Dave Gerr - Boat Mechanical Systems Handbook-How to Design, Install, and Recognize Proper Systems in Boats

as a home project (a fairly big one) with
pretty much everything in one location.
Since they are generally simpler to install,
they are usually best for smaller boats—
under 45 feet (13.7 m) or so. Sometimes,
however, it might make sense to have a split
system serving most of the accommodations
of a large boat, but install one or two self-
contained units in areas far from the main
machinery space, such as in a wheelhouse
located fairly high up.

How Much Air-Conditioning?

The big question (and usually the first one)
is, how big or how much? Air-conditioning
capacity (like heating) is measured in
British thermal units (BTUs). Interestingly,
although you don’t want too little capacity,
you don’t want too much either. More than
you need is definitely not better. Not only
does this put excess drain on the boat’s
electric system, but also, such oversize air-
conditioners don’t work efficiently. Indeed,
a principal portion of the air-conditioner’s
job is not just to remove heat, but also to
reduce humidity by taking water out of the
air. Air-conditioners do this by condensing
the water out. This occurs because warm
air can hold more moisture—in the form of
vapor—than cool air can. As the warm
air cools in the evaporator/air-handler, it
reaches its dew point, the temperature at
which the water vapor condenses out of
the air into liquid water droplets. These
droplets are deposited on the outside of the
evaporator’s coils. Air leaves the evaporator
not only cooler but much drier, and the
dryness (even more than the coolness)
makes for comfort. Oversize systems run
too cold and are less effective at remov-
ing moisture, because air passes too quickly
over the cold evaporator coils, or because
it cycles on less often. Oversize systems
can also ice up, which makes them almost
totally ineffective.

The Condensation Problem

So much moisture condenses out of the air
(and then drips off the evaporator coils) that
the water absolutely must be collected care-
fully and routed overboard. It’s surprising

to me just how often this feature is over-

looked. Whether you’re installing a new air-

conditioning system or inspecting an exist-

ing one, make doubly certain that there’s a

good, large, deep drip-collector pan under

the evaporator. Then double-check to see

that it has a drain hose that leads to a sump,

which then pumps overboard.

It’s not uncommon to see the drip-

collector drain lead directly into the bilge.

This isn’t good practice. After all, your goal

is to keep water out of the bilge. On a hot,

humid day, you could easily accumulate

several gallons of fresh water in this way.

Such accumulated fresh water leads to dis-

agreeable smells, mildew, fungus, and simi-

lar unpleasantness.

Sizing Air-Conditioners

For preliminary design and as a check, you

can use Formula 15-1, which I worked up

to estimate Btus required based on boat

displacement.

Formula 15-1. Preliminary Estimate of

Required Btus of Air-Conditioning

Btu= 3,000+ (1,500× disp. tons)

(for southern waters add 10 percent)

Chapter 15: Air-Conditioning and Heating

Units for Measuring Air-Conditioning

Output

In the United States and Canada, the Btu is the standard measure

of heat energy. This is the heat energy needed to increase the

temperature of 1 pound of water by 1°F, which is very roughly

equal to the heat energy given off by burning one match. I guess

you could say that a 16,000 Btu air-conditioner is a 16,000-match

cooling system. A Btu is equal to 252 calories per hour.

When quantifying air-conditioner output, BTUs are really Btu

per hour (Btu/hr.). One Btu/hr. is 0.293 watts. Or

Btu/hr.×0.293=watts

watts ÷ 0.293 =Btu/hr.

1,000 Btu/hr.×0.293=kilowatts

kilowatts ÷ 0.293 =1,000 Btu/hr.

In Europe, air-conditioning units are often sold based on their out-

put in kW (kilowatts). You can use the preceding formulas to con-

vert the required capacity in Btu (really Btu/hr.) to kW.

Formula 15-1.