Learn how to diagnose and repair poor drainage and other ventilation issues in your home’s plumbing system. If you have issues such as foul odor, gurgling, and slow drainage it may be your vent line problem.

Since plumbers are expensive it pays to learn how to understand more about your mobile home’s plumbing system can help you save money when hiring a professional.

Learn more about manufactured home plumbing with our article Plumbing In Manufactured Homes: The Basics.

Note: Your state and local plumbing codes will determine the exact requirements that your manufactured home must meet when it comes to plumbing (and electricity, additions, foundations, decking, roofing). While HUD sets standard manufactured home building codes on a national level, your state and local laws will take precedence.


Manufactured Home Vs. Site-Built Home Plumbing

There are a few differences between manufactured home and site-built home plumbing systems.

Plumbing pipes, or lines, are typically placed under a manufactured home and stubbed up through the floor while site-built homes usually have their pipes placed inside the walls. Getting to the pipes is much easier in a manufactured home. The drawback is that pipes ran under a mobile home are more prone to freezing but that issue can be easily fixed.

Manufactured homes typically do not have clean-outs or overflows like a site-built home. The lack of clean-outs can be troublesome when tackling trunk line blockages.

Manufactured homes have played an important aspect in the evolution of modern plumbing. New products are often installed on factory-built homes long before they make it to the traditional housing market. PEX, an industry standard and favorite product of plumbers worldwide, was first tested and used in manufactured homes.


The 3 Basic Elements of a Plumbing System 

All home plumbing systems, whether in a manufactured or site-built home, have three main elements: supply lines, drainage lines, and ventilation.

Your supply lines are completely separate from your drainage and ventilation but your drainage lines and ventilation are typically together. All three of these elements work together – if one part of the system is broken the entire system is broken.

This video from Home Dept explains how a basic plumbing system works:


Supply Lines

Supply lines carry water into and throughout your home.

Leaks around connections will be your biggest concern with supply lines. Frozen water lines is another big problem that manufactured homeowners face. You can learn how to repair both issues on our manufactured home plumbing basics article. 


Drain Lines

Drain lines in your mobile home carry the waste away.

Ventilation Lines 

Drainage lines carry waste water out and away from your home. We explained drainage in our first plumbing article, Plumbing in Manufactured Homes; The Basics:

Drainage lines use gravity, traps, and ventilation to ensure the optimum waste removal and keep gases and fumes from building up and releasing. Think of this as a completely closed system with positive and negative vacuum or pressure. All the parts have to work correctly to allow the system to do what it is designed for. Without the proper positive or negative pressure acting as a vacuum in the pipes the waste won’t go where it’s supposed to, it can back-flow instead.



Diagnosing Poor Drainage and Ventilation Issues

There are a few signs that will clue you into drainage or ventilation issues.


Signs of Poorly Vented Plumbing Drain Lines


  • Slow Drainage
    • Slow waste water drainage is one of the first signs of ventilation issues.


  • Gurgling
    • If you hear gurgling come from your drains you likely have poor drainage caused by ventilation issues.


  • Foul Odors
    • A foul odor can be another sign of poor drainage or ventilation issues.

Water Heater Odors

Note: Not all foul odors are caused by venting problems. Foul water odor could be coming from your water heater.

  • Missing Water in your Toilet
    • Nope, the dog probably isn’t drinking all the toilet water! (Source) If you find your toilet is missing its water you may have either a drainage leak or a ventilation problem.
  • Air Bubbles in Your Toilet Water
    • Seeing or hearing bubbles rise from the toilet bowl is another sign that your drainage system isn’t getting enough ventilation.


All of these issues can be caused when your drainage system is air-locked or when the p-traps are emptied of their water due to poor ventilation. Learn more about drainage noises here. 

If your drain lines leak you’ll probably be able to tell. Water or dampness under your home is a big clue but foul odors and soggy ground can also be signed. Repairing a drainage line leak is fairly straightforward – you find the issue and replace that area. However,  if you have determined there is no leak but you are still incurring issues described above you likely have a ventilation problem.


Venting Your Drainage Lines

A healthy plumbing system must be able to breathe.

Think of a drain system as having two elements within the same pipe. The lower part of the pipe is where the wastewater goes and the upper part is how the ventilation enters to provide the lower drain lines with proper (neutral) air pressure.

A two-liter bottle may help explain this better:

When pouring soda out of a plastic 2-liter bottle you can see just how important ventilation can be. Turn the bottle completely up, where no air can enter the through the opening, and the soda gurgles and slows. If you don’t turn the bottle completely up and allow air to enter the soda comes out faster and more smoothly. That’s exactly what proper ventilation does for your drain lines.

Plumbing, Drainage, and Ventilation Terms

Plumbers, like all construction workers, speak a different language from the rest of the world.  Living with a master plumber for 18 years has given me just enough time to learn some of their language (not all of it by any means). Here are some of the more common plumbing words and their meaning.

Wet Vents

Wet vents are a drain system where one pipe serves as a vent for one fixture and a drain for another (source) or a waste pipe that also serves as a vent pipe (source).

Wet vents can consist of a toilet and sink (left image below) or a toilet, sink, and tub (right image below). It could also be the kitchen sink and the laundry sink, whichever is closest. Here’s how HomeTips.com explain wet vents:

With wet venting, fixtures that are relatively close to the soil stack (the main vertical drain-waste-vent pipe) are connected directly to it, even if the section of stack above the connection serves as a drain for another fixture.

[column size=one_half position=first ]

wet vent example - toilet and sink

[column size=one_half position=last ]

wet vent example - toilet- sink-tub

Stack Vents

A vent stack is a small pipe that you see coming out of your roof. It completes the wet vent system.

