Calculating Building Heat Gain

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If you got here from my how to select your Passive Cooling Method lens, then you are ready to get into the nitty-gritty of designing your passive cooling system. In order to do that, we need to know just how much heat do we have to get out of the building with our selected system. That is where building heat gain comes in. The estimated summer heat gain is determined by finding the sum of all the heat gain effecting each component of a building, ie, walls, windows, doors, roof, people and equipment.
And of course that means having a building design, or at least a start at one. We will use this simple office design.

What you need in order to do this is as follows:
-A calculator
-Paper and Pencil
-Mechanical and Electrical Equipment for Buildings

Now that you have the tools and a building to work with, you need to identify the location of the building. For our fictitious office building, we will use Sacramento, California for the location. The summer design dry bulb temperature is 98/70 (from appendix A).
Now about the building. You need to identify the following areas (my building's units will follow;
-Total Floor Area 1286 sf
-Floor Slab Perimeter 188 ft
-Total Roof Area 1616 sf

-Opaque Wall Area
- North 364 sf
- South 518 sf
- East 320 sf
- West 365 sf
Total 1567 sf

-Window Area
- North 144 sf
- South 108 sf
- East 12 sf
- West 0 sf
Total 264 sf

-Door Area
- North 0 sf
- South 0 sf
- East 42 sf
- West 0 sf
Total 42 sf

Once you have all of that, we can start to do the calculations.

Mechanical and Electrical Engineering for buildings has all of the factors and other data that we will use here. Check it out from the library or if you plan on doing this often, it is worth the purchase. I am using the 9th edition so page numbers may vary depending on the version you are using.

Okay, lets get started. We are going to calculate the heat gain per square foot per hour.

Part 1. People and equipment.
This is based on the function of your building. Using table 5.8, we find our building use in the left column and follow to the right to find the total sensible heat gain due to warm bodies and equipment. The table gives ranges and you can be as conservative or loose as you want, but it effects the end result, I aim high knowing that it will probably help me in the end. So, for an office the chart gives me a range of 1.7-3.4 btu/ft^2 hr. I will use 2.5, the office is small and does not have a lot of heat producing equipment.

Part 2 Electric Lights
First we determine the estimated Daylight Factor which is the window area divided by the floor area, mine is .21 or 21%. Table 5.8 part B gives me a sensible heat gain of .5 Btu/ ft^2 hr. That was easy wasn't it?

Part 3 The Building Envelope

WINDOWS
Now we do the real work starting with the windows. The design cooling load factor for windows varies with the direction the window faces if they are not shaded, which is the case for my building. If you windows have exterior shading, use the factor from Part C of table 5.8. As for the rest of us, let's move on to table 5.19 for those factors; North = 17, South = 25, and East/West = 47. This reveals the reason why architects recommend reducing glazing on the East and West, the heat gain factor is almost double that of the South and nearly 4 times that of the North.
Anyway, for the calculation we take the window area and divide it by the total floor area and multiply that number by the load factor from the chart. Add the results to get your total heat gain through the windows.
For my building it looks like this;
N - (144/1286)x 17 = 1.9
S - (108/1286)x 25 = 2.1
E - ( 12/1286)x 47 = .44
Total heat gain through windows = 4.44 Btu/ ft^2 hr

WALLS
A similar calculation is made on the opaque area of the walls, except now we need the U-value of the wall assembly. The U-value or transmisivity of an assembly is the inverse of the R-value or resistance of the assembly. For my wall the U-value is .081. I then calculate the total wall area divided by the floor area, this is then multiplied by the U-value, and this final number is multiplied by the heat load factor from part C of table 5.8, in my case 22. Thus,
((1562/1286)x .081)x 22 = 2.17 Btu/ft^2 hr

ROOFS
Roofs are calculated exactly like the walls, my U-value is .046 and my heat gain factor is 42. Thus,
((1616/1286)x .046)x 42 = 2.43 Btu/ft^2 h

AND TOTAL...
If we add up the sum total of Part 1-3 we get the total heat gain;
2.50 Btu/ft^2 hr
+0.50 Btu/ft^2 hr
+4.44 Btu/ft^2 hr
+2.17 Btu/ft^2 hr
+2.43 Btu/ft^2 hr
=12.04 Btu/ft^2 hr
That is your key number in determining the size and effectiveness of your passive cooling system. From here we will look at each of the methods mentioned in my other lenses linked below.