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Before I choose ducts and vents, I want to know the heater model and the path through the boat. The air distribution must be able to transport heat to the rooms without restricting the heater's airflow too much.
Start with a sketch of the rooms
Mark the location of the heater, the rooms you want to heat, and the possible path for the ducts. Measure the actual routing including bends and passages through bulkheads. The straight distance between the heater and the cabin is rarely the same as the length of the duct.
Also, decide which rooms are most important. A salon that needs to be warm all evening and a small bunk area may have different needs. The sketch makes it possible to assess distribution and operation as a whole.
The diameter must match the heater
AUTOTERM’s installation manual specifies at least 60 mm at the start of the air system for the Air 2D and at least 90 mm for the Air 4D. Reductions and branches must follow the manual's instructions. These figures are not a general rule for other brands or models.
An existing duct is therefore not automatically suitable just because an adapter can make it fit a new heater.
Give the air a simple path
For AUTOTERM, the manual describes that the first meter from the outlet must be as straight as possible and without 90-degree or other sharp bends. The air system must have at least one vent that cannot be closed when the warm air is distributed.
Plan the space for the duct at the same time as the heater itself. A spot where the unit just fits may still be unsuitable if the air has to turn sharply immediately.
Example: warm salon, cool forecabin
If the nearest vent provides ample heat while the forecabin is cool, I would first check the route of the ducts, branches, and openings. I would also check where the temperature sensor is located and how the heat is controlled.
That is a better basis for a decision than immediately choosing a larger heater. The fault may lie in the distribution, but this must be investigated on the specific installation.
Keep the different air paths separate
The warm air system for the living area and the heater's combustion air/exhaust have different tasks. Use only parts intended for the specific function. This guide is about warm air distribution; exhaust and fuel require a separate plan according to the manufacturer's marine instructions.
What to include in the shopping list
- Heater model and diameter at the inlet and outlet.
- Measured duct lengths and number of passages.
- Suitable branches, joints, and vents.
- Fastenings and any suitable insulation.
- Placement of controls and temperature sensing.
Send the sketch and photos of tight spots. Then I will help ensure the air distribution is included in the quote from the beginning.
Sources and manufacturer documentation
The guide was written by SejlStrøm. Technical information was verified on September 22, 2026. Always use the documentation for the specific model and version.