Diesel Heat
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A decade or so ago, I joined a friend for
a cruise to Newfoundland aboard his
47-foot cutter, Paloma. The passage
lasted four months and afforded me
an unforgettable plethora of
experiences and photographic images. One of the
more memorable components of that passage was
the varied and extreme weather in those latitudes.
Although we departed Long Island Sound in late
May—shirtsleeve weather for that region—once we
transited Massachusetts Bay, the thermometer
seemed to have a hard time getting out of the 60s.
By the time we reached Nova Scotia, a 50-degree
day was considered a balmy godsend, and June in
Newfoundland was sometimes reminiscent of
November in New England. It rained or was foggy
six days out of seven, and ice wasn’t an unusual
sight. For several years after the passage, I would
sometimes wake during the night, dreaming—it’s a
nightmare, really—of a failed passage of Cabot Strait,
the body of water that separates Nova Scotia from
Newfoundland (and through which nearly all of the
St. Lawrence Seaway’s marine traffic passes). Six
hours out of Louisbourg, Nova Scotia, an
unpredicted front—as they so often are in this
climatically tumultuous region—swept down upon us,
bringing 50-knot winds and torrents of bone-chilling
rain and spray that lasted all through the afternoon
and into the night. Ultimately, we were forced to
turn and run before it, seeking shelter in the
industrial port of Sydney, Nova Scotia.
Although the images of that four-month passage,
particularly the night spent in Cabot Strait, are
firmly embedded in my memory, one other image in
particular stands out. I was fortunate to sail aboard a vessel equipped with a diesel-fired heater. Coming
off the watch that night in Cabot Strait and other
watches and standing next to that cylindrical brassand-
stainless-steel heat source was salvation in
itself—a feeling I shall never forget.
And it never failed that wherever Paloma docked
or anchored in Newfoundland during that passage,
once the crews of other cruising vessels (granted,
there weren’t many) in the anchorage discovered we
had a diesel heater, they were soon aboard. Many
cruisers are unprepared for Newfoundland’s
“summer,” and thus, few of them had an adequate
heat source other than their galley stoves and diesel
engines. Some had been living in wet clothes and
berths for days.
WHY DIESEL HEAT?
Diesel certainly isn’t the only way to heat a
cruising vessel. Reverse-cycle heat and air
conditioning systems are de rigueur aboard nearly all
new and many older trawlers. These systems, akin
to shore-based heat pumps, work well in many
scenarios and are capable of taking the chill off
those fall passages. However, they suffer from two
shortcomings. One, many users complain that blown
air—particularly when it’s less than hot—often
produces a certain chilling effect. Two, heat pumps
are efficient only when the temperature of the water
surrounding the vessel is above about 40 degrees
Fahrenheit. Below that, the heat exchanging
capability of the heat pump system is no longer
effective, and the system rapidly loses efficiency and
its ability to produce heat.
If a vessel is already equipped with propane for
the galley stove, one may wonder, why not use propane as a heat source? While propane, or LP gas,
as it’s often known (not to be confused with CNG,
compressed natural gas or “street gas”), is an ideal
fuel with which to heat a shop or home, it, too,
has its drawbacks. Not the least of these is its
inefficiency compared to that of diesel fuel; pound
for pound, diesel fuel is difficult to beat as a heat
source.
Diesel fuel offers nearly 140,000 Btu per gallon,
compared to 91,000 Btu for propane. In my opinion,
LP gas is peerless where cooking appliances are
concerned. Many vessels use propane for cooking as well as heat, and in these roles it works well, but
it’s simply not as Btu-abundant as diesel fuel.
Additionally, the crew of a vessel heated with
propane must, during any passage, concern themselves
with obtaining two types of fuel—one for heat
and one for propulsion or power generation—a task
that can be difficult in remote regions. (Few filling
stations that carry LP gas are within walking
distance of fuel docks; even if they were, who wants
to lug 30 lb. of LP gas and a tank to and fro?) When
used for cooking only, LP gas lasts an especially
long time; thus, refilling becomes an occasional rather than a regular event. But when used for heat,
that same tank is not nearly as long lived.
