How to size the air conditioning system of a boat
- UPDATED 2026
- 8 MIN READ.
In this guide
Sizing a marine air conditioning system does not simply mean choosing the machine with the highest BTU/h. The design must find a balance between comfort, consumption, available space, onboard voltage, battery capacity, air distribution, and seawater circuit.
An undersized unit can operate for a long time at maximum power without reaching the desired temperature. An oversized unit, on the other hand, may take up more space, require higher currents and wiring, and not solve potential ducting or air distribution problems.
The correct method starts from the spaces and the actual use of the boat, continues with the calculation of the thermal load, and is completed by checking the electrical system, batteries, charging, seawater, condensation, and accessibility for maintenance.
In summary
The length of the boat is not enough to determine how many BTU/h are needed.
Cabins, salon, bathrooms, and other spaces to be air-conditioned should be analyzed separately.
Ducts and grilles must be sized together with the unit, not afterwards.
Voltage, wiring, protections, battery bank, and charging must support the maximum expected consumption.
Seawater circuit and condensate drain are essential parts of the project.
The first piece of data: which spaces to air-condition
A single cabin requires a different design from a lounge or a multi-cabin system. It is necessary to define which spaces need to be cooled or heated, whether they will operate simultaneously, and at what times of the day.
For each space, at least volume, window area, sun exposure, insulation, number of people, and the presence of equipment that produces heat must be collected. The intended use is also important: a night cabin may have different priorities than a lounge used during the hottest hours.
The layout also influences the routing of the ducts, the position of the grilles, the return air, and the distance between the unit and the battery bank. A good thermal sizing must therefore be compatible with an installation that can actually be carried out.
The power in BTU/h
The cooling capacity is expressed in BTU/h or in kW. The length of the boat can provide a first orientation, but two boats of the same size may require very different power levels due to layout, windows, insulation, deck color, sailing area, and usage habits.
The Blue-Airco range includes units from approximately 8,400 to 18,200 BTU/h. The choice of model should be based on the environmental load and not solely on the availability of space or the preference for a specific voltage.
Adequate capacity allows variable-speed technology to reach the required temperature and then modulate. If the load is higher than the available capacity, the machine will tend to remain at maximum power, resulting in less comfort and autonomy.
Do you need to size a system?
The Blue-Airco technical team helps you calculate the thermal load, power, and system configuration.
The factors to consider in the calculation
| Factor | Why it matters | What to check |
|---|---|---|
| Volume and layout | Determine the amount of air to be treated and the comfort distribution | Dimensions, connections between rooms and doors |
| Windows and exposure | Increases solar load during the hottest hours | Surface, orientation, curtains, and shading |
| Insulation | Affects the speed at which heat enters or dissipates | Bulkheads, deck, hull and thermal bridges |
| People and equipment | They produce heat and humidity | Expected occupancy and internal loads |
| Navigation climate | Temperature and humidity modify the load | Mediterranean, tropical, seasonal use |
| Simultaneous use | More active areas require greater overall capacity | Cabins and zones to serve at the same time |
Ducts and grilles
A properly sized machine can lose efficiency if the air does not circulate well. The supply duct must have adequate cross-section, as regular a path as possible, and limited bends. The grilles must provide a sufficient free area, without being obstructed by furniture or fabrics.
The supply is generally placed high to favor the distribution of cold air, avoiding a direct jet on people. The return must allow the unit to freely recover the room air and must remain accessible for filter cleaning.
When the duct is long or has high pressure losses, it is necessary to carefully evaluate the fan's flow rate and pressure. Reducing the diameter to save space can compromise performance and increase noise.
Voltage and electrical system
12V, 24V and 48V are not simple commercial variants. At the same power, the voltage determines the current and affects the cable sections, voltage drops, protections, and the structure of the battery bank.
Each unit must be connected with cables sized according to the maximum current draw and the actual length of the route. A fuse or protective switch must be chosen based on the technical data sheet of the model, including any associated loads planned by the project, such as the seawater pump.
