Choosing the right hot water system for a boat is not always straightforward. The best option depends on the type of vessel you own, how often you use it, the number of people on board and the power available. For some owners, a traditional storage tank provides a practical reserve of hot water for showers and washing up. For others, an on-demand system offers a compact alternative without the need to store heated water.
A marine tankless water heater and a traditional marine storage tank work in very different ways. Neither is automatically the right choice for every vessel. Understanding the advantages and limitations of both can help you make a more suitable investment.
How Does a Marine Tankless Water Heater Work?
A tankless water heater heats water as it passes through the unit. Instead of warming and storing a fixed quantity of water, the system activates when there is a demand for hot water.
Cold water enters the heater and passes through a heat exchanger or heating element. The water is heated and then supplied directly to the tap, shower or other outlet.
The main attraction is that there is no large cylinder of hot water taking up valuable space. As long as the system has sufficient power and is operating within its designed flow capacity, it can continue heating water without the stored supply running out.
However, the available electrical or fuel supply is particularly important on a boat. Heating water instantly requires significant energy, especially when a higher flow rate or greater temperature rise is required. The system therefore needs to be correctly matched to the vessel's electrical installation, generator, shore power or other approved energy source.
How Does a Traditional Marine Storage Tank Work?
Traditional marine storage systems, commonly referred to as calorifiers on many UK boats, heat a volume of water and keep it in an insulated tank until it is needed.
A calorifier can often use heat from the engine's closed cooling circuit through an internal heat exchanger. Many installations also include an electric immersion element for use when connected to shore power. This combination can be particularly useful because the engine can heat the water while the vessel is underway, while electricity can provide hot water when moored at a marina.
The main benefit is that hot water is already available once the tank has reached temperature. You can generally achieve a stronger flow without relying on the heater to raise the water temperature instantly at the same moment.
The limitation, of course, is capacity. Once the stored hot water has been used, the system needs time to heat another supply.
Marine Tankless Water Heater vs Traditional Storage Tank
Feature | Marine Tankless Water Heater | Traditional Storage Tank |
How it works | Heats water as it flows through the unit | Heats and stores a fixed volume of water |
Size | Compact and space-saving | Requires space for the tank |
Hot water supply | Continuous while adequate power and flow capacity are available | Limited to the amount of stored hot water |
Standby heat loss | Very low as water is not stored hot | Some heat loss occurs over time |
Power demand | Can require substantial power for instant heating | Can heat water gradually using lower-power sources |
Flow rate | Limited by the heater's output and incoming water temperature | Can provide a strong flow until stored hot water is depleted |
Engine heat integration | Depends on the specific system and installation | Well suited to engine-heated calorifier systems |
Installation | May require electrical or plumbing upgrades | Requires tank space and suitable plumbing connections |
Multiple outlets | Performance must be carefully sized for simultaneous use | Can serve several outlets until the stored supply is used |
Hot water availability | Produced when there is demand and sufficient power | Immediately available after the tank has been heated |
Weight | Generally lighter due to the absence of a large water store | Heavier, particularly when full of water |
Maintenance considerations | Heat exchanger and internal components require appropriate maintenance | Tank, immersion element, valves and associated components require inspection |
Space and Weight on Board
Space is one of the strongest arguments in favour of a tankless system. On a small yacht, motor cruiser, narrowboat or other compact vessel, finding room for a large hot water cylinder can be difficult.
A traditional tank not only occupies physical space but also adds weight when full. A 30-litre tank, for example, carries around 30 kilograms of water before the weight of the tank and associated equipment is considered.
A tankless unit can therefore be an attractive choice where every cupboard, locker and machinery space has to be used carefully.
For larger vessels, however, the size of a calorifier may be less of a concern. In that situation, having a stored reserve of hot water can offer greater convenience, particularly where several people are on board.
Energy Use and Efficiency
Tankless systems avoid the standby losses associated with keeping a tank of water hot. Water is heated only when there is demand, which can make sense for a vessel that is used occasionally or where hot water consumption is relatively low.
