Re: [Phys-l] Air Conditioner Sizing (was Another Try)

[John Barrer <forcejb@yahoo.com>]

In many parts of the country HVAC contractors
routinely oversize residential units since "success"
to them is no callbacks when it hits 115-118 like it
did here in Fresno last month. Even our old (20 year)
unit serving a poorly insulated condo with single pane
windows and high ceilings did not run continuously on
the hottest days. On normal (ie, 95F) days, it's duty
cycle is <20%. Quite inefficient. And local
contractors were proposing even higher BTU/SF capacity
systems for a new, energy-efficient house were
contemplating building. More up-front cost and higher
energy bills to boot.

It seems to me that a properly sized unit would run
continuously on the hottest day to maintain 80F. I'd
be curious to know the duty cycle of Roger's high
speed compressor when it's REALLY hot.

John Barrere
Fresno, CA
(where it's usually, but not always, a dry heat)

--- Roger Haar <haar@physics.arizona.edu> wrote:

> Dear Mike,
>
> 	I am from NW Ohio, so I remember it a bit.  I am
> now in AZ and late 
> summer is hot and fairly humid.  Two years ago I
> replaced our 20 year 
> old AC and swamp cooler with the best  ( highest
> SEER rating) I could 
> get.  The unit has two compressors.  The low speed
> pulls about 1600 
> Watts.  The air conditioning part of my electric
> bill dropped from 
> almost $200 to less than $50 per month.  But the
> thing was pricey.  I 
> figure over the life of the unit, that at current
> power prices, the 
> savings with the higher SEER just balanced the
> couple thousand more it cost.
>
> ENJOY
>   Roger Haar
>
> Edmiston, Mike wrote:
> >  
> > Brian Whatcott makes a very good point that HVAC
> units in the USA are
> > often meausured in tons.  However, his estimate of
> a 5-ton AC unit for a
> > 2000 sq ft home seems high to me.
> >
> > I am cooling about 3000 sq ft in northwest Ohio
> with less than 2 tons.
> > My house is an old Victorian house with walls that
> are three bricks
> > thick, so it is fairly well insulated, and also
> has a lot of thermal
> > mass.  Since it has hot water heat, there is no
> duct work, so I am using
> > window AC units which fortunately have gotten much
> quieter, more
> > efficient, and lighter.  I have four 6000-BTU
> units.  At 12,000 BTU per
> > ton, that means I have exactly 2 tons.  However, I
> only run two units
> > (one upstairs and one downstairs) unless the
> temperature gets over 85 F
> > oustide.  Today it is about 90 F and quite humid,
> so I have all four
> > running.
> >
> > If I were to install central air, which I have
> considered doing, I think
> > I would only install a 2-ton unit; not a 5-ton
> unit.  I think AC is
> > often oversized, and then it cycles on and off. 
> The problem with that
> > is poor dehumidification, which is important in
> Ohio.  I get good
> > humidity control by running the required number of
> units continuously.
> > If one of the units begins to cycle (because the
> temperature setting has
> > been reached), I just turn it completely off.
> >
> > I'd like to have a two-stage central air system
> constructed with two
> > one-ton units.  I would run one continuously and
> bring on the second as
> > needed.
> >
> > The science building at Bluffton University just
> had its 28-year-old AC
> > unit replaced.  A 25-ton compressor was replaced
> with a two-stage unit
> > consisting of a 10-ton compressor and a 15-ton
> compressor.  For most of
> > the summer we have been running the 10-ton only
> and continuously.  It is
> > cooling fine and keeps the humidity much lower
> than when we ran the
> > previous 25-ton that cycled on and off.  Only on a
> few days when the
> > outside temp went above 95 F did the 15-ton stage
> kick in.  We're saving
> > electricity and keeping drier with the two-stage
> unit.
> > Michael D. Edmiston, Ph.D.
> > Professor of Chemistry and Physics
> > Bluffton University
> > Bluffton, OH 45817
> > (419)-358-3270
> > edmiston@bluffton.edu
> > _______________________________________________
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> >
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