Chapter 15: Air-Conditioning and Heating
air-conditioner operation underway and at
anchor, however, you simply must have a gen
set. Regardless, all air-conditioning systems
should be equipped with GFCIs (ground-fault
circuit interrupters) to reduce shock hazard.
NOTE: Though you can run air-
conditioners under 7,500 Btus off 12 volts
DC, I’m not crazy about this arrangement.
You will need to add at least a 200-amp high-
output alternator and a “smart” voltage
regulator in addition to the standard engine
alternator. You will also need a generously
large house battery bank. The current draw at
12 volts will be so big that running off the bat-
teries only—with the engine off—will quickly
draw these down flat. Similarly, at these high
currents, thick, heavy, expensive electric ca-
bles are required. I feel it’s usually best to in-
stall a small 2-kilowatt AC gen set to power a
120-volt AC electric air-conditioner; however,
these small DC-powered air-conditioners can
work acceptably if you are aware of the draw-
backs and make allowances for them.
Air-Conditioning Generator
Size
How big a generator do you need? The mini-
mum power in kilowatts (kW) required is
kW= installed A/C Btu ÷6,000(This includes and allows for the higher start-
ing loads from the compressor motor.)
Accordingly, for good old Kool Kat—
fitted with 16,000 Btu—we would need 2.7,
say, 3 kW (16,000 Btu ÷6,000= 2 .7 kW). You
really want to install a bit more than the mini-
mum generator capacity, however, because—
once you have a gen set—you’ll use it to
power other things as well. As a rule of thumb,
use about 1.5 times the minimum generator
size required for the air-conditioner alone. For
Kool Kat, with 16,000 Btu, a 4-kilowatt gen set
would be about perfect. Of course, if you also
had a large AC electric fridge, a dishwasher, a
microwave, and a washer/dryer, you might
very well want a 5- or 6-kilowatt set, but that’s
an awful lot of gear to cram into our poor old
32-foot (9.7 m) Kool Kat.
Don’t go overboard on gen-set size. Just
as with air-conditioners, excess generator
capacity is not good. Not only is such a gen
set larger, heavier, and more expensive, but
it will be run underloaded for long periods
of time, which will cause carbon fouling—
bad news.Reverse Flow Equals Heat
An excellent option for many boats is
reverse-cycle air-conditioning. Reverse-cycle
means exactly what it says. The refrigerant-
flow cycle can be reversed: Rather than pump-
ing warmth from the cabin air and depositing it
into the ocean, it takes heat from the seawater
and deposits it into the cabin interior. This is
surprisingly effective. You can get reliable
heating even in water as cold as 40°F (4.5°C)
and in outside air temperatures near freezing.
In other words—for most boats operating
south of Maine during the regular boating
season—reverse-cycle air-conditioning will
not only cool you in summer but also warm
you in the early spring and into the late fall
when you haul out. The drawback—and an
important one—is that you must have the gen
set running whenever you want heat.
If you’re going to operate farther north or
right into winter, or if you want heat without
the gen set on, then reverse-cycle air-
conditioning won’t do the job. In this case,
you’ll need an independent diesel-fired heat-
ing system.Compressor Refinements
In the old days, all air-conditioning com-
pressors were piston motors (or in very
large units, scroll machines). Over the last
15 years or so, rotary compressors have
been introduced, and—after some initial
problems—they are now quite reliable.
Rotary-compressor air-conditioners have
several good qualities: They are smaller,
lighter, quieter, and—most important—
they use roughly 20 percent less electric
power both in continuous operation and for
the higher engine-start loads. This is signifi-
cant. In fact, if you install rotary-compressor
air-conditioner(s) in your Kool Kat, you can
take advantage of this to reduce the mini-
mum required generator size by 15percent.
Indeed, if we’d installed an all-rotary air-
conditioning system in Kool Kat, we could
have used just a 3.5-kilowatt gen set rather
than the slightly larger 4-kilowatt, while