The important consideration is the energy needed to heat water quickly. Instantaneous electric water heating requires a high power input to achieve a useful temperature rise at a reasonable flow rate. A system that performs well for hand washing may not necessarily provide the same performance for a full shower.
This is why the power supply should be considered before selecting a unit. Shore power may provide a different level of capacity from a generator, inverter or battery-based electrical system. The installation should be designed around the actual electrical capability of the vessel rather than simply choosing a heater based on its physical size.
A storage calorifier works differently. It can heat water over a longer period and retain it for later use. When connected to an engine's cooling circuit, it can also make use of heat produced while the engine is running. This can be a very practical arrangement for vessels that spend regular time cruising.
For boat owners looking for a marine water heater in UK conditions, the way the vessel is normally used is often more important than choosing a system purely on the basis of efficiency figures.
Hot Water Demand and Number of People on Board
A tankless heater can be a good option for one or two people with modest and predictable hot water use. It is particularly useful where space is limited and hot water is required at one outlet at a time.
The heater must still be correctly sized. If someone is showering and another person opens a hot tap elsewhere, the available output may be divided between the two demands. Depending on the system capacity, this can reduce flow or affect water temperature.
A storage tank can be more forgiving during periods of higher demand. Once heated, the stored water can be supplied to several outlets, subject to the tank size and plumbing arrangement. The disadvantage is that a busy morning on board can empty the available hot water supply.
For families, charter vessels or boats with regular guests, it is sensible to estimate realistic usage before deciding. Consider the number of showers, galley use and simultaneous demands rather than simply looking at the heater's headline specification.
Installation Considerations
Marine installations should never be treated in exactly the same way as domestic installations. Vibration, movement, corrosion, confined spaces and the vessel's electrical and plumbing arrangements all need to be considered.
A storage calorifier requires secure mounting and correctly designed pipework. Pressure relief and expansion arrangements are also important because water expands as it is heated. Where the system uses engine heat, the installation must be compatible with the engine and cooling arrangement.
A tankless unit also requires careful installation. The power supply, cable sizing, protective devices, water pressure, ventilation requirements where applicable and manufacturer instructions must all be considered.
It is particularly important not to assume that a heater designed for a house, caravan or outdoor use is suitable for installation on a vessel. Marine suitability, the fuel or power source and the relevant installation requirements should be checked before purchase.
Maintenance and Reliability
Both systems need routine attention.
A storage tank system may require checks of the immersion element, pressure relief valve, expansion arrangements, hoses and other plumbing components. Water quality and periods of inactivity should also be considered as part of the vessel's general maintenance routine.
A tankless system has no large body of stored hot water, but that does not mean it is maintenance-free. Scale, debris and water quality can affect heat exchangers and internal components. Electrical connections and controls should also be inspected in line with the manufacturer's recommendations.
The simplest system is not always the most suitable system. Reliability depends greatly on selecting properly rated equipment and having it installed and maintained correctly.
Which Option Is Better for Your Vessel?
A marine tankless water heater is often the better choice where space and weight are priorities, hot water demand is moderate and the vessel has sufficient power available to support instantaneous heating.
It can also suit owners who do not want to keep a reserve of water heated between uses. For occasional use and smaller hot water demands, the compact design can be particularly appealing.
A traditional storage tank or calorifier is often better suited to vessels that regularly run their engines and can benefit from recovered engine heat. It can also be a practical option for boats with several users because a reserve of hot water is available once the tank has been heated.
If your vessel spends long periods underway, an engine-heated calorifier may make excellent use of energy that would otherwise be wasted. If your vessel has limited space but strong electrical capacity, a tankless system may be the more sensible arrangement.
The right choice should be based on your vessel's available space, power supply, cruising habits, number of users and expected hot water demand. A properly sized system will generally provide a better experience than simply choosing the smallest or highest-powered unit available.
To discuss suitable hot water solutions for your vessel, email us at sales@tanklesswatersolutions.
