On ships and boats, a heat pump mostly takes its heat from seawater, and in summer it rejects the heat drawn from the cabins back into the sea. Seawater is a good heat source because its temperature stays almost constant through the day and in winter it is often milder than the air: the same unit heats and cools cabins and saloons and prepares domestic hot water. What sets it apart from an installation on land is that the design has to suit salt water, tight spaces, vibration and the ship's electrical grid.
We have no series-produced marine model. In ship and boat projects we handle the heat pump as OEM or project-specific production, to the specification of the shipyard or of a brand that supplies marine climate systems. This article covers what needs to be considered when preparing such a specification.
Why is seawater a good heat source?
As the outdoor air gets colder, an air-source heat pump has to draw heat from a lower temperature; its efficiency falls and it has to defrost from time to time to melt the ice that builds up on the evaporator. With seawater this problem largely disappears. Volume for volume, water carries far more heat than air and passes heat to the exchanger surface much better, and its temperature hardly changes between night and day.
In summer the same advantage works the other way round. At midday the seawater is cooler than the air on deck; because the heat pump rejects the heat it draws from the cabins into a cooler medium, cooling efficiency stays high. We explain the differences between heat sources in detail in our article on heat pump types.
The seawater circuit
Seawater is taken in through a sea chest in the hull, passed through a strainer and pumped to the heat exchanger. Because salt water quickly corrodes ordinary copper and steel surfaces, the exchanger is chosen in a seawater-resistant material such as titanium or copper-nickel alloy. The position of the intake and the size of the strainer matter too: water drawn in harbours and shallow waters carries far more sand, sediment and marine life than water in the open sea.
In many projects the heat pump is not connected to seawater directly but through an intermediate circuit. The seawater passes only through a plate heat exchanger, while fresh water or a glycol mixture circulates on the heat pump side. The heat pump's own exchanger thus never meets salt water. In heating mode the seawater is cooled by a few degrees in the exchanger; in cold waters this drop is limited and the intermediate circuit is protected against freezing with glycol. The seawater flow rate and the allowance for the sediment and marine growth that build up on the exchanger surface over time are part of the design from the start.
Cabins, saloons and hot water
On large boats and ships, heat is usually distributed by water. The heat pump prepares hot or chilled water at a central point, and this water goes to fan coils in the cabins, saloons and on the bridge. Each cabin has its own thermostat, so passengers or crew set their own temperature. On small boats, a separate compact unit for each compartment is more common.
A heat pump heats in winter and cools in summer by reversing its cycle with a four-way valve. In the mid-seasons, however, cabins on the sunny side may call for cooling while those in the shade call for heating. In systems with a water circuit, this can be balanced by moving the heat drawn from one cabin to another, so that part of the heat is used on board instead of being rejected to the sea.
For domestic hot water the heat pump feeds a storage tank. While under way, waste heat in the main engine's cooling water can also heat the tank; in port or at anchor the heat pump takes over. The tank volume is chosen for the crew showers and the galley, whose use peaks at the same hours. Because the tank has to be raised above 60 °C at regular intervals to prevent bacterial growth, a separate electric heater is planned for this heating.
Limits on board: space, vibration, power and refrigerant
- Space and weight: Every centimetre counts in the engine room. The unit has to be compact, with its connections grouped on an accessible side. On boats, weight and the centre of gravity enter the calculation too.
- Vibration and motion: The compressor has to keep running despite the vibration of the main engine and propeller and the vessel's rolling and pitching. Anti-vibration mounts, flexible pipe connections and a circuit design that secures oil return even when the unit is tilted are required.
- Power: The ship's grid is fed by generators, and voltage and frequency can vary from vessel to vessel. The inrush current a compressor draws at start-up strains the generator. An inverter compressor limits this load with a soft start and adjusts its capacity to demand. Switching to shore power in port is taken into account as well.
- Refrigerant and safety: In enclosed spaces on board, a refrigerant leak is a more critical matter than on land. Flammable refrigerants such as R290 are subject to additional safety requirements, and the refrigerant is chosen together with the rules of the vessel's classification society.
- Noise: Cabins are bedrooms as well, so the sound of the unit and the fan coils is selected against a stricter limit than in a home. We explain how to read sound figures in our article are heat pumps noisy.
Which layout for which vessel?
The size of the vessel and the way it is used decide how the system is built:
- Yachts and gulets: In the Aegean and the Mediterranean, cooling dominates during the summer season. With few cabins, each cabin gets its own compact unit; with more, a central chilled and hot water circuit is installed. At anchor, the generator's running time and noise at night matter; an inverter compressor unit can lower its capacity as the load drops and run quietly on a smaller generator.
- Ferries and passenger vessels: Saloon volumes are large and the fresh air demand is high because of the number of passengers. A significant part of the heating and cooling load comes from the outdoor air drawn in; passing the heat of the exhaust air to the fresh air reduces this load.
- Work and research vessels: For a crew that stays at sea for weeks, cabin comfort and a constant supply of hot water come first. While under way, engine waste heat is used; in port and while holding station, the heat pump takes over.
Air-cooled units mounted on deck are also used on small boats, but because of salt spray, limited space and the need to defrost in cold weather, seawater-cooled units are more common.
Marinas and coastal facilities: the shore side
The marina buildings where boats berth, shipyard offices, yacht clubs and coastal hotels, on the other hand, are served by products made for land. The main issue here is the corrosion that salt and humidity cause on the outdoor unit. The condenser of V series outdoor units is made of O-bent, epoxy-coated copper tubes and suits buildings by the sea. In coastal regions with mild winters and long, hot summers, the Vaha series, tuned for cooling, is the preferred choice.
Custom production for marine projects
When a ship or boat needs a heat pump or a control board, we handle the work under our OEM solutions, starting from the specification: technical proposal, sample and, after approval, production. One example of designing a custom heat pump around a specific machine is the SarMakina project. Because we develop the control board and its software ourselves, the communication and control features the specification asks for are also assessed at the technical proposal stage.
The main information a specification needs is: the type of vessel and its sailing area, the number and volume of cabins and saloons, seawater temperatures in winter and summer, the ship's electrical grid, the space that can be set aside in the engine room, the hot water demand and the class rules that apply. Send these through our contact form and we will assess together which arrangement fits.
