Using two air filters at the same time is a configuration that exists for a variety of reasons. The lifespan of filters varies, so the number of times it is necessary to replace the air filters in the return duct compared to the air controller varies due to their different sizes. Technically, both components can be referred to as “ventilation grilles” since they are both places where air circulates in your home. The key difference is that the intake grilles “draw air out of the room” and the duct grilles “push” the air conditioner into the room.
Visually, you can usually tell the difference since the entrance grilles are larger and less numerous than those for the ducts, which are often found in every room in the house. Stacking two pleated filters is not recommended as it will increase static and affect system performance. As for stacking two disposable filters, it would probably be fine for the system, but it seems a bit unnecessary. The cabinet may be configured to accept either a one- or two-inch filter.
To insert a two-inch size, you would have to remove the side tracks. This was likely easy at the time of installation. A single good quality Merv 8 filter is sufficient. Stacking filters is not recommended as it is likely to cause the system to malfunction, wear out more quickly, or work less efficiently.
The main filter, which is closest to the air controller, does all the work and collects debris just before it collides with the fan motor. Experiments with several Merv filters 1, 2 and 4 from low to high were conducted and it was found that they managed to get their ovens running at 1600 CFM with the cheap Merv 2 fiberglass filter and, while using a Merv 12, they reduced the CFM to 1500. In many cases, the first filter was a looser fabric that trapped larger particles and the second one trapped smaller particles. There are several types of HVAC units and each may require a different installation of the filter.
This not only helps cleanse the air that circulates throughout your home but also helps your central air unit operate efficiently. A MERV rating of 8 means that the filter has a minimum efficiency of 70% in capturing particles with a size of 3.0-10.0 microns. Interestingly, while filter manufacturers are pushing filters with a high MERV, most experienced HVAC professionals call them oven killers. I think there would be a difference between stacking two filters versus using one filter as two layers will provide more filtration than one layer alone. Upon closer inspection, there are rails to accept and securely hold a one-inch filter which looks like they would also firmly hold a two-inch filter. Two filters would increase rather than decrease air noise and it is likely that people think doubling up on filters will increase filtration or extend life.
Because air controller filters have more filter material due to their larger size, they can collect and retain more dust, pollen and smoke before needing to be replaced. In conclusion, using two air filters at once is not recommended as it can cause your system to malfunction or work less efficiently than if you were using just one good quality Merv 8 filter.
