Heat Load Calculation
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This document provides an overview of calculating heating loads for buildings. It discusses determining heat loss through building envelope components like walls, windows, floors, and infiltration. The heat loss equation and assumptions are explained. Methods for calculating U-factors and R-values of walls, floors, windows, and doors are given. Corrections for factors like framing, metal studs, and cavity depth are also covered. Sample heating load calculations are worked through as examples.
Heat Load Calculation
- 1. ME 425 – Heating Load Calculation Heating Load Calculation ME 425 Air Conditioning System Design Keith E. Elder, P.E. Heating Load Calculation The heating load calculation begins with the determination of heat loss through a variety of building envelope components and situations. Walls Windows Exposed Floors Basement Walls Infiltration Duct Loss Roofs Doors Slab-on-Grade Floors Basement Floors Ventilation Building Pickup 1
- 2. ME 425 – Heating Load Calculation The Heat Loss Equation q = Uo x A ( Ti - To ) Where: Uo = the overall heat transfer coefficient, Btu/hr/ft2/°F A = the surface area, ft2 Ti = the indoor design temperature, °F To = 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 2
- 3. ME 425 – Heating Load Calculation 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 of the elements in the path of the "heat flow.” Properties of typical construction materials can be found in Table 3 of ‘05F25. Construction Material Properties 3
- 4. ME 425 – Heating Load Calculation Air Film Properties From ’05F25.2 Air Gap Properties From ’05F25.4 4
- 5. ME 425 – Heating Load Calculation Wall with no Framing Elements What is the U-factor of a 3-1/2 inch wall composed of 4 inch brick, R-11 insulation and 5/8 inch gypboard? Outside Air Film 4" Brick R-11 Batt Insulation 5/8" Gypboard Inside Air Film Ri 0.17 0.40 11.00 0.56 0.68 Total R = 12.81 Uo = 1/R = 1/12.81 = 0.078 Btu Hr-SF-deg F Correcting for Cavity Depth The R-value of batt insulation is rated by manufacturers for predetermined depths. Corrections must be made to calculations for installation 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 Standard Thickness Nominal Actual Lumber Depth of Cavity, Sizes, Inches Inches 2 x 12 11-1/4 2 x 10 9-1/4 2x8 7-1/4 2x6 5-1/2 2x4 3-1/2 2x3 2-1/2 2x2 1-1/2" 2x1 3/4" 38 30 12" 9-½ " 22 21 19 15 13 11 8 5 6-¾ " 5-½ " 6-¼ " 3-½ " 3-5/8" 3-½ " 2-½ " 1-½ " 3 ¾" Insulation R-Values when Installed in a Confined Cavity 37 32 27 ------ -30 26 21 ----- ---20 14 ---- ---21 ----- ---18 13 ---- ----15 ---- ----13 9.8 6.3 -- ----11 -6.0 -- ------5.7 -- ------5.0 3.2 -------3.0 5
- 6. ME 425 – Heating Load Calculation Correcting for Wood Framing What is the U-factor of the previous example if the wall is framed with Douglas Fir studs spaced 16" on-center. Outside Air Film 4" Brick R-11 Batt Insulation 3-1/2" Fir Stud 5/8" Gypboard Inside Air Film Through Insulation 0.17 0.40 Through 16" OC Stud 0.17 0.40 11.0 0.56 0.68 3.50 0.56 0.68 Total R = 12.81 5.31 Uo = 0.078 0.188 Uavg = 0.20 x 0.188 + 0.80 x 0.078 = 0.100 Btu/(hr-S.F.-F) 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 6
- 7. ME 425 – Heating Load Calculation Correcting for Metal Studs ASHRAE Standard 90.1 Correction Factors Size of Members 2x4 2x4 2x6 2x6 Framing 16 in. O.C. 24 in. O.C. 16 in. O.C. 24 in. O.C Insulation Correction R-Value Factor, Fc R-11 0.50 R-11 0.60 R-19 0.40 R-19 0.45 Using Metal Stud Factors What is the heat loss of the wall in the first example if the wall is framed with 31/2" metal studs spaced 16" on-center? Ri Outside Air Film 0.17 4" Brick 0.40 R-11 + Mtl. Stud (11.0 x 0.50) = 5.50 0.56 5/8" Gypboard 0.68 Inside Air Film Total R = Uo = 1/7.31 = 7.31 0.137 Btu Hr-SF-deg F 7
- 8. ME 425 – Heating Load Calculation ASHRAE Window Performance Because of conductive spacers, frames & new coatings available, window Ufactors cannot be calculated accurately. ASHRAE Window Performance Reasonable estimates can be found in Table 4, ‘05F31.8. 8
- 9. ME 425 – Heating Load Calculation ASHRAE Window U-factors ASHRAE Window U-factors 9
- 10. ME 425 – Heating Load Calculation Door U-Factors Door U-factors can be found in Table 4, ‘05F31.11. Floors on Grade qslab = F2 x Perimeter x ∆T 10
- 11. ME 425 – Heating Load Calculation Floors on Grade - F2 Factors Infiltration The uncontrolled inward air leakage through cracks and interstices in any building element and around windows and doors of a building, caused by the pressure effects of wind and/or the effect of differences in the outdoor/indoor air density. qinfiltration = 0.018Q∆T Q = air flow due to infiltration, ft3/hr 11
- 12. ME 425 – Heating Load Calculation 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 & doors weatherstripped In-Class Heating Load Exercise What is the heat loss for a Seattle office space 12 feet deep by 50 feet long, with 10 foot ceilings and floor-to-floor spacing of 12 feet? The office zone is located on the third floor of a five story building. The wall is constructed of 6 inch concrete, with R-11 batt insulation installed between metal studs 16 inch on-center, and 5/8 inch gypsum board. The window is insulated 1/4 inch glass with 1/2 inch air space and composes 40 percent of the gross wall area. The window framing is metal and has no thermal break. 12
- 13. ME 425 – Heating Load Calculation Building & Space Sketch 50’ 12’ 10’ 12’ Section Sketch Outside Air Film 6 Inch Concrete R-11 Batt Insulation between mtl studs 16” O.C. 5/8 Inch Sheet Rock Inside Air Film Total R = Uwall = 13