Almost all air-conditioning seawater
pumps are magnetic-drive centrifugal pumps.
Such pumps don’t have shaft seals to worry
about, but they’re also not self-priming.
Accordingly, the pump must be located be-
low the waterline in the bilge—it must have
a positive suction head (see Chapter 19). The
seawater circuit must run from the seacock
to the pump below the waterline and then
gradually and continuously rise to the con-
denser located above the waterline. From
there the water drains through the exit
through-hull, which is located below the con-
denser but a bit above the waterline so you
can look to see that there’s flow. Don’t locate
this exit through-hull too high, because the
noise of the water will annoy both you and
your neighbors. Six inches (15 cm) above the
waterline is ideal—8 inches (20 cm) is the
maximum, 4 inches (10 cm) the minimum.
Do not try and dump the condensation
drain water into this same seawater exit hose
or through-hull. If you do, debris in the con-
densate can block the seawater outlet. Then
the seawater will back up and empty into the
drip-collector pan, and from there empty into
bilge, giving your bilge pump a good work-
out and possibly sinking the boat! Also, be
sure the sea suction follows the recommen-
dations in Chapter 17.
A simple test to see if the heat exchanger
is working properly is to put your hand on it
after the air-conditioner has been on for a
while. If the heat exchanger is too hot to
touch, it’s not functioning properly.
Recirculating Air to the Air-
Conditioning Air Handler
All too often air-conditioners are installed
where the return air (the warm air coming
back in for cooling) can be sucked in from
outside the cooled area. Sucking in air from
a crack or hole leading to the bilge is a com-
mon mistake. The bilge is humid and not part
of the cooled accommodations area.
To remove humidity from the air, the air
in the compartment must be recirculated
through the air-conditioning air handler on
repeated passes. Each successive pass draws
out more moisture and further drops the
temperature. If you allow outside air into
the return-air inlet, you continue to add
humidity and thus fail to obtain the decreased
temperature and humidity achieved from
recirculation. If this air comes from the bilge,
it may also disperse foul smells and can
potentially suck in deadly carbon monoxide.
The exception to this is make-up air, which
may be deliberately added from a dedicated
intake duct to a large air-conditioning system
to ensure sufficient fresh air in the cabin. As
noted earlier, most boats have sufficient per-
meability (air leaking in) so that make-up air
is not required.Avoiding Carbon Monoxide
Dangers
Oh yes, an improperly installed air-conditioner
can kill you! How? By causing you to ingest
carbon monoxide, carbon dioxide, and other
exhaust gases into the air ducts. Do not ever
run the air ducts through engine spaces, and
make very, very sure the ducting can’t swal-
low exhaust gases from the main engine, the
generator, or from nearby boats. Do not ever
install a self-contained air-conditioner unit
inside an engine compartment so it can
takein warm (return) air from an engine
compartment. Check, double-check, and
triple-check this. If any of these gases do find
their way in, well, you could go to sleep one
night and wake up dead.Smoothing the Flow
A final consideration is that the air ducts
themselves should be as large as will fit and
should have as few and as gentle bends as
possible. Narrow ducts, many tight bends,
and tiny outlet grilles can smother your air-
conditioner. You’ll have all those BTUs in-
stalled and wonder where they went to. Keep
in mind that cool air sinks and warm air rises.
Accordingly, the cool-air outlet grille should
be sited as high as possible, and the return-air
inlet grille should be as low as possible.
Most air-conditioners come with
manufacturer-supplied ducting, distribution
and junction boxes, grilles, and such. You can,
however, keep in mind that 12,000 Btu of air-
conditioning is about equal to 400 cfm (680
m^3 /hr.) of airflow. You can use this and the
limit of 600 to 800 feet per minute (fpm) or
3 m/sec. to 4 m/sec. (as described in control-
ling noise in vent ducts) to estimate duct size.Chapter 15: Air-Conditioning and Heating