Cabin Heater Safety Aboard: Diesel, Propane and Electric Heaters

Before the heating season starts, make sure every sleeping area and the main cabin has a working marine-rated CO alarm (look for the UL 2034 listing with marine approval). The BoatUS Foundation suggests mounting them where people spend time and sleep, at about eye level, and away from hatches, doors and dead-air corners. Press the test button on each one. Replace alarms when they reach the end-of-life date printed on the unit or the alarm signals end of life. The Coast Guard's advice is simple: install alarms, maintain them, and never ignore one. See our CO detector guide for details.

Portable propane "camping" heaters, kerosene heaters, charcoal, barbecue grills and the gas stove are not cabin heaters. Even "indoor-safe" portable propane heaters burn cabin oxygen and release combustion gases into the space. Practical Sailor points to one popular portable LPG heater that carries a prominent warning against use in unventilated areas, and notes that safe LPG cabin heaters need a room-sealed design, where the air intake, flame and exhaust are all isolated from the cabin. The Coast Guard recommends using only fuel-burning devices that are certified or designed for marine use. Portable diesel heaters made for tents or workshops are just as unsuitable.

In December 2019, two men died sleeping aboard the motor cruiser Diversion in York, England. The UK Marine Accident Investigation Branch found CO had leaked into the cabin from the diesel cabin heater's exhaust. The exhaust silencer was not designed for marine use and its joint was not gas-tight, the owner-fitted installation had never been checked by a professional heater installer or serviced, cabin ventilation was poor, and no CO alarm was fitted. Every one of those is something you can check on your own boat.

With the heater off and cool, follow the exhaust pipe from the heater to the through-hull:

• Every joint should be clamped tight with no gaps, soot marks or black stains around it
• The pipe should be the maker's marine exhaust pipe, with insulation or lagging where it passes near wood, wiring, fuel lines or people
• Look for crushed, kinked, cracked or rusted-through sections
• Any silencer must be the maker's approved marine part
• Outside, the through-hull fitting should be clear, not cracked, and not near an opening hatch, port or cockpit seating where exhaust can blow back in

If there is soot inside the boat, a smell of exhaust when the heater runs, or a loose or damaged section, do not use the heater until a technician fixes it.

Diesel heaters take combustion air from outside or from a separate space through an intake pipe. Make sure it is not blocked and is well away from the heater's own exhaust and from engine or generator exhaust; the MAIB has investigated other cases where engine exhaust was drawn into a heater intake and blown through the boat. Keep the boat's fixed vents (dorades, vented hatches and cabin vents) open when any fuel-burning heater runs, even in cold weather. Check that warm-air outlets and return grilles are not blocked by bedding, cushions or gear.

• Diesel heaters: look along the fuel line from the tank pickup to the heater pump and heater for drips, weeps, cracked hose or diesel smell. A heater that keeps failing to start or smokes may have air or water in its fuel line.
• Propane heaters: Practical Sailor's LPG safety guidance includes a tank solenoid tied to a gas sensor, a locker that drains overboard, one continuous gas line per appliance, and periodic leak checks with soapy water on joints. Brush soapy water on each joint you can reach; growing bubbles mean a leak. See our propane safety guide for the full routine, and never use a flame to look for leaks.

Forced-air diesel heaters draw a lot of current while starting and need good voltage to burn cleanly. With the heater running, measure voltage at its supply if you can reach a safe point. Low voltage can cause failed starts, smoky burns or error shutdowns. Make sure the heater has its own fuse or breaker, wires are secure with no chafe, and the control panel shows no stored fault codes. If it does, look the code up in the manual or call a dealer.

Start the heater from its control and stay with it for the first half hour of the season:

• A little smoke or smell on the first start after storage can be normal, but it should clear quickly
• Watch the exhaust outside: heavy or continuous smoke means a burner or fuel problem
• Inside, there should be no smell of exhaust or fuel
• Check the CO alarms stay quiet

If something seems wrong, shut it down and call a technician. Don't keep restarting a heater that is smoking or throwing faults.

For natural-draft diesel or propane bulkhead heaters:

• Check the flue pipe for rust, gaps and soot at joints, and the deck fitting for cracks and leaks
• Make sure the chimney cap is fitted and clear, and that it is the type your heater maker specifies for your location
• Keep the clearances to woodwork, curtains and cushions your manual requires
• Practical Sailor notes that heater flues can get hot enough to damage running rigging; fit a guard where lines can touch it

Never run a natural-draft heater with all hatches sealed tight; it needs the air supply its manual describes.

A BoatUS Magazine writer who installed a natural-draft diesel bulkhead heater explains why: the hot chimney creates the draft, so the cabin needs enough fresh air coming in to replace what goes up the flue. His heater's instructions called for a fresh-air opening that stays open whenever it runs, plus marine smoke and CO alarms in the main cabin and every sleeping area. Other points from his install worth checking on yours:

• Woodwork closer than the maker's clearances should be shielded (insulation or a metal heat shield with an air gap behind it)
• The deck fitting needs the air gap around the chimney pipe that the maker specifies, and cored decks should be sealed where the hole was cut
• Flue length and elbows matter: a short flue or several bends can weaken the draft, so follow the maker's minimum length and keep the run as straight as you can
• On diesel models with an overflow fitting, the overflow line should be connected the way the manual describes, not capped off
• On the first firing of the season, feel nearby woodwork and anything that can burn as the heater reaches full temperature

Have fuel-burning heaters serviced by a qualified technician on the schedule your maker recommends (yearly is common for boats used often in winter). Service usually covers the burner, glow plug, fuel pump and filters, exhaust and safety controls. As one example, Webasto's operating instructions ask for an inspection by an authorized Webasto dealer at regular intervals of no more than two years, say repairs need special knowledge and tools, and recommend running the heater about once a month for at least 20 minutes, even in summer, so it stays ready. They also say never to clean the heater with a pressure washer or compressed air. Other makers set their own schedule, so check your manual. The MAIB's follow-up warning after Diversion was clear: diesel cabin heaters need regular checks and servicing, and CO alarms must be fitted. Keep service receipts in BoatMend.

BoatUS's claim files include boats lost when a space heater tipped over in a wake, when a heater ignited something nearby, and when the damaged extension cord powering a heater shorted, destroying the boat and damaging another boat and the dock. To use electric heat safely:

• Use a heater with a tip-over switch and an overheat shutoff, set on a flat surface on the cabin floor
• Keep it well clear of bedding, curtains, cushions and anything that can burn. The U.S. Fire Administration's home rule is at least 3 feet from anything flammable, and space on a boat makes that hard, so choose the spot carefully
• Plug it straight into a boat outlet, not into a household extension cord or power strip
• Don't leave it running with nobody aboard, and never use a heater instead of winterizing
• A space heater draws a lot of power. Check the shore power cord ends and the boat's inlet for heat, discoloration or burn marks, and replace damaged cords. See our shore power safety guide

BoatUS also notes that heaters can increase mold and mildew because warm air condenses on cold portlights and hatches, so ventilate as well as heat.

If an alarm goes off or someone feels unwell: turn off the heater and any engine or generator, open hatches, get everyone into fresh air, and get medical help. Do not go back below until the source is found and fixed. Have the heater and exhaust inspected before using it again. Check on anyone sleeping aboard nearby boats too; CO from a neighbor's exhaust can drift into your cabin.