I understand about thermal mass and I agree with you on that. My
question is - where is the thermal mass? Is it the bricks? What heat
sources are there - water heaters inside, etc.

Obviously, if the system is working correctly and it is coming on all
night, then something is adding heat to the house. This I can't
dispute. And possibly the nights are not 58F long enough to deplete
the latent heat inside the walls, bricks, etc.

If it were 58 for a long enough time, the heat would disipate outside
faster than on the inside where it is 78, only because of the higher
gradient. This depends on where the mass is in comparison to the
insulation, of course. But one would think that over an 8 hour period
of outside temps being 58, and inside temp stable at 78, the thermal
mass of the house would be at or below 78 by morning.

"CBHvac" wrote in message ...
Your degree wont do you a thing, if you cant understand thermal mass...
All the calculations you need, are in Manual J and T...understand your U and
V values, and you got it made...
They are NOT assumptions, and anyone in the HVAC field, with more than 3
weeks in can stand by the simple statement...

Your degree aint worth the paper its printed on if you cant understand
thermal mass...how about transferance?

Have you ever looked at what the average computer system adds to load? How
about a water heater indoors? TV? Refrigerator?
It all adds up, add some solar gain to that...and bingo...you got a heat
bank....

BTW, for giggles and grins today, I checked outdoor temp at 6am, and killed
the AC last night so it would not cool down the home..

It was 71F outside, 89F inside...the unit was cut off at midnight lst
night...now...where in hell did that extra heat come from? I mean...its only
about 3400SF.....brick..AHHHH....there is part of it..brick homes tend to
hold more heat, and while part of that heat radiates back outside, some is
transfered indoors as well..





"L. Borne" wrote in message
om...
I've had a few courses in Thermodynamics when I got my aerospace
engineering degree, and use thermo often. However, you don't need
more than common sense to know that if the entire volume of the house
is 78 and the outside air temp is 58, then something is heating the
inside of the house. There is no attic or other volume with high
enough temp to raise the inside of the house temp. Show some
calulations and explain your assumptions please.

"CBHvac" wrote in message
...
Learn what a manual J and T are, and latent heat is included...and
yes..it
happens...

Hell..it was 65 this morning and my air was on at 75 and the air kicked
on.....it happens...but then, you learn that after years in the biz, and
a
few dozen courses in thermal dynamics..storage....and such...

"L. Borne" wrote in message
om...
Am I the only one who thinks this is not normal, other than
LickyDucky? His house has no attic - just A/C rooms. The entire
house is cooled to 78 and it is 58 outside? Any heat they can create
should be going to the big heat sink outside. How does the house heat
back up to 79 to make the A/C kick back on? Where is the big heat
source? A fridge and couple of people can't create that much heat. Of
course, eventually those sources and all the other heat sources will
cause the A/C to kick back on sometime depending on insulation. But
it should not be very frequent. Is the A/C running long, or just a
little. Could it be off 90% of the night and is just coincidence it
is running when you wake?

In my experience with A/C in South Louisiana and Florida, all houses
I've lived in, (all with non A/C attics), did not have the air running
when the thermostat was set at 78 and the outside temp was below 65 -
aproximately.

As has been mentioned, check for a humidity control.

If the A/C is coming on and off, then it is probably working. To
check, just take the temp of the air coming out of a register as
another poster suggested. It should be at least 15-20 F cooler than
the return air. Also, see if water is dripping from the coils? It is
usually routed outside or to a drain.

Another problem which is not common is the placement of the
thermostat. It could be near a heat source, or have a vent blowing
right on it, causing the A/C to shut down too early.






"LuckyDucky" wrote in message
...
We have moved into a house that has central AC. We've never had or
used
central AC before but it's very humid here in the South so we've set
the
thermostat at 78 degrees to dehumidify the house.

I'm baffled as to why the AC is running in the morning even when the
outdoor
temperature is significantly cooler. This morning, for example, it
was
58
degrees outdoors and the indoor temperature was 78 degrees with the
AC
running. The only source of heat that I can think of is from the
refrigerator. The hot water heater is in the basement.

I understand why a house requires cooling during the day even when
it
cooler
outdoors but I'm totally clueless as to why the house would need AC
in
the
early morning after the house has been cooling down for over 10
hours.
I'm
especially baffled because yesterday it was mostly cloudy and cool.