Finally, storage, transport (through hoses,
plumbing, and valves), and combustion of a
compressed, heavier-than-air gas such as propane
presents its own set of safety challenges. An LP gas
leak is not a matter to be trifled with aboard any
vessel. Diesel fuel, on the other hand, with its flash
point of 140 degrees Fahrenheit, is especially safe.
(Conversely, the flash point of LP gas is negative
156 degrees Fahrenheit, which means it is always
flammable.) And while leaks cannot be ignored, a
diesel leak does not present the same explosion
hazard as does LP gas.
THE BASICS
Before continuing our discussion of diesel heat, two
subjects are worthy of mention. The first is safety
related and involves the formation of carbon
monoxide, or CO, gas. Carbon monoxide is a
colorless, odorless gas that is nearly the same density
as air. (This means it doesn’t accumulate in bilges or
vent through cabin hatches or ports; it moves around
the cabin the same way air does.) The leading cause
of poisoning in the United States, carbon monoxide is
created whenever carbon-based fuels—diesel, gasoline,
kerosene, propane, compressed natural gas, wood,
and coal—are burned incompletely. Carbon monoxide
interferes with red blood cells’ ability to transport
oxygen throughout the body. In high concentrations,
death can occur in just a few minutes.
A future PMM article will delve into this
important subject. But for the purposes of discussing
heating systems for vessels—diesel or otherwise—the
following warning must be given: Unless you use
wind or electricity alone to propel and heat your
vessel, you run the risk of carbon monoxide
poisoning. While it is true that diesel engines
produce less CO than their gasoline brethren, they
still produce CO—as do LP gas ranges and heaters—
and a diesel-fired heater typically does not burn fuel
as completely as an internal combustion engine.
Additionally, even if your boat isn’t equipped with
diesel or LP gas heat, the vessel docked adjacent to
you may use this fuel for heating (or propulsion or
power generation), so you and your crew are at risk.
Cases have occurred where fumes from one vessel
have entered the open ports or hatches of a nearby
vessel, poisoning its occupants. Therefore, every
vessel—whatever fuel she uses for propulsion or
heat—should be equipped with at least one ABYCcompliant
CO detector. (A detector should be installed in each stateroom and the main saloon.)
It is also vital that these detectors are installed in
such a way that they cannot be inadvertently or
intentionally turned off. My preference is to wire
them directly to the load or output side of the house
battery switch using an appropriately sized fuse.
Wired in this manner, the CO detector will be active
whenever the vessel’s house battery switch is in the
on position.
The second item, while not life threatening, is
worthy of discussion: the definition of a Btu, or
British Thermal Unit. This term is used frequently in
heating system literature, and it serves the reader
well to have a general understanding of its meaning,
if only for comparative purposes. In scientific terms—
and there’s really no other way to put it—a Btu is
the amount of thermal energy required to raise
the temperature of one pound of water 1 degree
Fahrenheit from the temperature at which water
has its greatest density—39 degrees Fahrenheit (in
metric terms, 1 Btu is equivalent to about 1055
joules). Don’t worry if you can’t wrap your brain
around this definition; just keep it in mind when
comparing one fuel or heating system to another.
It’s an apples-to-apples sort of thing.
A RANGE OF DIESEL-FIRED SYSTEMS Bulkhead Heaters
Diesel-fired heating systems range from the simple,
manually controlled “drip-pot” bulkhead-mounted
heaters pioneered by Scandinavian fishermen to the
sophisticated, electronically controlled, multizone hotwater
(“hydronic”) systems that would rival those
found in the most modern homes and offices.
Bulkhead heaters are available in a number of
sizes and styles from a score of manufacturers both
here in the United States and abroad. A diesel
bulkhead heater kept Paloma’s saloon toasty and dry
through her wet, cold Newfoundland passage.
