Water systems

Most enclosed boats will have some kind of water system on board, ranging from a single manual pump operated tap to a system featuring hot and cold water with mixer taps and showers. Marine plumbing can be a daunting subject but most pleasure craft designed for river use have surprisingly similar water systems and understanding the function of just a few key common components can really help to gain a good understanding of marine plumbing.

Electric Water Pumps

Jabsco Par Max Water Pump

Jabsco Par Max Water Pump

Unlike domestic plumbing where water pressure is generally created by gravity, boats require an electric pump to generate water pressure as tanks are often level with or below the
outlets they serve. The pump is therefore the heart of the system and works by cycling on and off to maintain the desired pressure and a good flow when you open a tap. Most pumps are switched on and off automatically via a pressure switch inside the pump, though some smaller pumps rely on micro switches within the taps themselves. Pumps do differ in the pressure they provide (usually in PSI) and the flow rate (usually in GPM or LPM). The size of your boat, the number of outlets the pump will serve and the complexity of the water system will dictate which pump you will need.

Pressure (PSI): One of the first things you will notice when comparing domestic water pumps is the variation in pressure (PSI) between different models. This figure refers to the pressure the pump will create in the system before it cuts off. For example a “25PSI” pump might typically begin to run when the system is at 15PSI and pressurise the water up to 25PSI before cutting off. Higher pressure pumps are required for larger craft i.e. narrowboats with long runs of hose or systems with a demand to pump water vertically.

An average inland motor cruiser (say 27ft) with 2 outlets (e.g. galley tap and shower) would typically use a 25-30PSI pump. (Jabsco Par Max 1.9′ or 2.9′ pressure-controlled pump)

Flow (GPM/LPM): The next consideration is the flow rate of a pump. This is advertised in either gallons or litres per minute (GPM/LPM) and indicates speed at which the pump can discharge water; the higher the flow rate the more water will pass through the taps at any given time. In our chandlery we carry pumps with a flow rate ranging from around 7LPM up to around 13LPM and this is sufficient to cater for a wide variety of craft. The flow rate required for a boats water system depends largely on the number of outlets served and how strong a flow is desired.

Using the above example, a 27ft motor cruiser with serving two outlets would typically have a flow rate of around 8-12 LPM. (Jabsco Par Max 1.9′ or 2.9′ pressure-controlled pump)

When replacing a pump within an existing water system it is important to consider the pressure. If you were happy with the pressure the old pump was providing, the unit can be replaced in a like for like manner. However if you desire to upgrade to a higher pressure pump this can cause problems. The increased pressure is likely to stress the system, so be prepared for leaks to appear around existing joints and fittings and you may have to chase these leaks around the system each time you bring the pressure back up. Flow rate does not cause such a headache and can be adjusted or upgraded with little to worry about. The pump will alert you to leaks by cycling periodically; even when the outlets are not in use.

View our Jabsco pumps

View our Jabsco pumps

Although most modern pumps are a similar size and weight it is important to consider how a new pump will fit; some pumps must be mounted horizontally, others vertically etc. Also consider the fittings on your existing pump; quickfit fittings are becoming more common than hose tails. The Jabsco ‘Par Max’ range of pumps comes with connections to fit either flexible ½” hose or quickfit.

Accumulator Tanks

Most electric pumps have an integral pressure switch which regulates when the pump should cycle in accordance with the pressure in the system. This can cause the pump to cycle on and off when taps or showers are running as the pump seeks to maintain a constant pressure; this in turn can cause spurts of pressure and uneven flow as well as unnecessary wear to the pump and preventable battery drain. The role of an accumulator tank is to help sustain the pressure and prevent this from happening.

Although accumulator tanks aren’t essential, they do add many benefits to a water system, both in terms of the quality of the flow and the longevity of the pump. Tanks vary in size, in our chandlery we carry tanks with a capacity of between 2 and 8 litres and these are sufficient for the majority of craft on our part of the river. The main factors to consider are the number of outlets served, the flow rate of your pump and the space available for the tank.

