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