In this article we explain first the theory of summer cooling and
winter heating and then the available systems and finally a practical
example of an installation
Carlo Formenti and Umberto Cranchi
Nautica Magazine 420, April 1997
AIR CONDITIONING ON BOARD
Cruising is always a nice thing, but in summer the inside of the
boat often gets too hot; We are already used to air-conditioning
in our car, in offices, in hotels: why waste a nice day? Good
air-conditioning can be a "do it yourself" installation. In this
article we explain first the theory of summer cooling and winter
heating and then the available systems and finally a practical
example of an installation.
Everybody who owns a boat with a small or medium-size cabin or has
a minimum of sailing experience knows that the warmest place of
the boat in summer is the cabin, where the temperature is always
few degrees higher than the external temperature on hot summer
days: The sensation of heat created by the warm still air
increases if we are not moving with air flow through the vessel
which may be limited by small accesses and hatches. It is obvious
that as boats are made from different materials and have different
shapes and configurations, we can find more or less comfortable
cabins; another variable is the possibility of ventilation and
therefore the quantity, size and position of doors, hatches etc.
We have so far considered the problems of the summer season, but
if you use the boat in the cooler periods as well, even if the
cabin feels slightly warmer than outside, it will never reach a
comfortable temperature if you do not install a heating system.
Many boat owners give up using their boat in the hottest or
coldest period of the year exactly for this reason. Or in summer
they simply cannot use the cabin as it is more comfortable staying
outside. Cabins become very uncomfortable when the sun shines, no
wind and air gets heavier and heavier....
In the picture above, tools (not many nor specific
tools (indeed) needed for the installation of a marine air-conditioner.
In the picture below, some of the components
used for the air distribution of the air-conditioning system.:
flexible duct of different sizes, splitter plenums and
distribution plenums, duct nipples and air grill in different
shapes and sizes to match the cabin decor.
May be you still remember 'that July 199... when I could not sleep
for an entire week because of heat, mosquitos, and marina noise'.
There are an infinity of types and models of boats but all, even
in different ways, have the same problems.
Nowadays we can find in the market a wide choice of systems which
will solve this summer inconvenience satisfactorily. The market
offers air-conditioning units specifically designed for marine
use. The range of these systems includes units for any size of
boats, with cabins of small, medium or large size. We will deal
here of the small units and boats, but it must be said that the
same manufacturers make very large units for very large vessels.
The most interesting thing, which is the good news of recent
years, is that the 'small' systems are quite easy to install and
reasonably inexpensive. Considering the advantages of the air-conditioning
on a boat, the installation of an air-conditioning
system is quite simple and anybody with a bit of "do it yourself"
experience and a with few, non specific tools, will be capable of
a successful installation of a compact system. This article will
give you all the necessary information to complete your installation.
Below you will find a short theoretical chapter with tables,
numbers and coefficients: it is obvious that in order to air-condition
a space, as each boat has different sizes and
characteristics, it is necessary to combine the different
components supplied by the manufacturers, and to do this, you need
positive figures of the requirement.
The capacity needed to air-condition a room is expressed in Btu/h.
In order to calculate the value you must calculate the room
surface of each cabin (drawing above) and consider is position in
the boat. This table gives the capacity value expressed in btu/h
from the surface value.
a) below deck cabin
b) under deck or deck saloon
c) upper deck saloon
When more rooms are to be conditioned, the addition of each
individual capacity gives the total capacity needed for the boat.
When more rooms are to be conditioned, see the paragraph
You do not need to go back to
school, nor to look for the physics school book: with these tables
and formulas you will know immediately the capacity you need. Our
purpose is in fact clear your doubts and answer your questions.
Air-conditioning is quite common already in any application, lets
make it easy also for onboard air-conditioning. We find it already
on even small cars, in houses it is more and more common, not to
say of the offices which are widely air-conditioned for a better
quality of life. Then we ask why a leisure boat, which is the long
dreamed place to spend our vacations must be not livable in summer
which is when we like to have holidays. This first part of the
article deals with the evaluation of the boat characteristics and
the needs, which will then be used to choose the correct system
and model of air-conditioner more suitable to our boat.