Bulkhead heaters have the advantage of simplicity
on their side. Depending on how they are installed,
they may be rigged to operate without using any
electricity at all. A “day tank” placed above the level
of the heater will supply fuel without the need for a
pump. If this isn’t practical, a small electric fuel
pump can be used to supply the heater. In either
case, these heaters offer the greatest bang for the
buck: They are relatively inexpensive—most cost
under $1,000—and, provided they are installed
properly, they work exceptionally well. The average
bulkhead heater produces between 6,000 and 15,000
Btu, and a few of the largest units are capable of
producing an impressive 18,000 Btu.
The drawback of this type of heater is that its
output is centralized to the compartment in which it
is installed, usually the main saloon. Leave this area
and the cabin temperature can drop precipitously.
Paloma’s saloon was warm and cozy, but when it
came time for me to retire to my aft cabin, the
temperature difference was considerable…and
unwelcome. This temperature differential is especially
noticeable because the temperature in the area where
the heater is installed tends to be a little on the high
side. It’s something like the feeling you get when you
stand in front of a campfire for a while—your tent feels
a little colder than it did before you warmed yourself
by the fire’s glow. Strategically placed fans can go a
long way toward mitigating this problem.
Perhaps the greatest challenge faced by bulkheadheater
installations is the flue or exhaust. Because
the flame in these heaters is passive, there’s typically
no forced draft, which means they are especially
sensitive to flue or chimney arrangements and back
drafts. Many chimneys incorporate an elaborate
anti-backdraft device known as a Charlie Noble.
(The story behind the unusual name is that the
device was invented by a ship’s cook named Charlie
Noble, who was frustrated with his stove filling the
galley with smoke and soot.) The device has to be
strategically placed on deck so the heater’s exhaust
doesn’t cause any heat or soot damage to
surrounding gear or structures. Most manufacturers
provide elaborate instructions to guide this
installation—and with good reason. The exhaust
temperature of a diesel heater can be in the region
of 700 or 800 degrees Fahrenheit; if it’s placed too close to combustibles such as fiberglass, wood, or
inflatable dinghy material, it will quickly cause
damage and possibly lead to a fire.
A few caveats are in order where bulkhead units
are concerned. According to ABYC guidelines, a
“sealed” combustion system is one in which the
incoming air used for burning the fuel, the combustion chamber, and the
exhaust by-products are sealed
off from the vessel’s interior.
This definition relegates most
bulkhead heaters to the “nonsealed”
category. While this is
not a condemnation of their
design or function, it must be
borne in mind when installing
and using this type of system.
What is important about this
classification is that ABYC guidelines require an
oxygen-depletion sensor when a non-sealed system
is used. This sensor, when properly installed, will shut off the diesel fuel supply to the heater if the
ambient oxygen level (the oxygen level within the
cabin or space where the heater is installed) falls
below 95 percent of normal.
Additionally, most bulkhead heaters are also
defined by ABYC as “attended devices,” which
means they require action and attention, including
operator-initiated ignition. Thus, once again
according to ABYC guidelines, these heaters are
designed to be operated while a [fully awake] crew
member is in attendance. (I’ve added “fully awake”
to emphasize that the guidelines envision that a
crew member will monitor the heater whenever it is
in operation—day or night.)
The final ABYC guideline worthy of note where
bulkhead units are concerned addresses the
temperature of fixtures surrounding the heater.
When the heater is operating, the temperature of
the surface below and immediately surrounding
vertical combustion surfaces must not rise more than
150 degrees Fahrenheit above the cabin’s ambient
temperature.
