Cooling load calculation method

[frvivi]

I am doing some research in finding the best method for manual and computer technique cooling load calculation.
I read books and papers for this topic and am confused in comparing the theory and practical use.
Some people do not seem to use the method at all and use the rules of thumb instead.
I am wondering when is the best time to use which methods or should I just use the rules of thumb instead.
Do you mind sharing your professional experiences and suggestions with me?
Thank you so much

 

[ZDR1985]

Rules of thumb comes with experience. If you are still beginning with your career, I would suggest to first understand the HVAC fundamentals, and learn manual calculations before dealing with softwares.

 

[IRstuff]

The issue is whether it's worth the effort to crank some complicated spreadsheet to get a load that's only going to accurate for the EXACT condition you analyzed and for the EXACT weather condition. When you work for a company, it needs to make money from your work. If you spend hours cranking calculations to then arrive at a number that's barely 50% accurate, you've wasted the company's money. Rules of thumb generally are "good enough," since they usually cover 90% of the cases with margin, which is what the customer needs anyway.

The only real question is WHICH rule of thumb. For residential A/C, I've seen salesmen use 1 ton/500 sq.ft. and 1 ton/700 sq.ft. That'll be the difference between gross overkill on margin vs. "good enough" for the customer. I wound up buying from the 1 ton/ 700 sq.ft. guy, mainly because he said that there really wasn't a need for 33% extra tonnage that the other guys would have sold me. Over the subsequent 15 yrs, the nice guy was proven correct; I never needed the extra tonnage, and I believe that my system was more energy efficient than the one I might gotten. Nevertheless, an on-site survey should be conducted to see if the the site is consistent with the rule of thumb being used. Obviously, the guys with the marijuana grow-lamps are going to need the higher tonnage ;-)

TTFN
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[urgross]

I believe it would depend on what level of energy audit is being performed. Rules of thumb can be used for preliminary screening, pretty much like using scoreboards like SIR and simple payback. When you screen out the low lying fruit and the pie in the sky and get to an investment grade audit, then bin analysis is advised. Though bin analysis still only uses averages, so do implementation estimates, utilities escalation factors, M&V estimates, etc. Most ESCO's I've dealt with use a conservative contingency with their numbers in any case, not much different than rule of thumb.

 

[willard3]

You should always do a thorough heat gain/loss whether by hand or computer is a matter of choice. If you don't know what you can know, you are not doing the right job.

In my Engineering career, I have fixed a lot of "rule of thumb" projects.

Rules of thumb are for preliminary estimates, not for design.

 

[Drazen]

Even if you are low profile contractor that does not come in touch with more serious projects and too many legislative requirements, you should use at least some basic load estimating software offered by some equipment manufacturers, and for everything above that you need good calculation.

"Real" climatic conditions are only part of the story. On one hand, there is opportunity that you tailor any used software with actual climatic data in your area. On the other, usage profile, internal loads, exposures and building elements inertia affect cooling loads so much that you would need to calculate them even if your overall climatic data are not precise at all.

Rules of thumb would not apply even for the very same building rotated so that lounge is moved from south to north. Forget about them as well as about manual calculation.

The one and only way for rules of thumb to work is to heavily oversize the equipment.

 

[dbill74]

Such good advice already, not sure what else I can add beyond saying that there is a time and place for using rules of thumb, and a time and place for doing a detailed analysis.

Rules of thumb are good for small jobs and select applications such as IRstuff's residential project. They are useful on larger jobs for establishing preliminary estimates, often used by architects and owners for space planning purposes.

Detailed computations are needed on bigger jobs for economic reasons. (By bigger I mean anything bigger than a 2,000 SF house.)

So when to use which? It depends. What are you trying to accomplish? How much time do you have to do it? Do you have the right tools? The answers to these questions are different for every job and comes with time and experience.

 

[frvivi]

Thank you very much for the insights