A misconception on how air admittance valves (AAVs) and carbon filters are used has led to Studor setting out the differences between the two. Deborah Andrich takes an introductory AAV class. 

“Do you have a carbon filter air admittance valve,” said a customer to Studor area sales manager Grant Weymouth.

And just like that, a can of worms was opened.

“We have an air admittance valve (AAV) or an active carbon filter, there is no such product as a carbon filter AAV. What exactly is the plumber trying to do?” asked Grant.

The response was that a supermarket’s plumbing design specified AAVs to be installed to ventilate over distance branch lines on a grease trade waste system, which is normal procedure. However, the peer review of the design deemed that carbon filters should also be used, which according to Grant isn’t necessary.

So what is the difference between an AAV and a carbon filter?

Essentially, an AAV is a one-way valve to allow air into a drainage system but prevent sewer gases from escaping. By using an AAV, it allows the pressure differentials between air and water in the pipe to find a happy balance. When a drain system is used (for example, water is flushed) it can create a negative pressure. The AAV allows air to enter to regain normal pressure, without letting any odour out. The best example of negative pressure is the ‘glup, glup, glup’ noise a sink makes when the system is trying to suck air through the water seal. If this negative pressure is not balanced quickly, it may lead to the depletion of this trap seal, in turn releasing sewer gases into living areas.

An active carbon filter – a Maxi-Filtra is the Studor version – is a two-way vent that lets air in and out of a drainage system to help balance negative pressure issues and the release of positive pressure. The carbon filter is the component that absorbs the foul odours. Most of the time, according to Grant, the filters are used with septic tanks and where traditional venting may be physically challenging or cost prohibitive, or under sink pump outs in bathrooms, laundries and kitchens where an AAV won’t work.

“An AAV is a ‘set and forget’ device, it requires no servicing, adjustment or maintenance, and they may be used on stack or drainage systems,” says Grant.

“A carbon filter on the other hand does attract maintenance, as the carbon filter itself has a fi nite lifespan and needs to be regularly replaced so as to ensure no foul odours are escaping to atmosphere.”

In homes for under sink pump outs, a filter might last as long as two years and can be easily examined and replaced by a plumber. However, in the supermarket example, the filters may only last a month or two before requiring a new filter. To achieve that means that the filters would need to be placed on the maintenance schedule – it adds unnecessary labour and cost.

“The other point to keep in mind is the carbon filter airflow. The maximum airflow passed by a Maxi-Filtra is about 5L/s, so if the pipe has an airflow of 10L/s, then this would require more than one unit to be installed to ensure airflow requirements are met.”

A good example of the use of a carbon filter is in New Zealand where the Maxi-Filtra was used for a heritage and cultural centre for a Maori tribe at Bay of Plenty on the North Island. The brief was to comply with stringent criteria under ‘The Living Building Challenge’, which defines the most advanced sustainability measures for the built environment. Mindful of the specification, filters were installed to ensure no escape of bad odours from the septic tanks. Installed either vertically or horizontally, the filters act as a two-way vent, to filter air in both directions. Given that the building was based on a septic system, this was seen as the most suitable solution to any foul odour issues that may arise.

“Having said that, carbon filters can be retrofitted into a system with AAVs if it is absolutely necessary,” says Gary.

“But they are not a replacement for an AAV. It should be reiterated that the filters are effectively a consumable item and will require ongoing maintenance.”