Diesel Cabin Heaters: What a Forced-Air or Bulkhead Heater Install Involves

• Forced-air suits most cruising boats: it hides away, heats several cabins and dries the air.
• A bulkhead heater suits a sailboat with one main cabin and a good spot for a chimney; it draws almost no battery power. Dickinson notes its natural-draft heaters can be lit and run without the small start-up fan, which matters on a boat with a weak battery bank.
• Hydronic suits larger boats that want even heat in many cabins and hot water.

Check your power budget: forced-air heaters draw much more current while starting and glowing the igniter than while running. Make sure your batteries and charging can handle it on nights at anchor.

For a forced-air unit, look for a dry, accessible locker with room for service, where the duct runs can be short and simple. Many sailboat installs use dead space under a cockpit seat or coaming. Keep it away from fuel and water tanks that could be heated by the exhaust.

For a bulkhead heater, the BoatUS installer chose a spot that radiates heat into the main cabin, is out of the traffic path (so nobody falls against it) and has room for a straight chimney above. Many makers want it facing fore or aft on a sailboat. Check the clearances to combustible materials listed in your manual.

This is the most safety-critical part.

• Forced-air exhaust: a short run of the maker's stainless exhaust pipe to a special exhaust through-hull. In the Good Old Boat install, the pipe sloped downward to the fitting for drainage, was wrapped with heat lagging along its full length, and was clamped and sealed at each joint. The through-hull went in with the clearances and heat-resistant washers the fitting maker specified.
• Bulkhead chimney: a vertical flue pipe to a deck fitting with an air gap around the pipe and a cap on top. The manual gives a minimum flue length and prefers a straight run; elbows reduce draft. Dickinson says the chimney's height and shape matter more than anything else when choosing the heater's spot, because the chimney creates the draft that pulls in fresh air and pushes exhaust out. Its Newport bulkhead heater, as an example, lists a chimney of roughly 4 to 8 feet, a straight run as best, and a barometric damper on longer chimneys; your heater's numbers may differ. Seal any cut core in the deck with thickened epoxy.
• Placement: keep the outlet away from hatches, opening ports, ventilators, docklines, fenders and the dock itself, and (for chimneys) clear of the boom, sheets and anything a crew member might grab. Exhaust gas must not find its way back inside.

• Combustion air for a forced-air heater usually comes through its own short intake pipe. The Good Old Boat owner got clouds of white smoke until he cut that pipe to the length in the manual; these heaters are very sensitive to intake and exhaust length.
• Air to be heated should come from the cabin (recirculation) or from outside, never from the engine compartment. Drawing it from a locker shared with the engine can pull fumes into the cabin and can starve both of air; that owner had a flameout running heater and engine with the lockers shut.
• Ducting: keep runs short with few bends, use the maker's heat-rated ducting and outlets, and close outlets only as the manual allows.
• Bulkhead heaters need a permanently open fresh-air supply into the cabin (the manual gives the minimum size). The BoatUS writer counted his open dorade vents toward it. Without enough fresh air, draft drops and smoke or fumes can spill back.

• Most installs use a separate pickup tube through a spare fitting in the top of the diesel tank, reaching near the bottom. Do not tee into the engine's fuel supply line unless the heater and engine makers both approve it; you can starve the engine or let air into it.
• Forced-air heaters use a small metering pump with strict rules on mounting angle, line size and run length. Bulkhead heaters typically use a low-pressure pump or a gravity day tank, plus an inline filter. Dickinson describes its natural-draft heaters as fed by gravity or an impulse pump, with a float-type metering valve that keeps a steady fuel level, and recommends a fuel filter.
• Use the fuel the heater is set up for. Dickinson's diesel heaters are calibrated for #2 diesel (other metering parts exist for kerosene or #1 diesel), and it says biodiesel cannot be used. Check your heater's manual before trying any other fuel. Approved fuels really do vary by maker and model: Webasto's product page for its Air Top Evo marine heater lists diesel and paraffinic fuels (such as renewable diesel), which is not the same list as Dickinson's.
• Many bulkhead heaters have an overflow line: it returns to the main tank, or (with a day tank) drains to a small container below the heater.
• Use fuel-rated hose and fittings specified in the manual, routed away from heat and chafe and well supported.

Give the heater its own fused or breaker-protected circuit at the panel, sized as the manual says, with wire heavy enough to limit voltage drop (low voltage causes failed starts). Mount any separate control module where it stays dry; the Good Old Boat owner oriented its wire entries downward so water could not collect. Run the controller or thermostat to a convenient spot in the cabin.

Where combustible surfaces are closer than the listed clearances, makers call for insulation or millboard plus a metal shield on standoffs with an air gap behind it. The BoatUS install used a stainless plate on spacers behind the heater and on nearby woodwork. Keep wiring, fuel lines and stowed gear clear of the exhaust and chimney.

Fit marine CO alarms covering the main cabin and every sleeping area, plus a smoke alarm, before you run the heater overnight. Test them, note their expiry dates and keep spare batteries aboard if they use them. See the CO detector guide for placement.

On the first run, with the installer present:

• Watch the exhaust outside: after warm-up it should be nearly clear. Persistent white or black smoke means a fault.
• Use an infrared thermometer on nearby bulkheads, liners and stowed items as the heater reaches full output. Nothing combustible should get too hot to touch comfortably.
• Look and sniff along the exhaust or chimney joints and the fuel line for leaks.
• Make sure the CO alarm stays silent and that air flows from every outlet.
• Ask for the manuals, a list of what was installed and the service schedule.

Webasto recommends switching the heater on once a month for at least 20 minutes, even in seasons when you don't need heat, so it stays ready to work. Some controllers remind you when a maintenance run is due. Run it with the boat attended and ventilated.

• Look at the exhaust through-hull or chimney cap: clear of nests, soot, salt and damage, and the gelcoat around it not discolored.
• Check the exhaust lagging, clamps and duct connections; ducts should not be crushed.
• Look for diesel weeps at the pump, filter and heater.
• Check the intake screens are clear.
• Test the CO and smoke alarms.

Webasto says the heater should not be cleaned with high-pressure water or compressed air.

Webasto recommends inspection and maintenance by an authorized dealer at regular intervals, at least every two years. Many owners have it serviced every year if they use the heater a lot or burn questionable fuel. Clean fuel matters: water or sludge in the tank is a common cause of failed starts. Record service dates in BoatMend.