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Environmental Controls I/IGEnvironmental Controls I/IGEnvironmental Controls I/IGEnvironmental Controls I/IG
Lecture 10Heat Flow in Glazing
InfiltrationVentilation
Window CharacteristicsWindow Characteristics
S: p. 1585, T.E.15
Super WindowsSuper Windows
Composed of subassemblies that control conductive and radiant heat exchange.
S: p. 198, F.7.15
Window CharacteristicsWindow Characteristics
S: p. 1585, T.E.15
Window CharacteristicsWindow Characteristics
S: p. 1585, T.E.15
Solar Heat Gain Coefficient Solar Heat Gain Coefficient (SHGC)(SHGC)
Percentage of incident solar radiation that passes through the entire window or skylight at a normal incident angle.
High SHGC desirable for passive solar
Low SHGC desirable for where cooling is dominant issue
Window CharacteristicsWindow Characteristics
S: p. 1585, T.E.15
Visible Transmittance (VT)Visible Transmittance (VT)
Percentage of the incident amount of visible light transmitted through the glazing
High VT means better daylighting quantities indoor
Window CharacteristicsWindow Characteristics
S: p. 1585, T.E.15
Spectrally Selective GlazingSpectrally Selective GlazingSpectrally selective coatings reduce SHGC with little reduction in VT
Low-ε coatings: reflect radiant energy back towards source
LSG: light to solar gain ratio
High LSG is better for day-lighting in hot climates
Window CharacteristicsWindow Characteristics
S: p. 1585, T.E.15
Air LeakageAir Leakage
Rate of outdoor air infiltration between the window and its frame.
Example A: 3’x5’ window at 0.65 cfm/lf.Inf. = (3+5+3+5) x 0.65= 10.4 cfm
Example B: 3’x5’ window at 0.98 cfm/sf.Inf. = (3x5) x 0.98= 14.7 cfm
Note: use the larger value of the two results
Heat Flow in GlazingHeat Flow in Glazing
Conductive Heat Flow through glazing:
Q= U x A x ΔT
Q: heat flow (Btuh)U: transmission coefficient (Btu/h-ºF-ft2)A: area (ft2) [including frame]ΔT: temperature difference (Ti-To)
Solar Heat Gain FactorsSolar Heat Gain Factors
S: p. 1504, T.C.3
Heat Flow in GlazingHeat Flow in Glazing
Radiant Heat Flow through glazing:
Q= SHGC x A x SHGF
Q: heat flow (Btuh)SHGC: solar heat gain coefficient for window and frameA: area (ft2) [including frame]SHGF: solar heat gain factor (Btu/h-ft2)
InfiltrationInfiltrationUnintentional introduction of untreated air into the occupied space(s) of the building
Heat lost or gained becomes part of the building system load
InfiltrationInfiltration
Calculated by two means:
1. Air Change per Hour (ACH)2. Crack Method
Air Change per Hour MethodAir Change per Hour Method
Volume of infiltration:
V= (ACH)(volume, ft3) 60 min/hr
V: total air flow volume (cfm)ACH: Air changes per hourvolume: space volume (ft3)
Air Change per Hour MethodAir Change per Hour Method
Volume of infiltration:
V= (ACH)(volume, ft3) 60 min/hr
V: total air flow volume (cfm)ACH: Air changes per hourvolume: space volume (ft3)
Air Change per Hour MethodAir Change per Hour Method
Determine Construction Type
S: p. 1601, T.E.27A
Air Change per Hour MethodAir Change per Hour MethodDetermine Winter & Summer Conditions
S: p. 1601, TE.27B&C
Crack Length MethodCrack Length Method
Calculate crack length of windows on the windward side only.
Calculate crack length of doors on the windward side only.
Crack Length MethodCrack Length Method
Determine window and door “fit” andFind “k”
S: p. 1604, T.E.28C
Crack Length MethodCrack Length Method
1. Determine wind velocity.
2. Find “velocity head factor.”
3. Determine infiltration rate
4. Calculate total infiltration
S: p. 1603, T.E.28A&B
Crack Length MethodCrack Length Method
Find winter infiltration for average fitting windows
k=2.0
S: p. 1603, T.E.28C
Crack Length MethodCrack Length Method
Wind velocity=15 mph
Velocity head factor=0.11
Infiltration rate= 0.5cfm/lf
Calculate total infiltration
Infiltration = Rate x Crack length
S: p. 1604, T.E.28A&B
VentilationVentilationIntentional introduction of treated fresh air into the occupied space(s) of the building
Outside air is introduced via the building ventilation ductwork system
Residential buildings generally rely on infiltration
Non-residential buildings use ventilation
VentilationVentilation
Ventilation is determined according to:
ASHRAE Standard 62-2001 (S: p. 1597, T.E.25)
Estimates the number of people/1000 sf of usage typePrescribes minimum ventilation/person for usage type
ASHRAE 62-2001ASHRAE 62-2001
Example: (1) Determine the ventilation rate for 2,000 sf office space.
(2) Determine total ventilation volume.
S: p. 1598, T.E.25
ASHRAE 62-2001ASHRAE 62-2001
Example: (1) Determine the ventilation rate for 2,000 sf office space.
17 cfm/person
S: p. 1598, T.E.25
ASHRAE 62-2001ASHRAE 62-2001
Example: (2) Determine total ventilation volume.
=2000 sf x (5 persons/1000sf) x (17 cfm/person)=170 cfm
S: p. 1598, T.E.25