and pressure balance.

Question 30Residential HVAC System Design (Manual J, S, and D)

In Manual J, what does the term 'design conditions' refer to?

  1. The manufacturer's rated capacity of the equipment
  2. The outdoor and indoor temperature and humidity values used for load calculations Correct answer.
  3. The total effective length of the duct system
  4. The maximum occupancy expected in the home

Explanation

Design conditions are the specified indoor and outdoor temperature and humidity values used to calculate loads. They represent worst-case weather and desired indoor comfort.

Question 31Residential HVAC System Design (Manual J, S, and D)

Which of the following is a common unit for expressing heating load in Manual J?

  1. Pounds per square inch (psi)
  2. Inches of water column (in. w.c.)
  3. Cubic feet per minute (CFM)
  4. BTU per hour (BTU/h) Correct answer.

Explanation

Heating load is expressed in BTU per hour, representing the rate of heat loss. CFM measures airflow, in. w.c. measures pressure, and psi measures pressure.

Question 32Residential HVAC System Design (Manual J, S, and D)

What does the abbreviation 'ACH' stand for in infiltration calculations?

  1. Air Conditioning Heat
  2. Annual Cooling Hours
  3. Average Comfort Humidity
  4. Air Changes per Hour Correct answer.

Explanation

ACH stands for Air Changes per Hour, a measure of how many times the indoor air volume is replaced by outdoor air in one hour. It is used to estimate infiltration loads.

Question 33Residential HVAC System Design (Manual J, S, and D)

In Manual D, what is the purpose of a duct fitting's equivalent length?

  1. To determine the physical length of the duct run
  2. To calculate the weight of the duct material
  3. To account for the pressure loss of a fitting in terms of straight duct length Correct answer.
  4. To measure the insulation value of the duct

Explanation

Equivalent length converts the pressure drop of a fitting into an equivalent length of straight duct. This allows summing all pressure losses in terms of effective length.

Question 34Residential HVAC System Design (Manual J, S, and D)

A room has a sensible heat gain of 3,000 BTU/h and a latent heat gain of 1,000 BTU/h. What is the room's sensible heat ratio (SHR)?

  1. 0.75 Correct answer.
  2. 0.25
  3. 0.33
  4. 0.67

Explanation

SHR is sensible heat divided by total heat (sensible + latent). Here, 3,000 / (3,000 + 1,000) = 0.75.

Question 35Residential HVAC System Design (Manual J, S, and D)

A wall has an area of 200 sq ft and a U-factor of 0.10 BTU/(h·ft²·°F). If the design temperature difference is 30°F, what is the conduction heat loss?

  1. 200 BTU/h
  2. 6,000 BTU/h
  3. 60 BTU/h
  4. 600 BTU/h Correct answer.

Explanation

Conduction heat loss = U × area × temperature difference. 0.10 × 200 × 30 = 600 BTU/h.

Question 36Residential HVAC System Design (Manual J, S, and D)

In Manual J, which of the following is considered an internal sensible heat gain?

  1. Heat from lighting fixtures Correct answer.
  2. Heat from duct leakage
  3. Heat from solar radiation through windows
  4. Heat from infiltration of outdoor air

Explanation

Internal sensible gains come from sources inside the conditioned space, such as lighting, appliances, and people. Solar, infiltration, and duct leakage are external or system-related gains.

Question 37Residential HVAC System Design (Manual J, S, and D)

A duct system has a total effective length of 200 ft and a design friction rate of 0.10 in. w.c. per 100 ft. What is the total pressure loss due to friction?

  1. 2.00 in. w.c.
  2. 0.10 in. w.c.
  3. 0.20 in. w.c. Correct answer.
  4. 0.02 in. w.c.

Explanation

Pressure loss = friction rate × (total effective length / 100). 0.10 × (200/100) = 0.20 in. w.c.

Question 38Residential HVAC System Design (Manual J, S, and D)

Which of the following best describes the purpose of a load calculation in HVAC design?

  1. To calculate the payback period of a new system
  2. To select the brand of equipment to install
  3. To determine the amount of heating and cooling required to maintain comfort Correct answer.
  4. To determine the cost of equipment installation

Explanation

Load calculations quantify the heat gain and loss of a building to size equipment and ducts properly. They are not used for cost, brand selection, or payback analysis.

Question 39Residential HVAC System Design (Manual J, S, and D)

In Manual S, what is the maximum oversizing limit for heating equipment relative to the calculated heating load?

  1. 40% Correct answer.
  2. 25%
  3. 15%
  4. 10%

Explanation

Manual S allows heating equipment to be oversized by up to 40% of the calculated heating load. This accommodates pickup loads and ensures adequate capacity in cold snaps.

Question 40Residential HVAC System Design (Manual J, S, and D)

A room has a total heating load of 8,000 BTU/h. Using Manual S, what is the maximum allowable heating capacity of the selected equipment?

  1. 11,200 BTU/h Correct answer.
  2. 10,000 BTU/h
  3. 12,000 BTU/h
  4. 9,200 BTU/h

Explanation

Manual S permits heating equipment up to 40% larger than the load. 8,000 × 1.40 = 11,200 BTU/h.

Question 41Residential HVAC System Design (Manual J, S, and D)

Which of the following is a consequence of undersizing a cooling system according to Manual S?

