Holding Tank vent sizing

Holding tank vent system - considerations over size and location

The diameter of the vent system has been aprox. 20mm and the ISO 8099 documente for the CE specification on holding tank systems, continues to use this figure.

When it comes to holding tanks it is easy for people to consider the size of the inlet connection and the outlet connection. This is probably because it is dictated by the size of the outlet of the toilet pump and/or the size of the pipe connected to the hull gland. What is harder to imagine is the size of the vent connection.

There are a number of problems with vents of this size, some brought about by the very specification itself which promotes the use of deck pump-out and other problems associated with power flush types of toilets and general over-pumping/overfilling

1) The marina pump-out equipment is very strong and a blocked or too small a vent can cause a total inward collapse of the tank. Possible result – contents in the bilge

2) Overfilling the tank can cause contents to be forced up the narrow vent tube, blocking it. The toilet paper when placed under pressure can become a very effective stopper. Result – messy job to clean it out and the possibility of a ruined filter

3) Further filling or over-pumping can then place extreme pressures in the tank, exploding it. Possible result – contents in the bilge

4) Power flush toilets pushing contents into the tank faster than the air can be vented resulting in explosion of the tank. Possible result – contents in the bilge

5) Heeling of a sailing boat causes the contents to reach the vent pipe and if the toilet is used in this condition then point 2 and point 3 could follow.

The answer is to fit a vent system that is as large as possible. The same size as the inlet and outlet is recommended. The benefits are that the vent is able to allow the tank to fully breathe when filling or emptying using either marina pump-out or power flush. The vent is much less likely to become actually blocked either by overfilling, heeling, etc.

The downside of this approach is that in many boats it is already difficult to run the inlet and outlet hoses through the boat, a 38mm vent pipe is just a step too far to contemplate for many people.

If it is difficult to run a 38mm vent through the whole boat then seriously consider running a 38mm vent for as far as you possibly can, until a bulkhead, partition, difficult bend, etc. This then acts as “extra tank capacity” in the vent system immediately next to the tank and can be an enormous help in preventing the blockages which cause so many people so many problems.

In summary

Vent 16mm - Very inadequate. Very high likelihood of problems

Vent 20mm - Inadequate. High likelihood of problems

Vent 25mm – Only just acceptable. BoatWide do not make tanks with vent size connections less than 1" (25mm).

38mm for a run of a metre or two then reduce to 25mm – Just acceptable.

38mm through the total vent system including 38mm filter. Best practice.

The location of the vent and the level sender

Location of the vent in the centre of the tank is always best for motor boats and sailing boats because the boat does not always stay exactly horizontal. Of course sailing boats move port to starboard more, and with motor boats there is more movement bow to stern. In both cases with the vent in the centre of the tank, there is less probability that the contents of the tank will go up the vent pipe, although the tank cannot always be situated in the centre of the boat.

When designing the tank, consideration must also be taken over the position of the level sensor. For example this sailing boat has the holding tank situated on one side of the boat with the shape of the hull to maximise capacity.

The level sensor location is also very important. The level sensor is ineffecive in the centre of the tank, especially if the boat is a sailing boat with most movement port to starboard.. If the sensor is on one side of the tank and the boat is heeled to the opposite side, the sensor will not feel anything..

Better to have two sensors, one on each side of the tank.

The best design for prevention of over filling the tank and suffering the consequences of a blocked vent system and possible explosion of the tank or parts of the vent system.

Further recommendations
Vent the tank with as short, straight, and level a line as is possible, with no sags, no arches, and no bends.

Install an anti odour vent filter between the deck skin-fitting and the tank to further reduce smells.

The vent fitting should not be the same type as a fuel vent fitting, a cap with a slit in it, but should be a completely open type.

Specify a tank level sender and gauge to notify the user the tank is full. Fit the tank full indication light in a place which is easy to monitor, if it is not visible to the user and the tank is full, the contents can only go one way, up the vent.

When the vent has been blocked and it continues to be over pumped but gets a reprise as the tank is emptied, the tank gets air through the toilet joker valve. The vent still remains blocked and the tank explodes on the next filling, maybe when it is only half full. For this very reason BoatWide are developing a Holding Tank Pressure Relief Valve. This is a valve designed to prevent explosion of the tank when the tank is in use and the vent is blocked. The other thing that might happen under the circumstances of a blocked vent is the tank implodes when emptied with marina pump out as the suction is very strong and the vent is blocked, the joker valve cannot provide sufficient air fast enough, in this instance having a Tanksaver saves the tank from implosion

It is a good idea when specifying your tank to put in a connection for deck pump out, we can put a blank plug on this connection which can remain in place, available to use at a later date if the need arises. The deck fitting is for 38mm hose. The deck fitting should not have a chain linking the cap, (you can't connect a pump-out with a chain in the way), and should be clearly marked 'Waste." Although the best quality deck plates are cast stainless steel with stainless steel caps. Colour coded plastic caps, blue for water, red for fuel and black for sewage are quite suitable and are far less likely to mistake one deck plate for another.