A/C contractor quotes - need to evaluate

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For round ducts, 6" is roughly 100 CFM, 7" is roughly 150 CFM, and 8" is roughly 200 CFM. Go around and add them all up, and this will give you the total CFM your ducts can handle, assuming your trunk duct is big enough. You may need to rip out your whole duct system if you go with a larger unit. Rarely have I seen a house require more heating or cooling than it was originally designed for unless there was an addition. Quite often, it will require less BTUs because of new insulation, windows, etc. A 3 ton unit will require ROUGHLY 1200 CFM. If you don't have enough duct work, your system isn't going to operate properly, and might be noisy.

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If someone is designing the size of you r system based on the size of your current duct work then this person is an idiot or a crook. You need to do a "manual J" to size a system correctly. Run from this person.

13 seer or greater is the law. period.

Probably true.

Bullshit, buy a rheem, get rheemed. Trane is top of the line.

This is correct.

This guy is an idiot or a crook. Run.

This is correct.
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Al Moran wrote:

Manual "J" is a great start but it is only as good as the information being entered. In other words garbage in garbage out.

Not necessarily true. R22 is partly preferred because the industry is familiar with the product. Regarding "unproven" it is going to make or brake a contractor if they do not install R410 products correctly.

That is a matter of opinion. I have seen Rhuud/Rheem, Janitrol/Goodman units last as long as Trane. Again many factors must take place for proper installion.

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Rule of thumb in south Texas is 800sq ft / per ton for a/c sizing. Other parts of the country will be different.
My guess is if you had a 2 ton and now added your basement to the a/c load, you probably need a 2.5 ton. Basements usually don't get as hot as above ground so I'm thinking you don't need a lot more than what you had before. Of course I'm guessing because I don't know the location nor size of your basement but assuming the old one was doing a decent job then 2.5 should work. No need to over power the house. I suppose if you got a greater ton a/c, you could cool off the house faster if all else is sized properly. And no I'm not an expert just a guy who deals with a lot of real estate over the years in Texas.
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Madhatter, it is correct that the home had (and still does) a 2-ton unit for the lower (main) level at the time of construction. At the time of and just prior to purchase, a fifth 8" duct was installed at my request, and I was way more ignorant then than I am now. The builder did not point out the downfall of such a plan; he wanted to unload the house. As I mentioned in the earlier thread, I have never been happy with the cooling. Call me an idiot, I then added a basement and another 8" duct.
So, it appears that 2.5 is definitely called for, and perhaps 3, given the two additional demands on the original unit as explained above. Thoughts, anyone?
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kck posted for all of us... I don't top post - see either inline or at bottom.

Yes, get the Manual J
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