Diagnose and Repair Common Vent Issues in Your Plumbing System

There is a lot more to a mobile home plumbing system than just water lines and drain lines. The ventilation system is vital to a healthy plumbing system. Without a healthy vent system, your home can suffer a variety of annoying problems.

A lot of venting issues are mistakenly thought to be drainage issues in mobile homes but they are absolutely caused by improper or imbalanced venting design.

There are a few venting and drainage issues in mobile homes that can be diagnosed and repaired by the average DIY mobile home owner. This article will help you better understand and diagnose those problems, and hopefully, repair them.

The 3 Parts of a Mobile Home Plumbing System





Every home plumbing system is made up of 3 elements, or systems. All three of these systems work together so a homeowner must understand all three before they can attempt to diagnose any issue.

Water Supply System

Supply lines carry water into and throughout your home. The supply lines, or water lines, need healthy pipes with tight connections (called fittings) that must handle high-pressure water. You turn the handle on a sink and the pressure forces the water out. The more you open the faucet the more water you get. For the hot water, supply lines go through a water heater first. Leaks, pressure, and the water heater cause the most common issues for supply lines.

Leaks around connections is a common concern with supply lines. Frozen water lines is another big problem that manufactured homeowners face. Our article about heat tape will help keep your supply lines from freezing.


Drain – Waste System

A Drain-Waste System flushes away the used wastewater. Drain lines also need healthy pipe and tight fittings that can carry wastewater to the sewer lines or septic tank. It needs the help of gravity and vent lines to keep the correct pressure in the pipe and push sewer smells up and away from the home.

Drain lines are the big (usually white) pipes under a mobile home that go from the water fixtures to the sewer or septic. The pipes are installed at a slope so gravity can push the waste away.

Mobile home plumbing repair rachellhough
Image Source: Rachell Hough

Ventilation System

Yes, all mobile homes have ventilation systems. I’ve seen some articles claim mobile homes didn’t have vents but it’s just another mobile home myth.

A ventilation system is part of the drain-waste system but it’s still considered to be a separate system. Vents do 2 things:

  • maintain pressure in the drain lines and help wastewater to drain smoothly
  • act as a planned route for sewer smells to follow so no one smells the foul odors in the drain waste pipe.

A healthy plumbing system must be able to breathe and it does that via the vents.





The ventilation system allows air to enter your drain lines so it can keep the right atmospheric pressure in the pipe. Without the ability to breath, draining water would create a vacuum or suction that pulls all the water out of the P-traps (at all fixtures and before sewer line connections).

The ventilation system is called several different things: ventilation pipes or ventilation lines, or just vents for short. Vent lines or drain-waste-vents (or DWV) are also used. And they all mean the same thing.

Elements of a Mobile Home Ventilation System

Mobile and manufactured homes typically use 2 kinds of vents in their ventilation system: the direct vent, and the auto vent.

Direct Vents or Vent Stack

The most common kind of vent is called a direct vent also referred to as a vent stack, soil stack, or VTR which is short for ‘vent through the roof.’ You can see these vents coming out of the roof or running up the side of the home where the drain line meets the sewer line (if needed).





These vents tend to get clogged with leaves, bird or bee nests, or any other debris that can find its way into a pipe sticking out of a roof.

Wet Vent

When one pipe serves as both drain pipe and vent pipe it’s called a wet vent. They can save a lot of pipe and money. Plumbing codes vary with these and there are a lot of restrictions.

Wet vents can consist of a toilet and sink or a toilet, sink, and tub (right image below). It could also be the kitchen sink and the laundry sink, whichever is closest.

Auto Vents or Air Admittance Valves

An auto-vent is a small device that is usually placed under sinks to let air into the drain waste line so it can keep the right vacuum or pressure when draining. Auto-vents do not let the air out, only in. We’ll cover them more below.

A lot of plumbers will use auto vents instead of running a wet vent or a direct vent in new home construction and remodels. It’s cheaper because it uses less pipe and quicker because you don’t have to anything but screw an auto vent.

P – Traps

A trap is a curved pipe under your fixtures but you usually only see the traps under your sink. Traps are a part of your ventilation system and they are called a trap because it actually traps water.

There are two main kinds of traps used in a mobile home: P-traps and running traps. There used to be S-traps but those aren’t used anymore.

Traps use water to close, or seal, the drainage-waste pipe so sewer smells in mobile homes don’t occur. The water seal forces the sewer smells to exit out of the roof vent instead of the fixture. Every time you run water it flushes the trap but gravity does its magic to keep some water in the lowest part of the traps.





The Reason Manufactured Homes Have so Many Vent Issues (or Why Plumbers Don’t Like Working on Mobile Homes)

Below you’ll see a photo of some plumbing that has been installed in a new manufactured home being built in a factory. It’s a brand new home with brand new plumbing and seeing it will make a master plumber with 22 years experience chuckle and walk away shaking his head.

Let me make it clear that manufactured homes only have to meet HUD code – no local codes at all. I’m sure everything in the image passes HUD code but sometimes HUD codes need to be, umm, fixed.

In the photo below, you are looking at the backside of the utility room where the washer will go in the manufactured home.

The white box is the washer box. That’s the control center for your washing machine – it’s how you connect the washer to your home’s plumbing system. You tie in the washing machine drain hose into the home’s drain system and attach the water supply lines from there. The water will flow through the p-trap and then down into the drain line. The small black thing on top of the right pipe (to the right of the P-trap) is an auto vent.

What’s Wrong with this Image?

In ‘real’ plumbing, the air vent should always be above the washer box and the washing machine drain lines would be larger to accommodate the very powerful new-age washers we have nowadays. Oh, and it’s always smart to use a real stack vent for your washing machine because that’s a lot of water leaving a large appliance very quickly.