Forced-Air Heating Systems
In many ways, the forced-air system is an easier
concept for vessel owners and consumers to
understand because it is much like the heating
systems used in many homes and businesses. In
simple terms—for both the marine and the shorebased
versions—air is drawn from the space being heated
(the cabin), usually through a filter of some sort, then
passed through a chamber or heat exchanger that is
heated—in this case with a forced-draft diesel-fired
flame. (This is what gives these systems that turbine
or “jet engine” noise.) The design and construction of
this heat exchanger is critical; the cabin air and the
exhaust by-products in the combustion chamber must
never be allowed to mix. (This was a common
problem on air-cooled automobile engines such as
the old Volkswagen Beetle and the Corvair.) If they
do, the heated air that is now destined for the vessel’s
cabin will become contaminated with exhaust byproducts,
including the potentially lethal carbon
monoxide. Most manufacturers of marine and
domestic forced-air heating systems go to great
lengths to prevent this from occurring, so the risk is
no greater than that from gas- or oil-fired forced-air
heating systems used in homes. In most cases, if a
leak does occur, it’s the result of an installation error
rather than a design or material flaw.
Once the air is heated, it is then distributed
throughout the vessel using metallic ducts that are typically 4 or 5 inches in diameter, making for a true
central heating system. These ducts can be directed
into each cabin, the head, the engine compartment,
and the drying lockers, and onto the windshield.
As mentioned above, for efficiency’s sake, the
intake air should be drawn from the accommodation
or cabin spaces rather than from outside the vessel.
To prevent the possibility of introducing combustion
by-products or exhaust gases into the cabin, this air
must never be drawn from an engine compartment.
If the intake air is ducted through an engine
compartment, great care must be taken to ensure
the intake ductwork is absolutely airtight.
The output of the average forced-air system ranges
from a low of 2,000–3,000 Btu in the smallest units to
a high of 45,000 Btu in larger models. These systems,
unlike bulkhead heaters, are not designed to operate
continuously; they cycle on and off using an
electronically controlled central thermostat, and they
may be operated at varying output levels. While this
is especially convenient, providing heat very much
like that found in a modern home, it usually cannot
be zoned. Therefore, while these systems typically
work very well, the temperature in the saloon and
galley—where the thermostat is located—may vary
widely from that in the staterooms or head.
Additionally, running 4- or 5-inch ducts
throughout a vessel after it’s been built may be
challenging. (Depending on the design, this may be
true even while the vessel is under construction.) I’ve done this a number of times, and the process usually
involves moving through the proposed duct runs
using a plywood template of the duct cross section
to determine routing feasibility. Once that process is
complete, a hole or reciprocating saw can be used to
cut a path for the various duct runs. Directional
registers are typically installed at the terminus of
each run, providing directable, warm, blown air at
each location.
Hydronic Heating Systems
This system, too, has a shore-based cousin familiar
to millions of home owners and apartment dwellers.
The marine version uses a diesel-fired furnace to
heat water that is then pumped throughout the
vessel through hoses—usually no larger than one
inch in diameter. (In fact, what is pumped is usually
a mixture of water and antifreeze, much like that
found in your engine or genset.) These hoses are
arranged throughout the vessel in series or in daisychain
fashion. The water circulates through the
entire run before returning to the furnace. The
source of heat at each selected location may be
either a passive radiator such as those found in
homes (typically, the marine versions are relatively
flat sheet steel rather than cast iron) or a fan-assisted
“radiator.” The latter consists of a small box, usually
smaller than a shoe box, through which the hot
water is passed. Within the box is a small heater
core, similar to those found in automobiles, over
which air is blown using an electric fan. One or
more short ducts are attached to the box and are
then “plumbed” to a nearby cabin bulkhead. The
heat available from or produced by each fan unit is
approximately 7,000–10,000 Btu, roughly the same
as a 1,500-watt electric cube heater. Thus, if a single
cube heater will heat a given space, it’s likely that a fan unit will as well. For spaces that don’t require
the output of a full unit, the output may be split—one
duct running to a head, for instance, and the other
to an engine compartment. (Additionally, the hot
water that runs through the system can be used to
passively heat or warm a hanging or drying locker,
or dish or towel rack. By passing the heated water
through a pipe rather than a hose in these locations
alone, heat is effectively radiated into these spaces.)
