Almost
everyone knows that residential air conditioners and heat pumps come in
approximately seven different sizes: 2 tons, 2-1/2 tons, 3 tons, 3-1/2 tons, 4
tons, 4-1/2 tons, and 5 tons… but do they really? No,
they really don’t, for those are just nominal sizes (close approximations), and
often contractors get themselves in trouble when they rely on nominal sizes in
real-world conditions.
I remember, for example, one job in Arkansas that I went out to look at many
years ago, which just didn’t get the house temperature down to accepted nominal
conditions, although the total load had been carefully calculated - and
re-calculated - and what I thought to be a 3-ton system had been selected and
installed. And of course, I (as the distributor’s service manager) went out to
look at the job to find out what had gone wrong, suspecting that the contractor
had made some sort of a mechanical mistake.
So, I carefully examined the ductwork and the refrigerant charge; I checked the
refrigerant pressures and for air in the refrigerant; I checked the refrigerant
superheats and the subcooling; then I went back and re-checked the load
calculations, the airflow, and the entering and leaving air wet-bulb
temperatures (plotting a psychometric chart); and found absolutely nothing
wrong!
And I’ll admit, that coming from the service side of the business, this job had
me stymied! I was looking for a mechanical problem, and it took me a long time
to get around to looking at the equipment matchup. After all, both the outdoor
unit and the air handler had 036 in their model numbers, so I knew that it was
a 3-ton system (12,000-Btu/ton into 36,000-Btu = 3 tons)… but it wasn’t! As it
turned out, the particular matchup only provided 34,000-Btus, which under
maximum-load conditions (Arkansas on a sunny, hot July day) just didn’t get the
house cool enough.
Do you think that your customers ever run into this problem? Well, listen to
their service guys when they come in to buy a replacement condensing unit.
You’ll hear them saying “I need a 3-ton condensing unit.” Never mind what
output the unit they buy actually produces when matched with the existing
airhandler and coil!
So, many contractors, to make sure that they don’t undersize the equipment,
specify a half-ton (or more) larger than their loads show is actually needed.
And what does this do? Yes, their customer can get the system to run cool
enough, but on milder days the house is humid and uncomfortable, because the
system isn’t running long enough to properly dehumidify the house! Why, I’ve
seen molded wallpaper, molded shoes, and even water dripping down the insides
of windows!
So, remember (and advise your customers) that one of many good reasons for
replacing both the indoor and outdoor units at the same time (rather than just
replacing the condensing unit by itself or the airhandler by itself) is to make
sure that the matchup will meet the existing load conditions.
And when it comes to their choosing whether to oversize a system (for a little
CYA), you might suggest that they ask their customers whether their first
priority is getting the house cool enough, or if they want better humidity control.
You’d be surprised to find that such a simple question clearly shows consumers
the options they have. And if they want both, then an upgrade to a
variable-speed indoor fan should be suggested.