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Heat Loss and Gain in Buildings - Lecture Slides | ARE 346N, Study notes of Architecture

The University of Texas at Austin Architecture

Prof. Jeffrey A. Siegel

Material Type: Notes; Professor: Siegel; Class: BUILDING ENVIRONMENTAL SYSTEMS; Subject: Architectural Engineering; University: University of Texas - Austin; Term: Spring 2007;

Typology: Study notes

Pre 2010

Uploaded on 08/26/2009

koofers-user-kqs 🇺🇸

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Heat Loss/Gain in Buildings

• Walls, roofs, etc. (1-D conduction)

• Finish Thursday’s lecture and example

• Infiltration and ventilation (convection)

• Ground contact (3-D conduction)

• Solar gains (radiation)

• Building individual pieces

• We will bring them together

Infiltration (Convection)

• Air carries sensible energy

•q = M × C × ΔT [BTU/hr, W]

•M mass flow rate = ρ × Q [lb/hr, kg/s]

•ρ air density (0.076 lb/ft3, 1.2 kg/m3 @ STP)

•Q volumetric flow rate [CFM, m3/s]

•C specific heat of air

• 0.24 BTU/(lb °F), 1007 kJ/(kg K) @ STP

• For similar indoor and outdoor conditions

•ρ and C are often combined

•q = 1.08 BTU min/(ft3 °F hr ) × Q × ΔT

• NOTE THAT THIS FORMULA IS UNIT SPECIFIC

Discover Study notes of Architecture The University of Texas at Austin

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Heat Loss/Gain in Buildings

Walls, roofs, etc. (1-D conduction)
- Finish Thursday’s lecture and example
Infiltration and ventilation (convection)
Ground contact (3-D conduction)
Solar gains (radiation)
Building individual pieces
- We will bring them together

Infiltration (Convection)

Air carries sensible energy
q = M × C × ΔT [BTU/hr, W]
- M mass flow rate = ρ × Q [lb/hr, kg/s]
  - ρ air density (0.076 lb/ft^3 , 1.2 kg/m^3 @ STP)
  - Q volumetric flow rate [CFM, m^3 / s]
- C specific heat of air
  - 0.24 BTU/(lb °F), 1007 kJ/(kg K) @ STP
For similar indoor and outdoor conditions
- ρ and C are often combined
  - q = 1.08 BTU min/(ft^3 °F hr ) × Q × ΔT
  - NOTE THAT THIS FORMULA IS UNIT SPECIFIC

Potential Confusion

Most of the world uses Q for both heat transfer

(i.e. BTU/hr) and for volumetric flow (i.e.

CFM)

Your textbook uses Q and CFM
- Unsatisfying from a units perspective
So, I will use q for heat transfer and Q for

volumetric flow

Look at context in case of confusion

Latent Infiltration and Ventilation

Can either track enthalpy and temperature and

separate latent and sensible later

q = M × Δ H [BTU/hr, W]
Or, track humidity ratio for just latent portion
q^ =^ M^ ×^ hfg ×^ Δ W
hfg = ~1076 BTU/lb, 2.5 kJ/kg
M = ρ × Q [lb/hr, kg/s]

Ground Contact

Receives less attention:
- 3-D conduction problem
- Ground temperature is often much closer to indoor air temperature - Only use for heating loads
Use F- value (from simulations) [BTU/(hr °F ft)]
- Note different units from U-value
- Multiply by slab edge length
- Add to Σ UA
- Still need to include basement wall area
WA State Energy Code heat loss tables
- In references section of website

What is the heat loss for a 20 × 20 ft slab 3.5 ft

below grade with R-19 insulation on the walls

and a thermal break at a 50 °F temp. difference?

A. q = 80 ft × 0.553 BTU/(hr °F ft) × (70 °F - 50 °F) = 885 BTU/hr B. q = 400 ft^2 × 0.042 BTU/(hr °F ft^2 ) × 50 °F = 842 BTU/hr C. q = 80 ft × 0.042 BTU/(hr °F ft^2 ) × 50 °F = 168 BTU/hr D. q = 80 ft × 0.553 BTU/(hr °F ft) × 50 °F = 2212 BTU/hr E. q = 400 ft^2 × 0.553 BTU/(hr °F ft) × (70 °F - 50 °F) = 4264 BTU/hr

Glazing

q = UA Δ T+A×SC×SHGF
Calculate conduction normally q = UA Δ T
- Use U-values from NFRC Certified Products Directory
  - ALREADY INCLUDES AIRFILMS
- http://cpd.nfrc.org/pubsearch/psMain.asp
Use the U-value for the actual window that you are going to use - Only use default values if absolutely necessary (Tables 4, and 6, Chapter 31 ASHRAE Fundamentals )
USE ROUGH OPENING AREA, NOT GLASS AREA

Solar Gain Through Windows

Add to conduction:
- A × SHGF × SC ( or A × SHGF × SHGC)
- SHGF = solar heat gain factor
  - Measure of how much energy comes through an average “perfect” window
  - Depends on
    - Latitude
    - Orientation
    - Time of Day
    - Time of Year
  - Tabulated in ASHRAE Fundamentals 1997 Chapter 29 Table 15
  - Tao and Janis Table 2-15A for 40° latitude (July 21 @ 8 am)

Shading Coefficient (or SHGC)

Ratio of how much sunlight passes through relative to a clean 1/8” thick piece of glass
Depends on
- Window coatings
- Actually a spectral property
- Frame shading, dirt, etc.
- Use the SHGC value from NFRC for a particular window
  - Lower it further for dirt, blinds, awnings, shading
- Only use default values if necessary (ASHRAE Ch. 31, Table 13, Text Table 2-16) - http://cpd.nfrc.org/search/searchdefault.aspx

What is the total cooling load in Austin associated

with the following window with a 40 °F ΔT?

10 ft^2 , south facing, ATI Windows^ Dimension

4 Horizontal Slider ALU-A-12-

U = 0.3 4 BTU/(hr °F ft^2 )
SHGC = 0. 3 2 (typically divide by 2 for dirt

etc.) = 0. 16

SHGF = 215 BTU/(hr ft^2 ) – Oct 20th^ at noon
q = UA Δ T+A× SHGC ×SHGF
q = (~ % from solar gain)

Summary

Include:
- 1-D conduction through components (walls, roofs, etc.)
- Convection (ventilation and infiltration)
- Ground contact
- Solar gain on walls and roofs and glazing
- Reading Assignment: Section 2.5 - 2.

Heat Loss and Gain in Buildings - Lecture Slides | ARE 346N, Study notes of Architecture

Related documents

Partial preview of the text

Download Heat Loss and Gain in Buildings - Lecture Slides | ARE 346N and more Study notes Architecture in PDF only on Docsity!

Heat Loss/Gain in Buildings

Infiltration (Convection)

Potential Confusion

(i.e. BTU/hr) and for volumetric flow (i.e.

CFM)

volumetric flow

Latent Infiltration and Ventilation

separate latent and sensible later

Ground Contact

What is the heat loss for a 20 × 20 ft slab 3.5 ft

below grade with R-19 insulation on the walls

and a thermal break at a 50 °F temp. difference?

Glazing

Solar Gain Through Windows

Shading Coefficient (or SHGC)

What is the total cooling load in Austin associated

with the following window with a 40 °F ΔT?

4 Horizontal Slider ALU-A-12-

etc.) = 0. 16

Summary