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 ...
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.
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
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.
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.
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
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
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:
Window type (single pane, double pane, low-E)
Window exposure (direction, overhangs, shade)
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
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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