Proper ventilation is crucial for an efficient and problem-free plumbing system. There are two main types of plumbing vents used to maintain air pressure and prevent sewer gases from entering buildings: air admittance valves (AAVs) and traditional passive vents. Both AAVs and vents allow air into the plumbing system to facilitate proper drainage and avoid siphonage, but they work in different ways.
So which is better – AAVs or vents? Both have their pros and cons and are suited for different applications. This article will compare air admittance valves and vents in depth, looking at how they work, their benefits and drawbacks, key differences, and when each is most appropriate to use. Read on to learn whether an AAV or vent is the best option for your plumbing ventilation needs.
Table of Contents
Here is an in-depth comparison table contrasting air admittance valves (AAVs) and passive vent stacks:
|Point of Comparison
|Air Admittance Valves (AAVs)
|Passive Vent Stacks
|How They Work
|AAVs have an internal flap or diaphragm that opens when drainage occurs, allowing air in to prevent siphonage. The flap then closes to prevent sewer gases from escaping.
|Vent stacks have an open pipe that always allows airflow. Air enters to prevent siphonage during drainage. Sewer gases naturally rise and escape through the vent.
|Install on branch drains only, no need for penetrations through upper floors or roof. Install with specialty saddle fitting or simple pipe clamp.
|Must be tied to main drain line and run vertically through all floors to penetrate the roof. Use standard pipe, elbows, and roof flashing.
|Size based on total drainage fixture units (DFUs). Typical residential capacity is 8-10 DFUs.
|Must be sized according to plumbing code based on DFUs and drainage pipe size.
|Ideal for smaller residential buildings and retrofits. Used when penetrating roof is impractical.
|Required by code for all larger commercial buildings. Also used when reliable operation is critical.
|Can freeze closed and block airflow. Special freeze-resistant designs must be used in cold climates.
|Less prone to freezing problems, but still possible. Must prevent exterior vent pipe from collecting condensation.
|Product cost is higher, but overall installed cost is often lower due to reduced labor. No roof penetrations.
|Product cost of pipe and fittings is low. Labor cost is higher, especially for tall buildings. Roof work required.
|Contains moving components that wear out and eventually fail after 5-10 years. Maintenance is required.
|No moving parts lead to reliable operation indefinitely. Virtually no maintenance needed.
|Allows flexibility in layouts, no alignment of vent stacks between floors required.
|Consumes interior space for vent stack alignment. Restricts layout flexibility. Roof penetrations.
|Works well to prevent trap siphonage during drainage.
|Very effective at preventing siphonage.
|Sewer Gas Venting
|Mainly vents gases during drainage events when flap opens. Not effective at consistent venting like open vent pipe.
|Naturally vents sewer gases consistently and safely above roof level.
|Often prohibited by code except in limited applications. Local codes must always be checked.
|Required and approved by plumbing code for all larger buildings and as primary venting in most jurisdictions.
What is an Air Admittance Valve?
An air admittance valve (AAV) is a one-way mechanical venting device designed to allow air to enter the plumbing drainage system to prevent siphonage and backpressure. AAVs open and close based on air pressure differentials to allow air to enter while preventing sewer gases from escaping.
How an AAV Works
An AAV is usually installed on the branch drain line, replacing the need for a traditional vent stack going through the roof. Inside the valve is a diaphragm or flap that opens when there is negative pressure in the plumbing system to allow air in. This equalizes the pressure to aid water drainage.
The diaphragm or flap then closes when pressure is neutral to prevent sewer gases from venting into the building. AAVs are passive valves with no electrical or mechanical activation – the diaphragm simply responds automatically based on air pressure.
Benefits of AAVs
Compared to passive vents, AAVs offer several benefits:
- Eliminate need to penetrate roofs – No vent stacks through the roof are required when using AAVs. This makes them ideal for use in multi-story buildings.
- Reduced risk of leaks – With no vent penetrations needed through the roof, AAVs eliminate the potential for leaks from vent pipe seals.
- Flexibility in installation – AAVs can be installed in any orientation – vertically, horizontally, or at an angle. This offers more flexibility compared to passive vents.
- Prevent backflow of sewer gases – The one-way diaphragm or flap of AAVs stops gases from the sewer from backflowing into the building.
- Lower cost – In new construction, using AAVs can reduce costs by eliminating the need for vent stacks through upper floors and the roof.
Drawbacks of AAVs
However, AAVs also come with some potential disadvantages:
- Maintenance required – AAVs contain moving parts that may eventually fail. They require periodic inspection and replacement when they fail.
- Freeze risk – In extremely cold climates, the flaps or diaphragms in AAVs can potentially freeze in place, blocking airflow. Special freeze-resistant designs are needed.
- Only suitable for low-rise buildings – AAVs alone are not adequate for venting high-rise building drainage systems. Traditional vents must also be used.
- Not permitted in all jurisdictions – Some plumbing codes prohibit or limit AAV use, especially for larger buildings. Always check local regulations.
What is a Vent?
In plumbing, a vent refers to a vertical or sloped pipe that connects to the drainage system to allow air in and prevent siphonage. Vents utilize natural air pressure and airflow to maintain equilibrium in the drainage system and “vent” sewer gases out of the building safely above the roof.
