Sailing 101: Installing a Boiler

Ahoy Sailors, last time we talked about tips and tricks to decorate your boat and win you the grand prize in that boat parade. To stay on theme with sailing during the chilly months, this time, we will be discussing how to install a boiler for cold weather sailing. So stay tuned. Next time we will discuss something fun— new year’s resolutions for boaters!


Unless you plan on making a trip somewhere tropical, Southern California sailing is currently what we Southern Californians call “chilly.” With highs of 55 degrees and lows of 40, we need to find a way to stay warm, especially when we’re out on the boat. Therefore, a heating system is required to make your experience more comfortable while cruising in places where the mercury stays at the bottom of the thermometer.


Heat requires energy. This means natural gas or electricity on land, but neither is readily available at sea. As a result, most boats run off safe fuel in the form of diesel. Diesel is a safe and energy-rich fuel to burn for heat. 


Heaters come in a few forms: a fireplace style that radiates heat passively, a direct air version that blows hot air, or a boiler (hydronic) that heats hot water that transfers heat through radiators. The fireplace or hot air version works great for a smaller boat or a single cabin, but you’ll need a boiler to heat a larger boat.


With a boiler system, the boiler is typically installed in a lazaret or locker with a small expansion tank of water. Then it channels water to radiators strategically placed around the boat. Next, the water piping system is installed as a loop with a small pump. The pump moves hot water out to the radiators and returns it to the boiler to be reheated. The expansion tank adds the volume required to allow the water to expand as it heats and extra water volume to the system to store excess heat in the form of thermal mass.


Several manufacturers make these systems, most of them Canadian or European, likely due to the higher latitude sailing in those areas.


All successful projects start with a plan, and this one begins by planning the size of the boiler. Boiler sizing is essential: a unit that is too small won’t keep the boat warm and will prematurely wear out, but a boiler that is too large will cause short cycling and prematurely destroy it. Boilers are specified in BTUs delivered. You can approximate the BTUs required by calculating the interior volume of the boat (length x width x height) in cubic feet and multiplying by 12. This will give a rough calculation that you can adjust depending on the boat’s design. For instance, a deck saloon will require more BTUs than a low-slung cruiser.


Make space next to the boiler for the expansion tank. The expansion tank must be the highest point in the system to capture any air in the system. The exhaust should be routed as direct as possible. The shorter the passage, the better. Exhaust can be deadly, so plug all connections with a high-temperature sealant.


Exhaust systems on these units can heat up to several hundred degrees, so they must be completely insulated and routed away from flammable materials. Try removing the exhaust line at an exhaust through-hull fitting designed to keep hot gasses away from the fiberglass. 


The boiler is useless without fuel, so address that next. These systems sip fuel and are supplied with an easy-to-route 3/16-inch fuel line. The boiler should only use about a pint of fuel per hour. Like any diesel system, it’s important to bleed the air from the line. This is easy to perform by extracting the fuel line from the boiler and pumping fuel into a cup until all signs of air disappear.


The most challenging part of the installation is the piping and radiators. Depending on your boat’s design, we recommend designing the system with a small radiator in each cabin and a larger one in the main saloon. Try small radiators with 4-by-8-inch faces for the cabins and cut holes in lockers to mount them.


To maintain the water integrity of bulkheads and lockers, seal the cut surfaces of each hole with epoxy. While you route the hoses, try to avoid any high spots in the hose, as these are prone to collecting air and could impede flow. Any high points require a fitting to allow air to escape, each radiator has one, and they can be added if an elevated spot is inevitable.


The electrical parts of the system are relatively straightforward. The boiler must be powered with 12 volts of direct current. The main cabin requires a thermostat, and each radiator must be powered to drive its fan. It is important to note that the boiler should be powered with constant power from a breaker. The power must be able to cycle independently of the system power switch so that when the system power switch is turned off, the boiler maintains power to complete its cool-down cycle.


Now that the system is installed, you must commission it. This means filling the system with water, flushing air, and looking for leaks. After that, you can add water to the system and run it cold or pump water through it with a utility pump.


With the commissioning process is complete, turn the system on. These systems go through a defined start-up sequence. First, you’ll hear the fuel pump kick in. Next, the boiler exhaust fan starts, and the boiler will light. The boilers are not silent, especially outside the boat— it begins with a whoosh and settles into a faint buzz. Finally, after maybe 15 minutes, the boiler will heat the coolant. Once the coolant is hot, the system turns on the radiator fans, and the heat flows.


Now get out on the water and enjoy some warm cruising. 

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