  1. The system will have excess latent capacity
  2. The system will operate at a higher sensible heat ratio than needed
  3. The system will short-cycle and fail to remove humidity
  4. The system may not maintain the design indoor temperature on the hottest days Correct answer.

Explanation

Undersized cooling equipment lacks the capacity to meet the load at design conditions, so indoor temperature may rise above the setpoint. Short-cycling and excess latent capacity are associated with oversizing.

Question 42Residential HVAC System Design (Manual J, S, and D)

In Manual J, what is the purpose of applying a 'safety factor' or 'margin' to the calculated load?

  1. To reduce the equipment size for better efficiency
  2. To eliminate the need for a room-by-room calculation
  3. To ensure the equipment is always oversized by 50%
  4. To account for uncertainties in the load calculation and variations in weather Correct answer.

Explanation

Safety factors account for uncertainties such as construction quality, weather variations, and calculation assumptions. They are not meant to arbitrarily oversize equipment or replace detailed calculations.

Question 43Residential HVAC System Design (Manual J, S, and D)

A duct run has a total effective length of 120 ft and an available static pressure of 0.24 in. w.c. What is the available friction rate in in. w.c. per 100 ft?

  1. 0.20 Correct answer.
  2. 0.24
  3. 0.40
  4. 0.12

Explanation

Friction rate = (available static pressure × 100) / total effective length. (0.24 × 100) / 120 = 0.20 in. w.c. per 100 ft.

Question 44Residential HVAC System Design (Manual J, S, and D)

Which of the following is a key input to Manual J load calculations for a residential building?

  1. The color of the interior paint
  2. Window U-factors and solar heat gain coefficients Correct answer.
  3. The number of electrical outlets
  4. The brand of the thermostat

Explanation

Window U-factors and solar heat gain coefficients are essential for calculating fenestration loads. Interior paint color, thermostat brand, and outlet count do not directly affect loads.

Question 45Residential HVAC System Design (Manual J, S, and D)

In Manual D, what does a 'supply trunk' refer to?

  1. A small duct that serves a single register
  2. A duct that exhausts air to the outdoors
  3. The duct that returns air to the air handler
  4. The main duct that carries conditioned air from the air handler to branch ducts Correct answer.

Explanation

A supply trunk is the primary duct distributing conditioned air from the air handler to branch ducts. Branch ducts serve individual registers, and return ducts carry air back.

Question 46Residential HVAC System Design (Manual J, S, and D)

What is the purpose of a Manual J room-by-room calculation?

  1. To calculate the total building load only
  2. To design the duct layout for the house
  3. To determine the heating and cooling load for each individual room Correct answer.
  4. To select the equipment size for the entire house

Explanation

A room-by-room calculation provides the load for each room, which is necessary for proper duct sizing and register selection. A block load gives only the total building load.

Question 47Residential HVAC System Design (Manual J, S, and D)

A homeowner wants to reduce heating costs. Which Manual J input change would most directly lower the calculated heating load?

  1. Reducing the thermostat setpoint
  2. Increasing window area
  3. Increasing ceiling insulation R-value Correct answer.
  4. Adding a skylight

Explanation

Increasing ceiling insulation R-value reduces the U-factor of the ceiling, lowering conduction heat loss. Adding a skylight or increasing window area would increase the load, and reducing the thermostat setpoint is an operational change, not a load calculation input.

Question 48Residential HVAC System Design (Manual J, S, and D)

In Manual D, what is the recommended maximum velocity for a supply trunk to minimize noise?

  1. 600 feet per minute
  2. 900 feet per minute Correct answer.
  3. 1,500 feet per minute
  4. 1,200 feet per minute

Explanation

Manual D recommends a maximum velocity of 900 fpm in supply trunks to keep noise levels acceptable. Higher velocities can cause excessive noise, while lower velocities may require larger ducts.

Question 49Residential HVAC System Design (Manual J, S, and D)

A room has a sensible load of 4,000 BTU/h and a latent load of 500 BTU/h. The selected equipment has a rated sensible capacity of 4,500 BTU/h and a total capacity of 6,000 BTU/h at design conditions. Which statement is true?

  1. The equipment's sensible capacity is insufficient to meet the room's sensible load.
  2. The equipment's sensible capacity exceeds the room's sensible load, but its SHR is lower than the room's SHR, which may lead to overcooling and high humidity. Correct answer.
  3. The equipment is properly sized because total capacity exceeds total load.
  4. The equipment's latent capacity is insufficient to meet the room's latent load.

Explanation

The room SHR is 4,000/(4,000+500) = 0.89, while the equipment SHR is 4,500/6,000 = 0.75. The equipment provides more sensible capacity than needed but less latent capacity relative to the load, which can cause overcooling and inadequate dehumidification.

Question 50Residential HVAC System Design (Manual J, S, and D)

In Manual J, how does an increase in outdoor design temperature affect the calculated cooling load?

  1. It decreases the load because the outdoor air is warmer and less dense.
  2. It increases the load because the temperature difference across the building envelope is larger. Correct answer.
  3. It has no effect because the indoor temperature is fixed.
  4. It decreases the load because the equipment operates more efficiently.

Explanation

A higher outdoor design temperature increases the temperature difference between indoors and outdoors, raising conduction and infiltration loads. Equipment efficiency and air density changes are secondary and do not reduce the load.