Design Topics
June 2010
Heat Load Calculation Primer
Your contractor says you need 20 tons of cooling but your
brother in law says you only need 10 tons and you're being
charged for something you don't need. How is the lay person
to know?
Take a few minutes now to get a grip on the fundamentals
!
A British Thermal Unit (BTU) is the amount of heat energy
needed to raise the temperature of one pound of water by one
degree Fahrenheit. This is the standard measurement used to
state the amount of energy that a fuel has as well as the
amount of output of any heat generating device.
In performing heat load calculations one is surveying every
source of heat within the space that is to be conditioned
and expressing the total load as BTU/hour. In addition to
the BTU values of human occupants and that of exterior radiation
that penetrates the envelope of the building, electrical devices
must be accounted for.
Conversion formula - All
electrical devices produce heat but in order to express this
in BTUs one needs to convert watts to BTUs and the conversion
factor is 1 watt = 3.41 BTU. For instance
a 40 watt light bulb, when on, produces 40 x 3.41 or 136.4
BTUs and a tower workstation rated ( found on the information
plate on the back of the device) at 350 watts would produce
1,190 BTUs hourly.
Heat loads typical of commercial buildings
| No.
|
Item |
Watts |
BTUs/hr |
| 1 |
Workstation (each) |
200. |
682 |
| 2 |
Monitor (each) |
200. |
682 |
| 3 |
Human Body (each) |
130. |
442 |
| 4 |
2x4 Florescent light (each) |
107. |
365 |
| 5 |
Exterior Windows (S.F. of surface
Area) |
1.3 |
4.4 |
| 6 |
Exterior Walls (S.F. of surface
Area) |
0.65 |
2.2 |
| 7 |
Ceilings (S.F. of surface Area) |
0.7 |
2.4 |
| 8 |
Other heat generators - these would
be printers, copiers, or any heat producing device not
listed above. |
|
? |
| Use
the conversion formula above to convert watts (on device
name plate) to BTUs and add in the list - see example
below |
The sample workup below is based on a 5,984 s.f. space improved
as office. The building is rectangular (68' x 88') with walls
16 feet high and has 1,368 square feet of exterior glass.
The design occupancy load is 60. Light fixture count is consistent
the Title 24 Energy conservation Law
Item |
Quantity |
BTU
ea |
Total
BTUs |
Workstation
(each) |
60 |
200. |
40,920 |
Monitor
(each) |
60 |
200. |
40,800 |
| Human
Body (each) |
60 |
130. |
26,520 |
2x4
Florescent light (each) |
73 |
107. |
26,645 |
Exterior
Windows (S.F. of surface Area) |
1,368 |
4.4 |
6,019 |
Exterior Walls
(S.F. of surface Area) |
4,992 |
2.2 |
10,982 |
Ceilings
(S.F. of surface Area) |
5,984 |
2.4 |
14,361 |
*Other heat
generators |
10 |
4,501 |
45,010 |
| |
|
Total BTUs |
211,257 |
| * Freestanding
copier 1,320 watts x 3.41 = 4,501 BTUs |
One ton of cooling, the equivalent of melting
2,000 pounds of ice in a twenty four hour period, removes
12,000 BTUs – therefore in our sample above one divides the
sum of the BTUs by 12,000 yielding , in this case, a cooling
demand of 17.6 tons. or one ton/ per
341 square feet.
A rule of thumb that has been around for years is that
of 350-400 square feet per ton of cooling
- and if you've read this you know why. Keep in mind that
this article is presented in a "rule of thumb" context
and that in any specific case there will be variations in
the result.
Contributor: Coldcraft Inc.
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