So it started out as a way of making people comfortable. Right? Well, not exactly. In 1882, thanks to Thomas Edison, the first electric power plant opened in New York making it possible for the first time to have an inexpensive source of energy for residential and commercial buildings. And by 1889, central station refrigeration was used in large cities to preserve foods and documents. It was well known that a cool surrounding could preserve foods and other perishables for a long time. But what was not well known was how humidity and heat were related. Then in 1902, Willis Carrier built the first air conditioner to combat humidity inside a printing company. Controlling the humidity in printing companies and textile mills was the start of environment management.

02/07/12

As our computer technologies keep improving, we’re getting rid
of lots of old rules-of-thumb and instead making more decisions
based on detailed information.

One big improvement affecting home owners has been the old rule
of sizing an air conditioner or heating system, and I have
written many times about the importance of accurately selecting
the right-sized equipment for a home. The right way to do it is
with a computer calculation, not a form of guesswork.

This point was driven home to me recently by a report I read
about that concluded that the average furnace put into a new
home in Anchorage, Alaska, is oversized by 121 percent. The
study, sponsored by the Cold Climate Housing Research Center,
investigated 19 fairly new homes that had forced-air heating
systems. Contractors in the area typically used a rule-of-thumb
that called for 40 BTUH per square foot of floor area to
determine the size of the furnace needed in a home in that
climate. As it turned out, the homes studied here actually had
systems that were slightly larger than this general estimate.
However, after looking at the results of detailed monitoring of
the homes, the researcher found that the average heat load in
the houses was only 19.7 BTUH per square foot, not 40 or more.
The home with the furnace that was least oversized was still 66
percent too big. At the other extreme was one house that had a
furnace that was 223 percent too large.

Not only does a homeowner pay more for a bigger system that is
not needed, but a system that is too big runs with less
efficiency and will often have a shorter lifetime. Increased
airflow rates also mean reduced indoor comfort as more air leaks
into and out of the house.

The study noted that it is often hard to get a contractor to
downsize a system because of the lingering generalities on how
big a system is needed. Contractors often say that they won’t
guarantee the performance of a system that is a lot smaller than
the one they recommend, even though their recommendation is
often based on outmoded thinking.

I’m a lot more familiar with the sizing of air conditioners for
hot climates, and this problem is just as serious and just as
prevalent as what is found with heating in cold climates. Buying
an air conditioner that is larger than needed will cost more in
purchase price, cut down on system efficiency, increase energy
costs and lower comfort levels. And in a hot climate, a system
that is too large will short-cycle, leading to poor control of
the indoor humidity level.

There’s really only one way to properly size cooling equipment,
and that is to throw out old ideas on what sizes work best in
various-sized homes, and have your contractor perform a Manual J
calculation (the procedure of the Air Conditioner Contractor’s
Association of America). Once this calculation is performed,
there is a Manual S calculation that helps the contractor select
compatible equipment.

Many contractors say they don’t always follow the recommended
sizing guidelines and purposely oversize systems, citing such
reasons as their customers asked for a bigger system or they
feel a bigger system will do a better job. In the long run,
however, is is the homeowner, not the contractor, who pays for
decisions like this.

So let me give you one piece of advice for the next time you
need a new air conditioner in your home: insist that the
contractor do a “Manual J’ calculation and uses the right-sized
equipment for your home. This calculation takes into account
such factors as the home’s insulation levels, amount of window
glass and other features to ensure that the new system will do
the most effective job and be the most efficient. The contractor
should show you the completed form and explain the results. If
your contractor says he doesn’t do these calculations because he
knows the right size that is needed, find another contractor.

Over-sizing or under-sizing equipment will cost you money and
comfort. Make sure it is done right.