HEPA filter for furnace floor vent?


Has anyone seen pleated HEPA filter that fit into floor register vent?
I froogled but no luck. Maybe nobody makes such product.
The closest I found are sheets filters like this: http://www.webproducts.com/Detail.bok?no1
What I need is HEPA that filters not just dust but finer particles that causes allergy that somehow got into the HVAC ducts. There is a furnace filter at the intake duct but that does not help remove particles coming out of the vents.
Of all the different hepa filters made in this world (some for vacuum cleaners, some for air purifiers, etc), perhaps there is one that happens to fit in a 3 7/8 inch x 11 5/8 inch rectangular space (inside dimension of a vent register)?
Any ideas?
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You would severely restrict airflow, restricting airflow to much and the temp of the heat exchanger can easily go above the design limit your manual states and kill its life. Even shutting down a few registers can raise temps to much. What kind of filter do you have, the 4" Air Bear is well designed. Dust isnt made from the furnace it comes from your lifestyle.
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responding to http://www.homeownershub.com/maintenance/hepa-filter-for-furnace-floor-vent-424415-.htm Gravedouger wrote: Well i have an idea if all the air goes through one filter take out the furnace filter and put the register filters in then the air is all going through one filter again.I'm sure an Engineer would disagree but i live alone got and old furnace and i ain't skeered lol i'll try it if it blows it blows what a ride! ransley wrote:

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james wrote:

If there is noticeable particulate coming out despite having a good filter on the intake for a reasonable amount of time, you likely have a leak somewhere in the return ducting that is allowing dust to come in and bypass the intake filter. This will also clog up your A/C "A" coil if applicable as well as reducing the efficiency of your heat exchanger.
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Pete C. wrote:

How can a return air bypass intake filter whether leaking or not? Filter is right at the base of furnace?????
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Tony Hwang wrote:

In some installations the filter is right at the base of the furnace, but in a lot of installations the filter is remoted to the intake register where it is easier to service.
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