What air-conditioning means
Air-conditioning means to modify the temperature and humidity of a
room in order to achieve a more comfortable living condition. An
air-conditioning system has the capacity to take, treat and
deliver the air of room cooled and dehumidified to the correct
values. An air-conditioner has normally also the capacity to heat
the room when it is equipped either with the reverse cycle system
or with an electrical heater. an air-conditioner is also supplied
with a room temperature control (thermostat), a on/off switch and
a fan speed control with multiple speeds. If the air-conditioner
has also the heat function, then the control panel gives also
gives the choice between cooling and heating. This choice can
also be automatically made, in such a way that once you set the
desired temperature, the air-conditioner control will choose
automatically the functioning mode to reach and maintain the set
temperature. In this case the air-conditioner becomes an
environmental control system.
Btu/h: British Thermal Unit. The unit to measure the heat capacity. It is 1/4 of Calorie.
Calorie: Heat capacity measure. One calorie is the heat needed to increase of one degree C. one litre of water.
Reverse cycle: It is a technical way to modify the refrigeration circuit in order to produce heat.
Fan coil: It is a heat exchanger or evaporator equipped with a fan.
Evaporator: Heat exchanger between refrigerant and air or between refrigerant and water.
Heat exchanger: Technical component used to transfer heat from one fluid to another.
Climate control system: An air-conditioner with an automatic control which maintains the preset temperature.
A. Saloon delivery grill
B. Owner's cabin delivery grill
C. Guest's cabin delivery grill
D. Air intake grills
E. Compact self-contained air conditioner, installed under the seats of the dinette
Types of air-conditioners
There are three types of air-conditioners:
- Independent direct expansion units as self contained or two
part units which are used to air-condition one or two rooms close
to one another. The air-conditioner treats the room air and
delivers the air back directly to the room recirculation, through
air ducting with sizes from 75 to 175 mm which avoids making
complicated and long distribution systems. An interesting version
of the independent unit is the split model which is built in two
parts: a compressor assembly and a separate evaporator/fan
assembly which can be installed several metres apart from the
compressor, saving cabin space and permitting the air-conditioning
installation in boats where there is no space for both components
in one piece. The temperature control is made by stopping and
running the compressor and also by controlling the fan speed.
- Central units with direct expansion circuit to several
evaporators (fan coils). These are very common units used in land
installations where they can be called 'multi-split'. In marine
applications there are some installations made with this
configuration where one (large) compressor cools several
evaporators. Unfortunately this simplified configuration makes the
system inflexible and it tends to become unbalanced, particularly
when the thermal load is reduced at night and with the large
compressor balanced for the high load of the day, it becomes far
in excess of the reduced night load. This situation can cause an
increase in the fan coil noise into the cabins. In addition to
this, the piping for the refrigerant connection to each fan coil
could become a weak point if not correctly designed and installed:
as any leak will stop the entire system and the repair could be a
real hassle. With this type of system it is not possible to
connect several compressors in parallel on the same circuit; each
compressor must have its independent circuit connected to its
evaporators. The temperature of each room is controlled by
stopping the fan of that room or by stopping the refrigerant flow
to that fan coil. In either case again the system becomes
unbalanced if not properly designed as the compressor capacity is
still the same while the fan coil load is reduced.
- Central systems with chilled (or heated) water distribution to
several fan coils each installed in the room to be air-conditioned.
In this case the central system, which can be made
with one or more compressors, cools (or heats) the water of a
closed water circuit which is pumped to each fan coil. This type
of unit has several advantages: the distribution system of the
chilled (or heated) water has the same characteristics as a
heating circuit but instead of a boiler there is one or more
chiller compressors and at the place of the radiator in each room
there are the fan coils. Each fan coil is completely independent
from the central unit , which is set to keep the fresh water
circuit temperature at a preset value (normally +12C in summer
mode and +40C in winter mode); all the fan coils are connected in
parallel to the fresh water circuit and the room temperature.
The airconditioner, when used in cool mode (summer use), is a
refrigerating unit which subtracts the heat from the room air
(directly in the direct expansion systems, and indirectly with an
intermediate fluid in case of 'chiller' systems. The heat removed
together with the heat generated by the compressor working, must
be then dissipated outside the air-conditioned space. The marine
air-conditioner uses a special marine heat exchanger to dissipate
the heat to the sea water, which is circulated by a pump.