An aquastat installed at each fan unit, optional on
some systems and standard on others, prevents the
fans from turning on—and blowing cold air—until hot
water has reached each heater. If you recoil at that
initial blast of cold air (and who doesn’t?), then
aquastats should be a prerequisite
for any hydronic system.
Hydronic heating systems as a
whole are available in a range of
output sizes, from 8,000 Btu to
nearly 180,000 Btu. Hydronic
systems can be found in vessels as
small as 28 feet and routinely in
vessels 40 feet and longer.
Finally, although hydronic
systems tend to be slightly more complex than forced-air systems—and considerably
more complex than bulkhead heaters—they do
possess a number of advantages over these systems.
Hydronic systems can be easily zoned: A thermostat
can be installed in each cabin if desired. Hydronic
systems may also be integrated with the vessel’s
domestic water system, providing an almost limitless
supply of hot water for washing, showers, and so on.
(To be fair, some bulkhead heaters may be equipped
with optional water-heating units, and thus they can
be used to provide limited domestic hot water needs
as well.) Additionally, this hot water can be made
almost instantly and virtually silently because it does
not require that the engine or generator be started.
A hydronic system may also be used to preheat an
engine or keep it warm, a feature that may be useful
under certain operating conditions. (If you opt for
this capability, my recommendation is that this be
accomplished using a heat exchanger, which avoids
the need to intermingle engine coolant, and
pressure, with that of the heating system.)
INSTALLATION CONSIDERATIONS
Regardless of the type of diesel-fired heating
system you use, considerable care and attention must be given to each unit’s exhaust system. As
mentioned previously, the exhaust by-products of
nearly all carbon-based fuels contain carbon
monoxide, as well as other harmful constituents. The
installation instructions provided by each
manufacturer must be followed meticulously.
Unfortunately, this alone is not enough to ensure a
properly operating, gastight system. In installing
several diesel-fired heating systems, I have
discovered defects in new exhaust components
supplied by reputable manufacturers. These defects,
if not corrected, would have allowed exhaust gases
to leak into
the vessel’s
engineering
spaces and
eventually
into the
accommodation
area.
For this
reason, the
installer of
your system
must be
fully familiar
with the
components
and possess
the requisite attention to detail to notice flaws—or potential
flaws—in this equipment.
The exhaust gases produced by diesel-fired heaters
are, as one might suspect, quite hot. Forced air and
hydronic systems typically require the installation of
an “exhaust pipe,” a tube or conduit that safely carries
these 700- to 800-degree Fahrenheit gases from the
furnace to the exterior of the boat. These exhaust
pipes or tubes are often made of flexible yet gastight
stainless-steel tubing that is wrapped, or “lagged,”
with insulation. ABYC guidelines require that no
exposed portion of any vessel heating system, dieselfired
or otherwise, exceed 180 degrees Fahrenheit
when the system is operating at maximum output
and the ambient compartment temperature is 77
degrees Fahrenheit. Under the guidelines, the
insulated portion of the exhaust system is considered
an “exposed portion” of the heating system.
The exhaust port of forced-air or hydronic heating
systems is typically installed in the vessel’s hull side
or transom. If the system is operated while the
vessel is dockside or rafted, as it invariably will be,
you must take care to ensure that combustibles such
as fenders, lines, docks, and other vessels are not
overheated or burned by the exhaust gases. Transom
outlets tend to be the safest, preventing the
overheating of nearby objects as well as minimizing
the likelihood of allowing exhaust gases to enter
your cabin or that of a neighboring boat. Some
heating systems use a highly desirable combined inlet and exhaust gas port, which draws inlet combustion
air through the perimeter of the exhaust
outlet, effectively cooling it in the process. While the
metal port doesn’t get as hot, the gases still remain
capable of causing damage or a fire.
