I am looking for an A/C installer that uses the Manual J method to correctly size the new unit. Someone who is thorough, will use correct inputs, and won't cut corners on the formulas.
In Denver Metro, Colorado
TIA
Give Alex a call, tell him I sent you.
Alex Walter Furnaces, A/C & More
12255 E Harvard Dr Aurora CO 80014-1919 USAVoice: 303-695-1465 Fax: 303-695-4543 email: snipped-for-privacy@FurnacesACandMore.com
To what end is the need to be that precise? They will just size it to the nearest ton (or maybe half ton) anyway. It's not like it's going to make much difference if it comes out 38,052 btu/hr for the anal retentive guy, or 41,019 btu/hr for the good enough for government work guy.
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Might as well guess on everything, Eh?
And what if it's more like borderline - "back of napkin" calc will suggest 2-ton, but Manual J suggests 1.5-ton.
The point imo is more to find somebody who actually will do a reasonable calculation and size it reasonably closely rather than putting in a 3T where a 1.5 would do...
If it's new, more likely to be of value as if have existing unit that works reasonably well, that's a good clue. If, otoh, the current unit is obviously way over-sized, can't hurt...
A really good, experienced guy can probably do ok w/o it, but who knows one of them, either? :)
The customer gets a complete print-out of the Manual J and Manual D calculations that tell exactly what is required. Unless you do the math and run the calculations, your doing nothing but guessing. Do *you* want to base that kind of investment and cash outlay on a guess??
v would probably suggest a 3-ton unit in that example.
What I'm getting at is, how does one "cut corners on the formulas" when the contractor is using a commercial spreadsheet package where he puts in the parameters and the "formulas" are built in. I used to do these calcs "by hand". I doubt most contractors could, they just use their computer package and the answer is whatever it says. So what that they give a copy of the printout to the homeowner. Don't confuse accuracy with precision.
If a window is 4'-8" by 3'-4" but the contractor figured it as 3 x 5 feet, is it going to matter that he took a shortcut with the formula?
So you get an "exact" calc, but the units are not made in that "exact" size. Then you ask the contractor to use his experience, judgment and understanding of principles to tell you that you are better off with the one that is a little undersized even though the one that is a very little bit oversized is closer to the "exact" figure.
I'm not impressed with people who toss around an "exact" figure that just came out of a computer as if that means the person holding it knows something.
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The contractor can't "cut corners on the formula" because he likely doesn't have and doesn't know the formula anyway. He just punches the numbers in to something that he bought and then tries to impress you with the resultant printout.
I used to do these "by hand" (okay, pocket calculator) back in the day. Don't confuse precision with accuracy. Garbage in garbage out. You are better off with someone who can make a reasoned estimate here and there, than with someone who laboriously inputs precise fractions but uses the wrong conditions and assumptions because he is just doing data entry and not understanding principles.
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Yeah, well, we don't send out a rookie with a computer to perform a load calc.
If you'd send someone out that has HVAC knowledge, you would understand that a load calc *is* worthwhile.
Might as well install that 5-ton unit, Eh?
I used to be able to manually do load calcs, but that was before I had a computer and the software to do it with more precission, and a bit faster too. Without precission, you don't have accuracy.
In the grand scheme of things, probably not, or not enough to make a difference. FWIW, I *DO* use the exact measurements.
The load calc will tell you what btu capacity you need, and the manufacturers specs will tell you whats available. Somewhere you have to come to a happy medium.
Nobody asked if you were impressed or not....I just do the very best I know how to do. so why are you here trying to stir the pot??
Where I come from, rule of thumb for average constructed homes is 1 ton/500 sq ft (8ft ceiling). Of course it's not calculated but I bet it's close to accurate for the average home where I am.
Gee..... where I am at, there is no rule of thumb.... I have homes with
*correctly sized*(calculated) systems that range anywhere from 500sqft per ton up to 1500sqft per ton, with an average being 700sqft/ton with design temps of 98DB/77WB for cooling and 28DB for heating.
Well its good you have two thumbs. You can stick one up your butt while your sucking on the other with that mental brainwork for calculations. Bubba
Where I come from, I do things correctly. That means the client gets a Manual J to correctly size his or her new system. However, there are hacks everywhere that use "rules of thumb" for just about everything. Their clients are unhappy when they receive a quote to fix all the screw-ups.
Do it right, or do it twice. I garranty doing it right the first time is cheaper in the long run!
Wow, that would make for a huge appliance, sounds oversized by quite a bit. I have a 3 ton York AC for my 2,450 sf house, and it's pretty good, except that the attic is noticeably warmer than the rest of the house. Anything larger would be a waste as we don't have proper cold air returns everywhere we should on the upper floors (built in 1930, long before central air).
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