by Bill Smith, president of Elite Software

Contractors have many different ways for sizing HVAC equipment. Some can just look at a job and tell you the necessary size. Others take a little more trouble and maybe "weigh" the plans, count windows, or use the area of the house in an effort to estimate HVAC loads more accurately.

Whatever way is used, it's most likely a rule-of-thumb method. This is not to say that some contractors don't meticulously hand calculate loads or punch them into a computer. It's just that a majority still rely on quick, rough estimates. Quick is the key word; many contractors would use a more accurate method as long as there is no loss of speed.

Computer enthusiasts like to say that calculating loads on a computer is fast and easy, and that rules-of-thumb should never be used. In reality, though, nothing is quicker than using a rule-of-thumb. However, good load calculation software can perform load calculations almost as fast as rule-of-thumb methods, with much better accuracy. Upon hearing the virtues of computerized load calculations, many contractors nod their heads and say, "Sure it's great, if you're a computer whiz".

The software person scoffs and says, "Anybody can do it. The programs are so easy to use, there's even built-in help." The contractor responds, "Who said anything about using programs - I can't even turn on my computer."

Many contractors are still in the dark regarding computers. No matter how easy a program is to use, without some minimal knowledge of computers, a contractor can't even get started. Most will admit that if they could get past the introduction, then yes, a computer program would be the best way to perform load calculations.

Fortunately, getting started with computers has become increasingly easy. The cost of hardware has dropped dramatically, and software has become very user-friendly. Although HVAC contractors can opt to learn an HVAC load calculation program on their own, most software suppliers offer relatively inexpensive training, particularly some air conditioning equipment manufacturers that supply software.

If a contractor would bite the bullet and make a modest commitment to computerization, he may be surprised at how painless it can be. This is especially true if the contractor starts slowly and begins with applications he understands well, such as load calculations.

There are many other good reasons for using load calculation software. The first and foremost is accuracy. Rule-of-thumb calculations can give wrong loads on both the high and low side, but the tendency is to be conservative with high loads. In years past, with many contractors using the same rules-of-thumb, being conservative was safe, and also a competitive way to bid. However, today's extremely competitive market has forced contractors to size equipment very close to the actual loads of the building, so the smallest, least-expensive HVAC unit can be bid.

Trying to size equipment close to actual loads makes using rule-of-thumb methods dangerous. The contractor knows rules-of-thumb are conservative. So, when he wants to cut it close, can he go down one ton in size or only half a ton? One wrong guess can ruin the hard-earned reputation of a good contractor. Contractors want to size HVAC equipment carefully, so they can offer the most economical solution possible and still have the utmost confidence that the system will work as required.

Because there is always some uncertainty involving occupants' future needs and lifestyle, most contractors know that the proposed system should have some reserve capacity. Just how much is a matter of engineering judgement. However, with load calculation software, the contractor can make better decisions because more accurate information is available.

Rules-of-thumb can cause problems even when close sizing is not required to win the job. Oversized HVAC equipment not only costs more, it sometimes does not work as well as properly sized equipment. This is especially true during summer cooling months. An oversized air conditioner often cycles on and off excessively, so it doesn't operate continuously enough to properly dehumidify the air. Homeowners with oversized equipment often turn their thermostats down below normal in order to feel comfortable. Of course, this results in high air conditioning costs.

Clearly, it is best to size HVAC equipment to the actual loads encountered, with some allowance for reserve capacity. Predicting the actual loads is no small feat, and software vendors are the first to admit that even computerized load calculations are simply estimates. Depending on the load calculation methodology used, software calculated loads can only hope to be within plus 15% of the real loads. There are so many variables concerning construction methods, building materials, occupant lifestyles, weather, and other factors, that obtaining anything more accurate is almost impossible.

Still, loads calculated by software represent the most accurate information available, and software is continually being improved. As time passes, the fundamental calculation methods used by software will offer extreme accuracy in even less time than today's load calculation programs.

Rules-of-thumb load calculation methods are in their last days. As contractors become more computer literate, those methods will just become comical footnotes in the history of air conditioning. Unlike some contractor software applications, the calculation of HVAC cooling and heating loads is well-covered by many different suppliers. Just for residential and light commercial projects, there are at least 20 load calculation programs available, ranging in price from $50 to $395. Although the top software suppliers are too numerous to list, they advertise regularly and exhibit at the trade shows.

There are two basic kinds of load program suppliers: independent software vendors and HVAC equipment manufactures. The independent software companies are all small, with 10 employees or less. Many are just one-man offices. Although some HVAC manufacturers have a considerable number of people associated with software development and sales, most do not.

In terms of number of programmers, some independent companies are equal to the software departments of larger manufacturing companies. Essentially, there is a lot of good competition in software development, and neither type of supplier has an edge.

No matter what software source is considered, there are some features common to all good load calculation programs. For residential jobs, it is important that the software use calculation methods described in the seventh edition of ACCA.

Manual J; it is the industry standard. Fortunately, the majority of residential load calculation programs follow manual J. A few do not. For commercial projects, software should use either the methods from ACCA Manual N or from ASHRAE. Besides providing correct results and well-formatted reports, it is important the software be fast and easy to use. Calculation attempts with slow and complicated software are guaranteed to send a contractor back to quick and dirty rule-of-thumb methods.

Software should be fast in two ways. First, when given all the necessary input, it should produce answers in a matter of seconds. Most modern programs have no problem calculating this fast. The second aspect of program speed - the ability for quick data entry - is where most programs fail.

Computer experts generally agree that the fastest, easiest way to enter data is with full-screen, "fill-in-the-blank" procedures. This means that a program presents the equivalent of forms on the computer that can be rapidly filled in. A user should be able to quickly move backward and forward through the forms and screens. He should be able to move the screen "cursor" quickly to any input item desired, and have full insert and delete editing capabilities. Some load calculation programs use these techniques, but many do not.

There is more to program ease-of-use than having full-screen entry procedures. The layout and organization of a program greatly affects how easy it is to understand and enter data. Most programs offer "menus," where users select what they want to do. However, if there are too many menus, the program can be confusing. Good programs have a nice balance between the number of menus and input screens.

Program ease of use is a subjective matter. A contractor should try a program before buying it, to see whether it makes sense to him. Ideally, a contractor should try as many programs as practically possible to find one that works best for him. The literature on every program ever developed claims great speed and ease-of-use. Reviewing literature or even the user manual is not enough to evaluate a program. The only way to see how it really works is to see it run.

How does a contractor review a program without having to buy it first? A contractor can sometimes see software demonstrated at a trade show, computer store, or even a friend's office. In addition, some vendors offer low-cost, functional "demo" versions of their programs that allow the user to run the program on small jobs. Functional demos packages usually include full documentation, and are a great way to evaluate software.

Instead of functional demos, some suppliers allow the user to run a full version of the program on a trial basis. This usually requires that the contractor buy the program first and return it for a refund within 30 days if not satisfied.

Besides functional demos and trial versions, many software companies offer free "slide show" demo packages. These diskettes display sample program screens and reports. They are basically an extension of literature; unfortunately, they do not allow program operation.

Kick the rule-of-thumb habit and adopt a quality HVAC load calculation program. Computers are now less expensive, and help is available on how to get started. Carefully evaluate as many programs as possible, and review functional demos whenever possible.

Don't let cost influence your decision; all load programs are relatively inexpensive. Just remember, when evaluating software, do your best to try it before you buy it.

Mr. Smith welcomes your email about this article. - email

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