BEM class 3 Building Thermo 1 - Conduction Heat Loss Building - - PowerPoint PPT Presentation

BEM class 3 Building Thermo 1 - Conduction Heat Loss

Building Thermodynamics

How buildings lose/gain heat - 3 fundamental mechanisms of heat transfer

• Conduction
• Convection
• Radiation

How Loads are Calculated

Heating Load, Q = conduction + infil/ventil [– SG – IG] Cooling Load, Q = conduction + infil/ventil + SG + IG

SG = solar gain IG = internal gains For cooling load, infil/ventil includes Latent Heat

Conductive Heat Loss

• Heat flows along thermal gradient
• Flow proportional to gradient - delta T
• Rate of flow characteristic for any

material is its "k" or "U" value

• BTU / SF / degree dT
• QC = U A dT

Remember that this is just for conduction. Heat transfers from air flow and from radiation are equally important in overall heat load calcs.

U-values and R-values

• U = 1 / R
• Per thickness of

material

• Look-up tables for

tested values.

• Field testing with

infrared to measure heat flux.

• R-values are additive;

U-values are NOT.

Conduction in composite constructions

Construction elements are not necessarily uniform. fenestration

• Varied features
• frame characteristics

frame walls

• compute R at each distinct kind of

construction.

• Convert to U-values, pro-rating

contribution to overall U based on percentage of area (see procedure in table

• n next slide)

Interesting application problem - compute the dew point temperature within a wall construction

Conduction in composite constructions

• Brick is uniform -

add R-values of layers

• Frame wall has

different condition at stud - add up the R- values for each construction

• Calculate each U

and pro-rate (multiply) by percentage of surface area. Add these all up to get an

• verall U-value for

the construction.

Conduction Heat Loss Calculation

• Collect, compute, compile R and U values and

areas for various constructions (walls, roof, windows). Apply UAdT to each. Sum. (Hmmm, where does that dT come from??)

• Can also develop overall envelope U by adding

“UA”s -- this weights the U-values proportional to building surface areas. This is the basis of envelope trade-off procedures in Code.

• What about foundation and/or slab heat losses?

Code

• ASHRAE 90.1 and ICC
• State Energy Codes

– http://www.dos.ny.gov/dcea/energycode_code.html – http://publicecodes.cyberregs.com/st/ny/st/index.htm

• NYC Energy Code

– http://www.nyc.gov/html/dob/html/codes_and_reference_materials/nycecc_mai n.shtml

– Training Modules

http://www.nyc.gov/html/dob/html/codes_and_reference_materials/nycecc_trai ning_modules.shtml

• Sallan Foundation "Decoding the Code"

Code cont'd

• Focus on envelope insulation values
• Compliance pathways
• Prescriptive A - meet U values by component
• Prescriptive B- meet overall U
• "appendix G" modeling against baseline
• RESCHECK, COMCHECK - free tools,

download, widely used.

"Effective R-value”

What you calculate isn’t necessarily what you get

• Product and construction

flaws

• gas-filled windows
• insulated walls and insulation

by-passes

"Effective R-value”

What you calculate isn’t necessarily what you get

• Thermal Bridges
• “thermal break” construction

Thermal Lags

• Building dynamics, non-steady-state
• Effect of “thermal mass”
• Most important as weather conditions swing

daily

Weather data

• Not to be confused with

CLIMATE data

• Climate zones. Design-for-

climate.

• Design Conditions - dT
• Annual Usage - Degree-days.
• Bins. Hourly.

Weather data cont'd

• Sources
• "weather tapes"
• TMY