OK, I'm going to start this thread and as I get more info I'll update.
I have 1100 sf house with (I believe) 2.5ton AC unit. When the outside
temp gets above 80 or so the AC unit will not shut off cause it can't
keep up, even thought the I have the thermostat set on 80.
I have only 6" of insulation in the attice and have two-by walls. I
had a professional come out to inspect the ac and he said my problem is
the units too big. If the unit was too big for the house wouldn't it
run for short periods of time and the shut down and I would end up with
a cold/damp house. Instead it runs constant.
Any clues as what really needs to be done.
Sounds like the unit either isn't performing correctly or is undersized.
You can't use rules of thumb like "square feet per ton" to size air
conditioning, so I don't know how a tech would know that a unit was
oversized just by looking at it.
If you have an accurate probe-type thermometer you can do some
diagnostics by yourself. Start by measuring the temperature at a return
register, at a supply register, at the return plenum (attached to the
base / end of the air handler), and at the supply plenum (the output
side of the air handler). Also measure the ambient temperature in the
area of the condenser inlet (fins), and the outlet air temperature at
the condenser exhaust. Post those numbers and we'll see if that turns
up a useful hint.
I can quickly measure temps at every place but the supply side of the
How accurate will this need to be be. I have a kitchen type instant
read thermometer that should be good to within a few degrees.. Im
guessing that we are looking for the temp differences between these
point so any inaccuracies will be constant from one measurement to
I will also post the correct size of the unit.
Travis Jordan wrote:
Your kitchen thermometer should be OK for this application.
Is your supply plenum made out of duct board? If so you can poke a
small hole in it and insert the temperature probe. When you are done
simply cover the hole with a piece of *yes* duct tape.
First let me state one thing.. I didn't build this home or have it
built.. It was bought this way. I knew the problems going in.. This
statement will make sense as you read further.
Ok I'll try and answer a few question this morning and then this
weekend I can get the rest of the info.
I live in Southern Indiana with Bloomington about 10 miles away.
I didn't get any measurements last night when I got home since it only
hit about 77 yesterday. But that shouldn't be a problem this weekend
since it's predicted to hit the 90's by Friday.
I have duct board for all the plenums, and flex duct for the runs.
There is complete access in the attic and this is where my return lines
are. My supply lines are in the crawl space.. And when I say crawl I
MEAN CRAWL. I believe at most a two block space to work in, really
more like a block and a half. This means any plenum needs to be
measured and cut outside, taken into the crawl space and assembled
there. Not an ideal situation.
I did try something the othter day and am waiting to see what real
difference it makes. When they put the furnace and AC in almost 10
years ago they sort of jury rigged a setup so I can put my filters in
the return line. At the time I had the non-replaceable things that you
take out and wash. To accomplish this they cut a "door" in the side of
the plenum and would place two filters inside so they looked like a
"V". This access door is 18"x18" and somebody at work suggested to try
and just open that door to make sure I dont have any back pressure. I
did this the other night and the flow from the supply lines did seem to
increase some, but unless I do more evealuation I'm not sure just how
Well this is where I'm at for now. The contractor who came out is
suppose to give me an estimate for new/upgraded duct and different
Oh one more question.. So I dont sound like an idiot when I call other
contractors do I just say I'd like someone to do a heat loss/gain
evaluation on my house.
Time for work..
PS thanks for the advice so far.. I WAS going to post on alt.hvac, but
after reading some stuff there I didn't think it would be wise. (mom
always alwasy said if ya cant say something nice...)
I'd just tell the contractor what the problem is, which is you have a
system that won't cool the house below 80 on a hot day. And while it
shouldn't really be necessary, I'd try to get them out there on a hot
day, so they can see it for themselves.
If the system has a basic flaw, like low refrigerant, blocked coils, or
a detached duct, etc, what's the point in starting with a load calc?
The steps in sizing an air conditioning or heating system are:
ACCA Manual J - calculates heat gain and loss of the area to be cooled /
ACCA Manual S - adjusts the BTU requirements from the Manual J to take
into account the efficiency of the equipment proposed.
ACCA Manual D - provides information on airflow requirements and
individual duct sizes.
When you call a contractor I'd ask them how they size replacement
systems when the current system doesn't seem to be keeping you
comfortable during the season. The non-technical response should be "I
measure your house and then use a software program (or worksheet) to
calculate the size". If you get any other answer then go to the next
contractor on your list.
Note that if you change the size of your system you may also need to
change the ductwork - however, this assumes that your current duct is
sized for the system that was installed....which may or may not be true.
Not sure people are going to go this far back, but if I dont get any
replies I'll post a second time.
You asked for some temps.
I have 7 supply ducts in the floor, and 4 return ducts in the celing.
Since two are coupled to one line I guess that would be only 3 return.
Before I forget I also opened the side of the return at the furnace
where the filters go to allow more return air to flow.
Outside temp when I was taking measurements was hovering around 90.
62 in the kitched return - about two feet from furnace/AC/Blower.
62 in the Master Bedroom
62 in on of the living room ducts.
60 in the other living room duct.
80 at the return in the living room
80 at the return in the MBr
78 at the return plenum.
Since I opened the return plenum the system seems to function better..
Late yesterday late afternoon/early evening the AC kick on and didn't
turn off for about two hours.
Travis Jordan wrote:
First, have a heat-gain heat-loss done and do more to lower both!
Then do another heat-gain, and check the ductwork for tightness and
Also, check for hot air entering the return air stream.
Measure the heat rise off the outside condenser and tell all of use what
all those temp reading were.
Where located, what state and large city do you live near?
Two & half ton would normally be way too large, which indicates major
If you are drawing hot air from the attic or outside the heat-rise will
be large and the condenser overworked.
If there is little temp-rise through the condenser either it is not
getting an adequate heatload on the indoor coil or there are other
Measure temps everywhere you can and tell us the Relative Humidity if
possible. - udarrell
Air Conditioning\'s Affordable Path to the "Human Comfort Zone Goal"
Amazing when a homeowner has more brains than the AC company. The OP
is correct. If the unit were simply too big, it would cool the house
too quickly and it might result in excess humidity making for a cold
damp house. How any AC guy could tell you that your house is 80 deg
because the AC is too big is beyond me.
You don't say where you are, but 6" of attic insulation isn't much for
most areas. I'd look at ways to get more attic insulation in there.
And I'd get another AC company that knows what they are doing. Also,
what is the history on this? Did you just buy it? Did it ever cool,
You had a professional in your home ... a cheating and fooling
This slackjaws only purpose was like at least 50 percent of all firms
who refer to themselves as "heating and air contractor"..... to sell
Talk to friends family coworkers etc.... and find someone known to do
reliable hvac work....but remember this....no matter how many glowing
recomendations you get you still only have a 50 percent chance of
getting a professional and not an outright theif who will take your
money and run...leaving you with an inefficient system that is probably
going to need a compressor in a year...possibly a little longer.
I'm not so sure the guy was even competent enough to be trying to sell
something. If I walked into a house where the complaint was it's 80
degrees and the AC is running but can't get it any lower, the last
thing I'd tell them is the problem is the current system is too big. I
just makes no sense at all and would most likely result in the
homeowner calling another contractor. On the other hand, if they told
them it needed a new compressor, recharge, etc, that would be a lot
more likely to fly.
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