How stupid can I be?
wcworth
| Posted in General Questions on
My firm just spent hundreds for a load calculation program that takes in fenestration, wall and roof loads, lighting and other internal gains, outdoor air for ventilation, location, exposures, etc. and now I find out I could have used 1000 square feet per ton! It even divides nice in base ten, by moving a few decimal places.
But seriously, the blog regarding a rule of thumb of t ton per 1000 square feet is the most idiotic, misleading, and oversimplifying way to create a cooling load that I have heard to date. ASHRAE gives the standard of care as computer analysis and that’s what we do.
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Replies
Hey, don't candy coat it, tell us how it really is! :-)
Sure if you want precision you have to do the real math. At the granularity of equipment sizing and the loads for modest sized homes some rules of thumb almost work, but there are lot's of ways to screw it up, so do the real math, eh?
Warren,
You didn't specify which blog you are talking about. Was it this one?
"An AC Sizing Benchmark for High-Performance Homes"?
That's the one where Allison Bailes wrote, "Air conditioners should be sized to meet the cooling load of the house, and bigger is NOT better. If the AC is too big, it doesn't run long enough to dehumidify well, and the constant on-and-off cycles will shorten the life of the equipment. A Manual J load calculation is how you find out what size air conditioner the house needs. ... Now before you get all in a huff and accuse me of being a hypocrite for promoting a rule of thumb, let me explain that I'm not saying the rule of thumb above is a subsitute for Manual J. ... You still have to do a room-by-room Manual J load calculation to select equipment and design ducts properly."
Is that the blog that you think is idiotic?
Or maybe you were thinking of this blog: "Calculating Cooling Loads"? That was the one that said, "Using a rule of thumb to size an air conditioner is no substitute for performing a room-by-room cooling load calculation. Room-by-room calculations are necessary for many reasons: to properly size ductwork, for example, and to address unusual architectural features like rooms with large west-facing windows. Moreover, as air-conditioning guru John Proctor points out, rule-of-thumb sizing “does not account for orientation of the walls and windows, the difference in surface area between a one-story and a two-story home of the same floor area, the differences in insulation and air leakage between different buildings, the number of occupants, and many other factors.” ...Cooling load calculations are complicated, and are therefore best performed with computer software. The most commonly used residential cooling-load software programs follow the Manual J method developed by Air Conditioning Contractors of America (ACCA). The Manual J method is itself based on a method developed by ASHRAE, the Cooling Load Temperature Difference (CLTD) method first published in 1977."
Is that the idiotic blog you are talking about?