Plumbing and electrical cables in manufactured home wall

Yes, this design will work. It has worked in millions of manufactured homes for many years. However, it’s things like this that ultimately cause an issue and make master plumbers across the nation not want to work on our homes. There’s no easy fix to this once the walls are installed.

It’s things like this that give manufactured homes a stigma they just can’t seem to shake. There are 3 systems that should never be cheaped out on: plumbing, electrical, and HVAC (mobile home heating systems and ACs). Manufactured home builders are well-known to go cheap on all three.

With all that said, let’s learn about the common cause of venting issues issues in mobile homes, how to diagnose an problem, and how to repair it.

5 Common Causes of Mobile Home Venting Issues

Venting issues in mobile homes are usually caused by four things and we’ll cover them all in-depth:

  • The venting pipe sticking out of your roof can get clogged fairly easily
  • Ventilation pipes can become disconnected during transport or settling
  • Auto vents wear out
  • Poor initial design from the factory
  • Many homeowners don’t even realize that plumbing systems have ventilation lines, or pipes, in the first place.

Clogged Vent Stacks

Vent stacks can get clogged by leaves, birds, rodents, and countless other things. To repair this you will need an auger or drain king or a snake. A snake is just a coiled metal rod that twists through the vent lines and either pushes the clogs away of pulls them out. An auger is a high-pressure water hose that pushes the clog down through the drain lines.





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.

Disconnected Vent Lines

Vent pipes can get damaged by a slew of things. Mostly, though, it’s either transportation or poor installation. Accidental cutting is also common. There are three ways to fix disconnected vent lines: find where the disconnection has occurred (which is difficult), run new vent lines to tie-in to the main stack, or hope it can be cured by an auto vent.

Worn-Out Auto Air-Admittance Valves

Auto vents, or air-admittance valves are mechanical as in they open and close every time you use water. The cheap brands (used by a lot of manufactured home builders) aren’t so great and wear out quickly. Luckily, replacing auto vents is pretty easy.

Poor Design

The fourth most common reason for venting issues in mobile homes is poor design from the factory. See the image above.

The Unknown

Lastly, but not less important, is the fact that many homeowners have no idea there is such a thing as ventilation in a plumbing system. You can’t repair something if you don’t even know it exists.

Learn How to Winterize your Mobile Home Like a Professional





Troubleshooting 6 Common Ventilation Issues in a Mobile Home

These are the most common signs that will clue you into drainage or ventilation issues.

Problem 1: Foul Odors

Smelling foul odors will be a big sign that something is wrong. You shouldn’t smell anything from your plumbing system. It’s designed to keep that very thing from happening. A foul odor can be caused by poor drainage or ventilation issues or septic tank issues. There’s a few things that can cause foul smells but it’s a sure sign that your plumbing system isn’t working properly.

Problem 2: Slow Drainage

Slow waste water drainage is one of the first signs of ventilation issues. It’s also one of the first signs that you have a clog.

Snakes and augers work if you can find the clog.

If you have an auto vent installed and that drain is slow unscrew the auto vent and see if the water drains correctly. If it does, replace the auto-vent. If it’s still draining slow it’s likely a clog or blockage and you can try a snake or auger. If those don’t work you’ll need to call a plumber to use their industrial strength snake. Those things can get a clog out of just about any pipe (except grease).





Problem 3: Gurgling

If you hear gurgling come from your drains you likely have poor drainage caused by ventilation issues. Don’t confuse there noises with noises that occur when you are running water. That’s a whole different animal.

Problem 4: Water Heater Odors

Of course, not all foul odors are caused by venting problems. Foul water odor could be coming from your water heater.

Problem 5: Missing Water in your Toilet

Nope, the dog probably isn’t drinking all the toilet water! If you find your toilet is missing its water you may have either a drainage leak or a ventilation problem.

Problem 6: 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.

Read about 5 more common plumbing problems in mobile homes.

Air Admittance Valves

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

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.

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
Installing an air admittance valve under a sink

Studor Air Admittance Valves

We highly recommend the Studor brand air admittance vents.

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 valve.

(Note: Studor does not pay us for this recommendation. After 22 years as a plumber you tend to learn what works and what doesn’t.)

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. All you do is unscrew the old one and screw in the new one. If you are installing one where there has never been one it’s a little more work.

The white pipe is drain lines and it is sloped though you may not notice it.

Q & A for Common Mobile Home Plumbing Venting Issues

Question 1 – Backups, Odors, Overflows

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?

Answer – 3 Common Issues

You are experiencing three very common plumbing issues but it’s likely caused by one thing – poor or failed venting. 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)

Questions 2: Drainage Issues in a Mobile Home

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 of 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?

Answer: Blockage or Ventilation Issues

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.

Question 3 – Noises and Sewer Smells in Mobile Homes

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

Answer: Open Sewer or Venting Issue

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? 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 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 to act as a mini-vent and help rid your home of the odor but my money is on the dishwasher drain assuming you had none of these issues until the dishwasher was disconnected.

Plumbing is a Complex Topic

It’s difficult to explain venting issues in mobile homes. Plumbing seems simple but it isn’t.

Keep in mind that a licensed plumber is absolutely necessary for your more complex drainage issues in mobile homes. You should call them if you have a serious issue or can’t find a leak (water is one of a mobile home’s biggest enemies). They have ways of testing a system and narrowing down a problem in minutes.

Thank you so much for reading Mobile Home Living®!

Please Note: Your state and local plumbing codes will always determine the exact requirements for your manufactured home’s plumbing system. This article should be used as a reference only. Mobile home venting issues is a problem that requires an on-site inspection and testing most of the time.

This article was first published on Feb 11, 2016. It was completely updated on May 9, 2019.

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