How do I choose a diesel heater for my boat?

Choose a diesel heater to suit the space you need to heat, the boat's insulation and the coldest conditions you expect, then check that its minimum output suits milder nights. For most small and medium cruising boats, a blown-air heater is the simplest choice, provided the battery, ducting and marine exhaust installation are properly matched.

Size for heat demand

Check the accommodation volume, exposed windows and insulation. These determine heat loss more reliably than boat length alone. An uninsulated steel hull or a motorboat with a large glazed wheelhouse needs considerably more heat than a compact, insulated yacht.

Use these starting points for spring and autumn cruising in northern Europe, then check the manufacturer's sizing guidance:

• Up to about 28ft: a 2kW air heater often suits one main cabin and a short duct run.
• About 28 to 35ft: consider 2 to 4kW, depending on cabin layout, insulation and how many berths need heating.
• About 35 to 42ft: 4 to 5kW is a common starting point for several cabins.

On a typical 32ft yacht, 2kW can work for spring and autumn weekends, but heating both sleeping cabins through long ducts may justify the next size up. Sustained winter use needs a proper heat-loss assessment.

Compare minimum output as carefully as maximum output. An oversized heater that repeatedly stops and restarts can be noisier, use more starting power and develop combustion deposits. Ask whether it can run steadily at low output with your normal outlets open.

Blown air or hot water

A blown-air heater warms cabin air through a heat exchanger and distributes it through insulated ducts. It heats quickly and is usually the simplest retrofit. Established ranges include Webasto Air Top, Eberspächer Airtronic and Autoterm Air. Compare specific models and complete marine kits rather than brand names alone.

A hydronic heater circulates hot coolant through radiators or fan-assisted matrices. It suits larger boats, particularly where small coolant pipes fit more easily than bulky air ducts, or where heating a calorifier is useful. Installation adds circulation pumps, plumbing and bleeding points, with more components to maintain.

A diesel heater does not add combustion moisture to the accommodation, but ventilation is still needed to remove moisture from breathing, cooking and wet clothing.

Check power and fuel use

These heaters work without shore power, but their fans, controls and glow plugs need electricity. Check the specified starting current, running consumption and minimum operating voltage, especially if your batteries are ageing.

• A small 12V air heater commonly draws roughly 1 to 3A once running; larger units and higher fan speeds can draw more.
• Starting demand can reach roughly 8 to 15A, depending on the model.
• At an average 2A, ten hours of heating uses 20Ah, before lights, refrigeration and other loads.