You probably don’t think of HVAC load calculations as something that takes “skill” in the way that the word is usually used. Skill is usually applied to things like sports, music, games, or similar activities. But consider this question: if two engineers perform load calculations on the same building, will they get the exact same result? Almost everyone who has done HVAC load calculations would likely agree that the answer is No. Now consider a second question: with two differing results, wouldn’t one of them be closer to the correct result? The answer to this is clearly Yes, since there will be a value that is the maximum heating or cooling load that the building will actually experience. With two engineers getting different results, and one of those results being better than the other, wouldn’t it mean that if an engineer can achieve closer results all or most of the time that they are more skilled at load calculations?
Now, of course there may be other factors to consider, such as a client’s willingness to oversize equipment, desire for energy efficiency, or cost constraints when purchasing equipment, but that doesn’t change the fact that when performing the calculations, one can still display more skill at achieving the most accurate results.
As a professional designing HVAC systems, you choose only the best tools to deliver the best outputs to your customers. These tools include CAD software, spreadsheets (likely Excel), load calculation software, a high-performance computer, and two or three monitors. Consider a professional carpenter. Someone who has achieved a level of skill at carpentry to the point where others are willing to pay them for that skill won’t settle for dull saws, mediocre wood, or semi-accurate measuring devices. They will use high quality tools to ensure they don’t need to contort their body to make cuts, eyeball measurements, or awkwardly force joints into place.
As an HVAC designer, why would you use an HVAC load calculation software that you don’t fully understand or that doesn’t allow you full flexibility over your design. Consider inputs such as wall and building weight, lighting power density, and radiant/convective splits of internal loads. All these inputs can make a noticeable difference in load calculations, but if you’re providing highly generalized estimates as inputs because “that’s how you’ve always done it” or “that’s how you were taught”, then you’re sacrificing the opportunity to obtain better results, likely because it is too difficult to obtain the proper values on each specific project.
Using some input or software tool just because that’s how your boss does it like a carpenter refusing to use electronic measuring devices or power tools just because his boss didn’t use them. If it’s a better tool that provides more accurate results while saving you time, why not use it?
So much of what we describe as skill in a profession such as engineering really amounts to having the ability and understanding to use the best tools.
When performing load calculations, many engineers will simply ignore things like schedules, diversity factors, and internal and external window shadings. Ignoring these values may give a good way to estimate the worst case scenario of individual spaces, but will end up significantly oversizing air handling systems and potentially massively oversizing central plants.
The image below was taken in the middle of the day on a work day of a new office tower.
This image shows the southern exposure of a glass wall, and the internal shading devices can easily be seen in each office with exposure on the wall. It is almost certain that when all the air handlers and the central chilled water plant in this building were sized the calculations were based on the assumption that internal shadings were not being used anywhere. However, it appears that more than 50% of this exposure is shaded on a sunny day.
Exercising skill at the load calculations for this building would involve sizing each office as if the shading were not used, but then sizing the air handlers and central plant as if many of the shadings were being used, since that would replicate a much more realistic situation. By using a statistical approach to the load calculations of this building, the central plant could be downsized, saving money on equipment, floor space in the mechanical room, and operating costs on energy inefficiency of an oversized system.
As is well-known by HVAC designers and engineers, a refrigeration system has an efficiency curve based on how high of a load it is operating at relative to its peak design point. The image below shows how a typical refrigeration system loses efficiency (and therefore achieves a lower COP) as it operates at partial loads.
If your chiller or other refrigeration equipment is oversized to the point that peak loads are operating at only around 80% capacity, then the should seasons may only be at around 30% capacity. In some cases (depending on equipment manufacturer) that could be the difference between a COP of 8 and a COP of 4, doubling the amount of energy used for useful cooling.
By sizing room maximum airflow rates based on maximum possible load but making more realistic estimates for central AHU and central cooling plant calculations, you can achieve room satisfaction as well as properly sized equipment.
Although this is not generally a concern with heating loads because both electric and gas heat do not lose efficiency in the same way cooling equipment does, it is becoming more important as heat pumps gain popularity.
An aspect of skill in a profession is eliminating wasted time. Like a carpenter saving time and effort by using a buzz saw rather than a manual saw, and engineers using AutoCAD and Revit rather than drafting by hand, it only makes sense that the same should apply to load calculations. Using software that causes you to waste time because it’s too difficult to understand or requires intensive setup, or requires constant troubleshooting is not what an expert in the field should be doing.
Using tools to minimize wasted time and maximize productive output are signs of a highly skilled professional in their field, and no matter how skilled someone is in their profession, using better tools always results in improvement.
The aspect of time-saving is especially important as HVAC designers because of the nature of the consulting industry. You are not charging for materials or hard goods, you’re simply charging for your time. Saving time on menial and repetitive tasks means you can have higher confidence in sticking to your project budget, saving on your operating costs while still providing the expected output to your clients.
HVAC load calculations are a complicated process that involve a degree of skill. Using the proper tools for the job are essential for a professional who is charging for their services. By choosing the right tools for the job, you save time, money, and difficulty, while also providing the best results. Your load calculation software should not be the cause of lost time, poor estimates because accuracy would be too difficult or time consuming to attain, or unnecessary stress.
Choosing the right software for your load calculations and exercising skill in your profession can improve your operating margins on projects, reduce the likelihood of oversizing equipment (and costing your client unnecessary money), and reduce excess energy consumption of completed projects through inefficient central plant operation.
