Warm and Snug

The old cliché that there’s more than one way to skin a cat is spot on when it comes to marine heaters. There are many different systems that can warm your cabin on a cold night. These include heating stoves, hydronic systems, forced-air systems, engine heaters, electric heaters and reverse-cycle air-conditioning systems.
These systems all work differently, but the basic idea is the same. Either fuel is burned or another process used to create heat, which is then dispersed throughout the boat. Some systems have ducts and vents; others feature central stoves that create heat, which is moved through the boat with fans. Thermal output is commonly measured in British Thermal Units; one BTU is the amount of energy needed to raise the temperature of one pound of water by one degree Fahrenheit. The more BTUs a system produces, the warmer you can make your cabin; more BTUs also increase fuel and electrical demands.
Determining which system is right for you depends largely on what kind of boat you have, how well insulated it is, where you plan to sail it, and how much time you spend sailing in cold weather. Most boat manufacturers provide guidelines for what size heater you’ll need.

Heating Stoves
Marine heating stoves burn either diesel, kerosene, propane or solid fuels, such as wood or charcoal. Some stoves are direct draft, meaning that fresh feed air is pumped in via a fan. Other systems rely on natural draft, meaning air is pulled into the stove naturally via the combustion process.
According to Michelle Parry at Dickinson Marine, some propane stoves can be hooked up to the boat’s main propane tanks, while others use smaller dedicated tanks. Heating stoves are typically situated in the saloon; some hang off of a bulkhead, others are freestanding. Some units simply radiate heat, others have fans to circulate the hot air. In all cases, heating stoves require an exhaust pipe. Generally speaking, the higher the exhaust pipe, the better it will draw. Some exhaust pipes will protrude from the deck several feet. Others have a lower profile. Most solid fuel stoves are equipped with a damper, which allows you to control the airflow, how hot the stove burns and how much fuel it consumes. Ensuring that the cabin is well ventilated will dramatically improve performance.
Most heating stoves require no electricity to operate. A heating stove appropriate for sailboats up to 36 feet, such as Dickinson Marine’s Newport P-1200, puts out from 7,000 to 9,700 BTUs and consumes one pound of propane every 3.9 to 5.5 hours.

This WEbasto display shows what's involved in a forced-air heating system.Hydronic Systems
Marine hydronic heating systems work on the same principle as household hot water heating: a heated fluid runs through tubing to radiators or fan units that warm the air. On the average sailboat, tubing runs from the heater in a single circuit throughout the boat.  The boat is divided into a series of zones, with one or more radiators or fan units in each zone. Fins and louvers on the radiators facilitate heat exchange with the interior air. Some systems also use fans to disperse heat.
Some hydronic systems can also heat potable water for the boat. Depending on the make and model, a hydronic heater is usually significantly smaller than a water heater, which is a consideration for anyone needing a generous supply of onboard hot water.  
Hydronic heating units are typically installed in the lazarette or the engine room. If the system is integrated with the engine’s cooling, the engine itself does not need to be running while the heater is in use; hydronic systems simply use the coolant to disperse heat. Depending on the model, the fuel can come from the vessel’s main diesel tank or from a dedicated tank. Different systems can burn #1 or #2 diesel, kerosene or 80/20 biofuel. A series of seals and pipes pump the combustion exhaust outside the boat, usually via a fitting at the stern.
Hydronic heaters need electricity to power the system’s coolant pump, a fan for combustion and a fuel-metering pump. They draw the most power when heating up the coolant during the first half hour of operation. Energy demand, then, drops precipitously once the boat is warm and the system only has to maintain cabin temperature. “The big advantage of a hydronic system is even heat throughout the boat,” says International Thermal Research’s (ITR) John Brooks. “You don’t have cold spots, [and] you’re not creating moisture in the boat because you’re exhausting outside.”
A hydronic heating system appropriate for a 40-footer, such as ITR’s Hurricane SCH-25, has a maximum output of 25,000 BTUs per hour while consuming 0.22 gallons of fuel and 6 amps of power.