The same airconditioner which produces 'cold' in summer, can
produce heat in winter. In order to produce heat the air-
conditioner must be equipped either by a "reverse cycle valve" or
by an electric resistor. The reverse valve is special 4 ways valve
which can "reverse" the refrigerating circuit so that the
evaporator becomes a condenser and the condenser becomes an
evaporator. In this way the heat is taken from the sea water
(which is consequently cooled) and given to the room air which is
heated. This heat is sufficient for Mediterranean climate, with
mild winter temperature and, more important, sea water temperature
above 0° C. The sea water temperature must be carefully
considered as the air-conditioner efficiency drops dramatically if
the sea water temperature drops below 10 c. If this happens the
air-conditioner looses efficiency and it can no longer be used.
In this case for cold seas it is advisable to install system
equipped with electrical heating, which doesn't loose efficiency
in cold waters. In the market are also available air-conditioners
equipped with electrical heating.
Sea water cooling of the air-conditioner
Because of the heat rejected overboard by an air-conditioner when
cooling, and the consequent problems in typical marine
installation, all marine air-conditioners are water cooled, in
other words the air-conditioner dissipates the heat into the sea
water, using a special marine heat exchanger in which the sea
water is circulated by means of a pump. Off course a "land" air-
conditioner could also be installed but due to external noise and
water ingress the result will-hardly be satisfactory. The pump
used to circulate the sea water should be rated for continuous
duty and built to-marine specifications. It is normally used a
marine centrifugal pump in 230V which is installed below the water
line as the standard centrifugal pump is not self priming.
The pictures show the use of the air distribution grills, their
shape and some of the possible installations. As the cold air is
heavier than warm air, the cold air falls therefore the cooled air
should be dispersed across the deck head or at least upwards. This
is achievable also by realigning the grill blades. The air grill
has also a high impact on the inside decor.
The marine air-conditioning has two aspects which must be well
considered for safety reasons:
- The system is connected to mains supply (normally 230V) and it
is essential that the connections follow the safety rules.
- The air-conditioner unit (or its fan coils) must recirculate
the cabin air and possibly a small percentage of external air. The
air intake should never come from contaminated compartment or even
worse, from the engine or generator room. In facts in case of a
problem in the exhaust system of the engine or generator, the
exhaust gas is lethal to man and if the air-conditioner takes and
delivers these gasses it will be extremely dangerous or even
lethal to the people on board.
Air distribution - Recirculation and air exchange
The air-conditioning of a compartment may work only by treating
the air of the room temperature. The majority of the air-conditioners
work with recirculation only as the air-conditioner
treats and delivers back to the ambient the same air taken from
the room. The use of fresh air is not normal practice, as there is
normally more than sufficient "natural" air exchange in the
original yacht project, from ventilators, hatches, doors and
generally "passage" to the external ambient for breathing and
odour removal. The air exchange becomes a must for yachts above
25-30 metres and for boats designed for personnel carriers. When
handling air (air-conditioning) it is necessary to keep in mind
the following rules:
Air delivery grills
The treated air must be delivered and diffused into the area to be
air-conditioned, in such a way that it doesn't bother people by
causing a draught and should be quiet, by avoid too high an air
speed, and for this reason air grills have to be installed of a size
adequate for the air-conditioner capacity. The market offers a variety
of grills in metal, wood and plastic; it is also possible to make custom
made grills incorporated in the yacht furniture. If well studied,
grills can be aesthetically pleasing and matching the internal decor.
The air-conditioner or fan coil must take the air from the room to
treat it. The space where the air-conditioner is installed must be
in direct communication with the room to be conditioned. In this
case it is not necessary for an intake grill as the air is drawn
freely. The intake grill is needed for the decor purpose only, as
the intake must be "masked" , but other solutions for the air
intake can be found with improved appearance. When the treated air
from an air-conditioner is delivered to several cabins, one must
consider that the air from these cabin must return to the air-
conditioner. Often cabin doors are not tight, but it must be
checked that the air passage is enough, otherwise the air-
conditioner will run at a reduced capacity due to a restricted
return. A passage for the return air can sometimes be found
through a locker which is common to both cabins.
To be continued...