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Old 09-16-2018, 10:36 PM   #1
Mini-Skoolie
 
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Plumbing Vents In Roof

Hopefully this is a simple questions for those knowledgable with basic plumbing.

I'm getting ready to close up the ceiling with insulation and wood. I've got my roof vents in and AC/Heat Pump is going in later this week.

I believe the last item that needs to be planned on going through the roof will be the Vent/s for the plumbing.

My pluming setup will be as follows....

Fresh Water Tank
Gray Water Tank
Shower
Sink
Kitchen Sink

Forgive my ignorance when it comes to plumbing. I understand that a vent is required but am not sure if all three fixture (sinks and shower) would share a single vent or if they have separates ones. I'm also not clear if the tanks below the bus need to be vented and if so do they need to be vented to the roof or just above where they are placed.

Any feedback would be greatly appreciated!
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Old 09-17-2018, 12:42 AM   #2
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For the shower, kitchen sink and bathroom handbasin you can use Air Admittance Valves instead of roof-penetrating vents. The fresh water tank needs only a small (1/2" or 3/4") vent to equalize pressure as you fill and empty it, and this can also serve as the overflow to let you know that it's full. The gray and poo tanks on the other hand require proper venting to allow the aerobic bacteria inside them to do their work, and to also let any methane harmlessly rise out of them. Most folk vent them up through the roof, but that's not the only way. My two waste tanks are next to each other but separated by the frame rail; each tank has two 2" vents, one for air in and the other for air out. The two tanks' front vents are connected together, the gray tank's rear vent goes down to ground level just ahead of the rear axle, and the poo tank's rear vent goes up to a louvered vent through one of the covered-over side windows just below the drip rail. This louvered vent points forward and the gray tank's vent points back. Why so much complexity, you ask? When driving, outside air is drawn into the louvered vent and forced down into the poo tank, out its front vent and into the gray's front vent, then is sucked out at ground level under the bus, well away from being sucked back inside by the high-pressure area towards the rear. In other words, I always have a strong flow of fresh air circulating through both tanks while driving, with no possibility of smells coming back inside the bus. When parked, the airflow is reversed, with cool fresh air drawn in from ground level under the bus, moving through both tanks, and exiting through the high louver well clear of any open windows. I've not heard of anyone else doing this, but to me it seems the best way to vent both tanks at all times. I also can't have any roof penetrations because I have 2kW of solar panels and a walkway on the roof, leaving no space for any vent pipes there even if I wanted them there.

Incidentally, you may want to consider completely separating your fresh water from your waste tanks by as much distance as possible, unlike so many RVs that have their fresh water fill and their dump valves close together or even in the same compartment. Cross-contamination, everyone? If we had plumbing like that in the healthcare facility where I work, we would be shut down immediately by our licensing authority and the local public health departments, for good reason. Keep them well separated - GI illness or food poisoning is not fun.

John
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Old 09-19-2018, 12:43 AM   #3
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My two cents: lived in a tiny home on wheels for about a year fully off-grid. I had a composting toilet vented down through the floor, and when the wind was right with the front door open...


I vote vent through the ceiling or high on the wall.
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Old 09-19-2018, 07:54 AM   #4
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Ditto what John said - two vent pipes for gray/black holding tanks (looks like only gray in your case). I have one out the roof, and one near ground level (and plan to move it to the roof).
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Old 09-21-2018, 08:25 AM   #5
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Wow, those Air Admittance Valves look like a great way to accomplish the basic plumbing vents! No roof penetrations needed.

For the gray and fresh water tank I'm still uncertain if any roof vents are required. These tanks will be separated by a couple of feet under the bus. Could a similar style vent be added to the tops of these tanks?

As for the composting toilet, my buddy ran his vent directly out the side of the bus and I believe that has not been an issue.
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Old 09-21-2018, 08:45 AM   #6
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For the gray and fresh water tank I'm still uncertain if any roof vents are required. These tanks will be separated by a couple of feet under the bus. Could a similar style vent be added to the tops of these tanks?
John said:
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The gray and poo tanks on the other hand require proper venting to allow the aerobic bacteria inside them to do their work, and to also let any methane harmlessly rise out of them.
So, yes. Gray tank requires a vent. It will stink. When vented properly, you won't smell it. (no different than a black tank)
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Old 09-21-2018, 11:38 AM   #7
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Can it just vent under the bus where itís mounted?
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Old 09-21-2018, 11:51 AM   #8
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Quote:
Originally Posted by Iceni John View Post
For the shower, kitchen sink and bathroom handbasin you can use Air Admittance Valves instead of roof-penetrating vents. The fresh water tank needs only a small (1/2" or 3/4") vent to equalize pressure as you fill and empty it, and this can also serve as the overflow to let you know that it's full. The gray and poo tanks on the other hand require proper venting to allow the aerobic bacteria inside them to do their work, and to also let any methane harmlessly rise out of them. Most folk vent them up through the roof, but that's not the only way. My two waste tanks are next to each other but separated by the frame rail; each tank has two 2" vents, one for air in and the other for air out. The two tanks' front vents are connected together, the gray tank's rear vent goes down to ground level just ahead of the rear axle, and the poo tank's rear vent goes up to a louvered vent through one of the covered-over side windows just below the drip rail. This louvered vent points forward and the gray tank's vent points back. Why so much complexity, you ask? When driving, outside air is drawn into the louvered vent and forced down into the poo tank, out its front vent and into the gray's front vent, then is sucked out at ground level under the bus, well away from being sucked back inside by the high-pressure area towards the rear. In other words, I always have a strong flow of fresh air circulating through both tanks while driving, with no possibility of smells coming back inside the bus. When parked, the airflow is reversed, with cool fresh air drawn in from ground level under the bus, moving through both tanks, and exiting through the high louver well clear of any open windows. I've not heard of anyone else doing this, but to me it seems the best way to vent both tanks at all times. I also can't have any roof penetrations because I have 2kW of solar panels and a walkway on the roof, leaving no space for any vent pipes there even if I wanted them there.