Accumulator tanks need to be fitted on the outlet side of your pump and as close to the pump as possible for the best efficiency. Most accumulator tanks can be fitted both vertically and horizontally though they must be securely mounted, for this job specific mounting kits are available or you can fashion your own mounting with straps, the important thing is that the tank is secure. The tank will already be pressurised upon purchase, check the instructions to see what pressure it is at (ours tend to be 35PSI). This pressure can be regulated via a valve in the top of the tank; you will need to set the pressure at around 2-3PSI below the cut in pressure of your pump. This can be done using the same pressure gauge you would use to check car tyres and although the gas in the tank will be nitrogen, it is fine to increase the pressure with air. When adjusting the pressure in the tank after installation, make sure the pump is switched off and an outlet is left open.

Expansion Tanks

Expansion tanks are only required on boats with a hot water system. When water is heated in a calorifier or hot water tank, its natural expansion can force a backflow of warm water out of the pressure relief valve or into the cold water line from which it has entered. Fitting an expansion tank just before the inlet of the calorifier can prevent this.

Expansion tanks and accumulator tanks are in fact mechanically identical; the name simply refers to where in the water system the tank is fitted. Expansion tanks need to be fitted on the inlet to the calorifier, just before the water enters the unit. A non-return valve must also be fitted on the pump side just before the expansion tank to prevent the warm water from flowing back into the system. Advice with regards to the installation of an expansion tank is nearly identical to that of an accumulator tank. The tanks pressure should be set the same as the cut off pressure of your water pump.

The table below was produced by Cleghorn Waring, who manufacture accumulator tanks, in order to help calculate the size expansion tank your require. Cleghorn Waring also have an excellent guide to accumulator tanks and expansion tanks here.



Hot water is perhaps the most intimidating concept for the marine plumbing novice, however calorifiers are fairly simple to understand. A calorifier is essentially an insulated tank, much like the one you find on a larger scale in your airing cupboard at home. It works by extracting heat from the engine cooling water; the hot engine water is passed through a coil in the middle of the tank which indirectly heats the cold water supplied from the domestic water system. An electric emersion heater is often incorporated to allow hot water when connected to shore power without running the engine.

Fitting a calorifer to an existing cold water system is not a straight forward procedure as it is not possible on all engines and often requires additional thermostats inline (as it can affect the operating temperature of the engine).

The calorifier is typically fed from a ‘t-piece’ off the existing cold water system. The intake is located at the bottom of the tank and the outlet at the top, and from here it is plumbed to any hot water outlet on board. A pressure relief valve is also required to protect the tank against natural expansion as the water is heated.

Bringing a water system together

The final hurdle is bringing all these units together into a water system that works. The location within the water system of each major component has been described above and can easily be understood with the help of a diagram.


An important decision is what to connect the components together with and there are a couple of options to consider.

The simplest method is to use a ½” braided pvc hose. Reinforced hose has the major advantage of being very flexible and easy to connect and disconnect to units fitted with hose tails. ½” hose tails are a common feature in marine plumbing and even if they are not present on a unit, in most cases they can easily be converted with attachments. Red (hot water) and Blue (cold water) reinforced hoses are readily available at chandlery stores; do not use clear hoses as they will allow algae growth.

Another option is the HEP20 or push fit system which is a fairly rigid plastic conduit. The advantage of this system is the ease at which it can be connected and disconnected (hence the name ‘push fit’, the connections are locking push-fits which can be clicked on and off with ease). The major disadvantage of speed fit is its rigidity, this makes it less versatile than pvc hose and more difficult to install; rather than bending round corners HEP20 needs elbows in order to turn corners.

It is common for boat owners to use a mixture of both systems.

This Guide was put together by Jones Boatyard with help from Jabsco and Cleghorn Waring.