motorhomes > technical > plumbing

Plumbing - the pressurised side

Here is a few schematics showing the basic elements of a motor home plumbing system on the pressure side of the appliances.

The diagrams start at a very simple system and progress to more complicated setups. After each figure there is a description of the elements that were introduced by the preceeding drawing.


Fig 1.

Two water tanks
This is partly for practical reasons, ie. we have certain places to put tanks, and partly because it's always a good idea to have redundancy. If one tank is holed then there's a chance you'll still have water in the other one.

It's also quite common to seperate drinking from fresh (non-drinking) water. You fill the drinking water tank only from known good sources whereas the fresh water might be sucked up from a creek.

The tanks should be vented to allow air to escape while filling. If not vented they may be very difficult to fill as the air will have to come back up the filling pipe. This may or may not be a problem according to the length and size of the filler pipe but you should also allow for expansion and contraction with changes in temperature.

Most motor homes use a Sureflo or Flojet pumps. They have both been around for years and do a good job, however they are a diaphram type of pump and can be quite noisy.

Most pumps used in this situation have built-in pressure switches that activate the pump when the water pressures drops. Thus the pump turns on automatically when you use a tap.

Inlets are usually are usually mounted on the outside of the rig somewhere and, obviously, allow you to insert a hose to fill the tanks. If open to the world they should be lockable so undesirables can't tamper with your water supply.

Hot water system
There's two main types of units used, storage and continuos. Storage heaters hold an amount of hot water in a tank (usually about 23 litres) while the continous (or instant) versions heat the water as it passes through the heater. There's arguments for and against each but as far as the plumbing is concerned they both have cold water going in and hot water coming out.

The rules about ventilation seem to make it more difficult to install the continous types in an out-of-the-way location as they must be flued properly. Also I find them to be noisy, especially as the aforementioned regulations often dictate that the unit is mounted smack in the kitchen. But then again you have instant hot water, you can pull up to camp and be showering within minutes.

Still I know many people storage units and there seems to be not practical limitations.

Fig 2.

Non-return valve
The input for the mains supply is all very well but when it's not connected it could become an output when the pump applies pressure to the system. I guess the cap for the input may stop water spewing out but I think a one-way valve is a more appropriate method.

Often there is a small delay between turning the tap and the pump realising that the pressure has dropped and starting up. Also the water flow can be uneven due to the pulsed pumping nature of a diaphragm pump and in some systems the action of turning off the tap can cause a shockwave to travel through the system resulting in a "water hammer".

These problems are solved with an accumulator.

An accumulator is a large tank with a bladder inside. One side of the bladder is pressurised with compressed air and the other with water from the system.

Because there is a constant pressure being excerpted on the water by the compressed air the response is immediate when you turn on a tap. Also the flow is smooth because the bladder expands and contracts quickly thus removing pulses/shock waves to a large degree.

Fig 3.

The internal motor home plumbing is not normally subjected to mains pressure however when set up in a caravan park for a few weeks it is a lot more convenient to plug into the water so you don't have to fill tanks all the time. The regulator will reduce the mains pressure to something more akin to that produced by the pumps, ie. around 50psi.

The regulators are be purchased as seperate units or you can buy mains inlets with builtin regulators.

Second accumulator
Not apparently used on most motor homes but I think if it's a good idea on the fresh water system then it must also be on the drinking water system.

Fig 4.

You are probably familiar with the KISS principle, but in true MIRC (Make It Really Complicated) tradition I present the above schematic. This is pretty close to the installation in our motor home.

Two fresh water tanks
Why? Because I have the room, but also the redundancy I mentioned above and the fact that I need a lot of water to be self-sufficient for long periods in the bush. The use of two fresh water tanks has meant that selection valves have been added to allow the pump to draw from either tank.

Pump control valves
In this setup the main pump can be isolated from the system to allow pumping from external water sources or transferring water between tanks (via an external connection).

Larger pump
This system features a much larger domestic-style centrifugal pump for extra flow and better ability to draw from creeks etc. when in the bush. This pump runs from 240v which is nowhere near as efficient as the 12/24v pumps but then it runs for such short periods I feel this is not an issue.

Non-return valves
Because the tanks are lower than the pumps, these have now been added to each tank to hold water in the pipes leading to the pump. The pump I'm using is not self-priming so the water must not be allowed drain from the pipes. For systems with self-priming pumps, ie. Sureflo and Flojet, these should not be necessary although it may depend on the length of line from the tank to the pump and the size of your accumulator.

For example, with long lines and no accumulator the pump would start pumping as soon as you operated a tap but there may be a delay, as it primed the lines, before you actually got any water.

There are two filters in the drinking water line and a third in the external input or the point of entry (POE).

The first drinking filter is a 1um (one micron) sediment filter. This will remove most nasties down to and including protozoan cysts (4um in size). These little fellows are bad news and you definitely want to get rid of them but we still have bacteria to worry about, they can be as small as 0.2um and will easily pass through the first filter.

The next filter is a silver impregnated activated carbon filter. The carbon will remove bacteria from the water passing through it but this bacteria will build up on the filter. This is where the silver comes in. Bacteria will suffocate in the presence of silver so any of the little darlings that get caught on the filter will also be killed.

Neither of these filters are washable and have to be replaced every year or so.

The POE filter is a simple 20um sediment filter used to remove the majority of sediment from the incoming water. You only start to see sediment at about 40um so this will produce water that is clear but not necessarily safe to drink. This is a washable filter as presumably it will clog up frequently.