How Vents Work
A vent pipe connects to the drain line and rises all the way through the building to terminate above the roof. It remains open to outside air pressure at all times. When fixtures drain, negative pressure can siphon water out of the fixture trap. The open vent allows air pressure to equalize so water drains properly.
Gases from the sewer or septic tank also drain through the vent, carrying them safely away from the building’s interior above roof level. Vent size and length must be properly engineered for the building’s drainage capacity.
Benefits of Vents
Traditional passive vents provide the following advantages:
- Reliability – With no moving parts, vents are a reliable means of ventilation that rarely need maintenance or replacement.
- Prevents siphonage – The open pipe equalizes pressure during drainage to prevent siphonage of water from fixtures.
- Vents sewer gases safely – Sewer gases naturally flow through the vent away from the building interior.
- Suitable for all building sizes – Vents are mandated by code for ventilation of larger high-rise buildings.
- Permitted by code – Vents are accepted by all plumbing codes and jurisdictions. AAVs are more limited.
Drawbacks of Vents
However, vents also come with some disadvantages:
- Roof penetrations can leak – A vent stack breaching the roof can develop leaks over time at seals.
- Blockages can occur – Birds, insects, or debris may enter and block passive vents.
- Vent freezing is possible – In very cold climates, water condensation or ice can block vents.
- More expensive for multi-story buildings – Running vents through upper floors and roof spaces adds significant expense.
- Interior space consumed – Vents require room within wall and ceiling cavities that could be used for living space.
AAVs vs Vents: Key Differences
Now that we’ve looked at how AAVs and vents work individually, let’s compare their major differences:
- Placement – AAVs install on branch drains, while vents tie into main drain lines and risers.
- Penetrations – Vents penetrate the roof but AAVs do not, installing on the pipe only.
- Sizing – Vents are sized based on drainage fixture units while AAV sizing is less complex.
- Fittings – Vents use elbows, tees, and roof flashings. AAVs use specialty saddle fittings or clamps.
- Operation – AAVs have an internal flap or diaphragm while vents are open-pipe systems.
- Leaking – Vent stack seals can leak over time but AAVs should not when operating properly.
- Blockages – Vents can be blocked by debris while AAVs only block if frozen or jammed internally.
- Mechanical parts – AAVs contain mechanical flaps that must function, while vents have no moving parts.
- Material cost – The products for venting are generally cheaper than the valve cost of AAVs.
- Labor cost – AAV installation takes less time than running full vent stacks in walls and ceilings.
- Multi-story buildings – Vents become extremely expensive for tall buildings while AAVs are simpler to install.
- Future maintenance – AAVs require eventual replacement when failed while vents should not.
- Preventing siphonage – Both vents and AAVs adequately prevent trap siphonage during drainage.
- Removing sewer gases – Vents naturally allow gases to escape, while AAVs only vent during drain events.
- Sensitivity – Vents function consistently while AAVs may stick closed with debris or ice.
- Durability – Vents last indefinitely if installed correctly, while AAV flaps wear out over time.
When to Use an AAV vs a Vent
Now that we’ve compared air admittance valves and vents, when is the appropriate time to use each method?
For new construction of smaller residential or commercial buildings under 6-7 floors, AAVs can provide a faster, lower cost installation without roof penetrations. This assumes local codes allow their use.
Retrofits and Remodels
When remodeling or renovating existing buildings and the option of adding roof vents is difficult or expensive, AAVs can provide a convenient alternative.
Most commercial building codes require traditional vent stacks, but AAVs may be allowed for some restaurant kitchens, small office buildings, etc. when permitted.
AAVs are commonly used in homes, apartment buildings up to 5-6 floors, and other small residential buildings where vent stacks are difficult to install.
What is the maximum number of AAVs that can be installed?
The maximum number of AAVs depends on the drainage fixture units (DFUs) served and plumbing code requirements. Typically 8-10 AAVs can serve a small residential building, but confirm local regulations on the limit.
Do AAVs need to be accessible for maintenance?
Yes, AAVs should be installed in an accessible location to allow inspection and replacement when failed. Installing them in wall cavities or other hidden spaces is not recommended.
Can I use both AAVs and vent stacks in the same building?
Yes, it is common to use AAVs to supplement vent stacks, such as venting branch drains with AAVs while still providing a main vent stack tied to the main drainage line.
Do I need a permit for installing AAVs?
Most jurisdictions require a plumbing permit for installation of AAVs. The applicable codes must be followed just like any other plumbing work. Never install them without checking permit requirements.
What is the expected lifespan of an AAV?
With proper installation, AAVs typically last 5-10 years before needing replacement. Hard water and debris can shorten lifespan. Higher quality AAV models may last longer.
Summary of Main Points
- Air admittance valves offer an alternative to traditional pipe vents, using a mechanical flap to open when air is needed in the drain system.
- AAV benefits include reduced costs, flexibility, and no roof penetrations, but they require maintenance and have some performance limits.
- Vents provide reliable, maintenance-free ventilation through open pipes but at higher costs for tall buildings and risk of leaks.
- AAVs are best for new construction of smaller buildings when allowed. Vents are required for larger commercial projects.
Recommendations on Choosing AAVs vs Vents
When choosing between air admittance valves or passive vents, consider code requirements, costs, building size, and reliability. Consult a qualified plumber to determine the best ventilation solution for your specific drainage system needs. Both AAVs and vents have a role in modern plumbing design when applied appropriately.