Air Conditioning Load Calculations (RACM-400)

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Questions and Answers

Which of the following is NOT a main source of heat loss in a building?

  • Windows & doors
  • Exterior walls
  • Interior walls (correct)
  • Roofs & ceilings

What is the relationship between absolute humidity and temperature?

  • Absolute humidity decreases with increasing temperature.
  • Absolute humidity increases with increasing temperature.
  • Absolute humidity is not related to temperature.
  • Absolute humidity remains constant regardless of temperature. (correct)

What is the maximum amount of moisture 1 pound of air at 80°F can hold before becoming 100% saturated?

  • It can hold an unlimited amount of moisture.
  • It depends on the relative humidity.
  • It can hold a specific amount of moisture, regardless of relative humidity. (correct)
  • It can hold more moisture than air at a lower temperature.

According to AHRI design specifications, at what outdoor temperature does an air conditioning system operate at its rated capacity?

<p>95°F (C)</p> Signup and view all the answers

What is the recommended indoor dry bulb temperature for cooling load calculations, according to industry standards?

<p>75°F (D)</p> Signup and view all the answers

What is the design temperature difference calculated for?

<p>The difference between the indoor and outdoor dry bulb temperatures. (A)</p> Signup and view all the answers

Which of the following is NOT a factor considered in AHRI's design specifications for air conditioning equipment?

<p>Air leakage in the building (D)</p> Signup and view all the answers

If a 12,000 BTU air conditioner is operating outside the design conditions specified by AHRI, what can be said about its performance?

<p>It will perform at a lower capacity than its rated capacity. (C)</p> Signup and view all the answers

What is the design temperature difference (TD) for cooling in Modesto, California?

<p>23 F (B)</p> Signup and view all the answers

Which of the following statements about R Value is true?

<p>R Value is a measure of thermal resistance in insulation materials. (A)</p> Signup and view all the answers

How is U Value related to R Value?

<p>U Value is the inverse of R Value. (C)</p> Signup and view all the answers

What is the formula used to calculate Net Wall Area?

<p>Gross Wall Area - Door Area - Window Area - Area of Other Components (C)</p> Signup and view all the answers

During the wintertime, heat is lost through what process?

<p>Exfiltration (C)</p> Signup and view all the answers

Given an R Value of 4.12, what is the corresponding U Value?

<p>0.24 (D)</p> Signup and view all the answers

What is the significance of the total R Value for a wall constructed of different materials?

<p>It indicates the wall's ability to resist heat transfer. (A)</p> Signup and view all the answers

What is the unit for measuring U Value?

<p>btus/hr/f/sq.ft (B)</p> Signup and view all the answers

What is the correct insulation value used for the ceiling in the given scenario?

<p>R-30 (C)</p> Signup and view all the answers

What is the indoor design temperature used for cooling?

<p>75 F (D)</p> Signup and view all the answers

In the heat loss calculation for partition walls, what is the temperature difference (TD) used in the example?

<p>30 F (C)</p> Signup and view all the answers

What is the calculated heat gain through a window facing East with a multiplier of 63 btu/hr/sq.ft and an area of 48 sq.ft?

<p>3024 btu/hr (C)</p> Signup and view all the answers

What formula is used to calculate the area of above-grade walls?

<p>height x perimeter (A)</p> Signup and view all the answers

What is the total heat loss (Qtotal) calculated in the example?

<p>5544 btu/hr (B)</p> Signup and view all the answers

What is the btu load per hour for full-time occupants?

<p>430 btu/hr (B)</p> Signup and view all the answers

When determining the design temperature for walls, what is the first step?

<p>Identify the wall construction (B)</p> Signup and view all the answers

For the heat loss through floors above unconditioned spaces, what was the temperature difference (TD) in the example given?

<p>60 F (B)</p> Signup and view all the answers

How much btu/hr does lighting contribute per watt?

<p>3.412 btus/hr per watt (B)</p> Signup and view all the answers

What is the F value used for calculating heat loss through slab floors in the example?

<p>0.049 (B)</p> Signup and view all the answers

In the example, what is the perimeter (P) used for calculating the area of slab floors?

<p>240 ft (D)</p> Signup and view all the answers

If the design temperature difference for a wall is 20 F and the actual temperature difference determined is 17.7 F, what formula is used to calculate heat loss?

<p>Q = 0.052 x 17.7 x 1000 (B)</p> Signup and view all the answers

What factor increases or decreases heat gain through windows based on occupant behavior?

<p>Solar load (B)</p> Signup and view all the answers

What determines the difference in U values when calculating heat loss through above and below grade walls?

