We're talking two kinds of vents here, drain vents and waste tank vents.
In most RVs, my conversion included, these are one and the same. Sure, you could have separate tank and fixture vents, but why run the extra pipe or cut another hole in the roof? For that matter, for most RVs, wet-vented systems are permitted (there are limits on the number of fixtures, and size limits), unlike in most fixed construction. So careful design lets you get by with a single stack for both waste and vent.
I repeat, the primary purpose of a building drain vent is to take air in, not vent methane.
You keep saying this, but it's only partly correct. Properly constructed vents do both, and I can assure you that the bulk of the gas flow in a sewer vent is out, not in.
Traps keep methane out of a building. The reason vents go through the roof is because they must be kept higher than the highest drain, not because they vent methane. This is a common myth and may even be believed by some plumbers.
Again, the reason vents go through the roof is to allow sewer gas to escape at a level higher than the living quarters. If what you say was true, we could simply vent waste pipes into cabinets, or even the attic -- yet the UPC is exceedingly clear on this issue: vents must proceed continuously to above the roof line. Period.
Drain vents in an RV serve the same purpose, without them the traps would be sucked dry every time water went down the drain system.
Yes. But since they are, by definition, also connected to the waste line, they ALSO VENT SEWER GAS. There is simply no way to avoid this and have the vent work properly.
These vents could go out the side of the RV if desired since they take air in, not out.
Again, gas flows BOTH WAYS in a vent. And the gas that flows out is sewer gas.
Check valves can also be used which take air from inside the bus and avoid more roof holes and long vent pipes. I use these valves.
What you call "check valves" are referred to in the trade as Air Admittance Valves (AAV's), and discussed in the code as "anti-siphon trap vent devices." They are indeed, permitted in certain situations. However, THEY CAN NOT SUBSTITUTE FOR A VENT THAT GOES ALL THE WAY TO THE ROOF. You still must vent the fixture at the earliest opportunity:
7.7.5 Anti-Siphon Trap Vent Devices. An anti-siphon trap vent device shall be permitted to be used only as a secondary vent ...
(underline emphasis mine).
If you do not know or understand what a "secondary vent" is, I refer you to the UPC.
The reason for large building roof vent pipes is that the vents are so long and the drain pipes so large that a small vent pipe would not allow enough air to enter the drain pipe as the liquid flowed out. My bottom vent pipe is only 1.25" and perfectly adequate because it is so short.
I am not sure what you mean when you say "large building roof vent" -- are we back to buildings, or still talking RV's? Because my code says the vent must be a minimum of 1.25", the same size you used, although adding enough fixtures or certain wet venting requires one trade size higher -- 1.5". But, again, it must go through the roof, as I cited earlier.
Any pressure of any kind in a waste tank will evacuate methane if it is present, including incoming liquid.
Precisely what I said. And, if that evacuation happens below the coach, then methane and other sewer gases can then rise right back into the coach.
The theory that it will accumulate in the loop is just a theory, nothing more. The theory that there is enough methane in an RV waste tank to be hazardous is also just theory, nothing more.
Just to be clear, while I have cited the problems involving methane for completeness, my bigger concern (as well as the code's) is not methane itself, but other sewer gases, including hydrogen sulfide, and any pathogens that might be along for the ride. Sewer gases entering residential spaces is a proven problem from which many people have become ill -- it is not a "theory." This is one reason why the codes mandate that vents proceed directly to the roof. A simple Google search will turn up hundreds of cases of people becoming ill and even being hospitalized when sewer gas entered an occupancy space, including school and office closures, residential condemnations, etc.
If anaerobic bacteria is allowed to grow in an RV waste tank there won't be any significant methane,
Now, this one takes the cake, because the primary byproduct of sewage digestion by anaerobic bacteria IS methane. (By contrast, aerobic bacteria would produce carbon dioxide.)
or solid waste for that matter. The problem is that most RVers seem to insist on cleaning the waste tank at every dumping or adding strong chemicals. Bad. This kills the little ab who will do the job much better if only allowed to live. I never completely empty my waste tank.
Here, I agree with you -- the health of the tank will be best served by keeping the bacteria alive, which means don't put harmful bacteria-killing chemicals in the tank. But please don't tell us this does not produce methane and other sewer gasses -- this is exactly what it produces.
There is nothing to indicate that methane in an RV waste tank is a problem.
I would say the code itself indicates that.
I've never heard of an RV explosion caused by methane?? I've also never heard of a methane explosion in a home for that matter. Just because it is posted here that it is a danger doesn't make it so. These are opinions.
A simple Google search revealed dozens of homes that have suffered methane gas explosions or fires from sewer problems. You can certainly look them up. That's fact, not opinion. As for RV's, well, I confess I could not find any reference to RV explosions due to methane in a brief search. But, again, the primary safety concern is not methane, per se
, but other sewer gases along with airborne biologic pathogens.
Saying that a bottom vent is incorrect or unsafe is baseless. Saying it is doesn't make it so.
I'm always amazed when a poster says another poster is wrong with no base for such a statement.
It's not "baseless" -- it's in the code. The code is the "basis" for my statements. And, the code makes it "so," whether I say it here or not.
Disagreeing is one thing, saying another poster is wrong is another thing altogether and not necessary. I was under the impression that we post here for the information of the questioner and that it was up to him to decide what is right and what is wrong?
Gus, I did not "decide" that it was "wrong" -- the code says you should not do it. That makes doing it unlawful. That's what I said -- it was you who contradicted that, and asked for an explanation. Now that I have acceded to your request and explained why the code says what it does (bearing in mind that I did not write the code, I only quoted it), you are accusing me of making things up.
Codes are very often more political than technical or safety based. Copper vs PB water supply pipes is the best example I can think of. PB is superior in every way to Cu yet it is not approved in many location. Why? Because it protects the Cu suppliers.
Setting aside, for the moment, the fact that I do not buy into these sorts of conspiracy theories, and assuming you are correct, please explain to me who is benefiting "politically" from the requirement to extend RV vents to the roof.
One can make a choice, blindly follow mindless codes or think! It is easy to follow mindless rules and regulations but a bit more challenging to think.
As I have said on this board many times, I don't buy this notion that any of us can ignore all rules and regulations and do whatever we want simply because we can "think" and that thinking leads us to believe that we know better than the law. That is a slippery slope. Right now, it's plumbing. Later, maybe it's headlights, or air brakes, or propane.
I'm not a fascist, but neither do I want to live in anarchy. I don't want to live in the world you propose, where anyone at all, regardless of qualification, gets to build anything they want.
I would also like to know where you got the notion that the codes are "mindless." That flies in the face of reality, and is disrespectful of the people who have spent countless hours working on them.
... My ole Grandpappy told me long ago to beware of experts and engineers!!
Great. I invite you to take a ride on an airliner, roller coaster, or elevator designed by people who did not finish engineering school. If you could even find one here in the U.S. -- doubtful, precisely because our concern for the safety of the general public here generally keeps such things from entering service.
Actually, for that matter, you should give up bus ownership right now -- those darn buses were designed by, gasp, engineers.