What size diesel heater does my boat need?

A 2 kW diesel air heater usually suits a small cruising boat with one main cabin, while a yacht with several cabins commonly needs 4 to 5 kW. Base your choice on the enclosed space you heat, insulation and expected outside temperature, then check that the heater's minimum output can keep the boat warm without repeatedly switching off.

Measure the heated space

Start with the accommodation, rather than overall boat length. A narrow 30ft sailing yacht and a broad 30ft motor cruiser can have very different heating demands. Include every cabin you intend to heat with its door shut.

Measure the approximate length, average width and average height of each space, then multiply those measurements to estimate cubic metres. Allow for the narrowing hull rather than treating the boat as a rectangular box.

Check where heat escapes. Large windows, an uninsulated hull, draughty companionway boards and an exposed wheelhouse increase demand. Keeping a cabin at 18°C when it is 8°C outside takes much less heat than maintaining the same cabin temperature when it is near freezing outside.

Keep essential ventilation open. Reduce unwanted draughts, but never block ventilators to make a smaller heater cope.

Use these starting ranges

For spring and autumn cruising, these diesel air heater outputs offer a starting point, rather than guaranteed limits:

• About 1.5 to 2 kW: compact accommodation, typically a 20 to 28ft sailing yacht or small enclosed motorboat, with one principal heated space.
• About 2 to 4 kW: roughly 28 to 35ft boats, depending on cabin volume, insulation and whether you want heat throughout the accommodation.
• About 4 to 5 kW: roughly 35 to 42ft cruising yachts or roomy motorboats with several cabins and longer duct runs.

A well-insulated boat may need less than these ranges suggest; a glazed wheelhouse or regular winter use can push demand above them. For larger boats and demanding winter installations, ask the supplier for a heat-loss calculation.

On a typical 32ft sailing yacht used for spring and autumn weekends, 2 kW can heat the saloon satisfactorily and provide some warmth forward. If all three cabins need to warm quickly and stay comfortable with their doors closed, around 4 kW is often a better starting point, subject to the heater's minimum output and ducting requirements.

Compare minimum and maximum output

A bigger heater warms a cold boat faster, but maximum output tells only half the story. Once the cushions, joinery and hull lining are warm, the heat needed to maintain the temperature drops considerably.

Check the lowest continuous heat setting in the manufacturer's specifications. If minimum output exceeds your overnight requirement, the heater may repeatedly stop and restart. That means changes in fan noise and extra electrical demand during ignition.