Central A/C Selection

call someone reputable in the business, verify nate certs and require manual j calcs to comply with utility comp requirements for energy rebates.

if that is correct, why did ashrae waste the time and money doing heat loss heat gain studies and publishing them for use by design engineers.

It's a rough estimate. It will get you in the ballpark and some of the time on the money. Another thing to realize is that AC equipment comes in standard sizes. 5 ton, 7.5 tons, 10 tons, 15 tons and so on. You can do a detailed analysis using the best software to determine that on a peak design day you need 12.13 tons of cooling -- but you're going to wind up buying a 15 ton unit.

I can be careful and buy "right sized" equipment, or just take a semi- intelligent guess and pay the utility for something I hope is only slightly oversized.

Rules of thumb are meant to be quick and easy, but also have some degree of overkill for a "safety factor".

Pay me now or pay me later.

I only use the "Rules of Thumb" sizing if I need to make a fast budget estimate. There is a book you might want to get called "HVAC Equations, Data, and Rules of Thumb by Arthur A Bell, Jr. put out by McGraw Hill. You need to do the detail load analysis to select the nominal tonage unit. I would be in real trouble trying the same rule of thumb for a classroom, bowling center or a precision measurment lab.

I do agree with you. for cost estimate we can use the thumb rules. rough esitmate approx capacity is ok,

But definetly not for "sizing the Equipment"