Environmental Controls I/IG Lecture 10 Heat Flow in Glazing Infiltration Ventilation

Window Characteristics S: p. 1585, T.E.15

Super Windows Composed of subassemblies that control conductive and radiant heat exchange. S: p. 198, F.7.15

Window Characteristics S: p. 1585, T.E.15

Window Characteristics S: p. 1585, T.E.15

Solar Heat Gain Coefficient (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 Characteristics S: p. 1585, T.E.15

Visible Transmittance (VT) Percentage of the incident amount of visible light transmitted through the glazing High VT means better daylighting quantities indoor

Window Characteristics S: p. 1585, T.E.15

Spectrally Selective Glazing Spectrally 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 Characteristics S: p. 1585, T.E.15

Air 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 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 Factors S: p. 1504, T.C.3

Heat 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 frame A: area (ft2) [including frame] SHGF: solar heat gain factor (Btu/h-ft2)

Infiltration Unintentional introduction of untreated air into the occupied space(s) of the building Heat lost or gained becomes part of the building system load

Infiltration Calculated by two means: Air Change per Hour (ACH) Crack Method

Air Change per Hour Method Volume of infiltration: V= (ACH)(volume, ft3) 60 min/hr V: total air flow volume (cfm) ACH: Air changes per hour volume: space volume (ft3)

Air Change per Hour Method Volume of infiltration: V= (ACH)(volume, ft3) 60 min/hr V: total air flow volume (cfm) ACH: Air changes per hour volume: space volume (ft3)

Air Change per Hour Method Determine Construction Type S: p. 1601, T.E.27A

Air Change per Hour Method Determine Winter & Summer Conditions S: p. 1601, TE.27B&C

Crack Length Method Calculate crack length of windows on the windward side only. Calculate crack length of doors on the windward side only.

Crack Length Method Determine window and door “fit” and Find “k” S: p. 1604, T.E.28C

Crack Length Method Determine wind velocity. Find “velocity head factor.” Determine infiltration rate Calculate total infiltration S: p. 1603, T.E.28A&B

Crack Length Method Find winter infiltration for average fitting windows k=2.0 S: p. 1603, T.E.28C

Crack 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

Ventilation Intentional 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

Ventilation Ventilation is determined according to: ASHRAE Standard 62-2001 (S: p. 1597, T.E.25) Estimates the number of people/1000 sf of usage type Prescribes minimum ventilation/person for usage type

ASHRAE 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 62-2001 Example: (1) Determine the ventilation rate for 2,000 sf office space. 17 cfm/person S: p. 1598, T.E.25

ASHRAE 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