A vent stack and a stack vent are two terms that most folks used interchangeably but they shouldn’t. So, what is the difference between a vent stack and a stack vent?
A vent stack is a vertical pipe parallel to the waste/soil to which all floor drain vents are connected. On the other hand, a stack vent is the uppermost pipe section of the waste stack above the highest fixture.
Vent stacks and stack vents can be connected together at the highest levels of the house (as shown below) or each pipe can exit through the roof separately.
The main difference between a vent stack and a stack vent is that vent stacks are only used to vent the drainage system and found in multi-storied buildings while stack vents can be used for both venting and drainage and are found in single and multi-storied buildings.
In short, the vent stack is connected to the waste stack and drain stacks, and sometimes back to the waste stack via the stack vent. The drain and waste stacks channel wastewater out into the municipal sewer line or septic tank.
Vent stacks on the other hand allows sewer gases to exit the drainage system (instead of the drain lines) and also introduces air into the drainage system so that fixtures can drain fast and toilet can flush better.
The definition of a stack vent changes depending on the type of the house being vented. And that is where most people get lost in the definition.
A house can afford not to have a vent stack but it should always have a stack vent. Houses can also have both a vent stack and a stack vent.
Most residential houses in the United States (and pretty much elsewhere in the world) have a stack vent. Commercial buildings on the other hand have a vent stack and stack vent.
Do You Have/Need a Vent Stack or Stack Vent?
In the above section, I defined a stack vent as the uppermost pipe of the waste stack just above the highest fixture. Some people also define it as the bottommost part of the vent stack.
I also said that a stack vent is used for both venting and draining. But how is that possible considering that I have mentioned that it is the uppermost pipe of the waste stack above the highest fixture?
What can it drain then at that level? That is where the confusion is for most people and I will try to explain it.
A house that has both a vent stack and a stack vent is a multi-storey house while a single-storey house will only have a stack vent.
In a single-storey house, all fixtures are usually at the same level. In this case, a vent stack will not be needed and that brings us back to the definition of a vent stack.
As I mentioned, a vent stack is a section of vertical pipes which are installed parallel to the waste stack to vent the drainage system. That means that on one hand you have vertical sections of pipes where all fixtures drain lines are connected to at different levels.
On the other hand, you have a vertical section of pipes connected to the bottom of the waste stack and running through the roof of the house, or connected back to the waste stack via the stack vent where then only a single pipes exits the house’s roof.
A vent stack is very important in ensuring that atmospheric pressure is maintained through the drainage system. What would happen if we only used a stack vent to vent a multi-storied building?
Assume you have a bathtub at the highest floor level of the house and you have toilets and sinks on the lowest levels of the house.
Now, if you do not already know it, sinks have P-traps (U-bend underneath the sink) that is always full of water. Just like the water at the bottom of the toilet bowl.
The water creates a barrier, preventing sewer gases from coming up into the bathroom or entire house. More on that in this post.
If you attempt to drain water from the bathtub, since the bathtub holds a large volume of water, the wastewater will fill up the waste stack meaning that at that point the fixtures on the lower levels will not be vented.
As the wastewater drains out, it will siphon water from the P-traps of the fixture in the lower levels due to creation of negative air pressure and as a result sewer gases will exit the drainage system and enter your house without any restriction.
And that is where a vent stack and a stack vent come in. The vent stack is installed parallel to the waste stack where each fixture can be vented through.
By using a vent stack, air will be introduced into the drainage system (remember the vent stack runs through the roof of the house) and therefore balances the pressure avoiding the creation of a negative air pressure.
A stack vent in this case as I mentioned would be the section of pipe at the uppermost part of the waste stack above the highest fixture.
A vent stack can exit through the roof independently or it can be connected back to the stack vent. Both of those connections are code-compliant.
If you therefore see 2 vents at the top of your house, just know that one is vent stack while the other one is the stack vent.
Plumbing Vent vs Vent Stack vs Stack Vent
Now that we know the difference between a stack vent and a vent stack, what is the difference between them and a plumbing vent?
Well, both vent stacks and stack vents are plumbing vents. They serve the same function, which is to introduce air into the drainage system and allow sewer gases to exit the system.
For toilets to flush strongly and sinks, tubs and showers to drain fast, there has to be air at atmospheric pressure inside the drain lines.
Pressure build up inside drain lines results in slow draining fixtures while negative air pressure in drain lines results in siphoning of water from drain traps.
Whenever you have a clogged plumbing vent, you can hear you bathtub/shower drain gurgling when the toilet is flushed. That is as a result of the water in the shower/tub drain trap being siphoned out by the negative pressure created by the draining toilet.
Also a clogged plumbing vent will result in sewer gases accumulating inside the drain lines and when they can’t take it any longer the gases will be forced out through the drains.
A good sign of the above is a bubbling toilet when it is flushed or when the shower, bathtub or sink is draining.
And basically that is the difference between a vent stack and a stack vent. I hope this guide was more helpful than confusing.