<p>The grade level of the walls (B)</p> Signup and view all the answers

What is the default btu load for a house with a refrigerator and a vented cooking surface?

<p>1200 Btu/h (D)</p> Signup and view all the answers

What is the formula for calculating Net Wall Area?

<p>Gross Wall Area - Door Area - Window Area (A)</p> Signup and view all the answers

What is the U-value for a window based on the given information?

<p>0.49 (B)</p> Signup and view all the answers

What is the heat transfer rate through the window?

<p>352.8 Btu/h (A)</p> Signup and view all the answers

How is the total heat loss for the entire wall calculated?

<p>QNet Wall + QWindow + QDoor (C)</p> Signup and view all the answers

What are the dimensions used to calculate the area of a triangle for the roof?

<p>Base = 8 ft, Height = 6 ft (C)</p> Signup and view all the answers

What is the U-value assigned to the roof in the calculations?

<p>0.30 (D)</p> Signup and view all the answers

Given a house with 400 sq.ft of footprint, how is heat loss through ceilings to unconditioned attics determined?

<p>By calculating area and U-value for the ceiling (C)</p> Signup and view all the answers

What is the calculated value of Q for the roof?

<p>10,500 Btu/h (A)</p> Signup and view all the answers

What is the formula used to calculate heat loss through infiltration?

<p>Qinfiltration = 1.08 x cfm x TD (C)</p> Signup and view all the answers

Which factor does NOT directly affect heat loss through ducts?

<p>Humidity level of the house (A)</p> Signup and view all the answers

How is CFM calculated for infiltration in a structure?

<p>CFM = ACH x (House Volume / 60) (C)</p> Signup and view all the answers

Which of the following temperature differences would yield the highest potential heat loss through ducts?

<p>100 F (B)</p> Signup and view all the answers

What term describes the total heat loss in the building structure?

<p>Envelope load (B)</p> Signup and view all the answers

Which type of correction factor corresponds to R-4 insulation in duct walls?

<p>1.15 (D)</p> Signup and view all the answers

What factors could contribute to heat gain in a building? (Select all that apply)

<p>Occupants (B), Appliances (C), Lighting (D)</p> Signup and view all the answers

Flashcards

Heat Loss Sources

Key areas where heat escapes from a building, such as walls, roofs, and ducts.

Absolute Humidity (AH)

Total water vapor in a given volume of air, independent of temperature, measured in grains or pounds.

Relative Humidity

Comparison of current water vapor in air to maximum capacity at given temperature; indicates how full raw air is.

Design Conditions

Optimal settings for HVAC system performance, including temperature and humidity levels.

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Duct Losses

Heat loss from air ducts located in unconditioned spaces, affecting indoor temperature.

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Temperature Difference (TD)

The temperature gap calculated from inside the structure to the outside for load calculations.

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Indoor Comfort Levels

Preferred indoor temperatures for comfort: 75°F for cooling and 70°F for heating, with 50% humidity.

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Heat Loss through Windows

Heat transfer due to conduction and air leakage at windows and doors.

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Heat Gain via Infiltration

Heat gained into a structure during the summertime through air leaks.

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Heat Loss via Exfiltration

Heat lost from a structure during the wintertime due to air escaping.

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R Value

A measure of thermal resistance of insulation materials; higher R value means better insulation.

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Calculating R Value

Summing the R values of all construction materials for a wall to find total thermal resistance.

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U Value

The thermal conductivity of a material; higher U value means easier heat transfer.

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Net Wall Area

The area of a wall after subtracting areas of doors, windows, and other components.

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Net Wall Area Formula

Mathematically calculated as Gross Wall Area minus areas of doors and windows.

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R-30 Insulation

A specific insulation rating indicating thermal resistance in ceilings.

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Qceiling Calculation

Formula to calculate heat loss through the ceiling: Q = 0.033 x TD x Area.

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TD (Temperature Difference)

The difference in temperature between two environments, used in heat loss calculations.

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Partition Wall Heat Loss

Heat loss through a wall separating conditioned from unconditioned spaces, using the TD for each area.

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Area of Above Grade Wall

Measurement calculated as height of the wall times the perimeter of the wall above grade.

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Qtotal Calculation

Total heat loss calculated by adding heat loss through all building components.

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Heat Transfer Rate (Q)

The rate at which heat is lost through a material, depending on U-value, temperature difference, and area.

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Heat Loss Through Doors

Calculated by the same principle as windows, factoring in U-value and area of the door.

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Q Entire Wall

Total heat loss through all components of a wall including the net wall, windows, and doors.

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Roof Area Calculation

Calculating the area of a roof triangle and doubling it for total area.

