Trane Condenser Selection - 3.1 Ton XL16 vs 3.5 Ton XL15

I'm about to get my central AC replaced and was hoping to get some opinions on sizing. I currently have a 4-ton unit that feeds 13, 6" round ducts (appx 1300CFM capacity). My house is very well insulated. I recently recieved 3 estimates from authorized Trane dealers who recommened the XL16i and XL15i models. My dilemma lies in determining the correct size of the XL16 if I were to select it. All contractors indicated that the 4-ton unit is a little large and that the existing ductwork dosent support the CFM required for a 4-ton unit. None the less, 2 of the 3 recommended 4-ton units. One of the 3 recommended a 3.5 ton XL15 or a 3.1 ton XL16 (likely because the 4 ton is not eligable for the tax credit). I agree that a somewhat smaller unit (<4 tons) is appropriate because of the existing ductwork and the fact that my existing system could use to run a little longer. Turns out that the XL15 is available in .5 ton increments, while the XL16 is only available in 1 ton increments. I'm inclined to go with the XL16i, knowing that it will switch between 3 tons and 2 tons (70%) as required, however I'm afraid that if it is slightly undersized it will be spending most of it's time at 3 ton (losing the real 2-stage advantage). If I go with the 4-ton, I have the same concern - that it will may never run at the 4-ton level.
I suspect that the correct sizing is 3.5 tons given my discussions with the contractors.
One more piece of info - all units will be matched to the appropriate variable speed air handler.
Any comments or recommendations?
Thanks in advance, Joe
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Think about checking into a 2-speed compressor.

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1300 CFM? I doubt it. Tell me what your static is then you can decide what CFM you are running.You have done a static pressure test, havent you? Bubba
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Bubba - None of the contractors have performed a test. They just indicated that each 6" duct is rated at 100 CFM (nominal).
Bubba wrote:

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Exactly my point. They are guessing and assuming. So are you. I have many "thumbs" too. Which would you like? I can blow 40 cfm through a 6" run and I can shove 400 or more CFM through a 6" pipe. It all depends on how your ductwork was done and what static it's operating at. Get someone who has a clue or dont waste your money on a high eff system. On the other hand, dont be expecting to get all that info for free. Bubba

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So you want to run 4 tons of A/C through ductwork that will only support 3 tons..... Who spec'ed 4 tons?? How did they arrive at that size?? Maybe you should have a complete room-by-room Manual J heat load and loss calculation done to determine just exactly what you need, then go from there.
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The information you have: Three contractors say 4 tons is too large. You agree. But only one contractor recommended a smaller unit. He sounds like the best of the three. A Manual J calc would give you more info if you can find a competent person to do it. But keep in mind the results can be fudged to get the answer desired. If it were my choice with only the info you gave, I would get the 3.1 ton unit recommended by the best of the three contractors. I like my unit to run and do not mind taking some time to cool down to 75F.
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esquimeauxpie wrote:

Your 4-ton system with that ductwork may have only been delivery 3-ton of cooling. Did it cool okay? None the less, 2 of the 3 recommended 4-ton units, wow!
We don't know what climate you live in, but I am still betting a 3-ton unit with a scroll compressor and a TXV refrigerant control, and 4-ton airhandler will do the job! \\ Do a thorough manual J individual room calc and then do everything you can to reduce the heat-gain, and then refigure the equipment sizing. No one knows what equipment size you will end up with.
Unless you live in Florida or southern Texas I wouldn't go any higher than 14-SEER. Click the link below and read everything you can on the subject before spending your money! - udarrell - Darrell
--
Air Conditioning\'s Affordable Path to the "Human Comfort Zone Goal"
http://www.udarrell.com/air-conditioning-total-heat-enthalpy-latent-heat.html
  Click to see the full signature.
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Thanks Darrell. I've been to your website and it is very informative. I'm going to re-visit a couple of the contractors and ask them some more questions about better determining the required tonnage.
Joe udarrell wrote:

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Thank you for your reponse. Some of the other posters provide more attitude than input.
Joe esquimeauxpie wrote:

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Proof positive that when you are snowed nicely, you accept it.
There is NO such thing as a 3.1 ton unit.
LOL

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