The intake air requirements for any diesel-fired
system—bulkhead, forced-air, or hydronic—are
considerable. It requires approximately 14,000 cubic
feet of air to burn a gallon of diesel fuel. For this
reason, it is advantageous for any diesel heating
system to be installed so that the combustion
air is drawn from outside the vessel. With this
arrangement, already heated cabin air is not used
for the combustion process and, perhaps more
importantly, cold air is not drawn into the vessel to
displace the air that has been burned. Nearly all
forced-air and hydronic heating systems are capable
of being installed in this manner, and a number of bulkhead units offer this advantageous feature as
well. It’s worth noting that if a forced-air or
hydronic system uses cabin air for the combustion
process, it must be considered non-sealed and
therefore requires an oxygen-depletion sensor to
make it ABYC compliant.
SIZING YOUR SYSTEM
Properly sizing your heating system will ensure
that the system keeps the vessel warm under the
conditions in which you use her. For self-regulating
systems such as forced-air and hydronic, proper
sizing will ensure that the cycling times—50 percent
is desirable—are appropriate. (Bulkhead heaters are
designed to run continuously, and they operate
more efficiently at higher settings.) If a forced-air or
hydronic system runs too infrequently, it may
increase carbon accumulation and require more frequent maintenance. If it’s too small, a system will
run continuously, which will also increase
maintenance requirements. While manufacturers of
individual systems typically provide their own
guidelines for sizing their respective systems, a
general rule of thumb does exist. For moderately
cold operating environments (fall cruising in New
England and late fall on the Chesapeake Bay, for
instance) calculate the volume of the
accommodation spaces within the vessel in cubic
feet and then multiply by 12 to determine the Btu
needed. For winter cruising, the multiplier will be
15 to 19. These figures will vary based on the design
and insulating properties of a vessel—for instance,
cored hulls have excellent inherent insulation—and
the number of glass windows, ports, and hatches, all
of which radiate heat more quickly than the hull or
cabin structure.
MANUFACTURER ISSUES,
SERVICE, AND SUPPORT
Regular readers of my articles and columns
know my feelings on the subject of product support
and service. Regardless of how well a product is
designed or manufactured, if it is not well
supported, it will represent a poor investment for
the user.
Check with the manufacturer of the product you
intend to install before writing your check. Where
are the service facilities? How easy or difficult is it
to order parts? How much do commonly used parts
cost, and are they likely to be back-ordered? How
often will the unit require an overhaul? How easy is
it to speak with a technical service representative
for troubleshooting assistance—for you or your
service yard? Does the unit you are interested in
include self-diagnostic capabilities? (This is an
especially valuable feature.) Are special tools or
equipment required for service or diagnostics?
Some of these questions can be answered
objectively by the manufacturer or dealer. For
others, you’ll have to poll existing owners or
boatyards that service and install the equipment.
Finally, place a call or send an email to the
technical service department for the make of heater
you are considering installing. Ask a few simple
questions about the unit, its service, and operation.
Is the responder polite and knowledgeable about
the product? One manufacturer that I’ve dealt with
off and on through the years would often say, when
confronted with a seemingly unsolvable service or
repair scenario, “Well, these systems are really designed for trucks and buses, not boats.” This, in
spite of reams of advertising information to the
contrary. While it may be true that a number of
heating systems, along with numerous other
shipboard products used on recreational vessels,
were originally destined for the over-the-road
market, it’s not the type of retort I expect to hear
when seeking service or repair guidance.
Diesel-fired marine heating systems can make the
difference between a pleasurable, comfortable
passagemaking experience and a cold, damp,
miserable one. While your experience may—and
hopefully will—differ from the one I had in Cabot
Strait, chances are good that once you’ve installed
one of these systems, you’ll feel warm and confident
about it, inside and out.
Steve D’Antonio is PMM’s Technical Editor and the VP of
operations for Zimmerman Marine, a custom boatbuilder
and full-service repair yard in Mathews, Virginia.
Reprinted with permission. Copyright 2006 © Dominion Enterprises (888.487.2953) www.passagemaker.com
You are reading the text-only copy of this article. To access the article as it appeared in PassageMaker Magazine, please log in to purchase and download the PDF version of this article.