One of the primary requirements to making sure a filter system works is to ensure that it is SEALED. Research has shown that MERV 15 filter elements can be reduced to as low as MERV 6 efficiency if the seal integrity of the installed elements is not properly addressed (Ward, Siegal). All too often we find filters installed without gasket and/or clips that only prevent the elements from falling out of the holding mechanism when the unit is turned off. Using the proper clips, gasket, and tape can improve the performance of the filtration system tremendously.
Front Loading Frames
Gasket should be applied to filters-not holding frames. Like rebuilding any pump or motor the gasket should be replaced every time the filter element is replaced. Old gasket on frames breaks down or gets damaged making bypass holes for the dirt to travel around the filter element. Thus applying the gasket to the filter ensures replacement every time. Clips should apply enough pressure to filters to make them compress the gasket against the frame. Gasket thickness and pore density can be selected based upon the filter element and style of clips to make sure the depth of the clip pressure points creates an effective seal. All filters should have four clips per filter to effectively seal the filter into the framing grid. When clips are old and no longer create any compression they should be replaced. Make sure the clip compresses the gasket rather than the filter element face.
Downstream Loading Frames
Downstream loading is difficult to effectively seal. Without Gasket it is impossible. Air velocity pressure is continuously trying to push the filter element away from the framing surface and push bypass air through the openings. Gasket for this application should be closed cell material to prevent pressure degradation and have strong adhesive to maintain the seal position.
Clips used in downstream applications should be spring type clips that have tension pressure to continuously maintain compression of the gasket against the framing. Filter elements should be of durable material to prevent the clip from distorting the filter and allowing the air pressure to create bypass.
Side Access Filters
It is common for air handling units to use side access housings with elements sliding in from one side. In this style housing it can be difficult to seal properly especially when the tracks are not necessarily sized to standard size filter elements. Most cardboard framed filters are not rigid enough to use side to side gasket as the filter distorts before compressing the gasket material enough to fit the housing. In addition, if the tracks are formed metal and do not have any method of sealing the top and bottom of the filter, such as pile seal, bypass will slip over or around the filter elements. In this case tape can be the best seal method for the installation and seal of filters. Taping between filters after they are inserted and taping the top and bottom of the filter to the tracks when there isn't pile seal is not aesthetically pleasant but effective. Tape should be similar to box tape so that it does not peel after a few months of operation. When tracks are not sized for common filters there are a couple options - either use a blank that can be fixed into the track or have special size filters made for the equipment.
High Efficiency Filters
The higher the efficiency the filter the more important the seal integrity becomes. Along with efficiency also come higher operating pressures and this requires that gasket be compressed properly with a method of maintaining tension. As a rule of thumb the gasket should be compressed at least half its thickness to ensure the seal will not fail or leak. Whenever clips or retaining springs are not strong enough to maintain this pressure they should be replaced.
The operational conditions and length of filter life will potentially have an effect on gasket material. Hot dry environments can degrade the material and cause it to crumble and lose its elasticity. When this happens the gasket is compromised which allows filters to lose performance with time. Many installations of air filters are being exposed to UVC lights. In recent research, ASHRAE has found that UVC energy will degenerate most filters in time. Gasket on filters is highly susceptible to UVC degradation in most cases so it becomes important to plan where and what material is used when UVC fixtures are added to AHU equipment.