Re: [Phys-l] cooling hints

["Dr. Richard Tarara" <rtarara@saintmarys.edu>]

Certainly with a poorly insulated hourse (circa 1950s) and less than robust 
cooling unit you would find limits on the delta-T possible.  As delta-T 
increases so does the rate of (dare I say it?) heat flow through the walls! 
There would have to be an equilibrium at some finite delta-T.

The whole issue of set-backs  for both heating and cooling seems to be 
reasonably complicated depending on things like the length of the set back, 
the magnitude of the setback, the insulation in the building, the humidity 
of the air, etc.  We have bounced this one around before with mixed 
conclusions.  I can personally relate that while using a setback thermostat 
seems to have some positive effects for heating, I saw no such effects when 
I tried it for cooling and have since simply set my interior temperature for 
an average 77 Fahrenheit.  I will qualify that I have a 2x6 constucted, well 
insulated home (with a common North wall with the townhouse next to me) and 
although 2800 square feet, I can cool the home for $20-$30 a month in extra 
electrical costs (that also factor in running a freezer in a hot garage). 
The ambient outdoor temps run in the 80s most of the summer.

Heating (gas) is more expensive but averages only around $50 a month 
(includes water heating) peaking at less than $200 in our reasonably cold 
winters (temps set to cycle between 65-72 degrees).

The one 'hint' given by heating/cooling companies that doesn't pay is 
running the furnace fan continuously.  While that does equalize the 
temperature throughout the house reasonably well, it also adds a good $20 to 
the electrical bill each month.

Rick

Richard W. Tarara
Professor of Physics
Saint Mary's College
Notre Dame, Indiana

*******************************************
Free Physics Instructional Software
www.saintmarys.edu/~rtarara/software.html
Updates and new multi-resolutions versions now available.
********************************************
----- Original Message ----- 
From: "John Clement" <clement@hal-pc.org>
To: "'Forum for Physics Educators'" <phys-l@carnot.physics.buffalo.edu>
Sent: Wednesday, August 24, 2011 9:28 AM
Subject: Re: [Phys-l] cooling hints


> That doesn't sound right.  Refrigerators can lower the temperature from 
> the
> 70s or higher to below freezing, and they work the same way an AC does.
> Actually the vents in our houst put out much colder air than the 
> temperature
> we wish, so obviously it is producing a much colder temperature than a 15
> degree difference.  On the really hot days, the indoor temperature is more
> than 20 degrees different from the outside.  The waste water coming out of
> the AC is quite cold.  Thirty years ago I know Houston houses could be
> cooled more than 15 degrees below outside ambient.  Without AC Houston 
> would
> probably be abandoned.
>
> The cooling power depends on the capacity of the AC, and it is possible to
> get one that will chill your house below what would be comfortable, but
> usually that is not optimal.  On the Gulf Coast one wants the moisture
> removal, so you want an AC that stays on for a long enough time to do 
> this.
> One that is too powerful will cool too quickly and you get a cold clammy
> home.
>
> I suspect that the 15 degree difference is what was achieved in the 
> average
> home with little insulation.  I also suspect that the temperature of the
> inside and outside coils is determined by the design of the AC and has 
> only
> a small dependence on the ambient temperature.  As long as the outside
> ambient is significantly lower than the outside coil temperature the AC
> should be able to maintain a very low indoor temperature in a perfectly
> insulated sealed home.
>
> I am pretty sure that even back then you could get a greater temperature
> differential if you just paid for a larger AC unit.  Most repair people 
> tend
> to tell you what the manuals say regardless of whether it is correct or 
> not.
> Advertising can also be totally incorrect.  For example I saw a tag on a
> lamp that claimed that its 60W halogen bulb saved you energy compared to a
> lamp with a conventional 60W bulb.  What were they thinking?  Since the 6
> inch fiberglass insulation has a better R value than the 4 inch insulation
> people have been told to install the 6 inch in the 4 inch space.  But
> squishing it just reduces the R value to the same as the 4 inch.  I am 
> sure
> we all have similar common misconceptions to report.
>
> John M. Clement
> Houston, TX
>
>
> > In regards to a/c cooling -- may have changed a bit but many,
> > many years ago
> > when I was married to my son's father, he was an a/c repair
> > person for part
> > of his working life.  He would always tell me that and a/c
> > can lower the
> > inside temp about 15 degrees and that is all.  So, if it is
> > 103 degrees out,
> > like we had this summer (NOT heat index, plain, ordinary
> > pre-meteorologists
> > temperature), you will be lucky to have your house cool at 88
> > degrees.  Of
> > course, a lot depends on insulation, sun through the windows,
> > children/pets
> > running in and out, etc.
> > Holly Priestley
> >
> > _______________________________________________
> > Forum for Physics Educators
> > Phys-l@carnot.physics.buffalo.edu
> > https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l
>
> _______________________________________________
> Forum for Physics Educators
> Phys-l@carnot.physics.buffalo.edu
> https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l