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.

V/R Greg

I can quickly measure temps at every place but the supply side of the
plenum.
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
another.
I will also post the correct size of the unit.
Thanks Travis.

Greg

Travis Jordan wrote:
wedge40 wrote:
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.

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.

wedge40 wrote:

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. V/R Greg


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
insulation.
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
problems elsewhere.
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
system problems.

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"
http://www.udarrell.com/air-conditio...tent-heat.html
http://www.udarrell.com/ac-trouble-s...ubcooling.html

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,
etc?

wedge40 wrote:
I can quickly measure temps at every place but the supply side of the
plenum.
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
another.

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.

wedge40 wrote:
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.

V/R Greg

You had a professional in your home ... a cheating and fooling
professional

This slackjaws only purpose was like at least 50 percent of all firms
who refer to themselves as "heating and air contractor"..... to sell
you something........

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.

wrote:
wedge40 wrote:
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.

V/R Greg

You had a professional in your home ... a cheating and fooling
professional

This slackjaws only purpose was like at least 50 percent of all firms
who refer to themselves as "heating and air contractor"..... to sell
you something........

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.









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.

wedge40 wrote:
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.



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?

wedge40 wrote:
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.

The steps in sizing an air conditioning or heating system a

ACCA Manual J - calculates heat gain and loss of the area to be cooled /
heated.
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.

Further reading:
http://www.fsec.ucf.edu/bldg/pubs/ACsize/index.htm

Call an AC guy out. He should clean the outdoor unit with chemicals
and a garden hose. He should check the freon, and also check the air
flow through the furnace.

--

Christopher A. Young
You can't shout down a troll.
You have to starve them.
..

"wedge40" wrote in message
oups.com...
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.

V/R Greg

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.

Supply temps
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.
Return measurements.
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.

Greg.


Travis Jordan wrote:
wedge40 wrote:
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.

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.