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ME 425 Heating Load Calculation

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Heating Load Calculation

ME 425

Air Conditioning System Design

Keith E. Elder, P.E.

Heating Load Calculation

The heating load calculation begins with thedetermination of heat loss through a varietyof building envelope components andsituations.

Walls Roofs

Windows Doors

Exposed Floors Slab-on-Grade Floors Basement Walls Basement Floors

Infiltration Ventilation

Duct Loss Building Pickup

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The Heat Loss Equation

q = Uo x A ( Ti - To )

Where:

Uo =the overall heat transfercoefficient, Btu/hr/ft2/F

A = the surface area, ft2

Ti = the indoor design temperature, FTo = the outdoor design temperature, F

Heat Loss Equation Assumptions

Steady State Conditions

No Thermal Storage

Heat Transfer is Instantaneous

One-Dimensional Parallel "Heat Flow"

Thermally Homogeneous Construction

Construction Anomalies can be Accounted for by

Transmission Weighting Corrections Heat flow path ignores temperature variations

due to construction anomalies

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Solving the Heat Loss Equation

Area Takeoffs (from plans with est. zoning)

Design Temperatures (05F28 CD & Std. 55)

Assembly U-factors, Uo:

Uo = 1/ RTotal

RTotal = R1 + R2 + Rn + ...

R1, R2 and Rn represent the thermal resistance of each ofthe elements in the path of the "heat flow.

Properties of typical construction materials can be foundin Table 3 of 05F25.

Construction Material Properties

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Air Film PropertiesFrom 05F25.2

Air Gap PropertiesFrom 05F25.4

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Wall with no Framing Elements

What is the U-factor of a 3-1/2 inch wallcomposed of 4 inch brick, R-11insulation and 5/8 inch gypboard?

Ri

Outside Air Film

4" Brick

R-11 Batt Insulation

5/8" Gypboard

Inside Air Film

Total R =

Uo = 1/R

= 1/12.81

= 0.078 Btu

0.17

0.40

11.00

0.56

0.68

12.81

Hr-SF-deg F

Correcting for Cavity Depth

The R-value of batt insulation is rated bymanufacturers for predetermined depths.Corrections must be made to calculations forinstallation in non-standard cavities.

TABLE 20-A

R-Value of Fiberglass Batts Compressed within Various Depth Cavities

Insulation R-Value at Standard Thickness

R-Value 38 30 22 21 19 15 13 11 8 5 3

Standard Thickness 12" 9- " 6- " 5- " 6- " 3- " 3-5/8" 3- " 2- " 1- " "

Nominal

LumberSizes,

Inches

Actual

Depth ofCavity,

Inches

Insulation R-Values when Installed in a Confined Cavity

2 x 12 11-1/4 37 -- -- -- -- -- -- -- -- -- --

2 x 10 9-1/4 32 30 -- -- -- -- -- -- -- -- --

2 x 8 7-1/4 27 26 -- -- -- -- -- -- -- -- --

2 x 6 5-1/2 -- 21 20 21 18 -- -- -- -- -- --

2 x 4 3-1/2 -- -- 14 -- 13 15 13 11 -- -- --

2 x 3 2-1/2 -- -- -- -- -- -- 9.8 -- -- -- --

2 x 2 1-1/2" -- -- -- -- -- -- 6.3 6.0 5.7 5.0 --

2 x 1 3/4" -- -- -- -- -- -- -- -- -- 3.2 3.0

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Correcting for Wood Framing

What is the U-factor of the previousexample if the wall is framed withDouglas Fir studs spaced 16" on-center.

Through

Insulation 16" OC Stud

Outside Air Film

4" Brick


3-1/2" Fir Stud

5/8" Gypboard

Inside Air Film 0.68 0.68

Total R =

Uo =

Uavg = 0.20 x 0.188 + 0.80 x 0.078

= 0.100 Btu/(hr-S.F.-F)

Through

0.56 0.56

3.50

11.0

0.40 0.400.17 0.17

12.81 5.31

0.078 0.188

What About Metal Studs?

Transmission weighting

is not appropriate if the

conductivity of the

penetration exceeds that

of the layer being

penetrated by more than

an order of magnitude.

Isotherms

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Correcting for Metal Studs

ASHRAE Standard 90.1 Correction Factors

Size of Insulation Correction

Members Framing R-Value Factor, Fc

2 x 4 16 in. O.C. R-11 0.50

2 x 4 24 in. O.C. R-11 0.60

2 x 6 16 in. O.C. R-19 0.40

2 x 6 24 in. O.C R-19 0.45

Using Metal Stud Factors

What is the heat loss of the wall in thefirst example if the wall is framed with 3-1/2" metal studs spaced 16" on-center?

Ri

Outside Air Film

4" Brick

R-11 + Mtl. Stud (11.0 x 0.50) =

5/8" Gypboard

Inside Air Film 0.68

Total R =

Uo = 1/7.31 =

0.56

5.50

0.400.17

7.31

0.137 Btu

Hr-SF-deg F

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ASHRAE Window Performance

Because ofconductivespacers,frames & newcoatingsavailable,window U-factors cannot

be calculatedaccurately.

ASHRAE Window Performance

Reasonable estimates can be found inTable 4, 05F31.8.

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ASHRAE Window U-factors

ASHRAE Window U-factors

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Door U-Factors

Door U-factorscan be found inTable 4,05F31.11.

Floors on Grade

qslab

= F2 x Perimeter x T

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Floors on Grade - F2 Factors

Infiltration

The uncontrolled inward air leakagethrough cracks and interstices in anybuilding element and around windowsand doors of a building, caused by thepressure effects of wind and/or theeffect of differences in theoutdoor/indoor air density.

qinfiltration = 0.018QT

Q = air flow due to infiltration, ft3/hr

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Infiltration Estimates

From the 1972 Fundamentals Manual:

Number of Air

Changes Taking

Kind of Room or Building Place per Hour

Rooms with no windows or exterior doors 1/2

Rooms with windows/exterior doors on one side 1

Rooms with windows/exterior doors on two sides 1-1/2

Rooms with windows/exterior doors on three sides 2

Entrance Halls 2

Note: Values may be reduced by 1/3 if windows & doorsweatherstripped

In-Class Heating Load Exercise

What is the heat loss for a Seattle office space 12feet deep by 50 feet long, with 10 foot ceilings andfloor-to-floor spacing of 12 feet? The office zone islocated on the third floor of a five story building. Thewall is constructed of 6 inch concrete, with R-11batt insulation installed between metal studs 16inch on-center, and 5/8 inch gypsum board. Thewindow is insulated 1/4 inch glass with 1/2 inch air

space and composes 40 percent of the gross wallarea. The window framing is metal and has nothermal break.

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Building & Space Sketch

50

12

10

12

Section Sketch

Outside Air Film

6 Inch Concrete


between mtl studs 16 O.C.

5/8 Inch Sheet Rock

Inside Air Film

Total R =

Uwall =

Documents

425-2-Heating Load-2007