Technology in Architecture. Lecture 7 Degree Days Heating Loads Annual Fuel Consumption Simple Payback Analysis. Heating Degree Days. Balance Point Temperature (BPT): temperature above which heating is not needed DD BPT = BPTTA. Sample Calculation. January TA=28ºF
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Lecture 7
Degree Days
Heating Loads
Annual Fuel Consumption
Simple Payback Analysis
Balance Point Temperature (BPT): temperature above which heating is not needed
DDBPT= BPTTA
January TA=28ºF
DD65=6528= 37 Degreedays/day
x 31 days
= 1,147 degreedays
S: p. 1562, T.C.19
Computed for worst case scenario:
Do not include:
SR3
Indoor Dry Bulb Temperature (IDBT): 68ºF
Outdoor Dry Bulb Temperature (ODBT): 8ºF
ΔT=IDBTODBT=68ºF  8ºF = 60ºF
Calculate ΣR and then find U for walls, roofs, floors.
Obtain U values for glazing from manufacturer or other reference
Perform area takeoffs for all building envelope surfaces on each facade:
gross wall area
window area
door area
net wall area
1200 sf
100’

368 sf

64 sf
768 sf
4’
12’
4’
8’
Elevation
For floor slabs at grade, there are two heat loss components:
S: p. 1624, F.E.1
Q=Uslabx 0.5 x Aslabx (TITGW)
TI=Indoor Air Temperature
TGW=Ground Water Temperature
Q=F2x Slab Perimeter Length x (TITO)
where,
TI= Indoor air temperature
TO=Outdoor air temperature
Building: Office Building
Location: Salt Lake City
ΔT=IDBTODBT=688=60ºF
Building: 200’ x 100’ (2 stories, 12’6” each)
Uwall= 0.054 Btuh/sfºF
Uroof= 0.025 Btuh/sfºF
Uwindow= 0.31 Btuh/sfºF
Uslab= 0.16 Btuh/sfºF
Udoor= 0.20 Btuh/sfºF
Determine Building Envelope Areas (SF)
Building: 200’ x 100’ (2 stories, 12’6” each)
N E S W
Gross Wall 5,000 2,500 5,000 2,500
Windows 1,000 500 2,000 500
Doors 20 20 50 20
Net Wall 3,980 1,980 2,950 1,980
Roof/Floor Slab 20,000
0.025 20,000 60 30,000 30,000
N 0.054 3,980 60 12,895
E 0.054 1,980 60 6,415
S 0.054 2,950 60 9,558
W 0.054 1,980 60 6,415 38,555
Insert roof values
Insert wall values
Insert glass values
Insert door values
Insert floor values
N 0.31 1,000 60 18,600
E 0.31 500 60 9,300
S 0.31 2,000 60 37,200
W 0.31 500 60 9,300 74,400
0.20 110 60 1,320 1,320
N/A N/A N/A N/A
SR3
Q=Uslabx 0.5 x Aslabx (TITGW)
TI=Indoor Air Temperature
TGW=Ground Water Temperature
Ground Water= 53ºF
ΔT=68ºF53ºF=15ºF
0.025 20,000 60 30,000 30,000
N 0.054 3,980 60 12,895
E 0.054 1,980 60 6,415
S 0.054 2,950 60 9,558
W 0.054 1,980 60 6,415 38,555
Insert floor values
N 0.31 1,000 60 18,600
E 0.31 500 60 9,300
S 0.31 2,000 60 37,200
W 0.31 500 60 9,300 74,400
0.20 110 60 1,320 1,320
N/A N/A N/A N/A
0.16 20,000 15 24,000
SR3
Method I
Interpolate to find F2
at 5983 DD
5350 5983 7433
0.50 F2? 0.56
S: p. 1624, T.E. 11/F.E.1
Find difference in Degree Days: 59835350=633 74335350=2083
Find difference in F2:F2?0.50=x
0.560.50=0.06
Set up proportion, solve for x: 633/2083=x/0.06
x=0.018
F2?0.50=0.018
F2?=0.518
Method I
Interpolate to find F2
at 5983 DD
5350 5983 7433
0.50 F2= 0.56
0.518
S: p. 1624, T.E. 11/F.E.1
0.025 20,000 60 30,000 30,000
N 0.054 3,980 60 12,895
E 0.054 1,980 60 6,415
S 0.054 2,950 60 9,558
W 0.054 1,980 60 6,415 38,555
Insert floor values
N 0.31 1,000 60 18,600
E 0.31 500 60 9,300
S 0.31 2,000 60 37,200
W 0.31 500 60 9,300 74,400
0.20 110 60 1,320 1,320
N/A N/A N/A N/A
0.16 20,000 15 24,000
0.518 600 60 18,648 42,648
SR3
Residential buildings use infiltration to provide fresh air
“Air change/hour (ACH) method” (see S: p.1601, T. E.27)
or
“Crack length method” (see S: p. 1603, T. E.28)
Prone to subjective interpretation
Vulnerable to construction defects
Provides a relatively approximate result
Nonresidential buildings use ventilation to provide fresh air and to offset infiltration effects.
