Upgrading central AC unit before adding onto house -- caveats?


Hello,
My wife and I are planning on enclosing our garage next summer, and this will bring our 1600 sq house up to about 2100 sq. We planned on upgrading the 23 year old central air/heat system in the house then, but a couple of weeks ago the unit died and needs replacing now.
We currently have a 3 ton system but in anticipation of convering the garage in we're upgrading to a 4 ton, 15 SEER system. I know if the system is too large it won't remove enough humidity from the house and mold can set in, plus shutting on and off alot is bad for the system, however will us having the system in place for 6-8 months hurt it like this? Winter is here, and I doubt we'll be using the air conditioner much until next March or April, just the heat if that, but just curious of what we need to look out for.
We live in Central Texas where the humidity generally stays around 50% or lower, and only rarely jumps up to 65% or higher. I figure worst case we can get a stand alone dehumidifier if humidity does become a problem. Will this help?
Just curious on what others think about this. Thanks for any input ...
Alex
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Alex wrote:

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buffalobill wrote:

There's no harm in having an oversize unit for a year until you do the addition. The bigger question is, did they do a manual J to figure out what size you needed? 4 tons sounds like a hell of a lot of capacity for 2100 sq ft. average construction house. Also, they should have verified that they can get proper ducting to the new area as well, figured out how it's gonna be done, etc, so that it can be integrated in. I would have made sure of that, so there are no surprises later on.
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snipped-for-privacy@optonline.net wrote:

Hi,
Ducting to the garage shouldn't be a problem... the attic is pretty much open and the duct work from what I can tell can be easily patched in.
As for the 4 ton for 2100 square feet, it's my understanding that 1 ton is suggested per 500 square feet, so 4 tons for 2000 square feet. Is this not correct? We're at 1660sqft now, and the AC is installed and working. It's blowing more air then I thought, but the intsallers said once the addition is added that will go away.
Thanks,
Alex
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This is a perfect example of a really, really bad way to size an A/C. Your contractor should have done Manual J and Manual D calculations which take into account things like insulation, windows, building orientation and duct size.
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My house is no doubt completely different than yours (built in 1930 with full basement, 2 floors, and full attic), but for the 2,450 square feet we have, our AC is a 3-ton unit, so I agree with Trader4 on the opinion that you may have bought an oversized unit for your ultimate house.
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Alex wrote:

No, 500 ft per ton is bogus. I have a 22 year old 3500 sq ft house, with vaulted ceilings and only have a 4.5 ton unit. The size of unit is dependent on many things, besides sq ft, like how well insulated the house is, number of storeis, how many and type of windows, exposure, local climate, etc. The way to size a unit is to do a manual J calculation.
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Alex wrote:

...
One ton per 500 sq. ft. is like a stopped clock: it is right twice per day. There is so much that differs from house to house to change the heat load other than floor area: Wall area Wall insulation Window area Window type (single pane, double pane, low-E) Window exposure (direction, overhangs, shade) Floor insulation Ceiling insulation Regional climate etc. The windows are probably where the most variation in the heat load can occur. Unfortunately, few AC contractors seem to want to heat load calculation, but just use that terrible rule-of-thumb. They don't care about wall or window area or insulation thickness. My last house had 4 tons for under 2000 sf. and it wasn't adequate (R-11 walls & ceiling, no floor insulation, northern california). I just got AC installed in this house and none of the contractors would do any real heat load calculations. This house has R-19 wall, R-30 floors, and R-30+ ceilings. We will see how this works.
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Alex posted for all of us...

Asked and answered MANY times - do your own research.
--
Tekkie Don\'t bother to thank me, I do this as a public service.

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