Expert Advice: Owen Cox's Top Technical Tips for Installing a Diesel Heater

A diesel heater is one of the best upgrades you can make to a campervan, overland vehicle or boat, providing efficient, dry heat for year-round adventures. But while modern heaters are remarkably reliable, their performance depends heavily on correct installation.

We asked Owen Cox of AUTOTERM UK to share the professional installation tips he believes every installer should know before fitting a diesel heater.

Start with the mounting position

If you're installing the heater inside the vehicle, use a turret plate for any floor penetration. The turret should pass completely through the floor, with the underside sealed using intumescent silicone sealant to prevent water ingress and provide additional fire protection.

Before drilling, always inspect beneath the floor for wiring, fuel lines and structural components. It's equally important to check for factory-fitted exhaust heat shields underneath the vehicle. These should remain in place wherever possible and only be trimmed if absolutely unavoidable.

Avoid mounting the heater where the exhaust or combustion intake will interfere with:

• Wheel arches

• Automatic side steps

• LPG floor drop-out vents

If the heater is mounted externally beneath the vehicle, it should ideally be housed in a protective enclosure unless a dedicated vehicle-specific mounting bracket is available, such as those designed for the VW Transporter beneath the factory undertray.

When side-mounting the heater, ensure the electrical cable exits from the uppermost side of the unit.

Exhaust and combustion air matter

Correct routing of the exhaust and combustion air intake has a significant effect on heater reliability.

Both the combustion air intake and the exhaust outlet should face towards the rear of the vehicle to minimise the chance of water, dust or road debris entering the system.

Where possible, both pipes should run continuously downhill. If this isn't achievable, drill a 2mm drain hole at the lowest point to allow condensation to escape.

The combustion air pipe should only be shortened if absolutely necessary, as altering its length can affect airflow characteristics.

For the exhaust:

• Maximum length is 2 metres when using a silencer

• Maximum length is 3 metres without a silencer

Keeping within these limits helps maintain correct exhaust back pressure and efficient combustion.

Fuel system installation

The fuel supply is another area where careful installation pays dividends.

The preferred method is a top-mounted fuel pick-up into the vehicle's fuel tank. On many modern vans, purpose-designed fitting kits allow push-fit connections to existing fuel modules.

Where suitable, the engine fuel return line may also be used, provided the vehicle has no pressure regulator in the return circuit and the fuel pressure does not exceed 0.3 bar.

Position the fuel pump:

• Within 1 metre of the fuel tank

• Angled upwards approximately 15 degrees towards the outlet

The fuel pipe should ideally climb gently from the pump towards the heater. Keeping rises and falls to a minimum reduces the risk of air pockets forming within the line.

When connecting the fuel pump wiring, ensure the terminals are fully seated inside the connector. Polarity is not important.

Electrical installation

Power supply is critical for reliable heater operation.

Power cables should be connected directly to the battery or to a suitably rated heavy-duty busbar before any battery isolation switch.

Avoid connecting the heater through switch panels or standard distribution boards.

Interrupting power before the heater completes its shutdown cycle can damage the heater and may present a fire risk.

Any cable joints for the power supply, controller or fuel pump should always be made inside the vehicle, where they are protected from moisture and corrosion.

Positioning the controller

Mount the controller around mid-height within the living area and away from direct hot airflow from the heater outlet.

Leave enough cable slack to allow the controller to be removed easily for servicing or replacement without stressing the wiring, and ensure the rear connector remains accessible.

From a usability perspective, Owen recommends positioning the controller where it can be conveniently operated from the bed.

Getting the ducting right

Airflow restrictions are one of the most common causes of poor heater performance.

Before fixing the heater permanently, confirm there is adequate space for the ducting and avoid tight bends, particularly within the first metre of the heater outlet.

For an AUTOTERM 2D, maintain one unrestricted 60mm outlet. A Y-piece can be added for additional outlets, but the primary airflow should never be restricted.

Larger installations using 90mm ducting can support up to four outlets, although adjustable vents are recommended to help balance airflow.

Always maintain one permanently open 90mm vent to prevent overheating. Additional outlets can be reduced to 60mm using suitable Y-pieces or reducers where appropriate.

For marine installations, insulating both the main 90mm duct and any 60mm branch feeding the forecabin helps deliver more consistent heat throughout the vessel.

One final detail: don't use Jubilee clips to secure ducting directly to the heater outlet, as overtightening can damage the heater casing. Heavy-duty cable ties are the preferred solution.

Special considerations for boats

Marine installations introduce additional challenges.

The exhaust skin fitting should be positioned as high as practical—ideally on the transom—to minimise the risk of water entering the system.

A correctly formed swan neck inside the boat provides further protection against water ingress.

The exhaust silencer performs best when mounted vertically rather than horizontally, and any nearby combustible materials should be protected using glass-woven insulation covered by 60mm ducting.

Where possible, install the heater running fore-and-aft rather than across the beam of the boat.

Don't forget carbon monoxide protection

No diesel heater installation is complete without a carbon monoxide (CO) alarm.

Even a perfectly installed heater should always be backed up by a quality CO detector. Exhaust damage, accidental blockages or component failures can allow carbon monoxide—a colourless, odourless and potentially fatal gas—to enter the living space.

Fit a certified carbon monoxide alarm in accordance with the manufacturer's instructions, test it regularly, replace the batteries when required and never ignore an alarm if it activates. It's a small, inexpensive addition that could quite literally save your life.

As Owen Cox puts it, the best diesel heater installation isn't just about maximum performance—it's about ensuring every journey is as safe as it is comfortable.