2018 Issue

41 Without the actual building to reference, you must make assumptions. • Exterior weather conditions * If you were asked to name a profession where it is accept- able, or even expected, to be wrong half of the time or more, most people would think of the infamous weatherman. With all the science we use for predicting the weather and all the tracking we do, we are still just guessing. Weather has a major impact on the heating loads for a building. These are only a few of the assump- tions required to calculate the heating loads for an office building. There are many more and this is a relatively simple task. Over the years we have become quite proficient at knowing what assumptions to make for these items so we can be fairly accurate with calculating heating loads on a build- ing. If you don’t know what assump- tions can be made and what they should be, it is unlikely that you will get accurate results. There are many other engineering tasks that are much more complicated with many more associated assump- tions. Most of us think we have been taught to avoid assuming anything. However, we make assumptions all the time. Assumptions are not always bad. They make it possible for us to find solutions to the complex problems presented in the real world. The art of science is knowing what assumptions can and should be made. Biochemistry professor and experienced sideburn cultivator Isaac Asimov said, “There is an art to science, and a science to art; the two are not enemies, but different aspects of the whole.” The more time we spend on a problem and the fewer assumptions we make, the more exact we can be. However, the law of diminishing returns applies. We will get to a point where spend- ing a large amount of additional time will result in only a small increase in accuracy. Knowing when you are as accurate as you need to be is an art. Some assumptions can cost a small amount of accuracy and save a great deal of time. Some assumptions may save time, but would reduce accuracy beyond an acceptable limit. I was once at my local hardware store trying to purchase a 12” length of rebar. The piece of rebar I brought up to the checkout stand did not have a sticker on it. The “checkout person” did not know what to type into the register to bring up the price. She opened her hard copy catalog and found a picture that sorta’ looked like the item I wanted. She started to ring me up for a 2’ length of rebar. I told her it was not 2’ long, it was only 1’. She would not take my word for it. I then held it next to my foot to show her it is only one foot long. I hoped she would recognize that my foot was not 2’ long. She still didn’t seem to believe me, the picture in her catalog looked too much like the item. I did not need to use highly accurate calipers to measure the length down to the ten-thousandth of an inch to realize that the rebar was not 2’ long. I just needed something close enough to 12” to know that it was not 2’ long. I was satisfied with the assumption that my foot was closer to 1’ than 2’ long. Unfortunately, the cashier was not. She needed to put in the extra effort and time to get a tape measure. The more information and experience we have, the more “freedom” we have with assumptions. This would be like having more colors to use in a painting. If knowing the approximate length of an average adult human foot is like having the color green to paint with, the cashier could not paint a tree. She had to go get a different green (i.e. tape measure). We cannot be just engineers, we must be artists. The astronomer Phil Plait said, “I have to smile wryly when I hear people try to distinguish art from science. The Universe is both, folks. You may try to tear them apart, but you can- not, for the artistry of the Universe is for- ever intertwined with how it works. They drive each other; the science is why the art is beautiful, and the art is one of the reasons we pursue the science.” Another example from my world (HVAC and plumbing) to illustrate that science is not an exact science is sizing a domestic water heater. I recently saw a domestic water heater design that was done by another engineer. The water heater was sized at least twice as large as I would expect for the building. I doubt the engineer finished the calculations and said, “OK, now I will double the size just for fun.” They probably sized it appropriately for the assumptions they used. His assump- tions were just different from my assumptions. There are many assump- tions that make a large difference in the result. Below is a list of assump- tions required for sizing a water heater. I have also listed how we decide what to assume. • Quantity of hot water fixtures in the building. * How to know what to use: count the fixtures from the architectural plans. • Water temperature entering the building (cold water inlet). * How to know what to use: ex- perience with other buildings in area; educated guess. • Temperature of water required at each fixture. * How to know what to use: Mix- ture of code requirements and owner preferences. • Number of building occupants. * How to know what to use: Wild, but semi-educated guess. • Duration of fixture use. * How to know what to use: personal preferences and wild guess. • When fixtures will be used. * How to know what to use: wild guess.