Air Conditioning for Humid Climates,
A device called the "user interface" (Figure 5) is the brain
center of the zone system: It continually sense temperatures in
the home, checks the outdoor temperatures, and tells the
control boards in the air handler and the condenser unit what
to do. It will also talk to the zone dampers, opening and
closing them as needed to condition each of the home's various
5.The "user interface"
circuit board is the brain of the comfort system; it
continually adjusts the air handler, outdoor heat pump,
and zone dampers based on sensor input and its
programmed instructions. The board shown, a Bryant
Evolution control, is capable of managing a wide
variety of mechanical systems and can serve as many as
eight independent zones.
The Bryant board I use can handle any combination of
residential heating and cooling. It can control heat pumps, gas
furnaces coupled with central air conditioning, or gas furnaces
with piggyback heat pumps. It can run variable-speed systems or
two-speed systems and is capable of independently managing as
many as eight zones.
The Bryant controller will work with any brand of equipment,
from any manufacturer. I use it with virtually every system I
install. I am qualified to sell other brands of control unit,
but this is the most flexible and capable zone-control product
I have ever used.
The thermostat for this device has a seven-day independent
control program. You can program it for four different periods
per day, per zone. When I install a system, I program the
boards on the equipment, I talk to the homeowners about what
conditions they want in the home, and I program the user
interface for the customer. Once everything is set up, no one
has to touch it again.
This component is one of the keys to the Bryant system's
effective humidity control. A typical thermostat senses room
temperature and initiates a call for cooling when the
temperature rises. But the Bryant Thermidistat, matched with an
air handler equipped with a variable-speed blower, offers an
option for those times when it is cool in the home but the
humidity is high, causing that "sticky" feeling.
This control can be set to maintain humidity levels in the
cooling mode as low as 50% RH. If the humidity level rises in
the home, the control initiates a call for cooling. During the
dehumidification mode, the indoor blower speed is reduced
approximately 40%. This allows the evaporator coil to become
much colder than normal; meanwhile, lower airflow velocity
allows the air to maintain contact with the coil longer,
allowing the cold coil to wring the humidity from the air. This
drastically increases the latent capacity of the comfort system
without overcooling the structure.
A Case Study
To monitor how the system works, I installed data-logging
devices in one home where conditions were particularly severe.
This house was built on totally saturated soil, downhill from a
pond. To provide accessibility, the floors had been set level
with the outdoor grade. During construction, the excavation had
to be continuously pumped in order to stay dry enough for the
foundation stemwalls to be built and for concrete to be placed
for a crawlspace slab.
Unfortunately, no vapor barrier was installed under the
crawlspace slab for this house, and the concrete constantly
wicks moisture into the house from the saturated soil beneath
it. My system was designed according to Manual J standards,
with no allowance made for this extreme and unusual moisture
source. But the data I collected show that the system I
installed, which is much like the one I've described in this
story, has been able to maintain the exact design conditions I
set it up for: 75°F temperature and 50% RH, with variation
of no more than one degree or 5% RH.
I was hoping that the data I collected would also verify the
energy performance of this system. But the energy used to pull
all that excess humidity out of this home threw those numbers
out of whack. The electrical bills for running the system were
not outrageous, but they were somewhat higher than I would
expect if the house didn't have that particular moisture
problem. In houses that have mainly atmospheric humidity to
contend with, my systems are typically very economical to
Workmanship Crucial to
Installation is the critical part of any comfort system. You
can go out and buy the best system on the market, but if it's
not properly installed, you've got a piece of junk. I can't
cover every aspect of workmanship in one short article, but
let's take a look at a few important details.
In Figure 6, I'm brazing a joint in a refrigerant line as I
attach a refrigerant dryer to the line. The dryer's purpose is
to clear oxidation and moisture out of the refrigerant. These
systems use a lubricant that is particularly vulnerable to
moisture; if the line isn't kept dry, there's likely to be
corrosion and excessive wear.
Figure 6.Careful installation work is key to any
system's success. Here, the author makes a brazed joint in a
copper refrigerant line as he installs a refrigerant dryer that
will protect the system against moisture and
The refrigerant dryer is a mandated standard item that has to
be installed on every new system. Any time the line is opened
for repair or modification, the dryer should also be replaced.
What's not standard, however, is the solder I'm using to braze
the tubing joints. I like to use a 15% silver formula, because
it is the most compatible formula with the copper in the lines.
Less expensive phosphorus-based formulas are more commonly
used, but they don't provide the strength and durability of the
15% silver joints. It's not critical in this particular case,
but when we place copper lines underground, as I do for some of
my geothermal installations, it's very important.
When I braze a joint, I clamp a set of tubing cutters onto the
line to hold the plastic insulation back from the area where
I'm applying heat. That keeps the insulation from melting. I've
also wrapped the valve in a wet rag, to keep it cool while I
apply my torch to the joint and melt the brazing compound. When
the joint has cooled, I'll take the cutter and the rag off, and
put the insulation back in place.
Figure 7 shows us flowing nitrogen through our refrigerant
loops to clear out any oxygen or water vapor before we load the
system with refrigerant. Not all contractors do this, but it's
a good idea — it eliminates the moisture and oxidation
that can cause corrosion, the same contaminants that the
refrigerant dryer is supposed to help protect against.
Figure 7.Before filling the system with
refrigerant, the author purges the loops with nitrogen to
eliminate moisture and oxygen that could corrode refrigerant
lines and mechanical components.
Proper duct installation is key. We use only metal ductwork
— no fiberglass duct board or flexible duct. We also seal
every duct joint with mastic (Figure 8). When ducts are leaky,
the losses don't occur just when the system is running. Air
never stops moving — you have leaks when the system is on
and leaks when it's off. It's a continuous loss, not a
momentary one. Air and moisture getting into the duct system
cost money, stress the system, and contribute to all kinds of
problems. Careful duct-sealing is well worth the trouble.
Figure 8.Thorough sealing of all duct joints is a
critical quality-control step. Leaks in the ductwork allow
moisture into the airstream, which will decrease system
efficiency and may lead to condensation and mold growth inside
the ducts. Applying sealant mastic with brushes or gloved hands
is the preferred method for achieving airtight
Duct sizing and layout are also critical to system
performance, but that topic deserves its own article. Here are
a few key points:
• ducts should be well supported
• runs should be kept as straight as possible
• sweeping curves are better than sharp bends
• ducts should be insulated to R-6 or more
All of these quality details help the system perform
effectively and reliably. But providing first-rate comfort
along with high efficiency is about more than workmanship
— it starts with good design. If your hvac contractor
makes a careful, room-by-room estimate of the sensible and
latent loads for the particular house, and if he chooses
equipment of the right capacity that can adjust to a changing
balance between those loads, you're well on your way to meeting
the homeowner's needs.
Dwayne Akersowns and operates Akers Custom Comfort,
Inc. (www.acchvac.com), a second-generation
residential and commercial hvac installation and service
contractor based in Stokesdale, N.C. Akers Custom Comfort
specializes in state-of-the-art energy equipment, including
direct-exchange geothermal heat pump systems.