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Heat Loss Through Roofs

Heat loss that occurs via roofs, calculated using Q = U x TD x Area for the entire roof.

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Design Temperature for Cooling

Optimal indoor temperature set at 75°F for cooling calculations.

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Group Determination

Process of classifying wall construction to find appropriate heat transfer values.

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Actual Temperature Difference (ATD)

The adjusted TD after looking it up in specific construction charts.

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Heat Gain through Windows

Heat transfer through windows that includes solar load considerations.

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Solar Load

Heat added to a space from sunlight entering through windows.

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Lighting Heat Load

Heat produced by lighting calculated as 3.412 Btu/hr per watt used.

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Occupants' Heat Load

Heat generated by full-time occupants, averaging around 430 Btu/hr.

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Qslab Calculation

Formula for calculating heat loss through a slab: Qslab = 0.178 x 60 F x 220'.

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Infiltration Heat Loss

The heat loss due to air entering a structure, calculated as Qinfiltration = 1.08 x cfm x TD.

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CFM Estimate

Cubic feet per minute; estimated by CFMinfiltration = ACH x House Volume / 60 min.

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ACH

Air changes per hour, a measure of how often indoor air is replaced with outdoor air.

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Duct Loss Factors

Percentage used to calculate heat loss from ducts based on temperature and insulation.

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Envelope Load

Total of all individual heat losses from a building's structure.

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Duct Insulation Values

Correction factors for duct losses based on the insulation R-value (R-2 to R-8).

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Heat Gain Sources

Factors that contribute to heat gain in buildings: lighting, appliances, and people.

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Study Notes

Air Conditioning Load Calculations (RACM-400)

  • Heat Loss Sources: Exterior walls, windows and doors, roofs and ceilings, partition panels, below-grade walls, slab and basement floors, floors above unconditioned spaces, ducts, and air leakage.

  • Absolute Humidity (AH): Total water vapor content in a given volume of air; measured in grains of moisture or pounds (7000 grains = 1 lb). Regardless of temperature.

  • Relative Humidity: Ratio of actual water vapor content to maximum water vapor content at a given temperature. Warmer air can hold more moisture.

  • AHRI Equipment Design Specifications: Design conditions include 95°F outdoor temperature, 80°F indoor temperature, and 50% relative humidity. System performance is rated based on these conditions.

  • Design Conditions for Load Calculations: General population comfort is at 75°F dry bulb for cooling and 70°F for heating, with 50% relative humidity.

  • Design Temperature Difference (TD): Difference between inside and outside temperatures. TD for cooling and heating calculations are different. This is for heat loss, heat gain into a structure. In summer, heat is gained through infiltration. In winter, heat is lost through exfiltration. This is location dependent.

  • Construction Materials - R-Value: Resistance to heat transfer. Higher R value means more insulation. Used to rate insulation materials or different layers of construction material.

  • Calculating R-Value: Sum of the R-values for all different materials. (Ex. R value calculation involving 4" brick, 5/8" drywall, 3/4" plywood and insulating board backed aluminum siding).

  • Calculating U-value: Inverse of R value; represents thermal conductivity. Higher U-value = easier for heat to pass through the material.

  • Heat Loss, Heat Gain Formula: Q = U x TD x Area; where Q is heat transfer rate, U is U-value of the wall, TD is temperature difference across the wall, and area is in square feet.

  • Net Wall Area: Gross exterior wall area minus the area of windows and doors.

  • Heat Loss Through Doors and Windows: Heat loss calculations using component U-values and temperature differences.

  • Heat Loss Through Roofs: Heat loss calculations based on roof geometry and material properties, including U-value and temperature difference.

  • Heat Loss Through Partition Walls: Calculate heat loss through walls in sections, above-grade and below-grade sections, separately.

  • Heat Loss Through Floors Located Above Unconditioned Spaces: Heat loss through floors located above unconditioned spaces.

  • Heat Loss Through Slab Floors: Calculation of heat loss using running feet of exposed slab edge with perimeter values.

  • Heat Loss Through Cracks, Openings, and Deficiencies (Infiltration): Heat loss due to air leakage. Uses factors such as CFM (cubic feet per minute) and the temperature difference (TD)

  • Heat Gain through Duct Runs, Attics, Cavities and Unconditioned Spaces: Heat gain due to warm air passing through ducts located in unconditioned spaces. Calculations include using factors such as correction factor, envelope load, and duct insulation.

  • Heat Gain Calculations: Heat gain calculations are similar to heat loss calculations, considering elements like exposed walls, doors, windows and infiltration. Separate calculations also include factors such as people (occupants), lighting, appliances and moisture factors.

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