BEM class 4
Building Thermo 1 –
Envelope 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 (people, lighting, equipment)
For cooling load, infil/ventil includes Latent Heat
Part 1 Conduction Heat Loss
High temp
• Heat flows through materials along thermal
Low temp
gradient
• Flow proportional to gradient – “delta T” (
T, dT)
• 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. Convective heat
transfer from air flow is equally important in overall heat load calc.
Conduction Heat Loss
Q = U A dT
Where,
Q is heat loss for the construction
U is value of material or composite, BTU/sf-hour
A is surface area, square foot
dT is term for temperature difference and may be
hourly (peak), bin, or annual.
U-values and R-values
• U=1/R
• R-values are additive;
U-values are NOT.
• Per thickness of
material
• Look-up tables for
tested values.
• Field testing with
infrared to measure
heat flux.
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
on 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 Rvalues for each
construction
• Calculate each U
and pro-rate
(multiply) by
percentage of
surface area. Add
these all up to get an
overall U-value for
the construction.
Code
• ASHRAE 90.1 and 62.1 and International Code Council
(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.
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?
"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
Part 2 – CONVECTIVE EFFECTS
•
Effect on conduction (surface effects)
•
Air movement into and out of buildings
•
Latent heat in outside air
•
Interior air flows and comfort
AIR MOVEMENT INTO & OUT OF
BUILDINGS
•
Wind and Stack
effects
• Variable impacts
• CFD
•
Ventilation exhaust
•
Supply air & building
pressurization
HOW MUCH OUTSIDE AIR?
•
All-natural draft ventilation
•
Function of wind, draft and openings
• estimation by crack-length or by air-changes per hour (ACH)
• 0.5 - 2 ACH. “Blower-door” testing.
•
Mechanically-driven systems
•
•
Count up CFM from fan ratings
Code for ventilation
•
NYC Building Code.
• ASHRAE 62.1
•
"15 CFM PER PERSON"
HOW MUCH OUTSIDE AIR?
•
ASHRAE 62.1 – NOT SO SIMPLE
HOW MUCH HEATING LOAD?
•
Infiltration/ventilation can be 50% or more
of building heating load
dT
Or, for air-change method,
Qventil = ACH x Volume x .018 x dT
Overall Building Heat Loss
Exercise
A 50’ x 150’ x 10 story free-standing building has an overall Rvalue of 3 (taking into account all walls, windows, roof). Each
story is 10’ tall. Ventilation, as calculated at 15 cfm per occupant
at design occupancy, provides .85 air-change per hour. Ignore
basement/foundation losses.
Calculate the design heat load at 10 dF outside temperature and
70 dF indoor temperature
[(50 x 2) + (150 x 2)] x 10 x 10 = 40,000 sf surface area
40,000 x 1/3 x (70-10) = 800,000 BTUH conduction
50 x 150 x 10 x 10 = 750,000 cf volume
750,000 x .85 x .018 x (70-10) = 688, 500 BTUH ventilation
Answer = 800,000 + 688,500 = 1,488,500 BTU