Heating Load Calculation

Transcription

1 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 The Heat Loss Equation q = U o x A ( T i - T o ) Where: U o =the overall heat transfer coefficient, Btu/hr/ft 2 / F A = the surface area, ft 2 T i = the indoor design temperature, F T o = 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 Solving the Heat Loss Equation Area Takeoffs (from plans with est. zoning) Design Temperatures ( 05F28 CD & Std. 55) Assembly U-factors, U o : U o = 1/ R Total R Total = R 1 + R 2 + R n +... R 1, R 2 and R n 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 Air Film Properties From 05F25.2 Air Gap Properties From 05F25.4 4

5 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 Total R = Ri Uo = 1/R = 1/12.81 = 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 12" 9-½ " 6-¾ " 5-½ " 6-¼ " 3-½ " 3-5/8" 3-½ " 2-½ " 1-½ " ¾ " Nominal Lumber Sizes, Inches Actual Depth of Cavity, Inches Insulation R-Values when Installed in a Confined Cavity 2 x / x / x 8 7-1/ x 6 5-1/ x 4 3-1/ x 3 2-1/ x 2 1-1/2" x 1 3/4"

6 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. Through Insulation Through 16" OC Stud Outside Air Film 4" Brick R-11 Batt Insulation 3-1/2" Fir Stud 5/8" Gypboard Inside Air Film Total R = Uo = Uavg = 0.20 x x = 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 Correcting for Metal Studs ASHRAE Standard 90.1 Correction Factors Size of Insulation Correction Members Framing R-Value Factor, F c 2 x 4 16 in. O.C. R x 4 24 in. O.C. R x 6 16 in. O.C. R x 6 24 in. O.C R Using Metal Stud Factors What is the heat loss of the wall in the first example if the wall is framed with 3-1/2" metal studs spaced 16" on-center? Ri Outside Air Film " Brick 0.40 R-11 + Mtl. Stud (11.0 x 0.50) = /8" Gypboard 0.56 Inside Air Film 0.68 Total R = Uo = 1/7.31 = Btu Hr-SF-deg F 7

8 ASHRAE Window Performance Because of conductive spacers, frames & new coatings available, window U- factors cannot be calculated accurately. ASHRAE Window Performance Reasonable estimates can be found in Table 4, 05F

9 ASHRAE Window U-factors ASHRAE Window U-factors 9

10 Door U-Factors Door U-factors can be found in Table 4, 05F Floors on Grade q slab = F 2 x Perimeter x T 10

11 Floors on Grade - F 2 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. q infiltration = 0.018Q T Q = air flow due to infiltration, ft 3 /hr 11

12 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 Building & Space Sketch 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 = U wall = 13