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Looking for a good A/C installer (Manual J) - Denver Metro bubbabubbs 06-11-2007
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Posted by Noon-Air on June 12, 2007, 9:11 pm

> On Jun 12, 4:07 pm, vze8f...@verizon.net (v) wrote:
>> On Mon, 11 Jun 2007 08:20:55 -0700, someone wrote:
>> >...Someone who is thorough, will use correct
>> >inputs, and won't cut corners on the formulas.
>>
>> 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.
>
> And what if it's more like borderline - "back of napkin" calc will
> suggest 2-ton, but Manual J suggests 1.5-ton.

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??



Posted by v on June 13, 2007, 4:36 pm
On Tue, 12 Jun 2007 20:11:54 -0500, someone wrote:

>
>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??
>
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.



Reply to NG only - this e.mail address goes to a kill file.

Posted by Noon-Air on June 13, 2007, 6:08 pm

> On Tue, 12 Jun 2007 20:11:54 -0500, someone wrote:
>
>>
>>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??
>>
> 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.

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.

> 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?

In the grand scheme of things, probably not, or not enough to make a
difference. FWIW, I *DO* use the exact measurements.

> 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.

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.

> 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.

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??



Posted by on June 12, 2007, 10:01 pm

> On Jun 12, 4:07 pm, vze8f...@verizon.net (v) wrote:
> > On Mon, 11 Jun 2007 08:20:55 -0700, someone wrote:
> > >...Someone who is thorough, will use correct
> > >inputs, and won't cut corners on the formulas.
> >
> > 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.
>
> And what if it's more like borderline - "back of napkin" calc will
> suggest 2-ton, but Manual J suggests 1.5-ton.


v would probably suggest a 3-ton unit in that example. <rolleyes>



Posted by v on June 13, 2007, 4:41 pm
On Tue, 12 Jun 2007 21:01:56 -0500, someone wrote:

>
>v would probably suggest a 3-ton unit in that example. <rolleyes>
>
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.





Reply to NG only - this e.mail address goes to a kill file.

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