Including self-containment
 #98442  by coldspider
 Fri Aug 21, 2015 9:11 am
Hi All

I am getting ready to order my tanks for my 11.5m bus conversion. I've scoured the forums for information on tank vent sizes and see conflicting information. I understand the minimum is 10mm and to run higher that the highest drain.

My fresh tank I'll likely run a 10 or 15mm vent and can't see a problem with that. My question relates to the grey and black tanks. I plan to fit 75mm (3") Valterra blade valves to both tanks for the drains. What size vent pipes should I run? Somewhere mentioned 32mm which I can do if I really need to but something a bit smaller that I could do in a flexible hose would be easier to route up the wall.

I need to lock these sizes in so I can order my tanks with the right fittings. Thanks a lot.
 #98448  by yachty
 Fri Aug 21, 2015 11:02 am
the Min is 10 mm but bigger better for ease of draining, i would suggest if you can do a 20+ easy then do it. with you vent height it must go higher than lowest drain and should be vented to exterior ie either up and back down or side vent through wall
 #98455  by mattn
 Fri Aug 21, 2015 12:50 pm
From the NZMCA guide :
All waste tanks shall be fitted with a vent of not less than 10 mm, preferably 25 mm, routed above the flood line of the lowest appliance and be vented to the outside of the vehicle. Preferably near the roofline away from windows and doors. Where the evacuation of the tank is by a valve and greater than 50mm an air admittance valve shall be fitted in the system to prevent the water seals being destroyed and/or tanks imploding and to speed up the evacuation of the tank.

As you have to have an air admittance valve, vent size is less critical.
 #98468  by coldspider
 Fri Aug 21, 2015 4:51 pm
Great thanks guys. I'll just fit the biggest I can and have an air admittance valve.
 #98479  by beanpole
 Fri Aug 21, 2015 10:32 pm
As you intend using 75 mm outlets you need a minimum of 32 mm venting for each tank.
7.8.3 NZ Standards. ------waste evacuation hose larger than 50 mm diameter is used to empty a waste tank/s the system shall be fitted with an automatic air admittance valve or a vent of not less than 32 m diameter.
You can use a minimum size vent of 10 mm if you have a HepVo valve on one of the appliances for each tank. This qualifies as a vent when discharging waste to prevent the tank/s imploding.
Alternatively, if you are running 32 mm pipe it is easy enough to fit an automatic air admittance valve in the line somewhere.
 #98484  by Teardrop
 Sat Aug 22, 2015 9:25 am
Out of interest, would it be easy with a 75mm outlet valve to contain all the flow in the dump station without splashing and overflow, and how do the dump stations cope with that flow level? Our first self contained van had a 25mm outlet, which was not enough. Our current one has 32mm which flows really well and has a huge flow difference from 25mm. I have just fitted a 50mm valve on a header tank for our sprayer. It has a bit of height, but it flows at a huge rate - fills about 1000 litres in a minute or so and the weight of the flowing hose is difficult to hold. I guess with a valve you can contain the flow to a point, but would a 50mm be a better proposition for handling purposes? Still fast and easier to control?? Just a thought because I don't really know as I have never experienced 75mm.

 #98486  by beanpole
 Sat Aug 22, 2015 9:45 am
The 75 mm hose is designed to have the bayonet couplings either end so no holding should be necessary. With the full flow "sealed" into the system, it is easy.
As all D/S are not equal with the Thetford bayonet coupling in them it can be a bit of a challenge at times. Especially when you can't get the 75 hose down into the waste outlet. Some have made up another fitting as a piece of pipe on the end and this is poked down the waste outlet and held.
Just hang on tight, it can be a two person job.
Any black tank where the waste is not macerated requires a 75 mm valve and discharge pipe. NZ Standards 7.6.2
All the NZMCA D/S have Thetford bayonet fittings. The Valterra 75 mm bayonet fitting is different externally but a fitting is available or can be made up to adapt to the Thetford. This doesn't work the other way round which is why all NZMCA D/S have the Thetford fittings.
 #98503  by coldspider
 Sat Aug 22, 2015 1:48 pm
I guess my main reason for wanting to use 75mm for both tanks is so I can just have 1 hose, empty the black first of course then switch over to the grey. I guess when emptying the grey tank I can just crack the valve half open so I don't make a mess.
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