The connection must comply with the manufacturer's instructions and the standards applicable to the vessel. Excessive voltage drop can limit performance, cause overheating, and lead to unit shutdowns or activation of protective devices.
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Battery bank, charging, and autonomy
The battery bank must support both the peak demand and the energy required for the expected hours of use. It is not enough to just sum up the watts of the air conditioner: the seawater pump, other onboard services, losses, allowable depth of discharge, and safety reserve must be considered.
Charging is also part of sizing. Alternator, shore power, auxiliary generator, and solar panels contribute to varying degrees. The desired autonomy at anchor must be compared with the energy that the system can replenish during navigation or at a stop.
For significant systems, it can be useful to provide a dedicated bank or an energy management system that prioritizes loads. The solution depends on the overall electrical design and should be defined with qualified technicians.
Seawater circuit
Water-cooled self-contained units require a circuit capable of ensuring the designed flow rate in all operating conditions. Sea intake, valve, filter, pump, piping, unit inlet and outlet, and overboard discharge must be compatible with each other and installed according to technical specifications.
The pump must be positioned so that it remains properly primed; filters and components must be accessible for inspection and maintenance. Pipes with restrictions, air pockets, or incorrect routing can reduce heat transfer and cause alarms or shutdowns.
In systems with multiple air conditioners sharing the circuit, Efficient Flow can modulate the distribution of water to the condensers based on the activity of the compressors, helping to optimize pump consumption.
Condensate drain and accessibility
Condensation must drain with a regular slope towards a suitable discharge point, avoiding stagnation, backflow, and interference with other systems. The route must be designed taking into account the movements and trim of the vessel.
The unit must also remain accessible for filter, connections, tray, electronics, and maintenance interventions. A very compact installation that is not inspectable can increase time and costs in case of control or assistance.
When to consider the Multizone Control Kit
If a single unit has sufficient capacity but you want to air-condition a second room, the Multizone Control Kit can create two independent climate zones through intelligent air distribution management.
The system uses a motorized damper and a BLDC fan controlled by a touch display in the second cabin. It can avoid the installation of a second unit, reducing weight, bulk, and interventions on the electrical system and the seawater distribution network. Compatibility and configuration must be verified on the specific project.
A seven-step design method
Define the environments. Establish which zones need to operate and whether they will be used simultaneously.
Assess the thermal load. Collect volumes, windows, insulation, exposure, people, and navigation area.
Select the capacity. Identify the necessary BTU/h without relying solely on the length of the boat.
Check the air. Design ducts, grilles, intake, and filter accessibility.
Check the electrical system. Choose voltage, cables, protections, battery bank, and charging.
Design seawater and condensate. Ensure flow rate, priming, drainage, and maintenance.
Coordinate the installation. Compare the layout with datasheets, manuals, and Blue-Airco support before carrying out the work.
Frequently asked questions
How many BTU/h do I need for my boat?
It depends on volume, windows, insulation, exposure, climate, and the spaces to be used simultaneously. The length of the boat alone is not sufficient.
Is it better to oversize for safety?
Not necessarily. A considered margin can be useful, but excessive capacity could cause unwanted increases in absorption during transient phases, bulkiness, and costs. The sizing must be proportionate to the actual load.
Can I use smaller ducts to save space?
Only if provided for by the project and the manual. Insufficient sections reduce capacity, increase pressure losses, and can compromise performance and quietness.
How do I choose between 12V, 24V and 48V?
Starting from the boat's electrical architecture, from the overall power and the length of the wiring. The voltage must be consistent with the battery bank and the charging system.
Can I air condition two cabins with a single unit?
It can be possible when capacity and air distribution allow it. The Multizone Control Kit is designed to create two independent climate zones with a single installed unit.
Who should carry out the final sizing?
The project must be checked by qualified technicians and installers, using the datasheets and manuals of the chosen model. Blue-Airco can provide support in the preliminary assessment.
Do you need to size a system for a new boat or for a refit?
Send Blue-Airco the layout, onboard voltage, spaces to be conditioned, and battery bank data for an initial technical evaluation.
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