Incidentally, you may want to consider completely separating your fresh water from your waste tanks by as much distance as possible, unlike so many RVs that have their fresh water fill and their dump valves close together or even in the same compartment. Cross-contamination, everyone? If we had plumbing like that in the healthcare facility where I work, we would be shut down immediately by our licensing authority and the local public health departments, for good reason. Keep them well separated - GI illness or food poisoning is not fun.

John
Great description, John. Do you have any photos of the setup, particularly what the exterior vents under the drip rail look like?

Thanks, Greg
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Old 09-21-2018, 12:45 PM   #9
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Quote:
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John said:


So, yes. Gray tank requires a vent. It will stink. When vented properly, you won't smell it. (no different than a black tank)
Quote:
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Can it just vent under the bus where itís mounted?
You could but you can have odor issues.

Better to vent waste tanks above the roof.
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Old 09-21-2018, 04:31 PM   #10
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Vents in a drainage system allow "air" pressure (whether atmospheric air or other gases, ie sewer gases) to equalize as water travels through the pipes. Sometimes that requires letting air into the drain pipes; other times that requires letting air out of the drain pipes without letting any water out.

A situation where we need to let air into the pipe is found at the downstream side of a P-trap. When a rush of water flows through the trap, such as when the plug is pulled from a full basin of water, it can actually siphon the water out of the trap. This leads to foul odors coming up through the trap later. A vent breaks the siphon by admitting air into the pipe. This is where the air admittance valve (AAV) is designed to work. It's a check valve; it allows air in but not out.

If a person is using a trapless solution like the HepVo Waterless Valve then in my opinion neither vent nor AAV is required there (though I haven't actually built and tested this myself yet).

A situation where we need to let air out of the pipes is further downstream of that rush of water. If we don't provide a vent then the water will go down the drain more slowly, or even not at all. This is why the RV holding tanks need a vent: water won't flow into the tank unless we allow air to escape somewhere. An AAV is no good here; it's designed to prevent letting air out. We need an open pipe venting some place where the odors won't be annoying, and terminating high enough that there's no possibility water put into a drain is going to cause the tank to overflow and spill out through the vent.

In the case of RV holding tanks the air vent also allows air back into the tank when the tank is drained.

It's common plumbing practice to combine vent stacks to reduce the number of roof penetrations. I'm interested to know the community's opinion on this. Suppose the tank vent and a sink vent were piped separately to an elevation a foot or so above the top of the sink, then combined with a wye so that only one vent pipe penetrates the roof. Can anybody suggest a reason why this might backfire?
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Old 09-21-2018, 09:59 PM   #11
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the hepvo valves are self venting. you can pass on the AAV with a hepvo. i've never had a problem dumping tanks from an air lock or anything.

i did run a 1/2" pex line to vent my waste tank. not thru the roof, but just under the drip rail away from any windows. i used a boat fuel tank vent for the vent cover

5 years, no problems.




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Old 09-22-2018, 12:44 AM   #12
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Great description, John. Do you have any photos of the setup, particularly what the exterior vents under the drip rail look like?

Thanks, Greg
Sorry, no photos, I'm photographically challenged! The upper louvered vent is a standard hose vent from West Marine that takes a Fernco rubber 90-degree elbow to connect it to the 2" stink pipe up from the poo tank: https://www.westmarine.com/buy/west-...50?recordNum=7 I wish it were stainless steel instead - I really didn't want plastic anything on the outside of my bus, otherwise it will end up looking like a POS plastic RV. Yuk.

John
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Old 09-22-2018, 01:31 AM   #13
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Marine fuel tank vents are generally all stainless steel. The visible looks like a chrome small hockey puck with two holes (rear and down). The inside is a sharp 90deg 1/2in push on tube for use with a hose clamp over rubber line.
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Old 09-22-2018, 12:59 PM   #14
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I would recommend that if you follow Turf's example that you put the vent on the drivers side.
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