Just after I moved into my (used) home, I had the AC unit cleaned & checked
over by a company I've been happy with for 22 years. It got a clean bill of
health. This was in September, though, so there was no really good way to
watch it do its thing for any length of time. I'm finding that on a very hot
day (92 outside today), the system runs almost constantly to keep the house
at 76. If I set it at 77, it cycles on for an hour, off for two, and the
house is comfortable. I don't NEED that extra degree, but I'm still curious:
For a given house, and a particular AC unit, is there usually a point where
it just won't do more? Or, does this mean that the previous owners didn't
have the right unit installed?
By the way, the attic is vented & insulated perfectly.
It could be low on freon or your coil could be dirty, it worked last
year you say, I would call a pro. Can you get to see your air handler
coil I would start there. You say your attic is insulated, but to what
standard or R value, accepted codes are outdated with presint energy
And ... insulation is (relatively) useless without good sealing. Else
it just adds a bit of turbulence to the flow-through. Potential inlets
and outlets should all be sealed. Around doors is where I'd look first;
note that near a low point of the house with cooled interior, the flow
might be out.
We used to say and think that 15-F below the outside temperature was okay.
It could be that below 77-F sensible temperature that you have an air
infiltration rate with a latent-load (humidity heatload) that consumes a
lot more than the average 30% latent capacity of the evaporator. The
"Combination of Both Sensible & Latent loads," could cause the sensible
air temperature to NOT drop below the shut-off when set at 76-F.
The higher the humidity infiltration load the higher percentage of the
E-Coils sensible capacity is lost to the latent workload!
You may have some negative pressure rooms drawing in these two heatload
It would appear to me that humidity infiltration could be the factor
that is stalling the sensible temperature drop under that specific set
There are far too many factors to cover here, that could be causing your
system to only handle the heatload at that temperature & humidity load
Under a specific set of conditions all units will reach a point where
they will stall at a specific TH setting.
Air Conditioning\'s Affordable Path to the "Human Comfort Zone Goal"
Yes. In most cases, the design is for a 20 degree temperature drop fron the
outside temperature. If that was the case, you should be good to about 72
degrees. OTOH, it may have been designed for a 15 degree drop.
Or, does this mean that the previous owners didn't
Depends on your definition of "right". There may be some problem that is
starting to show, such as a refrigerant leak. filter getting blocked, dirty
condenser coil, etc. To go from a 2 hour cycle to running near constant for
only 1 degree does not seem right. It may pay you to have a tech check it
out now under heavier loads.
Well, it's actually comfortable at 77-78, especially since I work at home,
so I'm sitting at a desk all day. If you're not up & moving around, 70 can
feel pretty chilly. I supplement the situation with a slow fan in the
office, too, so as the temp creeps up (before the AC comes on), it doesn't
I'm just curious, though.
On Wed, 26 Jul 2006 04:15:42 GMT, "JoeSpareBedroom"
SEER is used on Central Units and EER is used on window units, but
they measure 'Energy Efficiency Ratio'.
Lowes has some units that are 10.8 EER and use less electricity for
the same amount of cooling as a 9.8 EER unit.
I agree with the others that it is probably properly designed for a little
less performance than you would like on the very hottest days. You can
probably squeeze more out of it by improving insulation or shading high heat
gain areas of the house. Basically the same things you do to save heat.
Shading the A/C unit helps, putting silver mylar (emergency blankets from
the camping store) in the windows to reflect sunlight are a couple
inexpensive things you can do quickly and are reversable if they also turn
out to be cosmetically undesireable.
I agree with Edwin that it sounds like something isn't right. The same
system running under conditons where it can maintain 77 with 1 hour on,
2 hours off, shouldn't go to running continuously when it's set to 76.
But on the other hand, for a system to maintain 77 with it 92 outside
and only run 1/3 of the time sounds like the system at that point must
be running OK, as it's not much of a duty cycle.
Fortunately, the AC unit's in the shade for the entire
day after about 10:00 AM. I've never understood people
who say this isn't a factor.
There is considerable debate on the topic and there have
been some rather well done, well controlled studies.
Obviously, shade vs direct sun is a factor. The important
question is: "How much of a factor?"
Compute the hourly air flow through the compressor unit
on a very hot day when the unit may be running continually.
Better yet, measure the mean temperature of the air flowing
into the unit over the course of the day. I believe that the
studies have shown that there often is very little difference
in that average air temperature and the average air temp
for a unit in the shade. I would guess that the more the
unit cycles on-off, the more important sun vs shade would
Personally, I'm moving my compressor from the very sunny
east side of the house to the very well shaded north side of
the house when I replace the system. Primarily because
it will be further from my patio, but also because being in the
shade certainly must help some small amount. :)
A simple check. An AC system should have about a 20 degree drop across the
evaporator coil. Just measure the temperature difference of the incoming
and outgoing air of the unit. If it is much different something is wrong.
Since it seldom gets above 100 where I live, the delta T maybe less if the
outside air is approaching 120 as it is now in the west. Good to do this
for your furnace too in the winter. That way you can tell if there is a
Measure at a vent in the house where the air is entering to return to the AC
unit. Measure outlet air at the nearest outlet to the AC. For example on
my system inlet 78, outlet 59, outside temperature 90.
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