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How convenient is your convenience?

Conrad Anderson, the Birmingham based suppliers and installers of vehicle electronics, just installed the first retro-fitted Dometic vacuum toilet system with holding tank to a motorhome.

Status Indicator

Tank Level Indicator

Vacuum toilets have been a luxury afforded only for the American RV market or the marine industry, and even then only if they are in-built during manufacture. The standard sanitation feature of modern European motorhomes is the 'cassette toilet', all of which use chemicals and tend to smell. They invariably use small capacity tanks (cassettes) which require regular visits to the elsan point, and even if they are provided with a cassette of a reasonable size lifting it out can be a problem.

The function of a vacuum toilet system is really quite simple; vacuum is maintained in the system at all times. Before use, the water level in the bowl can be raised by simply lifting the flush lever. When the toilet is flushed (by a foot pedal or a push of a button) the vacuum energy stored in the system is released. The bowl is instantly cleared and the waste is moved down the vacuum pipe at 2 metres per second ending up in the waste tank. The change of pressure in the vacuum tank causes the integral pressure switch to activate the vacuum pump. After the flush lever is released, the vacuum pump continues to run until the vacuum level is recharged in the system. This process will occur each time the toilet is flushed until the holding tank is full. The Dometic system used in this fit has an added safety feature which causes the system to shut down when the full warning light is shown.

The discharge process is equally easy, just drive to within a few metres of the Elsan point, attach the provided hose extension to the outlet, direct it down the hole, flick a switch and either settle for a cuppa or watch the coloured lights change as the level in the holding tank drops.

A vacuum toilet system consists of a toilet bowl (obvious I know), a holding tank, a smaller vacuum tank with a vacuum pump and a discharge pump. The one used in this article is the Dometic 1600 series Vacuum Toilet System which features a household sized ceramic toilet bowl and a holding tank with level indicator lights.

L-shaped vacuum tank with vacuum pump and
discharge pump (in the foreground).

The Dometic 1600 vacuum toilet system utilises a Teflon Seal in the toilet bowl and a filter on the holding tank vent, so there is no smell and therefore no need to use any chemicals. This is more environmentally friendly and the lack of stored chemicals saves space within the van. The vacuum toilet bowl used is much smaller than a cassette toilet, it will therefore take up less space in the washroom and it does not have to be fitted near to an outside wall for cassette access, giving you more options on positioning. On the space saving theme the holding tank can be fitted in place of the present grey water tank so there is no need for an extra tank, just a different tank. This is also a good idea if you are worried about the weight of an extra tank.

The ease of using this system makes a vacuum toilet ideal for the older, disabled or the plain lazy; perhaps for families or vans with lots of people using the facilities, either way you can take this particular chore out of motorhoming.

Left: Old cassette. Right: New holding tank.

The Fit

The first job was to decide what size tank you can fit in your motorhome and, bearing in mind that the vacuum tank and pump assembly must be positioned above the level of the holding tank, where it is going to be fitted. In this case the vehicle, being a 27ft Autotrail Grande Frontier, it had room under the passenger side saloon bench seat for a 60-litre holding tank.

The holding tank was mounted sub-floor with the vacuum assembly mounting in the bench locker above. To fit the holding tank in this position the batteries were moved to one end of the battery box, which was shortened to make room.

There are various inlets, outlets and sensor mounting points on top of the holding tank. So a template is made to mark out where to drill holes for the various fittings. The fittings to run through the floor include the waste inlet pipe from the toilet to the vacuum tank, the down-pipe from vacuum tank to holding tank, waste discharge pipe, air vent pipes and electrical control cables.

To secure the holding tank steel strapping with coach bolts through the floor were used; the vacuum tank and discharge pump were screwed down to the floor using self-tapping bolts.

After the removal of the old toilet, the fitting of the new toilet required one hole through the floor for the waste pipe and 4 self tapping bolts into the floor to secure the housing. The toilet itself requires only a fresh water feed, which is already present from the previously installed cassette mechanism. The cassette toilet also has a far larger housing than the vacuum toilet so this created a large hole which needed to be filled. A 2 day search for some matching cherry faced ply board proved fruitful and our hole was filled.

Once the toilet and the tanks were installed we assessed which was the best, most straightforward route to run pipes and cables from the toilet to the tank and pump assembly. The pipe is an odour resistant strengthened pipe specifically designed not to collapse under vacuum. For this reason it is, although flexible, rather stiff requiring angled fittings for the tight bends. The van for this fit had a rear WC with the toilet on the same side as the holding tank, so the pipe run was a fairly straight forward affair with only one bulkhead, within a sub-floor rear locker, to run through. Some custom made heavy duty metal bracketry was used to pin the pipe to the chassis and the underside of the floor.

Holding tank and discharge outlet.

The discharge outlet is positioned inside what was previously the battery locker; this keeps all potentially odourous fittings and fixtures outside of the living quarters and also out of sight from the outside of the van. More heavy-duty metal bracketry was made to keep the discharge pipe in the correct position when attaching the extension. The extension for the discharge pipe was a tight push fitting to make sure it stays on when under pressure, but is also fairly easy to disengage when no longer needed.

The supplied pipe connections are push fit affairs so plastic cement, made for use with vacuum systems, was used to give extra oomph and make sure there are no vacuum leaks. The wiring loom supplied with the vacuum toilet system is a series of cables with four connectors, one for the vacuum pump, one for the holding tank level sensors and one for each of the indicator panels. The tank level indicator was positioned next to the living area door for easy viewing during discharge, along with a switch for the discharge pump which has its own live feed from the batteries. The vacuum level indictor is positioned within easy viewing of the toilet so the user knows if there is enough vacuum for the toilet to flush. The live feed for the vacuum pump and sensor system is connected to the tank level indicator control box and is earthed to a good convenient position.

Vacuum systems tend to lose their energy over a period of time, usually about 30min - 1 hour, so the vacuum pump will engage to retain the vacuum at that interval. So as an extra to this system we wired a shut down switch so the vacuum system can be turned off either at night so you are not disturbed or if you go out for the day to retain battery power.

This leads me nicely on to the economies of having a vacuum toilet. The system uses less than half a litre of water per flush, and the pumps draw a rather negligible 1.5 Amperes for 30-60 seconds after flushing depending on the length of the pipe run from the toilet to the vacuum pump.

Due to the complexity of the fit and the space constraints of modern motorhomes, all fittings are subject to an initial vehicle inspection. For details of supply and fitting the Dometic 1600 series vacuum toilet system please contact Conrad Anderson LLP.