Forced-Air Systems
The principle of a forced hot-air system is simple: a heater burns fuel (typically diesel) to heat air, which is then forced through ducts to a series of louvered vents in the boat’s interior. The ducts usually run behind panels and cabinets. Most forced-air heaters are located in the engine room, and a series of seals and exhaust pipes lead to the stern where combustion exhaust is discharged. When the system is turned on—either manually or via its thermostat—air and fuel are drawn into the combustion chamber. An ignition plug ignites the fuel/air blend, and a controlled flame is established inside the sealed combustion chamber. Clean air is then drawn (usually from outside the boat) into the heat exchanger where it is heated and forced into the duct throughout the boat.
Forced-air systems are easier to install or retrofit than hydronic systems, are less expensive than other heating options, and represent about 75 percent of the marine heating market. One of the great benefits of a forced-air system is that the ducts behind cabinets and panels also radiates heat. This makes it easy to heat cabinets and lockers for drying off foulies. Some systems can also be used to circulate fresh, cool air throughout the boat on hot days, helping maintain comfortable onboard temps and sweet-smelling air in all seasons.
An Espar forced-air systemA forced-air heating system such as Webasto’s Air Top 5500 produces between 5,000 and 17,000 BTUs per hour while consuming between 0.04 and 0.16 gallons of fuel and 1.2 to 8 amps of electricity per hour.

Engine Heat
Systems such as Volvo Penta’s QL Air Heater use the engine’s heated coolant fluid to transfer warmth into a boat’s interior. Coolant lines are run from the engine into the QL Air Heater’s heat exchanger. A fan blows cabin air into the heat exchanger, where it is heated; the hot air is then piped out into the cabin via a series of hoses. The upside of this system is that it’s quiet and takes advantage of an engine’s closed–circuit cooling system without requiring additional water pipes or radiators. The downside, of course, is that the system only works when the engine is running, which gets expensive, both in terms of fuel consumed and extra engine maintenance.  Therefore, they’re seldom seen on sailing boats.
An air heater appropriate for a 40-footer, such as Volvo Penta’s 5 kW QL Air Heater, produces 17,000 BTUs per hour, so long as the engine coolant remains hot.

Electric Heaters
One option for sailors who only occasionally venture out on cold days or who spend most of their time hooked up to shore power is an electric heater. These units, which typically run on 12-volt power, function much like a space heater in your home and can be portable or permanently mounted somewhere inside a cabin. The basic concept is simple: the unit uses electricity from the ship’s batteries to heat a series of coils. A fan draws ambient air over the coils, where it’s heated. Another fan blows the heated air back out into the cabin. According to Connie Beles at Family Safety Products, some systems, like the company’s Back Seat Heat, increase the temperature of air entering the heater by 30 to 40 degrees Fahrenheit. Since electric heaters continually re-circulate air, the warmer the boat’s cabin gets, the warmer the processed air becomes.
Engine heat systems, such as these Volvo QL units, use the engine's heated cooland fluid to transfer warmth into a boat's interior. Hydronic heaters, like this one from ITR, function much like home baseboard heating systems, delivering warm air to the entire cabinThe downside is the electrical demand. A heater like the Back Seat Heat draws 25 amps while producing a meager 1,100 BTUs per hour. “You want to make sure there’s a power supply recharging the batteries, as this will run them down pretty quickly,” says Beles.

Reverse-Cycle Air-Conditioning Systems
If you’re thinking of adding air conditioning to your boat, one consideration is to buy a unit that can run in reverse cycle, like a household heat pump. These systems—which are beyond the scope of this article—are more expensive than a simple heater, but they can help you extend your sailing season on both ends of the calendar. For sailors considering a circumnavigation, a reverse-cycle air conditioner could prove a worthy investment if you don’t plan to cruise too far north or south.


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