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
One more piece of info - all units will be matched to the appropriate
variable speed air handler.
Any comments or recommendations?
Thanks in advance,
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
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.
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.
Your 4-ton system with that ductwork may have only been delivery 3-ton
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
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"
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.
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