Regular Checkups of your Vent Stack 

Vent stacks can get clogged by leaves, birds, rodents, and countless other things. They can also settle, either due to poor installation or accidental cutting. Luckily, you can buy toppers that allow good air flow but keeps things from falling into the pipes. Vents can also get crushed or bent so a regular checkup is recommended to ensure there are no leaks around the seal and that nothing is blocking air flow.

repairing a plumbing vent stack


Air Admittance Valves

You’ve likely never noticed an air admittance valve or auto vent as some call it, under your sinks or inside your walls. Even if you have seen them, you probably didn’t realize how important it was to your home’s plumbing system. They are handy little things!

An air admittance valve is used in cases where connecting a drain pipe to a stack vent is difficult or impossible. They ensure that waste goes down to the sewer and gases stay out of your home.

Look under your sinks and see if you see something like this (it could be black or white, straight or domed):

Cheap air admittance valves

How do Air Admittance Valves Work?

Air admittance open and close by gravity. When anything moves along a pipe it essentially seals it and causes a negative air pressure. That negative pressure pushes the valve open so that air can enter the pipe. As the negative pressure eases the valve slowly falls back into place.


how an air admittance valve works

Essentially, an air admittance valve acts as a set of lungs for a weakly ventilated drainage system. It allows the pipe to suck in air when needed and closes off when it’s not needed.

installing an air admittance valve under a sink

Studor Air Admittance Valves

Studor is most plumber’s favorite brand for auto vents or air admittance valves. Joe, the master plumber of the family, always recommends that homeowners replace their original manufactured home’s generic air admittance valve with a Studor brand as soon as you notice signs of failure. He says there is a huge difference between a $5 item and a $35 item.

(Note: Studor does not pay us for this recommendation. After 18 years as a master plumber you tend to learn what works and what doesn’t.)

Read about 5 Common Mobile Home Plumbing Problems here.

This video from Studor explains how their air admittance valves work:


How to Install an Air Admittance Valve Under Your Sink

If you notice any of the above signs of drainage or ventilation issues you may want to install or replace an air admittance valve.

These images show the process of installing a valve under your sink: 

installing an AAV onto a drain pipe - step 1

installing an AAV onto a drain pipe - step 2

installing an AAV onto a drain pipe - step 3

Image source

Mobile Home Plumbing Q & A


I live in a 1987 mobile home. When the washer drains, water will back up into the kitchen sink and the fumes are horrible. Sometimes the water even overflows from the drain onto the laundry room floor. I see no vent stack thru the roof nor do I see any under sink venting devices. What do you suggest to correct this problem?


You are experiencing a very common issue in manufactured homes. It’s kind of a double whammy of not enough (or failed) ventilation and possibly too small of a drain line from the washer.

First, try adding a Studor auto vent under the kitchen sink. That should help the smell. Studor is the best name brand and well worth spending the extra money over the other auto vents.

If your washer drain line is only an inch and a half pipe you may want to consider re-piping it with a two-inch pipe. While doing that, tie it in under the house directly to the main drain/trunk line itself, separate from the kitchen drain. That will definitely fix your problem! Those 1.5″ pipes just aren’t meant to handle a lot of water, especially high-pressure water being released from a newer washer.

Try the auto vent first, especially if there is no stack connected to the immediate system (though there could be one in the walls that has failed – the cheaper vents seem to fail after only a few years)


I have a 1981 Hallmark 2 bedroom trailer. Our bathroom and kitchen sink won’t drain. The tub and toilet are fine. I’ve taken apart the plumbing under the sinks and ran a snake as far as i could, and still nothing. I’ve crawled all under the trailer, and the only lines I see coming down and connecting to the waste/septic is for the washer (also fine). The trailer is insulated very well underneath, and I’ve cut a couple spots looking for pipes under the sinks but for the life of me, I can’t find anything lol. I’ve even googled various phrases relating to the plumbing plan, to no avail. Any ideas?

It’s most likely a blockage or ventilation issues. Try a true professional plumber’s snake (those they sell at Lowe’s or Home Depot are not that great and are mostly for sinks, not drainage line blockages. You should be able to rent one. Poor ventilation could be air locking the system as well. Those are the most likely issues.



I have running water, but I have this horrible stench in the kitchen and it sounds like scratching in my walls first it was on the repair side of the house now alternate on both sides. The odor became so bad 4 days after we had water we left the house after raising 5 windows in the house. I returned the odor comes and goes and I haven’t heard the scratching noises. Please help. Asap

It sounds like you have an open sewer pipe or a ventilation issue. When you had your dishwasher uninstalled did the drain to the dishwasher get capped too? Maybe the dishwasher’s drain was accidentally left open. All you’ll need to do is find that and cap it like you did the water lines.

If it’s not an open waste line from the dishwasher, then it’s possibly a ventilation issue. The noise could be your pipes and that usually means the system isn’t able to breathe properly. You can either look for a broken vent line (poor glue jobs, settling, or accidental cutting are the usual culprits when it comes to disconnected ventilation lines) or you can try installing an under-the-sink Studor vent. It will act as a mini-vent and help rid your home of the odor.


My money is on the dishwasher drain assuming you had none of these issues until the dishwasher was disconnected.


To be honest, it’s difficult to explain such a complex issue as plumbing. It’s one of those cases where you can give too much information and too little very easily. If I haven’t made something clear please feel free to let me know.

If you have any plumbing issues feel free to ask questions in the comments below. I always forward your questions to a licensed master plumber with 18 years experience – he knows his plumbing (the hard part is putting his knowledge into the right words). On our first plumbing article we were able to help answer over 90 homeowners with their plumbing questions – maybe we can help you too! Click here to read them.

Thank you so much for reading Mobile and Manufactured Home Living!