ASHRAE Standard 622001 (S: p. 159799, T.E.25)
Estimates the number of people/1000 sf of usage type
Prescribes minimum ventilation/person for usage type
40,000 sf x 5people/1,000sf = 200 people
200 people x 17 cfm/person = 3,400 cfm
3,400 cfm x 60min/hr = 204,000cfh
0.025 20,000 60 30,000 30,000
N 0.054 3,980 60 12,895
E 0.054 1,980 60 6,415
S 0.054 2,950 60 9,558
W 0.054 1,980 60 6,415 38,555
Input Ventilation Load—Sensible
N 0.31 1,000 60 18,600
E 0.31 500 60 9,300
S 0.31 2,000 60 37,200
W 0.31 500 60 9,300 74,400
0.20 110 60 1,320 1,320
N/A N/A N/A N/A
0.16 20,000 15 24,000
0.518 600 60 18,648 42,648
204,000 60 220,320
SR3
Determine ΔW
WI= 0.0066 #H2O/#dry air
WO= 0.0006 #H2O/#dry air
ΔW= 0.0060 #H2O/#dry air
0.025 20,000 60 30,000 30,000
N 0.054 3,980 60 12,895
E 0.054 1,980 60 6,415
S 0.054 2,950 60 9,558
W 0.054 1,980 60 6,415 38,555
Input Ventilation Load — Latent
N 0.31 1,000 60 18,600
E 0.31 500 60 9,300
S 0.31 2,000 60 37,200
W 0.31 500 60 9,300 74,400
0.20 110 60 1,320 1,320
N/A N/A N/A N/A
0.16 20,000 15 24,000
0.518 600 60 18,648 42,648
204,000 60 220,320
204,000 0.0060 97308 317628
SR3
5.9
0.025 20,000 60 30,000 30,000
N 0.054 3,980 60 12,895
E 0.054 1,980 60 6,415
S 0.054 2,950 60 9,558
W 0.054 1,980 60 6,415 38,555
7.6
Total Load
504551 Btuh
or
505 MBH
N 0.31 1,000 60 18,600
E 0.31 500 60 9,300
S 0.31 2,000 60 37,200
W 0.31 500 60 9,300 74,400
14.7
0.20 110 60 1,320 1,320
0.3
N/A N/A N/A N/A
0.16 20,000 15 24,000
8.4
0.518 600 60 18,648 42,648
204,000 60 220320
63.1
204,000 0.0060 97,308 317628
SR3
504551
E= UA x DDBPT x 24
AFUE x V
where:
UA: heating load/ºF
DDBPT: degree days for given balance point
AFUE: annual fuel utilization efficiency
V: fuel heating value
QTotal= UA xΔT
UA= QTotal/ΔT
From earlier example:
QTotal=504,551 Btuh
ΔT= 60ºF
UA=504,551/60=8,409 Btuh/ºF
Annual Fuel Utilization Efficiency of an electric heating system is 100%
S: p. 262, T.8.7
Heat content is the quantity of Btu/unit
Note: Natural Gas is sold in therms (100 cf)
S: p. 259, T.8.5
What is the expected annual fuel usage for a house in Salt Lake City if its peak heating load is 39,000 Btuh?
UA=Q/ΔT
UA=39,000/60= 650 Btuh/ºF
Annual Fuel Utilization Efficiency of an electric heating system is 100%
S: p.262, T.8.7
E= UA x DDBPT x 24
AFUE x V
EELEC =(650)(5,983)(24)/(1.0)(3,413)
=27,347 kwh/yr
If electricity is $0.0735/kwh, then
annual cost = $2,010
E= UA x DDBPT x 24
AFUE x V
EGas =(650)(5,983)(24)/(0.8)(105,000)
=1,111 therms/yr
If gas is $0.41/therm, then
annual cost = $456
Heating SystemCost Comparison
First Annual Incremental Incremental Simple
Cost Fuel Cost First Cost Annual Savings Payback
($) ($/yr) ($) ($/yr) (yrs)
Electricity 6,000 2,010   
Oil 8,0001,152 2,0008582.3
Gas 8,900 456 2,900 1,554 1.9
If money is available, select gas furnace system