How to install a diesel heater on your boat

A properly installed diesel forced air heater will keep your boat warm for a long time, with little more than a service every couple of years.

Problems with diesel heaters are most often attributable to how it was installed.

Getting the installation right in the first place is key, therefore.

The exhaust (top) is lagged with insulation and then sheathed in protective ducting. The heater sits on a mounting plate that should be placed as close to horizontal as possible. The air intake (bottom) for combustion passes through a silencer

If you’ve already got heating on board, it’s worth giving it a once over to check that it was put in correctly to start with and that no parts of the system are damaged.

While a crushed vent might reduce the efficiency of your heating, a damaged exhaust could produce fatal carbon monoxide or pose a fire risk.

For those who don’t have heating, carefully planning how you would fit it is the first step.

Choosing the right diesel heater

Getting the right unit will be crucial for the long-term performance of your heating.

A unit that isn’t big enough for your boat will constantly be running at full capacity and will drain power, while a unit that is too big will be in idle much of the time and risks sooting up as a diesel engine would.

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A 2kW heater is sufficient for up to four outlets on boats up to 34ft, while a 4kW heater will be suitable for boats of up to 40ft or more, depending on how many areas you want to heat.

You also have the option of installing more than one smaller heater, so you can have separate heating for different areas of the boat, which may be a more efficient solution.

For this installation on a Sadler 29, an Eberspacher 2kW unit with two vents was plenty.

Locating the heater

The heater cannot be in any cabin space and must be in a separate locker, and depending on the make, should be fitted as close to horizontal in both planes as possible.

These requirements mean that the majority of heaters, as on this boat, are fitted in a cockpit locker, tucked up out of the way.

If you store lines and fenders in here, make sure they don’t rest on the system, though the exhaust should be lagged for its whole length.

Routing the ducting

The ducting leads the hot air around the boat and will lose some heat as it does so.

This isn’t significant where the ducting runs straight, and some residual warmth can help to keep lockers warm and dry, but on long runs or sharp corners, heat lost will be greater, making the heater work harder.

Fitting the ducting: Clip the ducting up out of the way. If it is at the bottom of a locked it is liable to be crushed. Credit: Theo Stocker

Ducting runs should therefore be as simple and as short as possible, and where sharp corners are unavoidable they should be properly insulated with lagging.

The fixed vent (which avoids the system overheating) on this boat was placed in the saloon by the base of the chart table, and the closing vent forward in the heads where the heat could be directed through to the forward cabin, or to the wet locker.

The correct vent: At least one vent must be fixed open to prevent the diesel heater from overheating. Credit: Theo Stocker

With the heater in the cockpit locker port aft, the ducting was routed along the engine bay, under the starboard quarter berth and then forward under the chart table into the locker below the starboard saloon berth, where the ducting was split with a Y connector.

Connecting it up

Power and fuel supply should be fairly straightforward on most boats as the batteries and fuel tank are usually close to the engine bay.

The exhaust outlet must be at least 300mm above the waterline; the higher the better, as long as there is still space for a swan neck in the exhaust to stop water backflow in rough conditions.

Fuel shut-off valve: The fuel line should have a shut-off valve at the top of the lift pipe. Credit: Theo Stocker

You then also need to think about where fuel, power and air ducting will be routed. the fuel pump needs to be within 2m of the fuel tank and within 4m of the heater, and it also needs to be fitted at between 15° and 30°, which helps prevent air locks.

Adding the fuel lift pipe to the tank, including a shut-off valve, required carefully drilling a hole into the metal tank.

This was done with a vacuum cleaner next to the drill bit to prevent debris entering the tank. the pump has its own small filter, and shouldn’t have an additional in-line filter as this can provide too much resistance to the pump.

Control system

The control unit can be located wherever it is most convenient, limited only by the length of the cable, though some wireless control units are available.

Connecting the controller: If the temperature reading is a little bit out, you can set an offset if necessary. Credit: Theo Stocker

Either way, the diesel heater should be within the area that is being heated, but not next to an outlet as this will affect the thermostat. if there is no other option, an external thermostat can be added.

Cutting the holes

Having the right tools for the job will make installation much easier and neater.

If you are having a professional installation, their experience will pay dividends here, but it’s worth asking if they have done a fitting on your type of boat before.

Exhaust safety: The double-walled skin fitting must be at least 30cm above the waterline and should vent well away from the air intake. Credit: Theo Stocker

Most ducting is a standard size, but check requirements for your system. this ducting required a 67mm (2 5/8ths in) hole saw, and the exhaust skin fitting needed a 38mm (1 1⁄2 in) hole saw, which was cut angled upwards, using the stainless skin fitting as a guide, before being fitting with plenty of sealant.

The only issue encountered was discovering some hidden cable ducting glassed into the forward bulkhead, so having a wiring diagram of your boat before you start is a good idea.

Insulation and running up

Thanks to David Cousins at Krueger Ltd, which are Eberspacher main dealers. www.krueger.co.uk

The only sharp corners that were unavoidable were under the quarter berth and again as it went through the forward bulkhead into the hanging locker, so both of these were insulated with lagging.

One area of ducting was also unavoidably exposed below the chart table, so a hardwood box was built to protect it.

Covers were screwed over the ducting apertures and the system was ready to use.

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